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Remote Detection of Hydrogen Leak Using Nd:YAG Pulsed Laser Induced Dual Line Detection Rayleigh Light Scattering

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PAGE 1

REMOTE DETECTION OF HYDROGEN LEA K USING Nd:YAG PULSED LASER INDUCED DUAL LINE DETECTION RAYLEIGH LIGHT SCATTERING By VENKATA SURYA RAGHURAM VEMPATI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

PAGE 2

Copyright 2005 by Venkata Surya Raghuram Vempati

PAGE 3

ACKNOWLEDGMENTS I would like to thank Dr. Jill Peterson for her support and guidance. I would also like to thank my fellow students (Sameer Paranjpe, Philip Jackson, Murray Fisher, Mathew Gabriel and Ryan Ferguson) for their assistance in various portions of the project. iii

PAGE 4

TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................iii LIST OF TABLES .............................................................................................................vi LIST OF FIGURES ..........................................................................................................vii LIST OF ABBREVATIONS AND SYMBOLS ..................................................................x ABSTRACT .....................................................................................................................xiv CHAPTER 1 INTRODUCTION........................................................................................................1 Project Goal..................................................................................................................1 Rayleigh Light Scattering.............................................................................................1 2 LITERATURE SURVEY.............................................................................................4 3 THEORETICAL BACKGROUND..............................................................................6 Light Scattering............................................................................................................6 Mechanism of Elastic Scattering...........................................................................7 Mie scattering.................................................................................................8 Rayleigh scattering.......................................................................................10 Angular scattering cross-section..................................................................12 Dual Line-detection Technique..................................................................................13 Buoyant Jets and Plumes............................................................................................15 4 EXPERIMENTAL METHODS AND APPARATUS...............................................17 Gas-flow Path......................................................................................................20 Varying Glare......................................................................................................20 Design of Plate Position..............................................................................................20 Experimental Methods................................................................................................31 Integrated Area Method.......................................................................................31 Peak Voltage Method..........................................................................................33 Peak Area Correlation.........................................................................................35 iv

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5 RESULTS AND DISCUSSION.................................................................................37 Numerical Results.......................................................................................................37 Experimental Results..................................................................................................38 Testing the Linearity of the Photomultiplier-tube Output Voltage.....................38 Eliminating Glare from the Rayleigh Light Scattering Signal............................42 Reference value method...............................................................................42 Assuming that the ratio of the glare at the two wavelengths is constant.....43 Theoretical and Experimental Photon-arrival Rates............................................45 Theoretical photon-arrival rate.....................................................................45 Experimental photon-arrival rate.................................................................46 Analysis of the Recorded Data............................................................................48 Raw photomultiplier-tube voltage variation................................................48 Variation of glare as a function of radial and plate position........................52 Variation of Rayleigh light scattering signal as a function of radial and plate position....................................................................................................53 Detection limits............................................................................................56 Correlation studies........................................................................................59 6 CONCLUSIONS AND RECOMMENDATIONS.....................................................61 LIST OF REFERENCES...................................................................................................63 BIOGRAPHICAL SKETCH.............................................................................................65 v

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LIST OF TABLES Table page 4-1 Percent variation in average peak voltage for 355 and 532 nm...............................35 5-1 Optical density and the corresponding transmission of neutral density filters.........41 5-2 Photomultiplier-tube voltages before and after using the 0.6 neutral density filter.42 5-3 Scattering cross-sections of helium, nitrogen, hydrogen and air at 532 nm and 355 nm.............................................................................................................................46 vi

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LIST OF FIGURES Figure page 3-1 Raman Scattering.....................................................................................................7 3-2 Creation of an induced dipole moment by an electric field.....................................8 3-3 Mie scattering geometry..........................................................................................9 3-4 Rayleigh scatterer is very small compared to the wavelength of the incident electromagnetic wave.............................................................................................11 3-5 Instantaneous and time averaged profiles of a typical buoyant jet........................16 4-1 Experimental setup used for collection of scattered light at 90 o ............................17 4-2 Mounting of nozzle, gas-flow meters and position of aluminum plate.................18 4-3 Collecting lens of radius R C and having focal point at (0,0).................................22 4-4 Ray tracing of the reflected rays from the aluminum plate at the collecting lens.23 4-5 Ray tracing of the reflected rays from the collecting lens at the focusing lens.....24 4-6 Solid angle subtended by the reflected rays from the aluminum plate..................25 4-7 Ray tracing output from MATLAB when the aluminum plate is directly over the nozzle.....................................................................................................................27 4-8 Ray tracing output from MATLAB when the aluminum plate is 3 cm from the nozzle.....................................................................................................................28 4-9 Ray tracing output from MATLAB when the aluminum plate is 7 cm from the nozzle.....................................................................................................................29 4-10 Glare for different plate positions as a function of number of rays.......................29 4-11 Glare as a function of plate position and wavelength............................................30 4-12 Typical waveform from a photomultiplier tube.....................................................31 4-13 Running average of area........................................................................................32 vii

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4-14 Percent variation in area.........................................................................................33 4-15 Running average of peak voltage...........................................................................33 4-16 Percent variation in peak voltage...........................................................................34 4-17 Correlation between peak voltage and area...........................................................35 5-1 Signal-to-glare ratio as a function of plate position...............................................37 5-2 Comparison of experimental and theoretical glare................................................38 5-3 Photomultiplier-tube and the photodiode voltage as a function of the power meter voltage....................................................................................................................39 5-4 Photomultiplier-tube voltage as a function of the power meter voltage when the glare is varied using an aluminum plate................................................................40 5-5 Theoretical photon-arrival rates.............................................................................46 5-6 Comparisons of theoretical and experimental photon-arrival rates.......................47 5-7 Percent error between theoretical and experimental photon-arrival rates.............48 5-8 Raw Photomultiplier-tube voltages as a function of radial and plate position for 532 nm...................................................................................................................49 5-9 Raw Photomultiplier-tube voltages as a function of radial and plate position for 355 nm...................................................................................................................51 5-10 Percentage errors in raw Photomultiplier-tube voltage for two typical plate positions as a function of radial position...............................................................51 5-11 Glare as a function of radial and plate position for 532 nm...................................52 5-12 Glare as a function of radial and plate position for 355 nm...................................53 5-13 Rayleigh light scattering signal as a function of radial and plate position for 532 nm..........................................................................................................................54 5-14 Rayleigh light scattering signal as a function of radial and plate position for 355 nm..........................................................................................................................55 5-15 Signal-to-glare as a function of radial and plate position for 532 nm and 355 nm56 5-16 Raw Photomultiplier-tube voltages as a function of radial and plate position for 532 nm and 355 nm................................................................................................57 viii

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5-17 Rayleigh light scattering signal as a function of radial and plate position for 532 nm..........................................................................................................................58 5-18 Rayleigh light scattering signal as a function of radial and plate position for 355 nm..........................................................................................................................58 5-19 Correlation of Rayleigh light scattering signal between two wavelengths............59 5-20 Variation in R 2 value as a function of plate position.............................................60 ix

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LIST OF ABBREVATIONS AND SYMBOLS Symbol Description a Particle radius (m) *A Area of the back ground surface visible to the detector(m 2 ) A Area under the voltage time curve c Speed of light (m/s) C Optical system calibration constant C Surface scattering parameter C PMT Photomultiplier tube calibration constant dV Control volume (m 3 ) d p Diameter of Rayleigh scattering particle e Electronic charge = 1.602E-19 Coulombs E 1 E 2 E 3 Energy states of a molecule. E Electric field vector B Magnetic field vector S Scattering field h Plancks constant = 6.626E-34 Js I Intensity (photons/m 2 -pulse) I o Incident laser power (photons/ m 2 -pulse) k Boltzmann constant (Joule/Kelvin) x

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n Refractive index of the gas n Number of moles N Molecular number density (molecules/m 3 ) *N Number of molecules N A Avogadros number P Pressure (N/m 2 ) r Radial distance from jet centerline (m) plr Distance between the collecting lens and the aluminum plate Re Reynolds number = vD/ R Universal gas constant (8.314 J/ mol K) R Ratio of the reflectivity at 532 nm and 355 nm R C Radius of the collecting lens (m) R p Photon arrival rate (photons/m 2 -pulse) S Ratio of the scattering cross-sections at 532nm and 355 nm T Temperature of gas Kelvin) v Velocity of buoyant jet (m/s) V Photomultiplier tube voltage (V) x Percentage of leaking fluid (80% nitrogen and 20% helium) in the control volume. z Downstream distance (mm) Greek Symbols Size parameter = 2a/ Spread angle xi

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Angle of observation measured from the forward to scattering directions. Scattering angle Wave function Optical efficiency of transmitting and collecting lenses Uncertainty d Solid angle of the collection optics Wavelength of laser light (nm) Scattering cross-section (m 2 ) Reflectivity Volume (m 3 ) Dynamic viscosity of gas (Pa-s) Frequency (Hz) t Time interval between two readings(s) Subscripts 1 532 nm wavelength line 2 355 nm wavelength line a Ambient act Actual air Air avg Average lensCollecting Intensity of the reflected light at the collecting lens FWHM Full width half maximum xii

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Focusing Lens intensity of light at focusing lens Glare Scattering intensity due to Glare i Species inc Incident leak Scattering intensity due to leaking fluid(80% nitrogen and 20% helium) min Minimum max Maximum Mie Scattering Mie scattering PMT Intensity of light at the photomultiplier tube r1 Energy state 1 r2 Energy state 2 Rayleigh Rayleigh reflected Intensity of the reflected light RLS Rayleigh light scattering ref Reference condition scat Scattering xiii

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science REMOTE DETECTION OF HYDROGEN LEAK USING Nd:YAG PULSED LASER INDUCED DUAL LINE DETECTION RAYLEIGH LIGHT SCATTERING By Venkata Surya Raghuram Vempati May 2005 Chair: Jill Peterson Major Department: Mechanical and Aerospace Engineering Our primary objective was to use laser induced Rayleigh light scattering to detect the presence of hydrogen leaks in the presence of high amounts of glare. A mathematical code in MATLAB was written to design the glare system and to compute the glare numerically. Experimental and numerical results corresponded well with a maximum error of 5%. Pure helium and a mixture of 20% helium and 80% nitrogen were used to simulate the hydrogen leak. The mixture of helium and nitrogen is used because the scattering cross-section of the mixture of 20% helium and 80% nitrogen is equal to that of hydrogen. The scattering cross-section of helium was 0.015 times that of air and the scattering cross-section of hydrogen was 0.23 times that of air. Major problems in using the Rayleigh light scattering as a diagnostic tool are uncertainty due to electronic shot noise and glare from the background surfaces. The uncertainty due to electronic shot noise was found to be less than 0.1% for an averaging time of 0.001 sec. The Nd:YAG pulsed laser (operating at wavelengths of 532 nm and 355 nm) was used. Intensity of the xiv

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scattered light due to the helium and nitrogen molecules in the jet was measured using a photomultiplier tube. Data were acquired using a high-speed digital oscilloscope. An aluminum plate was used to vary the glare. The ratio of the minimum to the maximum intensity of scattering light was 0.2 when a mixture of helium and nitrogen was used (for measurements at a downstream distance of 4 nozzle diameters). When pure helium was used (for measurements at a downstream distance of 8 nozzle diameters) the ratio of the minimum to the maximum intensity of scattered light was 0.14. It was possible to detect the hydrogen leak even with glare-to-signal ratios as high as 6:1. xv

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CHAPTER 1 INTRODUCTION Project Goal We designed and tested an experimental setup to measure hydrogen leaks in a buoyant jet in the presence of high amounts of glare or low signal-to-glare ratios. We used an Nd: YAG pulsed laser induced dual line-detection Rayleigh light-scattering technique to eliminate the glare. Rayleigh Light Scattering Rayleigh scattering is an attractive technique for non-intrusive measurements with high spatial and temporal resolution of gas-flow properties (such as density, temperature, concentration in a mixture of gases and velocity in the case of high-speed flows). All of these experiments measured the intensity of the scattered Rayleigh light to determine the property of interest. All of these applications required a laser beam passing through the gas. The laser beam is elastically scattered by the gas molecules when the incident electromagnetic wave reacts with the dipoles in the gas molecules and the light beam is scattered in all directions. Rayleigh light scattering is easy to set up compared to other scattering techniques (such as Raman scattering). The main difficulty of Raman scattering is the low scattered signal intensity, which requires relatively long integration times for adequate signal-to-noise ratios and requires very low background-flame radiation. The Rayleigh scattering cross-section is about 1000 times larger than the vibrational Raman scattering resulting in much larger intensity 1

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2 Two commonly encountered difficulties associated with Rayleigh scattering are the contamination of the scattered signal with background noise and scattering from particulates present in the gas (which is known as Mie scattering). Background noise comes from two sources: the surface-scattered laser glare; and the light from the test environment (which usually is broad band). Because the Rayleigh scattered light from the control volume is at the same frequency and wavelength as the laser beam, it is difficult to discriminate the surface scattered light from the Rayleigh signal. This problem is intensified when measurements are taken in closed enclosures. Glare can be controlled principally by blackening the surfaces. Our study used a dual line-detection technique to address the problem the background noise. We used an Nd: YAG pulse laser with a repetition rate of 10Hz. The signal was collected at two laser lines of wavelengths 532 nm and 355 nm with the energy of the laser being 200 mJ/pulse and 60-95 mJ/pulse at 532 nm and 355 nm respectively. Using the pulse laser provides a high level of Rayleigh signal because of the high energy densities at each pulse. The signal obtained from the two lines is analyzed simultaneously to eliminate the glare from the Rayleigh signal The dual line-detection technique greatly enhances the scope of the Rayleigh scattering technique as a measurement tool since it completely eliminates background noise from the Rayleigh signal; thus improving the Rayleigh signal. The system was tested and calibrated at various levels of background noise. Finally, measurements were taken using a mixture of helium and nitrogen issuing out of a nozzle. Changes in concentration were measured by traversing the nozzle in the radial direction (to and from the centerline); thereby changing the concentration of gas due to entrainment of air. Results indicate that accurate

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3 measurements are possible with the dual-line-detection technique in the presence of high levels of background glare. Our work is the first step towards the remote detection of hydrogen leaks.

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CHAPTER 2 LITERATURE SURVEY Lord Rayleigh was the first person to systematically observe light scattering, in 1871. The basics of Rayleigh and Mie scattering theory have been described (Van De Hulst, 1957, Kerker, 1969, Bohren and Huffman 1983). They gave the general solution of the Maxwell equations that describes independent scattering of incident electromagnetic waves by an isolated sphere. The solution is termed Mie scattering theory; and is a complicated series solution. For particles that are small compared to the wavelength of the incident light (r 0.03), the Mie series simplifies; and is termed Rayleigh scattering. The initial term in the series is dominant and is sufficient to express the intensity of the scattered light. McCartney (1976) gave a good description of Rayleigh light scattering. Laser-induced Rayleigh light scattering has been used for many purposes in the past. It has been used successfully for temperature measurements (Pitz et al. 1976; Dibble, Hollenbach and Rambach 1980; Bill et al. 1981). Pitts and Kashiwagi (1984) used Rayleigh light scattering to study turbulent mixing. (Namer and Scheffer, 1985; Namazina, 1989) used Rayleigh light scattering for combustion studies. Rayleigh light scattering has also been used to obtain temperature measurements in heated buoyant jet (Otugen and Namer (1988)). Otugen (1993) used a dual line-detection technique to simultaneously determine the glare and remove it from the Rayleigh signal of interest. Pitts, and Bryner, (1992) used Rayleigh scattering for investigation of free jets and 4

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5 plumes. It has also been used for temperature measurements in rapid thermal chemical vapor deposition reactor by J.E. Peterson and J.F.Horton, (1998).

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CHAPTER 3 THEORETICAL BACKGROUND Light Scattering Scattering is a process by which a particle in the path of an electromagnetic wave continuously abstracts energy from the incident wave and reradiates that energy into the total solid angle centered at that particle. For scattering to occur it is necessary that the refractive index of the particle be different from the surrounding medium. There are two types of light scattering mechanisms: elastic scattering and inelastic scattering. Inelastic scattering is also called Raman scattering and in this type of scattering the frequency of the scattered wave is different from that of the incident wave and there is a change in the energy of the incident wave. The molecule may either gain energy from, or lose energy to, the photon. In Figure 3.1, 3 energy states of the molecule are shown (E 1 E 2 and E 3 ). The molecule is originally at the E 2 energy state. The photon interacts with the molecule, exciting it with an energy h inc However, there is no stable state of the molecule corresponding to this energy, and so the molecule relaxes down to one of the energy levels shown. In doing this, it emits a photon. If the molecule relaxes to energy state E 1 it will have lost energy, and so the photon emitted will have energy h r1 where h r1 > h inc These transitions are known as anti-Stokes transitions. If the molecule relaxes to energy state E3, it will have gained energy, and so the photon emitted will have energy h r2 where h r2 < h inc These transitions are known as Stokes transitions. 6

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7 Figure 3-1. Raman scattering Mechanism of Elastic Scattering Consider an elemental scatterer such as a gas molecule in the path of an electromagnetic wave. The gas molecule can be considered as a mechanical oscillator of unequal masses carrying opposite charges at the center and the periphery according to McCartney (1976). The elastic scattering theory assumes that the molecule is isotropic, non polar and non-ionized. These assumptions mean that the molecule does not experience a net force in an electric field and the negative charge is uniformly distributed at the periphery and may be treated as though it were at the center. This means there is a negative charge at the center with equal positive charge. Hence the net dipole moment, which is equal to the product of the charge and the separation distance, is equal to zero. But when the molecule is subjected to external electric field of an electromagnetic wave, the charges are forced apart (Figure 3-2) and an induced dipole moment is created. This induced dipole moment oscillates synchronously with the field and emits a secondary wave with the same frequency as that of the incident electromagnetic wave and this secondary wave is the scattered wave. An oscillating dipole emits electromagnetic waves

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8 because it contains oscillating electric currents, and because the changing positions of the positive and negative charges make it harder for their electric effects to cancel. This elastic scattering of light can be explained by using two theories. -e Displacement Direction of electric field +e Shell of negative charge Figure 3-2. Creation of an induced dipole moment by an electric field which displaces the plus and minus charges of the molecule (Source: McCartney, Earl J., Optics of the Atmosphere, 1976, Figure 4.2, pp. 179-181, New York, John Wiley & Sons.) Mie scattering Scattering by particles of arbitrary size is called Mie scattering and is discussed in detail by Kerker (1969). Starting with very small particles, as the particle size relative to the wavelength increases, there is a gradual transition from Rayleigh to Mie scattering, which is characterized by A complicated dependence of scattered light intensity on the angle of observation, the complexity increasing with particle size relative to wavelength. An increasing ratio of forwarding scattering to backscattering as the particle size increases. Little dependence of scattering on wavelength when particle size relative to wavelength is large.

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9 Mie scattering has no size limitations and converges to the limits of geometrical optics for large particles. Mie theory may therefore be used for describing most spherical particle scattering systems (including Rayleigh scattering). However, Rayleigh scattering theory is preferred if applicable, due to the complexity of the Mie scattering theory. The criteria for Rayleigh scattering is 1 where is the dimensionless size parameter given by Equation 3-1, where is the spherical particle radius, and a is the scattering wavelength a2 (3-1) Figure 3-3 shows the spherical coordinate scattering geometry used for Mie light scattering for light incident on a single particle. Using this coordinate system, the scattering parameters may be defined as follows. Figure 3-3. Mie Scattering Geometry. (Source: http://plaza.ufl.edu/dwhahn/Light%20Scattering%20Theory.pdf Last accessed December 5 th 2004).

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10 For each scattering angle ( ), the Equations (3-2) and (3-3) represent the intensities of scattered radiation vertically and horizontally polarized with respect to the scattering plane, which is defined by the incident and scattered ray, 21222sin4i r IIo (3-2) 22222cos4i r IIo (3-3) In this formulation, Io is the incident intensity, and the intensity functions i 1 and i 2 given by Equations 3-4 and 3-5. 211coscos)1(12nnnnnbannni (3-4) 212coscos)1(12nnnnnbannni (3-5) Where n and n are the angular dependent functions and are expressed in terms of Lengendre polynomials by sin)(cos)(cos)1(nnP (3-6) ddPnn)(cos)(cos)1( (3-7) The constants a n and b n are obtained from the boundary conditions that the tangential components of the electric field and magnetic field of the incident wave are continuous over the entire surface of the sphere. Rayleigh scattering The Mie solution is a complex mathematical solution and for particles of size much less than the wavelength of the incident light, the Mie solution converges in one term and

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11 is called Rayleigh theory and is discussed in detail by McCartney (1976). Rayleigh scattering is elastic scattering of an electromagnetic wave by particles far smaller than the wavelength of the incident wave (Figure 3-4). The electromagnetic radiation is scattered in all directions different than that of the incoming light. pd p d Figure 3-4. The size of a Rayleigh scatterer is very small as compared to the wavelength of the incident electromagnetic wave Lord Rayleigh assumed that the particles were spherical, isotropic, far smaller than the wavelength of light, and denser than the surrounding medium. He showed through simple reasoning that the scattering varies as the square of the particle volume and inversely as the fourth power of wavelength of light. Rayleigh scattering is marked by several characteristics: The amount of light scattered varies nearly as the inverse fourth power of the wavelength. Spatial distribution of scattered light bears a simple relationship to the direction of observation. The light scattered at 90 degrees is almost completely polarized.

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12 In the domain of light scattering, the concepts of angular and total scattering cross-sections are basic concepts. These concepts lead to several coefficients and expressions having great practical utility. Angular scattering cross-section The angular scattering cross-section of a molecule is defined as that cross-section of an incident wave, acted on by the molecule, having an area such that the power flowing across it is equal to the power scattered by the molecule per steradian. The angular scattering cross-section represents the ratio of the scattered intensity to the incident irradiance. 4222)1(4)90( N no (3-8) This expression is derived for an ideal gas assuming that n-1<<1. The refractive index term, n-1, is proportional to the molecular number density. Therefore, the scattering cross-section for an individual molecule is not a function of N, and is independent of gas temperature, pressure and density. For a volume of gas, however, the amount of scattered light is proportional to gas number density. The random spacing and thermal motion of gas molecules are such that scattering is incoherent and independent. The result is that there are no discernable phase relationships between the separately fluxes except in the exact forward direction and thus the individual intensities are additive. The scattered intensity is therefore, is directly proportional to the molecular number density. ooscatoNII)90()90( (3-9) The constant of proportionality is defined by the solid angle of the collection optics, the length of the control volume and the optical efficiency of the collection optics.

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13 Hence the intensity of the Rayleigh scattered light from a control volume containing a mixture of gases is given by ioRLSNdVdII)())()(( (3-10) Dual Line-detection Technique The major problem in using Rayleigh light scattering as a diagnostic tool is background glare and Mie scattering. Mie scattering due to dust and aerosols can be reduced by filtering the incoming air. To eliminate the glare from the Rayleigh signal, dual line-detection technique is used in this project. It involves obtaining the scattered light intensity using two different lines of the laser and solving the set of simultaneous linear equations to eliminate the glare from the Rayleigh signal. It is known that the Rayleigh signal is proportional to the incident laser intensity and the gas number density and also to the scattering cross-section, where C is the optical system calibration constant and N is the gas number density. ooRLSoNICI9090 (3-11) The gas number density is the ratio of the number of molecules to the volume occupied by them. It can be written mathematically as *NmoleculesthebyoccupiedVolumemoleculesofnumberN (3-12) Where ANnNumbersAvogadrosmolesofnumberN*)')((* (3-13) From the ideal gas law it can be noted that the gas number density can be replaced by the pressure, temperature and Boltzmann constant. The ideal gas law is given by Equation 3-14, where n is the number of moles and R is the universal gas constant (8.3145 J/mol K).

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14 TRnP (3-14) Replacing n with gas number density in Equation 3-14, ideal gas law can be written as NkTP (3-15) Where is Boltzmann constant and is defined as k ANRk =1.38066E-23 J/K (3-16) N A = Avogadros number = 6.023E23 Hence the intensity of the Rayleigh signal can be written as oRLSIkTPCI (3-17) The scattered light intensity detected by the photomultiplier-tube is the sum of the Rayleigh signal and the glare. Hence the intensity of the scattered light incident on the photomultiplier-tube can be written as GlareRLSscatIII (3-18) GlareI is proportional to the incident intensity and the reflection from the background surfaces. LensoGlareAICI* (3-19) Where C is the surface scattering parameter and is the area of the background surface visible to the detector. Hence the scattered light intensity can be written as *A )()(*LensooscatAICkTPCII (3-20) The scattered intensity can be normalized with the incident intensity to account for the pulse to pulse variation in the incident intensity of the laser. Hence the scattered light intensity can be written as

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15 LensscatACkTPCII*0 (3-21) Since two different lines of wavelengths 532 nm and 355 nm are used, the intensity of the scattered light at both these wavelengths can be written as LensoscatACkTPCII11*111 (3-22) LensoscatACkTPCII22*222 (3-23) Hence by varying either the pressure, temperature or the scattering cross-section individually or a combination of any of the three parameters, the intensity of the scattered light can be obtained at different conditions and then by solving the simultaneous set of equations using linear regression the glare can be eliminated from the Rayleigh signal. The detailed description of how the glare is eliminated is given in Chapter 5 Buoyant Jets and Plumes The primary objective in this project is to detect a hydrogen leak. For this purpose a mixture of helium and nitrogen are used that issue out of a nozzle and since the densities of both helium and nitrogen are different from the surrounding ambient air, buoyancy forces arise in the jet. A fluid motion is called a jet if its primary source of kinetic energy and momentum flux is a pressure drop through an orifice. A fluid motion whose primary source of kinetic energy and momentum flux is body forces is called a plume and flows whose motion is in transition from a jet to plume are called forced plume or a buoyant jet. For a jet, due to conservation of momentum, the momentum flux is constant along the jet axis. Since the mass flux increases along the jet axis due to entrainment, the axial

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16 velocity must correspondingly decrease. For a plume, the buoyant force tends to accelerate the fluid in the plume in a vertical direction, turning the plume axis in the direction of the buoyant force. For a plume initially discharged upwards, and with an upwards buoyancy force, the deceleration (caused by entrainment) is hence less than for the case of a jet. Figure 3-5 shows the instantaneous and time averaged profiles of a buoyant jet. Figure 3-5. Instantaneous and time averaged profiles of a typical buoyant jet Inside the potential core the concentration of the flow fluid is 100% and at the edge of the shear layer the concentration of the flow fluid is 0%. According to Chen and Rodi (1980) the spread angle is 13 o for vertical round buoyant jet.

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CHAPTER 4 EXPERIMENTAL METHODS AND APPARATUS This chapter describes the design of experimental set up for detecting the hydrogen leaks by measuring the intensity of the scattered light at 90 degrees to the incident light. The leak was simulated using a jet of pure helium in some cases and a mixture of 20% helium and 80% nitrogen in others. The mixture of 20% helium and 80% nitrogen was used because the scattering cross-section of the mixture is equal to the scattering cross-section of hydrogen which is 1.89E-32 m 2 /sr. at 532 nm. Also, the data acquisition system used to measure the intensity is described. Figure 4-1. A schematic of experimental setup used for collection of scattered light at 90 o The incident laser beam was generated by an Nd:YAG pulsed laser, the fundamental wavelength being 1064 nm and the energy being 450 mJ/pulse at that 17

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18 wavelength. However in our work, the laser was operated at 532 nm and 355 nm wavelengths using 2 nd and 3 rd harmonic generators. The energy of the laser was 200 mJ/pulse and 60-95 mJ/pulse at 532 nm and 355 nm respectively. The laser was operated at a frequency of 10 Hz and the pulse width of the beam was 10 ns with a beam diameter of 6 mm and a beam divergence of 1.1 o The incident laser beam passed over the jet of helium coming through a nozzle. A diameter nozzle was used to simulate the leak, it was placed at a distance of 10 from the laser and hence the diameter of the laser beam at the nozzle is 8 mm. The leak was simulated at a Reynolds number of 100. The nozzle was mounted on 3 micrometer traverses for 3-dimensional motion (Figure 4-2) r z r y Aluminum Plate z Traverse 1 Traverse 2 Traverse 3 Plate position from nozzle Nozzle Mass Flow Meters Valve Laser Pulse y He 2 N Figure 4-2. Mounting of nozzle, gas-flow meters and position of aluminum plate The least count of each traverse was 0.05 mm. The mass flow rate of pure helium and the mixture of helium and nitrogen were monitored using the mass flow controllers. When no plate was used, the laser beam was trapped using a beam dump which was placed at a distance of 20 from the nozzle.

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19 The collection optics were oriented at 90 degrees to the incident laser beam (Figure 4-1) and consisted of a pair of 60 mm diameter lenses.(from TSI optics). The collecting lens has a focal length of 254 mm and this lens collects and collimates the scattered and reflected light. The focusing lens has a focal length of 124 mm and this lens focuses the collimated beam onto a 0.15 mm and 3 mm long slit which was mounted on the face of the photomultiplier-tube. The scattered light was separated into two different lines of wavelengths 532 nm and 355 nm using a beam splitter (from CVI lasers.) The beam splitter had a manufacturer coated transmissivity of 100% at 355 nm and 100% reflectivity at 532 nm. Band pass filters (from Newport) were mounted in front of both the photomultiplier tubes which allow only the respective wavelengths to pass through and eliminated any light of other wavelengths to be detected by the photomultiplier-tubes. The collection optics and the two photomultiplier-tubes were covered with a black drape to reduce any stray light to be detected by the photomultiplier-tubes. The photomultiplier-tube was Hammamatsu model HC120-01 and has a built in amplifier with adjustable gain. The spectral range of the photomultiplier-tube was 185 to 650 nm and has a frequency response of 23 kHz. The signal from the photomultiplier-tube was acquired using a high-speed digital oscilloscope. The oscilloscope is a LeCroy model LT 372. It was triggered externally using the pulsed-laser. The laser sends a trigger pulse to the oscilloscope exactly 100 ns before pulsing and after 100 ns the laser sends an output beam. The oscilloscope was set to record the data 100 ns after it received the trigger pulse. The signal from the two photomultiplier-tubes was acquired on two different channels of the oscilloscope.

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20 A portion of the incident laser beam was deflected using a piece of glass and was focused onto two different photodiodes (from Thor Labs) with the aid of a beam splitter to monitor the pulse-to-pulse variation in the incident beam power. The signal from the two photodiodes was analyzed using two different channels on a high-speed digital oscilloscope. A band pass filter was mounted in front of each photodiode to eliminate any light other than the signal at the respective wavelengths to be detected by the two photodiodes. Gas-flow Path The leak was simulated using a mixture of 20% helium and 80% nitrogen coming out of a nozzle. The gases used were pressurized in high pressure cylinders. The gases from these cylinders were connected to the nozzle using hoses and the mass flow rate was monitored using two gas-flow meters. Varying Glare To vary the glare an aluminum plate was used (Figure 4.2). The laser beam was reflected from the aluminum plate and the intensity of the reflected light was measured by the two photomultiplier-tubes. The plate was mounted directly in the line of the incident beam on a traverse so that it can be traversed in the horizontal direction (to and fro from the center of the control volume) and thus the glare could be varied. Design of Plate Position To calculate the glare numerically and to study how the glare varies as a function of the plate position, a ray tracing program was written in MATLAB. The rays were traced from the surface of the aluminum plate to the collecting lens, focusing lens and finally onto the photomultiplier tube. The code used simple co-ordinate geometry and Snells law to trace the rays

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21 Snells law states the ratio of the sine of the angle a particular kind of wave makes in one medium to the sine of the angle it makes in another medium is a constant This constant is also called the index of refraction. It can be represented in mathematical form as BBAAnn sinsin (4-1) The assumptions made in the numerical design of plate position are The light reflected from the aluminum plate was 100% diffuse. All calculations were made in 2-dimensional plane. The pulse-to-pulse variations in the incident laser beam were neglected. The reflectivity of aluminum plate is assumed to be 0.93 at 532 nm and 0.85 at 355 nm. The number of rays used to calculate glare ranged from 100 to 100000 The glare computed numerically was glare from the plate only and the glare from the background surfaces was neglected. The collecting lens was drawn using the general equation of the circle (Equation 4.2) with the focal point of the lens as the center and the radius of the collecting lens as the radius of the circle (Figure 4-3). 222CRyx (4-2) Only a fraction of the rays reflected from the aluminum plate reached the collecting lens. To trace the rays from the collecting lens to the focusing lens, Snells law was applied at both ends of the collecting lens (Equation 4-1). The reflected rays from the aluminum plate incident on the collecting lens bend toward the normal (Figure 4-4) as the refractive index of the lens is greater than the refractive index of air. The normal An

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22 An to the collecting lens at (p 1 q 1 ) is perpendicular to the flat surface of the collecting lens. (0, 0) CR Focal Point Figure 4-3. Collecting lens of radius R C and having focal point at (0,0). The angle of incidence is A and the angle of reflectance is B The refractive index of air is 1.0029 and that of the collecting lens is 1.523. The angle of incidence of the reflected rays at the second edge of the collecting lens is C and the angle of reflectance is D The rays exiting the collecting lens bend away from the normal (Figure 4-4). The normal to the collecting lens is drawn by differentiating the Equation 4-2 at (p Bn Bn 2 q 2 ) as given by the Equation 4-3. The transmitted rays from the collecting lens are incident on the curved surface of the focusing lens with an angle of incidence of E and have an angle of reflectance of F )2,2()/(1qpBdxdyn (4-3)

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23 Figure 4-4. Ray tracing of the reflected rays from the aluminum plate at the collecting lens Collecting Lens AnBn A B Rays rfrom AlumiPlate eflected num C D Aluminum Plate To trace the rays from the focusing lens to the photomultiplier-tube, Snells law was applied at both ends of the focusing lens (Equation 4-1). The rays incident on the focusing lens from the collecting lens bend toward the normal and the rays exiting the focusing lens will bend away from the normal (Figure 4-5). The intensity of the light reflected from the aluminum plate was calculated given by Equation 4-4. As mentioned in the assumptions, 100000 rays were used in the calculation of the glare and each individual ray was assumed to have equal intensity because the rays were reflected diffusely from the aluminum plate. Cn Dn

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24 oreflectedII (4-4) E 33,qp Figure 4-5. Ray tracing of the reflected rays from the collecting lens at the focusing lens The intensity of the reflected light at the collecting lens was calculated based on number of rays reaching the collecting lens from the aluminum plate and since each ray has equal intensity, the total intensity is the product of number of rays and the intensity of individual ray The incident laser light was assumed to be diffusely reflected in all directions. Only a fraction of the reflected rays reached the collecting lens which was calculated by computing the solid angle subtended by the reflected rays at the collecting lens. The solid angle d subtended by a surface is defined as the surface area of a unit sphere covered by the surface's projection onto the sphere. The solid angle was calculated according to the formula given by Equation 4-5, where is the radius of the hemisphere which is the plr Focusing Lens 44,qp 3'3',qp E' F'Cn^' Cn^ F Dn^' Dn^ G 4'4',qp I I' G' Transmitted rays from Collecting Lens

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25 distance between the aluminum plate and the collecting lens (Figure 4-6), and R C is the radius of the collecting lens 22cosplCplrRrradiusofHemispheretheofArealenscollectingtheofareaprojectedd (4-5) Figure 4-6. Solid angle subtended by the reflected rays from the aluminum plate at the collecting lens for two different plate positions. The intensity of the reflected light at the collecting lens is given by Equation 4-6. dIreflectedlensting I collec (4-6) the number of rays reaching thewhere Then the intensity of the reflected light at the focusing lens is calculated based on focusing lens from the collecting lens (Equation 4-7), the term 0.9 is the optical efficiency of the collecting lens. raofnumbertotallensingfocusthereachingraysofnumberlenscollectinglensgfocusin ysII9.0 (4-7)

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26 The intensity of the reflected light at the photomultiplier tube is calculated based on the number of rays reaching the photomultiplier tube from the focusing lens (Equation 4-8), wh ere 0.9 is the optical efficiency of the focusing lens. raysofnumbertotalPMTthereachingraysofnumberlensgfocuPMTsin II9.0 (4-8) Figure 4-7 shows the output of the MATLAB code for the case where the aluminum plate was directly over the nozzle. As the focal point of the collecting lens was acal point ns all the rays reflected by the collecting lens r was observed that none of the rays reflected by thelate glare. Using more than 10000e late t the nozzle, the reflected rays from the aluminum plate originated at the foof the collecting lens and were collimated. The collimated beam from the collecting lewas focused by the focusing lens onto the photomultiplier-tube. The photomultiplier-tube was at the focal point of the focusing lens. Figure 4-8 shows the output from the MATLAB code for the case when the aluminum plate was 3 cm from the nozzle. Not eached the focusing lens. Figure 4-9 shows the output from the MATLAB code for the case when the aluminum plate was 7 cm from the nozzle. It collecting lens reached the focusing lens and the glare from the aluminum pdetected by the photomultiplier-tube was zero for this case. Figure 4-10 shows the signal-to-glare as a function of number of rays used in the code. 100 to 100000 rays are used in the computation of the 0 rays was time consuming and the results obtained were not significantly accuratas compared to the results obtained with 100000 rays. The x-axis in the figure is the pposition in meters from the control volume. Theoretical Rayleigh scattered signal was used for the computing the signal-to-glare ratio.

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27 Figure 4-7. Ray tracing output from MATLAB when the aluminum plate is directly over the nozzle

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28 Figure 4-8. Ray tracing output from MATLAB when the aluminum plate is 3 cm from the nozzle.

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29 Figure 4-9. Ray tracing output from MATLAB when the aluminum plate is 7 cm from the nozzle. 1.00E-051.00E-041.00E-031.00E-021.00E-011.00E+000.000E+005.000E-025.900E-026.040E-026.081E-02plate position(m)Signal/Glare 100 rays 10000 rays 100000 rays number ofrays Figure 4-10. Glare for different plate positions as a function of number of rays.

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30 It was observed that when 100 rays were used the glare initially decreased as the plate was moved away from the nozzle and then the glare became constant at a plate position of 0.0603 m from the control volume. When 10000 rays were used the accuracy of the code increased and the glare decreased as expected when the plate was moved away from the nozzle and was not constant as was the case when 100 rays were used. When 100000 rays are used the accuracy of the code further increased and the glare decreased as the plate was moved away from the control volume. Figure 4-11 shows the glare from the aluminum plate as a function of plate position (for 532 nm, 355 nm and 1064 nm). It was observed that the glare decreased as the plate was moved away from the nozzle as expected and there was exponential decrease in glare starting from a plate position of 6.08 cm from the nozzle and reached almost zero when the plate was at 6.083 cm from the nozzle. 1.00E-101.00E-091.00E-081.00E-071.00E-061.00E-051.00E-041.00E-031.00E-021.00E-011.00E+000.000E+005.500E-026.010E-026.070E-026.083E-01Plate position(m)glare(mJ/pulse) 532 nm 355 nm 1064 nm Figure 4-11. Glare as a function of plate position and wavelength. 100000 rays are used for the computation of the glare.

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31 Experimental Methods To measure the intensity of the scattered beam two different approaches are adopted. The first method involved calculating the integrated area under the voltage time curve and the second method involved using the peak. We conducted these studies to establish the use of peak voltage as a repeatable and reliable method of acquiring data rather than integrated area. Integrated Area Method This method involved calculating the full width half maximum area under the voltage time curve measured as the area between two points where the voltage is 50% of the maximum voltage. Initially 1000 data points per pulse are captured from the oscilloscope. A typical waveform captured from the oscilloscope is shown in Figure 4-12. -0.0200.020.040.060.080.10.120.140.160.18160166172178184190196202208214220226232238244250256time (microseconds)raw pmt voltage (V) Figure 4-12. Typical waveform from a photomultiplier tube as captured by the oscilloscope. (Down stream distance of 4 nozzle diameters; Re=100; 100% Helium) The full width half maximum area was calculated from the voltage time curve using the trapezoidal rule given by Equations 4-9 and 4-10.

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32 tVVAiii21 (4-9) 10001iiAA (4-10) These measurements were done at a Reynolds number of 100 and at 4 nozzle diameters downstream with 100% helium flowing through the nozzle. These were done in the shear layer of the leak at a radial position of the nozzle corresponding to 60% helium. This is because it is at the shear layer; the maximum variation in the voltage was expected because jet fluctuations are greater there. The same procedure was repeated until the areas converged. Figure 4-13 shows the convergence studies for the full width half maximum area for both 355 nm and 532 nm wavelengths. 1.30E-061.50E-061.70E-061.90E-062.10E-062.30E-062.50E-062.70E-062.90E-063.10E-0612345678910number of pulsesarea(V-s) running average for 355nm running average for 532nm instantaneous area for355 nm instantaneous area for532 nm Figure 4-13. Running average of area for 355 and 532 nm It was observed that the area converged after 10 pulses. Figure 4-14 shows the percent variation of the instantaneous area from the average area. Average area is the average of the areas of the 10 pulses and is calculated as given by Equation 4-11. It was observed that the percent variation was a maximum of 3.5%.

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33 (4-11) 10101iiavgAA -4-3-2-10123412345678910number of pulses(Vi-Vmean)Vmean% 355 nm 532 nm Figure 4-14. Percent variation in area for 355 nm and 532 nm Peak Voltage Method To establish the convergence of the peak voltage and the repeatability, two methods were adopted. First method involved recording the peak voltage from the voltage time curve from which the full width half maximum area is calculated (Figure 4-12 and Figure 4-13). Figure 4-15 shows the convergence studies for the peak voltage. 0.440.460.480.50.520.540.560.5812345678910number of pulsespeak voltage(V) instantaneous peak voltage for355 nm instantaneous peak voltage for532 nm running average of peak for 355nm running average of peak for 532nm Figure 4-15. Running average of peak voltage for 355 nm and 532 nm

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34 It was observed that the peak voltage converged after 10 pulses and the percent variation between the instantaneous peak and the average peak was 3% (Figure 4-16). -4-3-2-10123412345678910number of pulses(Vi-Vmean)/Vmean% 355 nm 532 nm Figure 4-16. Percent variation in peak voltage for 355 nm and 532 nm The second method involved recording the average peak voltage from the oscilloscope after 0, 100, 200, 300,400 and 500 pulses and the percent variation in the average peak was calculated. Table 4-1 shows the voltage recorded for each measurement and also the percent variation between in the peak voltage after 100,200,300,400 and 500 pulses. It was observed that the percent variation in the average peak was less than 1% after 300 pulses. All these measurements were taken when 100% helium was flowing through the nozzle and in the shear layer at a radial position of the nozzle where the helium concentration was 60%

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35 Table 4-1. Percent variation in average peak voltage for 355 nm and 532 nm Pulse Number Peak voltage 355 nm Peak voltage 532 nm % variation in peak voltage between two measurements for 355 nm % variation in peak voltage between two measurements for 532 nm 1 460 552 100 443 533 3.837 3.564 200 447 525 0.894 1.523 300 444 526 0.675 0.190 400 441 529 0.680 0.567 500 443 531 0.451 0.376 Peak Area Correlation A correlation study was done between peak and integrated area and it was observed that the correlation coefficient between peak and area was 99.1% for both 355 nm and 532 nm (Figure 4-17). R2 = 0.9918R2 = 0.99160.40.450.50.550.60.651.30E-061.80E-062.30E-062.80E-063.30E-063.80E-06area (V-s)Peak voltage(V) 355 nm 532 nm Figure 4-17. Correlation between peak voltage and area for 355 nm and 532 nm

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36 So it was established that recording the average peak voltage after 300 pulses is reliable way of recording data and provides all the important information that is needed to analyze the readings.

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CHAPTER 5 RESULTS AND DISCUSSION Numerical Results As mentioned in chapter 4, a numerical code in MATLAB was written to compute the glare numerically. The glare was computed as a function of plate position for the two different wavelengths. Figure 5-1 shows the Signal to glare ratio for different wavelengths as a function of plate position. To compute the signal-to-glare ratio, first the theoretical Rayleigh signal was calculated as given by the Equation 3-3 in chapter 3. Then the Rayleigh light scattering signal was divided by the glare which was computed numerically as discussed in chapter 4. As expected the Signal-to-glare ratio increased as the wavelength decreased because of the inverse fourth power dependence of Rayleigh signal on wavelength and also the Signal-to-glare ratio increased as the aluminum plate was moved away from the control volume. 1.00E-061.00E-051.00E-041.00E-031.00E-021.00E-011.00E+000.000E+005.000E-025.900E-026.040E-026.081E-02plate position (m)Signal/Glare 532 nm 355 nm 1064 nm wavelength Figure 5-1. Signal to glare ratio as a function of plate position. 37

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38 To compare the experimental and theoretical results, the normalized voltage was calculated for each case. This was done because the numerical code computes the glare from the aluminum plate alone and does not take into account the glare from the background surfaces, where as the experimental results incorporated the glare from the aluminum plate and also the glare from the background surfaces. To account for this, the photomultiplier-tube voltage was normalized with respect to the maximum and minimum voltage as given by the Equation 5-1. The percent error in the theoretical and experimental results is a maximum of 5%. Voltage ratio= minmaxminVVVVi (5-1) -0.200.20.40.60.811.20.055000.060100.060300.060500.060700.060810.06083plate position(m)(Vi-Vmin)/(Vmax-Vmin) theoretical experimental Figure 5-2. Comparison of experimental and theoretical glare as a function of plate position. Experimental Results Testing the Linearity of the Photomultiplier-tube Output Voltage This section discusses how the output voltage from the photomultiplier-tube was tested to make sure that it was in the linear range. The laser power was varied from 0.9 kV to 1.31 kV and a power meter was used to monitor the incident laser power. As the

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39 laser power was varied, the corresponding photomultiplier-tube voltage and the photodiode voltage were recorded. Figure 5-3 shows the variation of the photomultiplier and the photodiode voltages as a function of the power meter voltage. These readings were taken at 4 nozzle diameters downstream and without any gas flowing through the nozzle. The voltages recorded from the photomultiplier-tube, photodiode and the power meter were normalized according to Equation 5-1. It was observed that the voltages of both the photomultiplier and the photodiode increased linearly as the power meter voltage increased. R2 = 0.9744R2 = 0.9984-0.200.20.40.60.811.200.20.40.60.811.2power meter voltage(V)(vi-vmin)/(vmax-vmin) photodiode photomultiplier tube Figure 5-3. Photomultiplier tube and the photodiode voltage as a function of the power meter voltage. The photomultiplier tube had a correlation coefficient, R 2 value of 0.9744 and the photodiode had a R 2 value of 0.9984. The next study done was to ensure the linearity of the output voltage from the photomultiplier tube when the glare was varied in the presence of the aluminum plate. To do this the laser power was varied from 0.9 kV to 1.31 kV. The glare was varied by placing the aluminum plate in two positions directly in line with the incident laser beam. The incident laser power was monitored with a power meter when there was no plate.

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40 These measurements were done at a nozzle position of 4 nozzle diameters downstream and with no gas flowing through the nozzle. The photomultiplier tube voltages are normalized using Equation 5-1. Figure 5-4 shows the variation of the photomultiplier tube voltage as a function of the power meter voltage. 00.20.40.60.811.20.000.130.290.430.570.680.800.901.00power meter voltage(V)(Vi-Vmin)/(Vmax-Vmin) no plate plate at 6 cm plate at 6.08 cm plate at 5.5 cm Figure 5-4. Photomultiplier tube voltage as a function of the power meter voltage when the glare is varied using an aluminum plate. It was observed that the photomultiplier tube voltage increased linearly as the power meter voltage increased even in the presence of the aluminum plate. The correlation coefficient value, R 2 when there was no plate is 0.9744 and the value of R 2 when the plate was at 6.08 cm from the nozzle is 0.9922 and the R 2 value when the plate was at 6 cm from the nozzle is 0.9939. When the aluminum plate was placed at a distance of 5.5 cm from the nozzle, the glare was too high and there was no change in the photomultiplier tube voltage. To test the linearity of the photomultiplier tube voltage when the aluminum plate was placed at a distance of 5.5 cm from the nozzle, neutral density filter was used. The main purpose of using a neutral density filter is to reduce the amount of light that can pass through a filter. Neutral density filters absorb or reflect

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41 fraction of light incident upon them. Neutral density filters are usually rated in optical density numbers. Optical density is the degree of opacity of a translucent medium. It is given by the Equation 5-2, where T is the transmission of the filter. TDO10log. (5-2) Table 5-1. Optical density and the corresponding transmission of neutral density filters (Source: http://www.evetar.com/product/6.asp Last accessed December 6 th 2004). Optical Density Transmission 0.1 80% 0.2 63% 0.3 50% 0.4 40% 0.5 32% 0.6 25% 0.7 20% 0.8 16% 0.9 13% 1.0 10% To check the linearity of the photomultiplier tube voltage, the photomultiplier-tube voltage was recorded when the aluminum plate was at 5.5 cm from the nozzle and with no neutral density filter. Then a 0.6 density filter was used and the voltage from the photomultiplier tube was recorded and the procedure was repeated as the laser power was varied. It was observed that the voltage from the photomultiplier tube decreased to 25% as compared to the voltage when there was no neutral density filter. Since the transmission of a 0.6 density filter was 25%, it was established that the photomultiplier tube is operating in the linear range.

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42 Table 5-2. Photomultiplier-tube voltages before and after using the 0.6 neutral density filter. Laser Power Photomultiplier-tube voltage before using neutral density filter for 532 nm Photomultiplier-tube voltage before using neutral density filter for 355 nm Photomultiplier-tubevoltage with neutral density filter for 532 nm (% decrease ) Photomultiplier-tube voltage with neutral density filter for 355 nm (% decrease) 0.9 1.562 1.012 0.389 (25.1%) 0.251 (24.8%) 1 1.562 1.325 0.390 (25%) 0.330 (24.9%) Eliminating Glare from the Rayleigh Light Scattering Signal This section discusses how the glare was eliminated from the Rayleigh light scattering signal. To eliminate the glare from the Rayleigh signal, two approaches wesre adopted. Reference value method This method involved recording the photomultiplier-tube voltage when there was no gas flowing through the nozzle for every radial position of the nozzle and for every plate position. The voltage recorded by the photomultiplier-tube was due to glare from the background surfaces, Mie scattering and scattering due to air molecules. It is written mathematically as airScatteringMieGlarerefVVVV (5-3) Next, the photomultiplier tube voltage was recorded when a mixture of 80% nitrogen and 20% helium was flowing through the nozzle. The diameter of the laser at the control volume was 8mm and the distance between the edges of the shear layers of the jet was 5.8mm. Hence there was entrainment of air in the control volume. Hence the voltage recorded by the photomultiplier-tube was because of the glare from the

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43 background surfaces, Mie scattering, and scattering due to the mixture of helium and nitrogen molecules (leaking fluid) and scattering due to air molecules. It is written mathematically as, where x is the percentage of the leaking fluid present in the control volume. xVxVVVVleakairScatteringMieGlareact 1 (5-4) When the two equations are subtracted, glare and the Mie scattering was eliminated from the Rayleigh signal. It is written mathematically as ScatteringMieairleakRLSVVVxV (5-5) Hence the glare was eliminated from the Rayleigh light scattering signal. But as there was entrainment of air in the control volume (Equation 5-5) the signal is always higher than the predicted signal (theoretically) as the scattering cross-section of air 1.32 times that of nitrogen and the entrained aerosols will scatter in the Mie regime. Assuming that the ratio of the glare at the two wavelengths is constant As discussed in chapter 3, an aluminum plate was used to vary the glare. This method takes into consideration that the irradiance from the aluminum plate was much higher as compared to the irradiance from other surfaces. From Equation 3-18, the scattering intensity is written as GlareRLSScatIII (3-18) From Equation 3-17, the Rayleigh light signal is proportional to the scattering cross-section of the gas and the incident intensity of the laser. Hence the ratio of the Rayleigh light signal at the two wavelengths is the ratio of the scattering cross-sections. 2,1,2,21,12,1,ooooRLSRLSIISIIII (5.6)

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44 The intensity of the glare is written as where A GlareI niiiAG1 i is the area of the background surface visible to the detector and is the irradiance from i iG th surface. The irradiance from the surface is proportional to the reflectivity of the surface. As the area of the surface visible to the two detectors is same, the ratio of the glare at two wavelengths was the ratio of the reflectivity of the surface at those two wavelengths. Since different surfaces have different reflectivities, the ratio of the glare is not constant. We used an aluminum plate to vary the glare. It was proved that the irradiation from the aluminum plate was much more compared to the irradiation from other surfaces. niiiplplAGAG2 (5-7) The ratio of the glare in this case was just the ratio of the reflectivity of the aluminum plate. Since the ratio of the reflectivity of the aluminum plate was constant, the ratio of the glare was constant and is written as 2,1,2,21,12,1,ooooGlareGlareIIRIIII (5-8) As discussed in chapter 4, the intensity of the light incident at the photomultiplier-tube at the two wavelengths is given by the Equations 5-9 and 5-10. 1,1,1,GlareRLSScatIII (5-9) 2,2,2,GlareRLSScatIII (5-10) From Equation 5-6 the ratio of the Rayleigh light scattering signal at the two wavelengths is S and from Equation 5-8, the ratio of the glare at the two wavelengths is R. Solving Equations 5-6, 5-8, 5-9 and 5-10 simultaneously, the Rayleigh light signal at the two wavelengths was written as

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45 ))((2,1,2,2,1,1,ooScatoScatRLSIRSIRIIISI (5-11) And 1,2,1,2,ooRLSRLSIISII (5-12) Equations 5-11 and 5-12 gave the Rayleigh light scattering signal with the glare decoupled from them. Theoretical and Experimental Photon-arrival Rates This section discusses how the theoretical and the experimental photon-arrival rates compare. Theoretical photon-arrival rate As discussed in Chapter 3, the theoretical photon-arrival rate was calculated according to the formula )]))1(()([)()((2NHeoPxxNdVdIR (5-13) Where oI 200 mJ/Pulse = 5.34E17 photons/pulse for 532 nm And 75 mJ/Pulse = 2.00e17photons/pulse for 355 nm 0.59049. dV 0.118 mm 3 d 22)250()60( N 2.25E25 molecules/m 3 x percentage of helium.

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46 Table 5-3. Scattering cross-sections (of helium, nitrogen, hydrogen and air) at 532 nm and 355 nm. Gas Scattering cross-section at 532 nm, 1 Scattering cross-section at 355 nm, 2 Helium 1.22E-33 6.17E-33 Nitrogen 6.15E-32 3.10E-31 Hydrogen 1.89E-32 9.53E-32 Air 8.16E-32 4.11E-31 The percentage of helium was decreased from 100 to 0 and the percentage of nitrogen was increased from 0 to 100 and the resulting photon-arrival rate for both 355 nm and 532 nm is shown in Figure 5-5. 0.00E+005.00E+081.00E+091.50E+092.00E+092.50E+093.00E+093.50E+091009080706050403020100% heliumphoton arrival rate(photons/pulse) 355 nm 532 nm Figure 5.5 Theoretical photon-arrival rates for 355 nm and 532 nm. Experimental photon-arrival rate To calculate the experimental photon-arrival rates, a mixture of helium and nitrogen was used and the measurements were done at a nozzle position of 4 nozzle diameters downstream and the Reynolds number used was 100. The percentage of Helium was decreased from 100 to 0 and the percentage of nitrogen was increased from 0

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47 to 100 and the corresponding waveforms of the photomultiplier-tube are recorded from the oscilloscope. The area under the voltage time curve was calculated as discussed in chapter 4 using the Equations 4-9 and 4-10. Then the area which has units of volts-sec was converted to photons per pulse using the Equation 5-14, where is the photomultiplier tube calibration constant and is obtained from the manufacturers specifications. It has a value of 121 V/nW for 532 nm and 244 V/nW for 355 nm. PMTC hcCARPMTPMTalExperimentp, (5-14) The glare from the experimental photon-arrival rate was reduced using the reference value method. Figure 5-6 shows the comparison of the experimental and the theoretical photon-arrival rates. 0.00E+005.00E+081.00E+091.50E+092.00E+092.50E+093.00E+093.50E+094.00E+091009080706050403020100% heliumphoton arrival rate(photons/pulse) 355 nm (experimental) 532 nm (experimental) 355 nm (theoretical) 532 nm (theoretical) Figure 5-6. Comparisons of theoretical and experimental photon-arrival rates for 355 nm and 532 nm. Figure 5-7 shows the percent error between experimental and theoretical photon-arrival rates for 532 nm and 355nm. The percentage uncertainty due to electronic shot noise was less than 0.1% for an averaging time of 0.001 sec. It is calculated according to the formula

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48 tRpRp1 (5-15) 10.0020.0030.0040.0050.0060.0070.0080.0090.001009080706050403020100% helium% error 355 nm 532 nm Figure 5-7. Percent error between theoretical and experimental photon-arrival rates as shown in Figure 5-6 It was observed that the maximum error is around 80% for 100% helium and the percentage error gradually decreased thereafter. This error is attributed to two facts. The diameter of the laser at the control volume was 8 mm and the distance between the edges of the shear layers of the jet was 5.8 mm. As a result of this there was entrainment of air in the control volume. As the scattering cross-section of air was more than helium and nitrogen, the intensity of the scattered beam as detected by the photomultiplier-tube was increased. The optical efficiency of the collection optics may deviate from the assumed 90%. Analysis of the Recorded Data Raw photomultiplier-tube voltage variation This section discusses how the recorded data was analyzed to predict the presence of hydrogen in the jet and to eliminate the glare from the Rayleigh light signal. Figure 5-8 shows the variation of the raw peak voltage as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm 6 cm and 5.5 cm from the nozzle for 532 nm wavelengths. The peak voltage was converted into mJ/pulse using

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49 the photomultiplier-tube calibration constant provided by the manufacturer. These measurements were taken at a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen was flowing through the nozzle. This mixture was used because the scattering cross-section of the mixture of 20% helium and 80% nitrogen is equal to that of hydrogen. The scattering cross-section of hydrogen was 0.23 that of air (Table 5-3). The Reynolds number was 100. The x-axis is the radial position of the nozzle normalized with the downstream distance. 0.00E+005.00E-101.00E-091.50E-092.00E-092.50E-09-2-1.5-1-0.500.511.522.53r/zPmt voltage(mJ/pulse)111 no plate plate at -r/z = 2.39 plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 1.96 plate at -r/z = 2.16 plate at -r/z = 2.16 Figure 5-8. Raw photomultiplier-tube voltages as a function of radial and plate position for 532 nm. The measurements were taken at 4 nozzle diameters downstream for a mixture of 20% helium and 80% nitrogen It was observed that the peak voltage was constant outside the shear layer as there was only ambient air outside the shear layer. The minimum peak voltage occurred at the jet centerline which corresponded to 100% hydrogen. The fall in voltage at the jet centerline occurred because the scattering cross-section of hydrogen was less than that of air and as discussed in chapter 3, the voltage output from the photomultiplier-tube is a function of the scattering cross-section of the gas. It was observed that the raw peak

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50 voltage was not symmetric about the jet center line when the aluminum plate was used where as it was symmetric about the jet centerline when there was no plate. This is because when the plate was used, the glare was not constant and was higher when the nozzle was closer to the plate and gradually decreased as the nozzle was moved away from the plate. The glare from the aluminum plate was higher when the plate was at 5.5 cm and 5 cm from the nozzle and a neutral density filter of 0.6 optical density was used to attenuate the intensity of the scattered beam. Figure 5-9 shows the variation of the raw peak voltage as a function of the radial position when there was no plate and when the plate was at 6.03 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 355 nm wavelengths. As observed from the Figure 5.8, there was a fall in voltage at the jet centerline corresponding to 100% hydrogen and the peak voltage was constant outside the shear layer. As seen in Figures 5-8 and 5-9, two measurements were taken when the plate was at 6.08 cm and 5.5 cm from the nozzle. This was done to test the repeatability of the data. Figure 5-10 shows the percentage error in the raw photomultiplier-tube voltages between the two measurements taken when the plate was at 6.08 cm and 5.5 cm from the nozzle for 532 nm and 355 nm. It was observed that the maximum error in the raw photomultiplier-tube voltage when the plate is at 6.08 cm from the nozzle is 5%. The maximum error when the plate was at 5.5 cm from the nozzle was 3%. Hence it was concluded that the data is repeatable and reproducible.

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51 0.00E+005.00E-101.00E-091.50E-092.00E-092.50E-093.00E-093.50E-094.00E-094.50E-095.00E-09-2-1.5-1-0.500.511.522.53r/zPmt voltage(mJ/pulse)111 no plate plate at -r/z = 2.39 plate at -r/z =2/39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 1.96 plate at -r/z = 2.16 plate at -r/z = 2.16 Figure 5-9. Raw photomultiplier-tube voltages as a function of radial and plate position for 355 nm. The measurements were taken at 4 nozzle diameters downstream for a mixture of 20% helium and 80% nitrogen 012345621.510.50-0.5-1-1.5-2-2.5-3r/z% error plate at -r/z = 2.16; 355 nm plate at -r/z = 2.16;532 nm plate at -r/z = 2.39;355nm plate at -r/z = 2.39;532nm Figure 5-10. Percentage errors in raw photomultiplier-tube voltage for two typical plate positions as a function of radial position.

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52 Variation of glare as a function of radial and plate position Figure 5-11 shows the glare as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 532 nm. 0.00E+002.00E-094.00E-096.00E-098.00E-091.00E-081.20E-081.40E-081.60E-081.80E-08-2-1.5-1-0.500.511.522.53r/zglare(mJ/pulse) plate at -r/z = 2.39 plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 2.16 plate at -r/z = 2.16 plate at -r/z = 1.96 Figure 5-11. Glare as a function of radial and plate position for 532 nm for a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen is flowing through the nozzle. It was observed that the glare was higher when the nozzle was closer to the plate and gradually decreased as the nozzle is moved away from the aluminum plate. The glare was calculated using the reference value method when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6cm and 5.5 cm from the nozzle. When the plate was at 5.5 cm and 5 cm from the nozzle, the glare was calculated taking into consideration that the ratio of the glare at the two wavelengths was constant. The percentage error in the glare calculated by two methods for the case when the plate was at 5.5 cm from the nozzle was a maximum of 0.17%. So it was concluded that the two methods of calculating the glare agree well with each other.

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53 Figure 5-12 shows the glare as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 355 nm. 00.0000000020.0000000040.0000000060.0000000080.000000010.000000012-2-1.5-1-0.500.511.522.53r/zglare (mJ/pulse) plate at -r/z = 2.39 plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 2.16 plate at -r/z = 2.16 plate at -r/z = 1.96 Figure 5-12. Glare as a function of radial and plate position for 355 nm for a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen is flowing through the nozzle. As observed from Figure 5-11, it was observed that the glare was higher when the nozzle was closer to the plate and gradually decreased as the nozzle was moved away from the plate. The percentage error in the glare calculated by two methods for the case when the plate was at 5.5 cm from the nozzle was a maximum of 0.25%. Variation of Rayleigh light scattering signal as a function of radial and plate position Figure 5-13 shows the Rayleigh light scattering signal as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 532 nm.

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54 Reference value method was used to eliminate the glare from the raw peak voltage shown in Figure 5-8. It was observed that the Rayleigh light scattering signal is symmetric about the jet centerline as the glare was eliminated. 0.00E+001.00E-092.00E-093.00E-094.00E-095.00E-096.00E-09-2-1.5-1-0.500.511.522.53r/zRLS signal(mJ/pulse) no plate plate at -r/z = 2.39 plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 2.16 plate at -r/z = 2.16 Figure 5-13. Rayleigh light scattering signal as a function of radial and plate position for 532 nm for a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen is flowing through the nozzle. Error bars are shown for the case when the plate was at 6 cm from the nozzle. The error bars are calculated using the formula 3 where is the standard deviation in the Rayleigh light scattering signal. This is because 99.9% of the data points fall in the range of 3 The standard deviation was of the order of 0.1 for a signal of the order 1. The ratio of the maximum to the minimum voltage was around 5 and the fall in voltage was 20% which corresponded to the ratio of scattering cross-sections of air and hydrogen which was 4.31. The scattering cross-section of hydrogen and air are shown in Table 5-3. Figure 5-14 shows the Rayleigh light scattering signal as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 355 nm.

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55 02E-104E-106E-108E-100.0000000011.2E-091.4E-091.6E-091.8E-090.000000002-2-1.5-1-0.500.511.522.53r/zRLS signal(mJ/pulse) no plate plate at -r/z = 2.39 plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 2.16 plate at -r/z = 2.16 Figure 5-14. Rayleigh light scattering signal as a function of radial and plate position for 355 nm for a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen is flowing through the nozzle. Reference value method was used to eliminate the glare from the raw peak voltage shown in Figure 5-9. It was observed that the Rayleigh light scattering signal was symmetric about the jet centerline as the glare was eliminated. Error bars are shown for the case when the plate was at 6 cm from the nozzle. The error bars are calculated according to the formula 3 where is the standard deviation of the Rayleigh light scattering signal. Figure 5-15 shows the signal-to-glare ratio as a function of the radial position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for 532 nm and 355 nm.

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56 signal to glare as a function of nozzle and plate position0.00E+002.00E-014.00E-016.00E-018.00E-011.00E+001.20E+00-2-1.5-1-0.500.511.522.53r/zsignal to gla r plate at -r/z = 2.39; 355 nm plate at -r/z = 2.39; 532 nm plate at -r/z = 2.395; 355 nm plate at -r/z = 2.395; 532 nm plate at -r/z = 2.36; 355 nm plate at -r/z = 2.36; 532 nm plate at -r/z = 2.16; 355 nm plate at -r/z = 2.16; 532 nm plate at -r/z = 1.96; 355 nm plate at -r/z = 1.96; 532 nm plate position Figure 5-15. Signal-to-glare as a function of radial and plate position for 532 nm and 355 nm for a downstream location of 4 nozzle diameters and when a mixture of 20% helium and 80% nitrogen is flowing through the nozzle It was observed that signal-to-glare ratio was higher outside the shear layer in the ambient and minimum at the jet center line. It was also seen that the signal-to-glare ratio is not symmetric about the jet center line. This was because the glare is higher when the nozzle was closer to the plate and gradually decreased as the nozzle was moved away from the nozzle. The lowest signal-to-glare ratio was observed when the plate was at 5 cm. It was seen that the hydrogen leak can be detected when the signal-to-glare ratio is as low as 0.166. Detection limits Figure 5-16 shows the raw peak voltage as a function of the radial position when there was no plate for 532 nm and 355 nm. These measurements were taken at nozzle position of 8 nozzle diameters downstream and when 100% helium was flowing through the nozzle. The Reynolds number was 100.

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57 00.10.20.30.40.50.6-3-2.5-2-1.5-1-0.500.511.522.53r/zpmt voltage(V) 355 nm 532 nm Figure 5-16. Raw photomultiplier-tube voltages as a function of radial and plate position for 532 nm and 355 nm at 8 nozzle diameters downstream and when 100% helium is flowing through the nozzle. It was observed that the fall in voltage occurred over a wider range as compared to the nozzle position of 4 nozzle diameters downstream. This was because when the nozzle was at 8 nozzle diameters downstream, the jet spreads out and the variation in voltage due to the presence of helium occurred over a wider radius. The drop in voltage was around 15%. To test for the detection limits, measurements were taken when the plate was placed as close as possible to the nozzle. Figure 5-17 shows the Rayleigh light scattering signal as a function of the radial position when the plate was at 5.5 cm from the nozzle for 532 nm. The measurements were taken at a nozzle position of 8 nozzle diameters downstream and when 100% helium was flowing through the nozzle. The Reynolds number was 100. To attenuate the intensity of the scattered beam a neutral density filter of optical density 0.6 was used.

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58 532 nm1.68E-081.70E-081.72E-081.74E-081.76E-081.78E-081.80E-081.82E-081.84E-081.86E-081.88E-081.90E-08-2-1.5-1-0.500.511.522.5r/z plate at 5.5 cm Figure 5-17. Rayleigh light scattering signal as a function of radial and plate position for 532 nm for a downstream location of 8 nozzle diameters and when 100% helium is flowing through the nozzle. Figure 5-18 shows the Rayleigh light scattering signal as a function of the radial position when the plate was at 5.5 cm from the nozzle for 355 nm. 355 nm5.80E-095.90E-096.00E-096.10E-096.20E-096.30E-096.40E-096.50E-096.60E-09-2-1.5-1-0.500.511.522.53r/z plate at 5.5 cm Figure 5-18. Rayleigh light scattering signal as a function of radial and plate position for 355 nm for a downstream location of 8 nozzle diameters and when 100% helium is flowing through the nozzle. The glare here was eliminated by considering that the ratio of the glare at the two wavelengths was constant. From Figures 5-17 and 5-18, it was observed that there was a

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59 fall in voltage at the jet centerline but the fall in voltage does not correspond to the ratio of the scattering cross-section of helium and air. This was because at 8 nozzle diameters, the jet spreads out and there will be greater entrainment of air. This caused the signal to be greater than expected since the scattering cross-section of air was greater than that of helium. The scattering cross-sections of air and helium are shown in Table 5-3. Correlation studies Figure 5-19 shows the correlation of Rayleigh light scattering signal between the two wavelengths: 355 nm and 532 nm when there was no plate, when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle. 0.00E+005.00E-101.00E-091.50E-092.00E-092.50E-093.00E-093.50E-094.00E-094.50E-095.00E-090.00E+002.00E-104.00E-106.00E-108.00E-101.00E-091.20E-091.40E-091.60E-09355 nm532 nm no plate plate at -r/z = 2.39 plate at -r/z = 2.395 plate at -r/z = 2.36 plate at -r/z = 2.16 Figure 5-19. Correlation of Rayleigh light scattering signal between two wavelengths. The correlation of the Rayleigh light scattering signals between the two wavelengths was drawn so that there are two sets of data to analyze and detect the hydrogen leak present in the jet. Figure 5-20 shows the R 2 value for the correlation of the Rayleigh light scattering signal at the two wavelengths as a function of the plate position. It was observed that the value of the correlation coefficient, R 2 is a minimum of 0.9567 when the plate is at 5.5 cm from the nozzle. Hence it was concluded that a decrease in

PAGE 75

60 voltage on one line will imply a decrease in voltage on the second line as well. Hence there are two sets of data available and by analyzing the two sets of data simultaneously; it is possible to predict the presence of hydrogen leak in the jet. 0.930.940.950.960.970.980.991no plate 6.083 cm6.08 cm6 cm5.5 cmplate positionR 2 value Figure 5-20. Variation in R 2 value as a function of plate position. It was observed that the R 2 value for the correlation was highest and is equal to 0.9944 when there was no plate and gradually decreased as the plate was moved closer to the nozzle. So it was concluded that as the plate was moved closer to the nozzle, the uncertainty in the Rayleigh light scattering signal increased.

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CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS A Nd:YAG pulsed laser induced dual line-detection Rayleigh light scattering technique was used to detect the presence of hydrogen in the jet in the presence of high amounts of glare. Also, a mathematical code in MATLAB has been written to compute the glare numerically, design the experimental system and study the variation in the glare as a function of the plate position. The MATLAB code computed only the glare from the aluminum plate and did not take into account the glare from the background surfaces. The results from the mathematical code are compared with experimental results by normalizing the glare and the results corresponded well. The percentage error between the numerical and experimental results was a maximum of 5%. Two methods were used to eliminate the glare from the signal. When multiple surfaces are considered, glare from the signal is eliminated taking reference values at each point in space and when the glare from one surface is dominant compared to the glare from the other surfaces; the data analysis to eliminate the glare was simplified. The ratio of the glare at the two wavelengths is proved to be constant and the glare is eliminated by solving a set of simultaneous equations Dual line-detection technique was used successfully to eliminate the glare from the signal and get accurate results even when the glare to signal ratio was as high as 6:1. When a mixture of 20% helium and 80% nitrogen was used for the measurements at 4 nozzle diameters, the voltage at the centerline of the nozzle was around 0.2 times that of 61

PAGE 77

62 voltage outside the shear layer. This indicated a reduction in the scattering cross-section of the gas in the jet centerline by an amount of 0.2 as compared to the scattering cross-section of the gas outside the shear layers. This reduction in the scattering cross-section corresponded well to the ratio of the scattering cross-sections of hydrogen and air. The scattering cross-section of hydrogen is equal to 0.23 times that of air. When pure helium was used for measurements at a downstream location of 8 nozzle diameters, a fall in voltage was observed in the centerline of the jet. The voltage at the jet centerline was observed to be 0.14 times that of the voltage outside the shear layers. The scattering cross-section of helium is equal to 0.015 times that of air. The decrease in the voltage at the jet centerline does not correspond well the ratio of the scattering cross-sections of helium and air. This is because at a downstream location of 8 nozzle diameters, the jet spreads out and there will be greater entrainment due to air. Future work: A study should be done to eliminate the glare from the signal at downstream distances greater than 8 nozzle diameters and establish the detection limits. Also, a study should be done to detect the hydrogen leaks in the presence of cross currents. The feasibility of the technique should be determined in the back scatter mode to enable remote detection.

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LIST OF REFERENCES Bohren, C. F., and Donald R. Huffman., Absorption and scattering of light by small particles, New York, John Wiley &Sons, 1983. Bryner, N., Richards, C.D., and Pitts, W.M., A Rayleigh Light Scattering Facility for the Investigation of Free Jets and Plumes, Review of Scientific Instruments, Vol. 63, No. 7, 3629-3635, 1992. Chen, C., Rodi W., The Science and Applications of Heat and Mass Transfer, HMT, 1980, Vol. 4. Dibble, R.W., Hollenbach, R.E., and Rambach, G.D., Laser Probes for Combustion Chemistry (ed. D.R. Crosley), 1980, Washington, DC, American Chemical Society, 435-441. Temperature measurements in turbulent flames via Rayleigh scattering, Dyer, T., Rayleigh scattering measurements of time resolved concentration in a turbulent propane jet, AIAA Journal, Vol. 17, No. 8, 1979, pp. 912-914. Geoff, A., Jason, K.B., Randall ,J.K., and Paul, H., Raman and Rayleigh Holographic Lidar, Applied Optics, Vol. 41, No. 9, 2002, pp. 1798-1804. Graham, S., Grant, A., Jones, J., Transient molecular concentration measurements in turbulent flows using Rayleigh light scattering, AIAA Journal, Vol. 12, No. 8, 1974, pp. 1140-1142. Horton, J., Peterson, J.E., Transient temperature measurements in an ideal gas by using laser induced Rayleigh light scattering, Review of Scientific Instruments, Vol. 70, No. 8, 1999, pp. 3222-3226. Kerker, M. The Scattering of Light and Other Electromagnetic Radiation, New York, Academic Press, 1969. Long, M., Chu, B., and Chang, R., Instantaneous two dimensional gas concentration measurements by light scattering, AIAA Journal, Vol. 19, No. 9, 1981, pp. 1151-1157. Matthew, A., Peterson, J.E., Flow visualizations and transient temperature measurements in an axisymmetric impinging jet rapid thermal chemical vapor deposition reactor, Journal of Heat Transfer, Vol. 124, 2002, 564-570. McCartney, Earl J., Optics of the Atmosphere, New York, John Wiley & Sons, 1976. 63

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64 Muller-Dethlefs, K., Weinberg, F., Burning velocity measurements based on laser Rayleigh scattering, Seventeenth Symposium on Combustion, 1979, pp. 985-992. Namazian, N., Kelly J., Shefer, R.W., Johnston, S.C., and Long, M.B., Nonpremixed Bluff-Body Burner Flow and Flame Imaging Study, Experiments in Fluids, Vol. 8, 1989, pp. 216-228. Namer, I., and Schefer, R.W., Error Estimates for Rayleigh Scattering Density and Temperature Measurements in Premixed Flames, Experiments in Fluids Vol. 3, 1985, pp. 1-9. Otugen M.V., Annen, K.D., and Seasholtz, R.G., Gas Temperature Measurements Using a Dual-Line Detection Rayleigh Scattering Technique, AIAA Journal, Vol. 31, No. 11, 1993, pp. 2098-2104. Otugen M.V., Kim, J., and Popovic, S., Nd:YAG laser based dual line Rayleigh scattering system, AIAA Journal, Vol. 35, No. 5, 1997. Otugen, M. V., and Namer, I., Rayleigh Scattering Temperature Measurements in a Plane Turbulent Air Jet, Experiments in Fluids, Vol. 6, No. 7, 1988, pp. 461-466. Pitts, W.M., and Kashiwagi, T., The application of laser induced Rayleigh light scattering to the study of turbulent mixing, Journal of Fluid Mechanics, Vol. 141, 1983, pp. 391-429. Pitz, R.W., Cattolica, R., Robben, F., and Talbot, F., Temperature and Density in a Hydrogen Air Flame from Rayleigh scattering, Combustion and Flame, Vol. 27, 1976, pp. 313-320. Robben, F., Noise in the Measurements of Light with Photomultipliers, Applied Optics, Vol. 10, No. 4, 1971, pp. 776-796. Robben, F., Comparison of density and temperature using Raman scattering and Rayleigh scattering using combustion measurements in jet propulsion systems, Proceedings of a Project SQUID workshop, Purdue University, 1975, pp. 179-195. Rosenweig, R., Hottel, H., and Williams, G., Smoke scattered light measurements of turbulent concentration fluctuations, Chemical Engineering Science, Vol. 15, No. 1, 2, 1961, pp. 111-129. Schlichting, Boundary Layer Theory, McGraw Hill Series in Mechanical Engineering, 1979. Van de Hulst, H.C., Light Scattering by Small Particles, London, Chapman &Hall, 1957. Zhu, J.Y., So, R.M.C., Otugen, M.V., and Hwang, B.C., Some Measurements in Binary Gas Jet, Experiments in Fluids, Vol. 9, 1990, 273-284.

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BIOGRAPHICAL SKETCH Raghuram Vempati completed his undergraduate degree in mechanical engineering from Jawaharlal Nehru Technological University, India, in April 2002. He is pursuing his masters degree in mechanical engineering at the University of Florida. He has been a research assistant under Dr. Jill Peterson since August 2002. 65


Permanent Link: http://ufdc.ufl.edu/UFE0008640/00001

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Title: Remote Detection of Hydrogen Leak Using Nd:YAG Pulsed Laser Induced Dual Line Detection Rayleigh Light Scattering
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
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Permanent Link: http://ufdc.ufl.edu/UFE0008640/00001

Material Information

Title: Remote Detection of Hydrogen Leak Using Nd:YAG Pulsed Laser Induced Dual Line Detection Rayleigh Light Scattering
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0008640:00001


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REMOTE DETECTION OF HYDROGEN LEAK USING Nd:YAG PULSED LASER
INDUCED DUAL LINE DETECTION RAYLEIGH LIGHT SCATTERING















By

VENKATA SURYA RAGHURAM VEMPATI


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Venkata Surya Raghuram Vempati















ACKNOWLEDGMENTS

I would like to thank Dr. Jill Peterson for her support and guidance. I would also

like to thank my fellow students (Sameer Paranjpe, Philip Jackson, Murray Fisher,

Mathew Gabriel and Ryan Ferguson) for their assistance in various portions of the

project.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ......... ......... .. ..................................................................... iii

LIST OF TABLES ........ .................... .......... .......................... vi

LIST OF FIGURE S ......... ..................................... ........... vii

LIST OF ABBREVATIONS AND SYMBOLS ........................ ...............x

ABSTRACT ........ .............. ............. ...... ...................... xiv

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

P roj ect G oal ..................................... ................................... .. ............. .
R ayleigh L ight Scattering ................................................................ .. ................... 1

2 LITER A TU RE SU RVEY .................................................. ............................... 4

3 THEORETICAL BACKGROUND.................................................................. 6

Light Scattering ........................................ .........6
M echanism of Elastic Scattering................................................. ............... 7
M ie sc atterin g .................................................................................. 8
R ayleigh scattering ............................................................ .. .... ........ 10
A ngular scattering cross-section ...................................... ............... 12
Dual Line-detection Technique ............................................................................ 13
Buoyant Jets and Plum es .......................................................................... 15

4 EXPERIMENTAL METHODS AND APPARATUS .............. ............... 17

G a s-fl o w P ath ............................................................................ ................ .. 2 0
V ary in g G large ...............................................................2 0
D design of Plate Position .................. ........................... .. ...... ................. 20
E xperim mental M ethods........................................................................ ..................3 1
Integrated A rea M ethod ......................................................... .............. 31
Peak V oltage M ethod ................................................. ............................. 33
P eak A rea C orrelation ........................................ ...................... .....................3 5










5 RESULTS AND DISCU SSION ........................................... .......................... 37

N u m erical R esu lts.......... .......................... ................................................... .. ...... .. 3 7
E xperim mental R results ................................ .......................... .. .... ..................... 38
Testing the Linearity of the Photomultiplier-tube Output Voltage ..................38
Eliminating Glare from the Rayleigh Light Scattering Signal ..........................42
R reference value m ethod ..................................................... .......................42
Assuming that the ratio of the glare at the two wavelengths is constant .....43
Theoretical and Experimental Photon-arrival Rates............................... 45
Theoretical photon-arrival rate ............... ............................................ 45
Experim ental photon-arrival rate ...................................... ............... 46
Analysis of the Recorded Data. ............................. .............................. ........ 48
Raw photomultiplier-tube voltage variation ............................................. 48
Variation of glare as a function of radial and plate position ...................52
Variation of Rayleigh light scattering signal as a function of radial and plate
position ................................................... 53
D election lim its ............................................... ............... 56
C orrelation studies........... ...... ...................................... .......... ... ......... 59

6 CONCLUSIONS AND RECOMMENDATIONS............................................. 61

L IST O F R E F E R E N C E S ........................................................................ .....................63

BIOGRAPH ICAL SK ETCH ...................................................... 65
















LIST OF TABLES


Table page

4-1 Percent variation in average peak voltage for 355 and 532 nm ............................35

5-1 Optical density and the corresponding transmission of neutral density filters.........41

5-2 Photomultiplier-tube voltages before and after using the 0.6 neutral density filter.42

5-3 Scattering cross-sections of helium, nitrogen, hydrogen and air at 532 nm and 355
n m ................... .......................................................... ................ 4 6
















LIST OF FIGURES


Figure page

3-1 Ram an Scattering ......... ...... .. ..... .............. .. .. .... .. .. .. ........ .. 7

3-2 Creation of an induced dipole moment by an electric field...............................

3-3 M ie scattering geom etry ........................................ ................................. 9

3-4 Rayleigh scatterer is very small compared to the wavelength of the incident
electrom agnetic w ave ................................................. .... ... .. .. .......... 11

3-5 Instantaneous and time averaged profiles of a typical buoyant jet........................ 16

4-1 Experimental setup used for collection of scattered light at 90............................17

4-2 Mounting of nozzle, gas-flow meters and position of aluminum plate .................18

4-3 Collecting lens of radius Rc and having focal point at (0,0) ...............................22

4-4 Ray tracing of the reflected rays from the aluminum plate at the collecting lens. 23

4-5 Ray tracing of the reflected rays from the collecting lens at the focusing lens .....24

4-6 Solid angle subtended by the reflected rays from the aluminum plate ................25

4-7 Ray tracing output from MATLAB when the aluminum plate is directly over the
n ozzle ........................ ......... ......... ........ ................ ............... 2 7

4-8 Ray tracing output from MATLAB when the aluminum plate is 3 cm from the
n ozzle. .............................................................................. 2 8

4-9 Ray tracing output from MATLAB when the aluminum plate is 7 cm from the
n ozzle. .............................................................................. 2 9

4-10 Glare for different plate positions as a function of number of rays....................29

4-11 Glare as a function of plate position and wavelength.......................... .........30

4-12 Typical waveform from a photomultiplier tube................... .................................31

4-13 Running average of area .............................................. .............................. 32









4-14 Percent variation in area...................................................................... 33

4-15 Running average of peak voltage.................................................. ....................33

4-16 Percent variation in peak voltage............................................... ........ ....... 34

4-17 Correlation between peak voltage and area ................................ ..................... 35

5-1 Signal-to-glare ratio as a function of plate position............................................37

5-2 Comparison of experimental and theoretical glare..............................................38

5-3 Photomultiplier-tube and the photodiode voltage as a function of the power meter
v oltag e......................................................................................... .. .. ..... 3 9

5-4 Photomultiplier-tube voltage as a function of the power meter voltage when the
glare is varied using an aluminum plate. .................................... .................40

5-5 Theoretical photon-arrival rates....................................... .......................... 46

5-6 Comparisons of theoretical and experimental photon-arrival rates .....................47

5-7 Percent error between theoretical and experimental photon-arrival rates .............48

5-8 Raw Photomultiplier-tube voltages as a function of radial and plate position for
5 3 2 n m .......................................................................... 4 9

5-9 Raw Photomultiplier-tube voltages as a function of radial and plate position for
355 nm ............................................................................5 1

5-10 Percentage errors in raw Photomultiplier-tube voltage for two typical plate
positions as a function of radial position. .................... ...................................51

5-11 Glare as a function of radial and plate position for 532 nm ..............................52

5-12 Glare as a function of radial and plate position for 355 nm ..............................53

5-13 Rayleigh light scattering signal as a function of radial and plate position for 532
n m ................................................................................ 5 4

5-14 Rayleigh light scattering signal as a function of radial and plate position for 355
n m ................................................................................ 5 5

5-15 Signal-to-glare as a function of radial and plate position for 532 nm and 355 nm56

5-16 Raw Photomultiplier-tube voltages as a function of radial and plate position for
532 nm and 355 nm ............. ........................................ ........... .. ..... 57










5-17 Rayleigh light scattering signal as a function of radial and plate position for 532
n m ................................................................................ 5 8

5-18 Rayleigh light scattering signal as a function of radial and plate position for 355
n m ................................................................................ 5 8

5-19 Correlation of Rayleigh light scattering signal between two wavelengths............59

5-20 Variation in R2 value as a function of plate position. ...........................................60















LIST OF ABBREVATIONS AND SYMBOLS

Symbol Description

a Particle radius (m)

A* Area of the back ground surface visible to the detector(m2)

A Area under the voltage time curve

c Speed of light (m/s)

C Optical system calibration constant

C* Surface scattering parameter

CPMT Photomultiplier tube calibration constant

dV Control volume (m3)

dp Diameter of Rayleigh scattering particle

e Electronic charge = 1.602E-19 Coulombs

Ei, E2, E3 Energy states of a molecule.

E Electric field vector


B Magnetic field vector


S Scattering field

h Planck's constant = 6.626E-34 Js

I Intensity (photons/m2-pulse)

Io Incident laser power (photons/ m2-pulse)

k Boltzmann constant (Joule/Kelvin)









n Refractive index of the gas

n Number of moles

N Molecular number density (molecules/m3)

N* Number of molecules

NA Avogadro's number

P Pressure (N/m2)

r Radial distance from jet centerline (m)

r Distance between the collecting lens and the aluminum plate

Re Reynolds number = pvD/ t

R Universal gas constant (8.314 J/ mol K)

R Ratio of the reflectivity at 532 nm and 355 nm

Rc Radius of the collecting lens (m)

Rp Photon arrival rate (photons/m2-pulse)

S Ratio of the scattering cross-sections at 532nm and 355 nm

T Temperature of gas Kelvin)

v Velocity of buoyant jet (m/s)

V Photomultiplier tube voltage (V)

x Percentage of leaking fluid (80% nitrogen and 20% helium) in the
control volume.

z Downstream distance (mm)


Greek Symbols

a Size parameter = 27na/k

0 Spread angle

















xl'






dco






p

V




v
It

V

At

Subscripts

Al

A2

a

act

air

avg

Collectinglens

FWHM


Angle of observation measured from the forward to scattering
directions.


Scattering angle

Wave function

Optical efficiency of transmitting and collecting lenses

Uncertainty

Solid angle of the collection optics

Wavelength of laser light (nm)

Scattering cross-section (m2)

Reflectivity

Volume (m3)

Dynamic viscosity of gas (Pa-s)

Frequency (Hz)

Time interval between two readings(s)



532 nm wavelength line

355 nm wavelength line

Ambient

Actual

Air

Average

Intensity of the reflected light at the collecting lens

Full width half maximum









Focusing

Glare

i

inc

leak


min

max

Mie Scattering

PMT

rl

r2

Rayleigh

reflected

RLS

ref

scat


Lens intensity of light at focusing lens

Scattering intensity due to Glare

Species

Incident

Scattering intensity due to leaking fluid(80% nitrogen and 20%
helium)

Minimum

Maximum

Mie scattering

Intensity of light at the photomultiplier tube

Energy state 1

Energy state 2

Rayleigh

Intensity of the reflected light

Rayleigh light scattering

Reference condition

Scattering















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

REMOTE DETECTION OF HYDROGEN LEAK USING Nd:YAG PULSED LASER
INDUCED DUAL LINE DETECTION RAYLEIGH LIGHT SCATTERING

By

Venkata Surya Raghuram Vempati

May 2005

Chair: Jill Peterson
Major Department: Mechanical and Aerospace Engineering

Our primary objective was to use laser induced Rayleigh light scattering to detect

the presence of hydrogen leaks in the presence of high amounts of glare. A mathematical

code in MATLAB was written to design the glare system and to compute the glare

numerically. Experimental and numerical results corresponded well with a maximum

error of 5%. Pure helium and a mixture of 20% helium and 80% nitrogen were used to

simulate the hydrogen leak. The mixture of helium and nitrogen is used because the

scattering cross-section of the mixture of 20% helium and 80% nitrogen is equal to that

of hydrogen. The scattering cross-section of helium was 0.015 times that of air and the

scattering cross-section of hydrogen was 0.23 times that of air. Major problems in using

the Rayleigh light scattering as a diagnostic tool are uncertainty due to electronic shot

noise and glare from the background surfaces. The uncertainty due to electronic shot

noise was found to be less than 0.1% for an averaging time of 0.001 sec. The Nd:YAG

pulsed laser (operating at wavelengths of 532 nm and 355 nm) was used. Intensity of the









scattered light due to the helium and nitrogen molecules in the jet was measured using a

photomultiplier tube. Data were acquired using a high-speed digital oscilloscope. An

aluminum plate was used to vary the glare. The ratio of the minimum to the maximum

intensity of scattering light was 0.2 when a mixture of helium and nitrogen was used (for

measurements at a downstream distance of 4 nozzle diameters). When pure helium was

used (for measurements at a downstream distance of 8 nozzle diameters) the ratio of the

minimum to the maximum intensity of scattered light was 0.14. It was possible to detect

the hydrogen leak even with glare-to-signal ratios as high as 6:1.














CHAPTER 1
INTRODUCTION

Project Goal

We designed and tested an experimental setup to measure hydrogen leaks in a

buoyant jet in the presence of high amounts of glare or low signal-to-glare ratios. We

used an Nd: YAG pulsed laser induced dual line-detection Rayleigh light-scattering

technique to eliminate the glare.

Rayleigh Light Scattering

Rayleigh scattering is an attractive technique for non-intrusive measurements with

high spatial and temporal resolution of gas-flow properties (such as density, temperature,

concentration in a mixture of gases and velocity in the case of high-speed flows). All of

these experiments measured the intensity of the scattered Rayleigh light to determine the

property of interest. All of these applications required a laser beam passing through the

gas. The laser beam is elastically scattered by the gas molecules when the incident

electromagnetic wave reacts with the dipoles in the gas molecules and the light beam is

scattered in all directions. Rayleigh light scattering is easy to set up compared to other

scattering techniques (such as Raman scattering). The main difficulty of Raman

scattering is the low scattered signal intensity, which requires relatively long integration

times for adequate signal-to-noise ratios and requires very low background-flame

radiation. The Rayleigh scattering cross-section is about 1000 times larger than the

vibrational Raman scattering resulting in much larger intensity









Two commonly encountered difficulties associated with Rayleigh scattering are the

contamination of the scattered signal with background noise and scattering from

particulates present in the gas (which is known as Mie scattering). Background noise

comes from two sources: the surface-scattered laser glare; and the light from the test

environment (which usually is broad band). Because the Rayleigh scattered light from

the control volume is at the same frequency and wavelength as the laser beam, it is

difficult to discriminate the surface scattered light from the Rayleigh signal. This

problem is intensified when measurements are taken in closed enclosures. Glare can be

controlled principally by blackening the surfaces.

Our study used a dual line-detection technique to address the problem the

background noise. We used an Nd: YAG pulse laser with a repetition rate of 10Hz. The

signal was collected at two laser lines of wavelengths 532 nm and 355 nm with the

energy of the laser being 200 mJ/pulse and 60-95 mJ/pulse at 532 nm and 355 nm

respectively. Using the pulse laser provides a high level of Rayleigh signal because of

the high energy densities at each pulse. The signal obtained from the two lines is

analyzed simultaneously to eliminate the glare from the Rayleigh signal The dual line-

detection technique greatly enhances the scope of the Rayleigh scattering technique as a

measurement tool since it completely eliminates background noise from the Rayleigh

signal; thus improving the Rayleigh signal. The system was tested and calibrated at

various levels of background noise. Finally, measurements were taken using a mixture of

helium and nitrogen issuing out of a nozzle. Changes in concentration were measured by

traversing the nozzle in the radial direction (to and from the centerline); thereby changing

the concentration of gas due to entrainment of air. Results indicate that accurate









measurements are possible with the dual-line-detection technique in the presence of high

levels of background glare. Our work is the first step towards the remote detection of

hydrogen leaks.














CHAPTER 2
LITERATURE SURVEY

Lord Rayleigh was the first person to systematically observe light scattering, in

1871. The basics of Rayleigh and Mie scattering theory have been described (Van De

Hulst, 1957, Kerker, 1969, Bohren and Huffman 1983). They gave the general solution

of the Maxwell equations that describes independent scattering of incident

electromagnetic waves by an isolated sphere. The solution is termed Mie scattering

theory; and is a complicated series solution. For particles that are small compared to the

wavelength of the incident light (r < 0.03k), the Mie series simplifies; and is termed

Rayleigh scattering. The initial term in the series is dominant and is sufficient to express

the intensity of the scattered light. McCartney (1976) gave a good description of

Rayleigh light scattering.

Laser-induced Rayleigh light scattering has been used for many purposes in the

past. It has been used successfully for temperature measurements (Pitz et al. 1976;

Dibble, Hollenbach and Rambach 1980; Bill et al. 1981). Pitts and Kashiwagi (1984)

used Rayleigh light scattering to study turbulent mixing. (Namer and Scheffer, 1985;

Namazina, 1989) used Rayleigh light scattering for combustion studies. Rayleigh light

scattering has also been used to obtain temperature measurements in heated buoyant jet

(Otugen and Namer (1988)). Otugen (1993) used a dual line-detection technique to

simultaneously determine the glare and remove it from the Rayleigh signal of interest.

Pitts, and Bryner, (1992) used Rayleigh scattering for investigation of free jets and






5


plumes. It has also been used for temperature measurements in rapid thermal chemical

vapor deposition reactor by J.E. Peterson and J.F.Horton, (1998).














CHAPTER 3
THEORETICAL BACKGROUND

Light Scattering

Scattering is a process by which a particle in the path of an electromagnetic wave

continuously abstracts energy from the incident wave and reradiates that energy into the

total solid angle centered at that particle. For scattering to occur it is necessary that the

refractive index of the particle be different from the surrounding medium.

There are two types of light scattering mechanisms: elastic scattering and inelastic

scattering. Inelastic scattering is also called Raman scattering and in this type of

scattering the frequency of the scattered wave is different from that of the incident wave

and there is a change in the energy of the incident wave. The molecule may either gain

energy from, or lose energy to, the photon. In Figure 3.1, 3 energy states of the molecule

are shown (Ei, E2, and E3). The molecule is originally at the E2 energy state. The photon

interacts with the molecule, exciting it with an energy hvino. However, there is no stable

state of the molecule corresponding to this energy, and so the molecule relaxes down to

one of the energy levels shown. In doing this, it emits a photon.

If the molecule relaxes to energy state El, it will have lost energy, and so the

photon emitted will have energy hvri, where hvri > hvinc. These transitions are known as

anti-Stokes transitions. If the molecule relaxes to energy state E3, it will have gained

energy, and so the photon emitted will have energy hvr2, where hvr2 < hvinc. These

transitions are known as Stokes transitions.











hvr hv, hv,


I__ iF 1_


Figure 3-1. Raman scattering
Mechanism of Elastic Scattering
Consider an elemental scatterer such as a gas molecule in the path of an

electromagnetic wave. The gas molecule can be considered as a mechanical oscillator of

unequal masses carrying opposite charges at the center and the periphery according to

McCartney (1976). The elastic scattering theory assumes that the molecule is isotropic,

non polar and non-ionized. These assumptions mean that the molecule does not

experience a net force in an electric field and the negative charge is uniformly distributed

at the periphery and may be treated as though it were at the center. This means there is a

negative charge at the center with equal positive charge. Hence the net dipole moment,

which is equal to the product of the charge and the separation distance, is equal to zero.

But when the molecule is subjected to external electric field of an electromagnetic wave,

the charges are forced apart (Figure 3-2) and an induced dipole moment is created. This

induced dipole moment oscillates synchronously with the field and emits a secondary

wave with the same frequency as that of the incident electromagnetic wave and this

secondary wave is the scattered wave. An oscillating dipole emits electromagnetic waves









because it contains oscillating electric currents, and because the changing positions of the

positive and negative charges make it harder for their electric effects to cancel. This

elastic scattering of light can be explained by using two theories.



Displacement



+e
-I --





Shell of

harge






Figure 3-2. Creation of an induced dipole moment by an electric field which displaces
the plus and minus charges of the molecule. (Source: McCartney, Earl J.,
Optics of the Atmosphere, 1976, Figure 4.2, pp. 179-181, New York, John
Wiley & Sons.)

Mie scattering

Scattering by particles of arbitrary size is called Mie scattering and is discussed in

detail by Kerker (1969). Starting with very small particles, as the particle size relative to

the wavelength increases, there is a gradual transition from Rayleigh to Mie scattering,

which is characterized by

* A complicated dependence of scattered light intensity on the angle of observation,
the complexity increasing with particle size relative to wavelength.

* An increasing ratio of forwarding scattering to backscattering as the particle size
increases.

* Little dependence of scattering on wavelength when particle size relative to
wavelength is large.









Mie scattering has no size limitations and converges to the limits of geometrical

optics for large particles. Mie theory may therefore be used for describing most spherical

particle scattering systems (including Rayleigh scattering). However, Rayleigh scattering

theory is preferred if applicable, due to the complexity of the Mie scattering theory. The

criteria for Rayleigh scattering is a << 1, where a is the dimensionless size parameter

given by Equation 3-1, where a is the spherical particle radius, and A is the scattering

wavelength


21m
a = ,
A


(3-1)


Figure 3-3 shows the spherical coordinate scattering geometry used for Mie light

scattering for light incident on a single particle. Using this coordinate system, the

scattering parameters may be defined as follows.


Ssc
scat


Figure 3-3. Mie Scattering Geometry. (Source:
http://plaza.ufl.edu/dwhahn/Light0%o20Scattering%20Theory.pdf, Last
accessed December 5th, 2004).









For each scattering angle (0, 0), the Equations (3-2) and (3-3) represent the

intensities of scattered radiation vertically and horizontally polarized with respect to the

scattering plane, which is defined by the incident and scattered ray,


I, = Io i, sin2 (3-2)

A2
I = Io i2 CS2 os (3-3)
4;r r

In this formulation, lois the incident intensity, and the intensity functions ii and i2

given by Equations 3-4 and 3-5.

2n I
2n+ 1 [a,,,,(cosO)+ br,,(cosO) (3-4)
n(n + 1)


i2 1 + [a, (cos+b (cos ) (3-5)
n (n + 1)

Where ,, and ,, are the angular dependent functions and are expressed in terms

of Lengendre polynomials by


.,, (cos 0) = (3-6)
sin 0

dP'1 (cos ()
r, (cos ) os 0) (3-7)
dO

The constants an and bn are obtained from the boundary conditions that the

tangential components of the electric field and magnetic field of the incident wave are

continuous over the entire surface of the sphere.

Rayleigh scattering

The Mie solution is a complex mathematical solution and for particles of size much

less than the wavelength of the incident light, the Mie solution converges in one term and









is called Rayleigh theory and is discussed in detail by McCartney (1976). Rayleigh

scattering is elastic scattering of an electromagnetic wave by particles far smaller than the

wavelength of the incident wave (Figure 3-4). The electromagnetic radiation is scattered

in all directions different than that of the incoming light.





dp << A











Figure 3-4. The size of a Rayleigh scatterer is very small as compared to the wavelength
of the incident electromagnetic wave

Lord Rayleigh assumed that the particles were spherical, isotropic, far smaller than

the wavelength of light, and denser than the surrounding medium. He showed through

simple reasoning that the scattering varies as the square of the particle volume and

inversely as the fourth power of wavelength of light.

Rayleigh scattering is marked by several characteristics:

* The amount of light scattered varies nearly as the inverse fourth power of the
wavelength.

* Spatial distribution of scattered light bears a simple relationship to the direction of
observation.

* The light scattered at 90 degrees is almost completely polarized.









In the domain of light scattering, the concepts of angular and total scattering cross-

sections are basic concepts. These concepts lead to several coefficients and expressions

having great practical utility.

Angular scattering cross-section

The angular scattering cross-section of a molecule is defined as that cross-section

of an incident wave, acted on by the molecule, having an area such that the power

flowing across it is equal to the power scattered by the molecule per steradian.

The angular scattering cross-section represents the ratio of the scattered intensity to

the incident irradiance.

4r2 (n 1)2
0(90) 42 (n 1)2 (3-8)


This expression is derived for an ideal gas assuming that n-l<
index term, n-1, is proportional to the molecular number density. Therefore, the

scattering cross-section for an individual molecule is not a function of N, and is

independent of gas temperature, pressure and density.

For a volume of gas, however, the amount of scattered light is proportional to gas

number density. The random spacing and thermal motion of gas molecules are such that

scattering is incoherent and independent. The result is that there are no discernable phase

relationships between the separately fluxes except in the exact forward direction and thus

the individual intensities are additive. The scattered intensity is therefore, is directly

proportional to the molecular number density.

I(90)s,,t = 0(900)NIo (3-9)

The constant of proportionality is defined by the solid angle of the collection

optics, the length of the control volume and the optical efficiency of the collection optics.









Hence the intensity of the Rayleigh scattered light from a control volume containing a

mixture of gases is given by

IRLS = I (do)(dV)(7) (No), (3-10)

Dual Line-detection Technique

The major problem in using Rayleigh light scattering as a diagnostic tool is

background glare and Mie scattering. Mie scattering due to dust and aerosols can be

reduced by filtering the incoming air. To eliminate the glare from the Rayleigh signal,

dual line-detection technique is used in this project. It involves obtaining the scattered

light intensity using two different lines of the laser and solving the set of simultaneous

linear equations to eliminate the glare from the Rayleigh signal. It is known that the

Rayleigh signal is proportional to the incident laser intensity and the gas number density

and also to the scattering cross-section, where C is the optical system calibration constant

and N is the gas number density.

I(90o)RLS = Ca(90 )NI (3-11)

The gas number density is the ratio of the number of molecules to the volume occupied

by them. It can be written mathematically as

number of molecules N*
N = (3-12)
Volume occupied by the molecules V

Where N* = (number of moles)(Avogadros's Number) = n NA (3-13)

From the ideal gas law it can be noted that the gas number density can be replaced

by the pressure, temperature and Boltzmann constant. The ideal gas law is given by

Equation 3-14, where n is the number of moles and R is the universal gas constant

(8.3145 J/mol K).









PV = nRT, (3-14)

Replacing n with gas number density in Equation 3-14, ideal gas law can be written as

P = NkT, (3-15)

Where k is Boltzmann constant and is defined as

R
k = 1.38066E-23 J/K (3-16)
NA

NA = Avogadro's number = 6.023E23

Hence the intensity of the Rayleigh signal can be written as


IRLS = r (3-17)
kT

The scattered light intensity detected by the photomultiplier-tube is the sum of the

Rayleigh signal and the glare. Hence the intensity of the scattered light incident on the

photomultiplier-tube can be written as

Iscat = IRLS + Glare (3-18)

IGlare is proportional to the incident intensity and the reflection from the background

surfaces.

IGlare = IopALe (3-19)

Where C* is the surface scattering parameter and A* is the area of the background

surface visible to the detector. Hence the scattered light intensity can be written as


IscO, = (CIl -c) + (CI opALe. ) (3-20)

The scattered intensity can be normalized with the incident intensity to account for

the pulse to pulse variation in the incident intensity of the laser. Hence the scattered light

intensity can be written as









I PU
Iat =C--a+C pALens (3-21)
I, kT

Since two different lines of wavelengths 532 nm and 355 nm are used, the intensity of the

scattered light at both these wavelengths can be written as

Isa = CI +C 2AlplALens (3-22)
o A


a = C, + C2 ALens (3-23)


Hence by varying either the pressure, temperature or the scattering cross-section

individually or a combination of any of the three parameters, the intensity of the scattered

light can be obtained at different conditions and then by solving the simultaneous set of

equations using linear regression the glare can be eliminated from the Rayleigh signal.

The detailed description of how the glare is eliminated is given in Chapter 5

Buoyant Jets and Plumes

The primary objective in this project is to detect a hydrogen leak. For this purpose

a mixture of helium and nitrogen are used that issue out of a nozzle and since the

densities of both helium and nitrogen are different from the surrounding ambient air,

buoyancy forces arise in the jet.

A fluid motion is called a jet if its primary source of kinetic energy and momentum

flux is a pressure drop through an orifice. A fluid motion whose primary source of

kinetic energy and momentum flux is body forces is called a plume and flows whose

motion is in transition from a jet to plume are called forced plume or a buoyant jet.

For a jet, due to conservation of momentum, the momentum flux is constant along

the jet axis. Since the mass flux increases along the jet axis due to entrainment, the axial









velocity must correspondingly decrease. For a plume, the buoyant force tends to

accelerate the fluid in the plume in a vertical direction, turning the plume axis in the

direction of the buoyant force. For a plume initially discharged upwards, and with an

upwards buoyancy force, the deceleration (caused by entrainment) is hence less than for

the case of a jet. Figure 3-5 shows the instantaneous and time averaged profiles of a

buoyant jet.






me Aeragcd Pfile

proe \ \ F Sisar la-r



Pontiial Sfcad angle
cre

Nzzle Blaikr cover





Figure 3-5. Instantaneous and time averaged profiles of a typical buoyant jet

Inside the potential core the concentration of the flow fluid is 100% and at the edge

of the shear layer the concentration of the flow fluid is 0%. According to Chen and Rodi

(1980) the spread angle is 13o for vertical round buoyant jet.
















CHAPTER 4
EXPERIMENTAL METHODS AND APPARATUS

This chapter describes the design of experimental set up for detecting the hydrogen

leaks by measuring the intensity of the scattered light at 90 degrees to the incident light.

The leak was simulated using a jet of pure helium in some cases and a mixture of 20%

helium and 80% nitrogen in others. The mixture of 20% helium and 80% nitrogen was

used because the scattering cross-section of the mixture is equal to the scattering cross-

section of hydrogen which is 1.89E-32 m2/sr. at 532 nm. Also, the data acquisition

system used to measure the intensity is described.

Incident B ea Nozzle Glass Pece






Collecting Len a a a

j _B and Pass filters
hotodiodes

Focu sing Le-, L-l io \

High Speed .J ijoscilloscope Beam Splirter




^// / ____ Phztomulttplilr tub,


Figure 4-1. A schematic of experimental setup used for collection of scattered light at
900

The incident laser beam was generated by an Nd:YAG pulsed laser, the

fundamental wavelength being 1064 nm and the energy being 450 mJ/pulse at that










wavelength. However in our work, the laser was operated at 532 nm and 355 nm

wavelengths using 2nd and 3rd harmonic generators. The energy of the laser was 200

mJ/pulse and 60-95 mJ/pulse at 532 nm and 355 nm respectively. The laser was operated

at a frequency of 10 Hz and the pulse width of the beam was 10 ns with a beam diameter

of 6 mm and a beam divergence of 1.1. The incident laser beam passed over the jet of

helium coming through a nozzle.

A 1/4" diameter nozzle was used to simulate the leak, it was placed at a distance of

10" from the laser and hence the diameter of the laser beam at the nozzle is 8 mm. The

leak was simulated at a Reynolds number of 100. The nozzle was mounted on 3

micrometer traverses for 3-dimensional motion (Figure 4-2)

Aluminum Plate
Laser Pulse

Plate position Nozzle
from nozzle z






He


Mass Flow Traverse 1
S Meters r



Valve

Figure 4-2. Mounting of nozzle, gas-flow meters and position of aluminum plate

The least count of each traverse was 0.05 mm. The mass flow rate of pure helium

and the mixture of helium and nitrogen were monitored using the mass flow controllers.

When no plate was used, the laser beam was trapped using a beam dump which was

placed at a distance of 20" from the nozzle.









The collection optics were oriented at 90 degrees to the incident laser beam (Figure

4-1) and consisted of a pair of 60 mm diameter lenses.(from TSI optics). The collecting

lens has a focal length of 254 mm and this lens collects and collimates the scattered and

reflected light. The focusing lens has a focal length of 124 mm and this lens focuses the

collimated beam onto a 0.15 mm and 3 mm long slit which was mounted on the face of

the photomultiplier-tube. The scattered light was separated into two different lines of

wavelengths 532 nm and 355 nm using a beam splitter (from CVI lasers.) The beam

splitter had a manufacturer coated transmissivity of 100% at 355 nm and 100%

reflectivity at 532 nm. Band pass filters (from Newport) were mounted in front of both

the photomultiplier tubes which allow only the respective wavelengths to pass through

and eliminated any light of other wavelengths to be detected by the photomultiplier-

tubes. The collection optics and the two photomultiplier-tubes were covered with a black

drape to reduce any stray light to be detected by the photomultiplier-tubes.

The photomultiplier-tube was Hammamatsu model HC120-01 and has a built in

amplifier with adjustable gain. The spectral range of the photomultiplier-tube was 185 to

650 nm and has a frequency response of 23 kHz. The signal from the photomultiplier-

tube was acquired using a high-speed digital oscilloscope.

The oscilloscope is a LeCroy model LT 372. It was triggered externally using the

pulsed-laser. The laser sends a trigger pulse to the oscilloscope exactly 100 ns before

pulsing and after 100 ns the laser sends an output beam. The oscilloscope was set to

record the data 100 ns after it received the trigger pulse. The signal from the two

photomultiplier-tubes was acquired on two different channels of the oscilloscope.









A portion of the incident laser beam was deflected using a piece of glass and was

focused onto two different photodiodes (from Thor Labs) with the aid of a beam splitter

to monitor the pulse-to-pulse variation in the incident beam power. The signal from the

two photodiodes was analyzed using two different channels on a high-speed digital

oscilloscope. A band pass filter was mounted in front of each photodiode to eliminate

any light other than the signal at the respective wavelengths to be detected by the two

photodiodes.

Gas-flow Path

The leak was simulated using a mixture of 20% helium and 80% nitrogen coming

out of a nozzle. The gases used were pressurized in high pressure cylinders. The gases

from these cylinders were connected to the nozzle using hoses and the mass flow rate was

monitored using two gas-flow meters.

Varying Glare

To vary the glare an aluminum plate was used (Figure 4.2). The laser beam was

reflected from the aluminum plate and the intensity of the reflected light was measured

by the two photomultiplier-tubes. The plate was mounted directly in the line of the

incident beam on a traverse so that it can be traversed in the horizontal direction (to and

fro from the center of the control volume) and thus the glare could be varied.

Design of Plate Position

To calculate the glare numerically and to study how the glare varies as a function of

the plate position, a ray tracing program was written in MATLAB. The rays were traced

from the surface of the aluminum plate to the collecting lens, focusing lens and finally

onto the photomultiplier tube. The code used simple co-ordinate geometry and Snell's

law to trace the rays









Snell's law states the ratio of the sine of the angle a particular kind of wave makes

in one medium to the sine of the angle it makes in another medium is a constant. This

constant is also called the index of refraction. It can be represented in mathematical form

as

nA sin OA = n sin 0 (4-1)

The assumptions made in the numerical design of plate position are
* The light reflected from the aluminum plate was 100% diffuse.

* All calculations were made in 2-dimensional plane.

* The pulse-to-pulse variations in the incident laser beam were neglected.

* The reflectivity of aluminum plate is assumed to be 0.93 at 532 nm and 0.85 at 355
nm.

* The number of rays used to calculate glare ranged from 100 to 100000

* The glare computed numerically was glare from the plate only and the glare from
the background surfaces was neglected.

The collecting lens was drawn using the general equation of the circle (Equation

4.2) with the focal point of the lens as the center and the radius of the collecting lens as

the radius of the circle (Figure 4-3).

x2 y2 = RC2 (4-2)

Only a fraction of the rays reflected from the aluminum plate reached the collecting

lens. To trace the rays from the collecting lens to the focusing lens, Snell's law was

applied at both ends of the collecting lens (Equation 4-1). The reflected rays from the

A
aluminum plate incident on the collecting lens bend toward the normal nA (Figure 4-4)

as the refractive index of the lens is greater than the refractive index of air. The normal










nA to the collecting lens at (pi, qi) is perpendicular to the flat surface of the collecting

lens.


Focal Point(0,


Figure 4-3. Collecting lens of radius Rc and having focal point at (0,0).

The angle of incidence is 0A and the angle of reflectance is O The refractive index

of air is 1.0029 and that of the collecting lens is 1.523.

The angle of incidence of the reflected rays at the second edge of the collecting lens

is 0c and the angle of reflectance is 0 The rays exiting the collecting lens bend away

A A
from the normal n, (Figure 4-4). The normal n, to the collecting lens is drawn by

differentiating the Equation 4-2 at (p2, q2) as given by the Equation 4-3. The transmitted

rays from the collecting lens are incident on the curved surface of the focusing lens with

an angle of incidence of OE and have an angle of reflectance of O.


n ( = (p2 ,12)
(dyl /dx) p2,q2)


(4-3)












Rays reflected
from
Aluminum
Plate


AIIIumu111 in Plate


Figure 4-4. Ray tracing of the reflected rays from the aluminum plate at the collecting
lens

To trace the rays from the focusing lens to the photomultiplier-tube, Snell's law

was applied at both ends of the focusing lens (Equation 4-1). The ray's incident on the


focusing lens from the collecting lens bend toward the normal nc and the rays exiting the

A
focusing lens will bend away from the normal nD (Figure 4-5). The intensity of the light

reflected from the aluminum plate was calculated given by Equation 4-4. As mentioned

in the assumptions, 100000 rays were used in the calculation of the glare and each

individual ray was assumed to have equal intensity because the rays were reflected

diffusely from the aluminum plate.









Reflected = oP


p4,q4


Figure 4-5. Ray tracing of the reflected rays from the collecting lens at the focusing lens


The intensity of the reflected light at the collecting lens was calculated based on

number of rays reaching the collecting lens from the aluminum plate and since each ray

has equal intensity, the total intensity is the product of number of rays and the intensity of

individual ray

The incident laser light was assumed to be diffusely reflected in all directions.

Only a fraction of the reflected rays reached the collecting lens which was calculated by

computing the solid angle subtended by the reflected rays at the collecting lens. The solid

angle dao subtended by a surface is defined as the surface area of a unit sphere covered

by the surface's projection onto the sphere. The solid angle was calculated according to

the formula given by Equation 4-5, where rpl is the radius of the hemisphere which is the


(4-4)










focusingg Lens


nf0,









distance between the aluminum plate and the collecting lens (Figure 4-6), and Rc is the

radius of the collecting lens


do projected areaof the collecting lens mei cosO
Area of the Hemisphere of radius r, Zr rpl2


(4-5)


Plate at position 1


Plate at position 2


Figure 4-6. Solid angle subtended by the reflected rays from the aluminum plate at the
collecting lens for two different plate positions.

The intensity of the reflected light at the collecting lens is given by Equation 4-6.

Icollechnglens = (Ireflected Xdo) (4-6)

Then the intensity of the reflected light at the focusing lens is calculated based on

the number of rays reaching the focusing lens from the collecting lens (Equation 4-7),

where the term 0.9 is the optical efficiency of the collecting lens.

Ifog (I c ) (0) number of rays reaching the focusing lens (47)
ls total number of rays









The intensity of the reflected light at the photomultiplier tube is calculated based on

the number of rays reaching the photomultiplier tube from the focusing lens (Equation 4-

8), where 0.9 is the optical efficiency of the focusing lens.

T (fsie) ( 9 number of rays reaching the PMT (48)
IPMT =(Ifocusm.glemn)( 0.9 4 -)
total number of rays

Figure 4-7 shows the output of the MATLAB code for the case where the

aluminum plate was directly over the nozzle. As the focal point of the collecting lens

was at the nozzle, the reflected rays from the aluminum plate originated at the focal point

of the collecting lens and were collimated. The collimated beam from the collecting lens

was focused by the focusing lens onto the photomultiplier-tube. The photomultiplier-

tube was at the focal point of the focusing lens.

Figure 4-8 shows the output from the MATLAB code for the case when the

aluminum plate was 3 cm from the nozzle. Not all the rays reflected by the collecting

lens reached the focusing lens.

Figure 4-9 shows the output from the MATLAB code for the case when the

aluminum plate was 7 cm from the nozzle. It was observed that none of the rays reflected

by the collecting lens reached the focusing lens and the glare from the aluminum plate

detected by the photomultiplier-tube was zero for this case.

Figure 4-10 shows the signal-to-glare as a function of number of rays used in the

code. 100 to 100000 rays are used in the computation of the glare. Using more than

100000 rays was time consuming and the results obtained were not significantly accurate

as compared to the results obtained with 100000 rays. The x-axis in the figure is the plate

position in meters from the control volume. Theoretical Rayleigh scattered signal was

used for the computing the signal-to-glare ratio.



















Incident Laser
Beam


Aluinum Plate -


Reflected
rays from
Aldutiumnu
Plate c 25.4 cm

i
i
Collecriig Lerns



r.,
I.
'i


30cm
I-
Iu


"-
FocMiiTi Lug I


12.4 cm

PMT


Figure 4-7. Ray tracing output from MATLAB when the aluminum plate is directly over
the nozzle


Nd Yag Pulsed laser














Nd:Yag Pulsed laser


Incident Laser
Beam


Alununum Plat


Reflected
rays from
Aluminum ;
Plate 25.4 cm




M1111E
NUIII
iiiiill
Ivl Ill
IIir ll
I ii ii i
Collect Len llll
Ifliii i I
IIiUI, II
:1,II1lI 3 'r|cm
Ifllll11

JIIIll I
II I III
111111 111
II III II
IIII11llll


II

PMT 4 cm


PMT =


Figure 4-8. Ray tracing output from MATLAB when the aluminum plate is 3 cm from
the nozzle.














Nd Yag Pulsed laser


Incident Laser
Beam


AJLiiuf iiilu Plate



Reflected
rays from
Aluinaiiun
Plate 25

AM
lliir
Mill
MMI




III ill
rIlll
1'Wllf







t Ili I
Collecting L euI, "

1If.1..













A ir
|..J'




Sului .I


.4 cm


I cm


Figure 4-9. Ray tracing output from MATLAB when the aluminum plate is 7 cm from
the nozzle.


1.00E+00 -O
0.0(0E+00 5.000E-02 5.900E-02 6.040E-02 6.081E-02
1.00E-01
w
- 1.00E-02

S1.00E-03

1.00E-04

1.00E-05

plate position(m)


number of
rays
- 100 rays
10000 rays
A 100000 rays


Figure 4-10. Glare for different plate positions as a function of number of rays.









It was observed that when 100 rays were used the glare initially decreased as the

plate was moved away from the nozzle and then the glare became constant at a plate

position of 0.0603 m from the control volume. When 10000 rays were used the accuracy

of the code increased and the glare decreased as expected when the plate was moved

away from the nozzle and was not constant as was the case when 100 rays were used.

When 100000 rays are used the accuracy of the code further increased and the glare

decreased as the plate was moved away from the control volume.

Figure 4-11 shows the glare from the aluminum plate as a function of plate position

(for 532 nm, 355 nm and 1064 nm). It was observed that the glare decreased as the plate

was moved away from the nozzle as expected and there was exponential decrease in glare

starting from a plate position of 6.08 cm from the nozzle and reached almost zero when

the plate was at 6.083 cm from the nozzle.


1.00E+00 -
1.00EB9COE+00 5.500E-02 6.010E-02 6.070E-02 6.083E-01
1.00E-02
1.00E-03
C 1.00E-04 f 3 ? 532 nm
1.00E-05 t A 355 nm
r 1.00E-06 AA A 1064 nm
S1.00E-07 t A
1.00E-08 A
1.00E-09 -
1.00E-10
Plate position(m)

Figure 4-11. Glare as a function of plate position and wavelength. 100000 rays are used
for the computation of the glare.










Experimental Methods

To measure the intensity of the scattered beam two different approaches are

adopted. The first method involved calculating the integrated area under the voltage time

curve and the second method involved using the peak. We conducted these studies to

establish the use of peak voltage as a repeatable and reliable method of acquiring data

rather than integrated area.

Integrated Area Method

This method involved calculating the full width half maximum area under the

voltage time curve measured as the area between two points where the voltage is 50% of

the maximum voltage. Initially 1000 data points per pulse are captured from the

oscilloscope. A typical waveform captured from the oscilloscope is shown in Figure 4-

12.


0.18
0.16
-^
0.14
0.12
s 0.1
-I

o 0.08-
-
0.06
S0.04
e 0.02
0


pap gas pp p p p


-0.02160 166 172 178 184 190 196 202 208 214 220 226 232 238 244 250 256
time microsecondss)

Figure 4-12. Typical waveform from a photomultiplier tube as captured by the
oscilloscope. (Down stream distance of 4 nozzle diameters; Re=100; 100%
Helium)

The full width half maximum area was calculated from the voltage time curve

using the trapezoidal rule given by Equations 4-9 and 4-10.











A = V 1At (4-9)


1000
A = A (4-10)


These measurements were done at a Reynolds number of 100 and at 4 nozzle

diameters downstream with 100% helium flowing through the nozzle. These were done

in the shear layer of the leak at a radial position of the nozzle corresponding to 60%

helium. This is because it is at the shear layer; the maximum variation in the voltage was

expected because jet fluctuations are greater there. The same procedure was repeated

until the areas converged. Figure 4-13 shows the convergence studies for the full width

half maximum area for both 355 nm and 532 nm wavelengths.


3.10E-06
2.90E-06 -
2.70E-06 -
2.0E-06- running average for 355
2.50E-06 -
nm
2.30E-06
-x-- running average for 532
| 2.10E-06 nm
1.90E-06 -
instantaneous area for
1.70E-06 *- --- tt-- 355 nm
1.50E-06 r"
instantaneous area for
1.30E-06 532 nm
1 2 3 4 5 6 7 8 9 10
number of pulses

Figure 4-13. Running average of area for 355 and 532 nm

It was observed that the area converged after 10 pulses. Figure 4-14 shows the

percent variation of the instantaneous area from the average area. Average area is the

average of the areas of the 10 pulses and is calculated as given by Equation 4-11. It was

observed that the percent variation was a maximum of 3.5%.











(4-11)


A


/2 3


//


4 5 6


--. 532 nm
7 9


number of pulses


Figure 4-14. Percent variation in area for 355 nm and 532 nm

Peak Voltage Method

To establish the convergence of the peak voltage and the repeatability, two methods

were adopted. First method involved recording the peak voltage from the voltage time

curve from which the full width half maximum area is calculated (Figure 4-12 and Figure

4-13). Figure 4-15 shows the convergence studies for the peak voltage.


U U


. .
h .8 .


2 3 4 5 6 7
number of pulses


* instantaneous peak voltage for
355 nm
instantaneous peak voltage for
532 nm
--running average of peak for 355
nm
-- running average of peak for 532
nm


8 9 10


Figure 4-15. Running average of peak voltage for 355 nm and 532 nm


10
A,
A -_
ag 10


0.58

0.56

0.54 i

0.52

0.5

0.48

0.46
0.44










It was observed that the peak voltage converged after 10 pulses and the percent

variation between the instantaneous peak and the average peak was 3% (Figure 4-16).


4
3


El
S--355 nm
-c-532 nm
i 3 4 5 6 7 9 0 532
E -1
2 -2
-3
-4 -
number of pulses

Figure 4-16. Percent variation in peak voltage for 355 nm and 532 nm

The second method involved recording the average peak voltage from the

oscilloscope after 0, 100, 200, 300,400 and 500 pulses and the percent variation in the

average peak was calculated. Table 4-1 shows the voltage recorded for each

measurement and also the percent variation between in the peak voltage after

100,200,300,400 and 500 pulses.

It was observed that the percent variation in the average peak was less than 1%

after 300 pulses. All these measurements were taken when 100% helium was flowing

through the nozzle and in the shear layer at a radial position of the nozzle where the

helium concentration was 60%










Table 4-1. Percent variation in average peak voltage for 355 nm and 532 nm
% variation in % variation in
peak voltage peak voltage
between two between two
Peak voltage Peak voltage measurements measurements
Pulse Number 355 nm 532 nm for 355 nm for 532 nm


1

100

200

300

400

500


460

443

447

444

441

443


552

533

525

526

529

531


3.837

0.894

0.675

0.680

0.451


3.564

1.523

0.190

0.567

0.376


Peak Area Correlation

A correlation study was done between peak and integrated area and it was observed

that the correlation coefficient between peak and area was 99.1% for both 355 nm and

532 nm (Figure 4-17).


0.65


0.55
oS
S0.5

0.45


R2 = 0.991i


R2 = 0.9916


* 355nm
* 532 nm


0.4 --
1.30E-06 1.80E-06 2.30E-06 2.80E-06 3.30E-06 3.80E-06
area (V-s)

Figure 4-17. Correlation between peak voltage and area for 355 nm and 532 nm






36


So it was established that recording the average peak voltage after 300 pulses is

reliable way of recording data and provides all the important information that is needed to

analyze the readings.















CHAPTER 5
RESULTS AND DISCUSSION

Numerical Results

As mentioned in chapter 4, a numerical code in MATLAB was written to compute

the glare numerically. The glare was computed as a function of plate position for the two

different wavelengths.

Figure 5-1 shows the Signal to glare ratio for different wavelengths as a function of

plate position. To compute the signal-to-glare ratio, first the theoretical Rayleigh signal

was calculated as given by the Equation 3-3 in chapter 3. Then the Rayleigh light

scattering signal was divided by the glare which was computed numerically as discussed

in chapter 4. As expected the Signal-to-glare ratio increased as the wavelength decreased

because of the inverse fourth power dependence of Rayleigh signal on wavelength and

also the Signal-to-glare ratio increased as the aluminum plate was moved away from the

control volume.


1.00E00
0.00 EO00 5.000E-02 5.900E-02 6.040E-02 6.081 Ei0
1.00E-01

2 1.00E-02 A
1 OOE-03 A wavelength
1.00E-03
S A 532 nm
1.00E-04 A A 355 nm

.OE-OSA A 1064 nm
A A A1.00E-05 A
1.00E-06
plate position (m)

Figure 5-1. Signal to glare ratio as a function of plate position.










To compare the experimental and theoretical results, the normalized voltage was

calculated for each case. This was done because the numerical code computes the glare

from the aluminum plate alone and does not take into account the glare from the

background surfaces, where as the experimental results incorporated the glare from the

aluminum plate and also the glare from the background surfaces. To account for this, the

photomultiplier-tube voltage was normalized with respect to the maximum and minimum

voltage as given by the Equation 5-1. The percent error in the theoretical and

experimental results is a maximum of 5%.


Voltage ratio= m (5-1)
max mm


1.2


E 0.8

E 0.6 theoretical
0.4 experimental

> 0.2 -
> 0
0.0 500 0.06010 0.06030 0.06050 0.06070 0.06081 0.06083
-0.2
plate position(m)

Figure 5-2. Comparison of experimental and theoretical glare as a function of plate
position.

Experimental Results

Testing the Linearity of the Photomultiplier-tube Output Voltage

This section discusses how the output voltage from the photomultiplier-tube was

tested to make sure that it was in the linear range. The laser power was varied from 0.9

kV to 1.31 kV and a power meter was used to monitor the incident laser power. As the










laser power was varied, the corresponding photomultiplier-tube voltage and the

photodiode voltage were recorded. Figure 5-3 shows the variation of the photomultiplier

and the photodiode voltages as a function of the power meter voltage. These readings

were taken at 4 nozzle diameters downstream and without any gas flowing through the

nozzle. The voltages recorded from the photomultiplier-tube, photodiode and the power

meter were normalized according to Equation 5-1. It was observed that the voltages of

both the photomultiplier and the photodiode increased linearly as the power meter voltage

increased.


1.2

1 R2 = 0.9984

S0.8 photodiode

c 0.6
S2 photomultiplier tube
= 0.4 R = 0.9744

E 0.2

0 0
0.2 0.4 0.6 0.8 1 1.2
-0.2
power meter voltage(V)

Figure 5-3. Photomultiplier tube and the photodiode voltage as a function of the power
meter voltage.

The photomultiplier tube had a correlation coefficient, R2 value of 0.9744 and the

photodiode had a R2 value of 0.9984.

The next study done was to ensure the linearity of the output voltage from the

photomultiplier tube when the glare was varied in the presence of the aluminum plate.

To do this the laser power was varied from 0.9 kV to 1.31 kV. The glare was varied by

placing the aluminum plate in two positions directly in line with the incident laser beam.

The incident laser power was monitored with a power meter when there was no plate.







40


These measurements were done at a nozzle position of 4 nozzle diameters

downstream and with no gas flowing through the nozzle. The photomultiplier tube

voltages are normalized using Equation 5-1. Figure 5-4 shows the variation of the

photomultiplier tube voltage as a function of the power meter voltage.


1.2

1 x x x x x x x x no plate

S0.8 plate at 6 cm
E A plate at 6.08 cm
0.6 -
x plate at 5.5 cm
"' 0.4
>I
0.2

0 --
0.00 0.13 0.29 0.43 0.57 0.68 0.80 0.90 1.00
power meter voltage(V)

Figure 5-4. Photomultiplier tube voltage as a function of the power meter voltage when
the glare is varied using an aluminum plate.

It was observed that the photomultiplier tube voltage increased linearly as the

power meter voltage increased even in the presence of the aluminum plate. The

correlation coefficient value, R2 when there was no plate is 0.9744 and the value ofR2

when the plate was at 6.08 cm from the nozzle is 0.9922 and the R2 value when the plate

was at 6 cm from the nozzle is 0.9939. When the aluminum plate was placed at a

distance of 5.5 cm from the nozzle, the glare was too high and there was no change in the

photomultiplier tube voltage. To test the linearity of the photomultiplier tube voltage

when the aluminum plate was placed at a distance of 5.5 cm from the nozzle, neutral

density filter was used. The main purpose of using a neutral density filter is to reduce the

amount of light that can pass through a filter. Neutral density filters absorb or reflect









fraction of light incident upon them. Neutral density filters are usually rated in optical

density numbers. Optical density is the degree of opacity of a translucent medium. It is

given by the Equation 5-2, where T is the transmission of the filter.

O.D = -log,0 T, (5-2)

Table 5-1. Optical density and the corresponding transmission of neutral density filters.
(Source: http://www.evetar.com/product/6.asp, Last accessed December 6th,
2004).
Optical Density Transmission
0.1 80%
0.2 63%
0.3 50%
0.4 40%
0.5 32%
0.6 25%
0.7 20%
0.8 16%
0.9 13%
1.0 10%

To check the linearity of the photomultiplier tube voltage, the photomultiplier-tube

voltage was recorded when the aluminum plate was at 5.5 cm from the nozzle and with

no neutral density filter. Then a 0.6 density filter was used and the voltage from the

photomultiplier tube was recorded and the procedure was repeated as the laser power was

varied. It was observed that the voltage from the photomultiplier tube decreased to 25%

as compared to the voltage when there was no neutral density filter. Since the

transmission of a 0.6 density filter was 25%, it was established that the photomultiplier

tube is operating in the linear range.









Table 5-2. Photomultiplier-tube voltages before and after using the 0.6 neutral density
filter.

Photomultiplier- Photomultiplier-
tube voltage before tube voltage before Photomultiplier-tubePhotomultiplier-
using neutral using neutral voltage with neutral tube voltage with
Laser density filter for density filter for density filter for 532 neutral density filter
Power 532 nm 355 nm nm (% decrease) for 355 nm (%
decrease)
0.9 1.562 1.012 0.389 (25.1%) 0.251 (24.8%)

1 1.562 1.325 0.390 (25%) 0.330 (24.9%)


Eliminating Glare from the Rayleigh Light Scattering Signal

This section discusses how the glare was eliminated from the Rayleigh light

scattering signal. To eliminate the glare from the Rayleigh signal, two approaches wesre

adopted.

Reference value method

This method involved recording the photomultiplier-tube voltage when there was

no gas flowing through the nozzle for every radial position of the nozzle and for every

plate position. The voltage recorded by the photomultiplier-tube was due to glare from

the background surfaces, Mie scattering and scattering due to air molecules. It is written

mathematically as

Vref = VGlare + VieScattering + ar (5-3)

Next, the photomultiplier tube voltage was recorded when a mixture of 80%

nitrogen and 20% helium was flowing through the nozzle. The diameter of the laser at

the control volume was 8mm and the distance between the edges of the shear layers of

the jet was 5.8mm. Hence there was entrainment of air in the control volume. Hence the

voltage recorded by the photomultiplier-tube was because of the glare from the









background surfaces, Mie scattering, and scattering due to the mixture of helium and

nitrogen molecules (leaking fluid) and scattering due to air molecules. It is written

mathematically as, where x is the percentage of the leaking fluid present in the control

volume.

Vact = VGlare + (ze Scatterng + V ar X x) + Vak (5-4)

When the two equations are subtracted, glare and the Mie scattering was eliminated

from the Rayleigh signal. It is written mathematically as

VRLS X(leak air MieScatterng ) (5-5)

Hence the glare was eliminated from the Rayleigh light scattering signal. But as

there was entrainment of air in the control volume (Equation 5-5) the signal is always

higher than the predicted signal (theoretically) as the scattering cross-section of air 1.32

times that of nitrogen and the entrained aerosols will scatter in the Mie regime.

Assuming that the ratio of the glare at the two wavelengths is constant

As discussed in chapter 3, an aluminum plate was used to vary the glare. This

method takes into consideration that the irradiance from the aluminum plate was much

higher as compared to the irradiance from other surfaces.

From Equation 3-18, the scattering intensity is written as

Iscat = IRLS +IGlare (3-18)

From Equation 3-17, the Rayleigh light signal is proportional to the scattering

cross-section of the gas and the incident intensity of the laser. Hence the ratio of the

Rayleigh light signal at the two wavelengths is the ratio of the scattering cross-sections.

IRLS,A1 CAl o,A1 o,
S=RLS2 (5.6)
IRLS,A2 C oA2 I.o,A2










The intensity of the glare Ilare is written as ZG,A, where Ai is the area of the


background surface visible to the detector and G, is the irradiance from ith surface. The

irradiance from the surface is proportional to the reflectivity of the surface. As the area

of the surface visible to the two detectors is same, the ratio of the glare at two

wavelengths was the ratio of the reflectivity of the surface at those two wavelengths.

Since different surfaces have different reflectivities, the ratio of the glare is not constant.

We used an aluminum plate to vary the glare. It was proved that the irradiation from the

aluminum plate was much more compared to the irradiation from other surfaces.


Z GAt >> GAA (5-7)
1=2

The ratio of the glare in this case was just the ratio of the reflectivity of the

aluminum plate. Since the ratio of the reflectivity of the aluminum plate was constant,

the ratio of the glare was constant and is written as


'Glare, 1 P11 1 o,\
R----- --- = R -- (5-8)
IGlare,.2 P21 o,A2 o,2

As discussed in chapter 4, the intensity of the light incident at the photomultiplier-

tube at the two wavelengths is given by the Equations 5-9 and 5-10.

Iscat, = IRLS,1 + IGlare,A1 (5-9)

Scat,A2 RLS,A2 Glare,Z2 (5-10)

From Equation 5-6 the ratio of the Rayleigh light scattering signal at the two

wavelengths is S and from Equation 5-8, the ratio of the glare at the two wavelengths is

R. Solving Equations 5-6, 5-8, 5-9 and 5-10 simultaneously, the Rayleigh light signal at

the two wavelengths was written as









RLS, S(Iscat1o, I,2 Scatr,A o, 2 )
RLS(S R)(I. 22)
(s


(5-11)


And


I girlsS, I oA2 (512)
IRLS,A2 = -I ~, (5-12)


Equations 5-11 and 5-12 gave the Rayleigh light scattering signal with the glare

decoupled from them.

Theoretical and Experimental Photon-arrival Rates

This section discusses how the theoretical and the experimental photon-arrival rates

compare.

Theoretical photon-arrival rate

As discussed in Chapter 3, the theoretical photon-arrival rate was calculated according to

the formula

R, = Io (r)(do))(dV)([N(xcrHe + (1- x)2 )]) (5-13)

Where

1, 200 mJ/Pulse = 5.34E17 photons/pulse for 532 nm

And 75 mJ/Pulse = 2.00el7photons/pulse for 355 nm

7 0.59049.

dV 0.118 mm3.

d) ;r7(60)2
(250)2

N 2.25E25 molecules/m3

x percentage of helium.










Table 5-3. Scattering cross-sections (of helium, nitrogen, hydrogen and air) at 532 nm
and 355 nm.
Scattering cross-section at Scattering cross-section at
Gas 532 nm, ua 355 nm, cA2
Helium 1.22E-33 6.17E-33

Nitrogen 6.15E-32 3.10E-31

Hydrogen 1.89E-32 9.53E-32

Air 8.16E-32 4.11E-31


The percentage of helium was decreased from 100 to 0 and the percentage of

nitrogen was increased from 0 to 100 and the resulting photon-arrival rate for both 355

nm and 532 nm is shown in Figure 5-5.


3.50E+09
3.00E+09

r 2.50E+09
2.00E+09 -355 nm
cac
0 1.50E+09 -532 nm

-.. 1.00E+09-
5.00E+08
0.00E+00
100 90 80 70 60 50 40 30 20 10 0
% helium

Figure 5.5 Theoretical photon-arrival rates for 355 nm and 532 nm.

Experimental photon-arrival rate

To calculate the experimental photon-arrival rates, a mixture of helium and

nitrogen was used and the measurements were done at a nozzle position of 4 nozzle

diameters downstream and the Reynolds number used was 100. The percentage of

Helium was decreased from 100 to 0 and the percentage of nitrogen was increased from 0










to 100 and the corresponding waveforms of the photomultiplier-tube are recorded from

the oscilloscope. The area under the voltage time curve was calculated as discussed in

chapter 4 using the Equations 4-9 and 4-10. Then the area which has units of volts-sec

was converted to photons per pulse using the Equation 5-14, where CpT is the

photomultiplier tube calibration constant and is obtained from the manufacturers

specifications. It has a value of 121 V/nW for 532 nm and 244 V/nW for 355 nm.

A A
p,Expenmental h (5-14)
PMThc

The glare from the experimental photon-arrival rate was reduced using the

reference value method. Figure 5-6 shows the comparison of the experimental and the

theoretical photon-arrival rates.


4.00E+09
3.50E+09 -
8 3.00E+09
S2. 50E+09 355 nm (experimental)
U) M m 532 nm (experimental)
o 2.00E+09 -
o o 2- 355 nm (theoretical)
S1.50E+09 532 nm (theoretical)
C-L 1.00E+09 *
cc *
5.00E+08
0.00E+00
100 90 80 70 60 50 40 30 20 10 0
% helium

Figure 5-6. Comparisons of theoretical and experimental photon-arrival rates for 355 nm
and 532 nm.

Figure 5-7 shows the percent error between experimental and theoretical photon-

arrival rates for 532 nm and 355nm. The percentage uncertainty due to electronic shot

noise was less than 0.1% for an averaging time of 0.001 sec. It is calculated according to

the formula











R 1 (5-15)
RRAt



90.00
80.00
70.00
60.00
500 355 nm
S50.00 -
S40.00 -
30.00 -
20.00 -
10.00 -
100 90 80 70 60 50 40 30 20 10 0
% helium

Figure 5-7. Percent error between theoretical and experimental photon-arrival rates as
shown in Figure 5-6

It was observed that the maximum error is around 80% for 100% helium and the

percentage error gradually decreased thereafter. This error is attributed to two facts.

* The diameter of the laser at the control volume was 8 mm and the distance between
the edges of the shear layers of the jet was 5.8 mm. As a result of this there was
entrainment of air in the control volume. As the scattering cross-section of air was
more than helium and nitrogen, the intensity of the scattered beam as detected by
the photomultiplier-tube was increased.

* The optical efficiency of the collection optics may deviate from the assumed 90%.

Analysis of the Recorded Data

Raw photomultiplier-tube voltage variation

This section discusses how the recorded data was analyzed to predict the presence

of hydrogen in the jet and to eliminate the glare from the Rayleigh light signal. Figure 5-

8 shows the variation of the raw peak voltage as a function of the radial position when

there was no plate and when the plate was at 6.083 cm, 6.08 cm 6 cm and 5.5 cm from

the nozzle for 532 nm wavelengths. The peak voltage was converted into mJ/pulse using







49


the photomultiplier-tube calibration constant provided by the manufacturer. These

measurements were taken at a downstream location of 4 nozzle diameters and when a

mixture of 20% helium and 80% nitrogen was flowing through the nozzle. This mixture

was used because the scattering cross-section of the mixture of 20% helium and 80%

nitrogen is equal to that of hydrogen. The scattering cross-section of hydrogen was 0.23

that of air (Table 5-3). The Reynolds number was 100. The x-axis is the radial position

of the nozzle normalized with the downstream distance.


2.50E-09


S2.00E-09 * *noplate
S+ plate at -r/z = 2.39
S150E-09 xxx + + + + + + + + 4 A plate at-r/z= 2.39
E x x plate at -r/z = 2.395
1O- a x mK x plate at -r/z = 2.36
S1.00E-09 a + + xx
S.0E-0 xx * plate at-r/z= 1.96
> + plate at -r/z = 2.16
E 5.00E-10 -plate at -r/z = 2.16

0.00E+00
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-8. Raw photomultiplier-tube voltages as a function of radial and plate position
for 532 nm. The measurements were taken at 4 nozzle diameters downstream
for a mixture of 20% helium and 80% nitrogen

It was observed that the peak voltage was constant outside the shear layer as there

was only ambient air outside the shear layer. The minimum peak voltage occurred at the

jet centerline which corresponded to 100% hydrogen. The fall in voltage at the jet

centerline occurred because the scattering cross-section of hydrogen was less than that of

air and as discussed in chapter 3, the voltage output from the photomultiplier-tube is a

function of the scattering cross-section of the gas. It was observed that the raw peak









voltage was not symmetric about the jet center line when the aluminum plate was used

where as it was symmetric about the jet centerline when there was no plate. This is

because when the plate was used, the glare was not constant and was higher when the

nozzle was closer to the plate and gradually decreased as the nozzle was moved away

from the plate. The glare from the aluminum plate was higher when the plate was at 5.5

cm and 5 cm from the nozzle and a neutral density filter of 0.6 optical density was used

to attenuate the intensity of the scattered beam.

Figure 5-9 shows the variation of the raw peak voltage as a function of the radial

position when there was no plate and when the plate was at 6.03 cm, 6.08 cm, 6 cm and

5.5 cm from the nozzle for 355 nm wavelengths. As observed from the Figure 5.8, there

was a fall in voltage at the jet centerline corresponding to 100% hydrogen and the peak

voltage was constant outside the shear layer.

As seen in Figures 5-8 and 5-9, two measurements were taken when the plate was

at 6.08 cm and 5.5 cm from the nozzle. This was done to test the repeatability of the

data.

Figure 5-10 shows the percentage error in the raw photomultiplier-tube voltages

between the two measurements taken when the plate was at 6.08 cm and 5.5 cm from the

nozzle for 532 nm and 355 nm. It was observed that the maximum error in the raw

photomultiplier-tube voltage when the plate is at 6.08 cm from the nozzle is 5%. The

maximum error when the plate was at 5.5 cm from the nozzle was 3%. Hence it was

concluded that the data is repeatable and reproducible.












5.00E-09
4.50E-09

4.00E-09
3.50E-09
3.00E-09 1

2.50E-09
2.00E-09
1.50E-09
1.00E-09

5.00E-10
0.00E+00


* no plate
* plate at -r/z = 2.39
A plate at -r/z =2/39
x plate at -r/z = 2.395
x plate at -r/z = 2.36
* plate at -r/z = 1.96
+ plate at -r/z = 2.16
- plate at -r/z = 2.16


-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z


Figure 5-9. Raw photomultiplier-tube voltages as a function of radial and plate position
for 355 nm. The measurements were taken at 4 nozzle diameters downstream
for a mixture of 20% helium and 80% nitrogen


* U


x
v


. A
. *.


1.5 1


* t

-


I
x


x


x



* plate at-r/z = 2.16; 355 nm

* plate at-r/z = 2.16;532 nm

A plate at -r/z = 2.39;355nm

x plate at -r/z = 2.39;532nm


0.5 0 -0.5 -1 -1.5 -2 -2.5 -3


XX x XX


Figure 5-10. Percentage errors in raw photomultiplier-tube voltage for two typical plate
positions as a function of radial position.


S- F + + + + +


Xx
X X K^^

X X X X x x x


m,,,


3 *










Variation of glare as a function of radial and plate position

Figure 5-11 shows the glare as a function of the radial position when there was no

plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for

532 nm.


1.80E-08
1.60E-08 -
+ + + ++ + +
1.40E-08 -+ + + + + + plate at-r/z=2.39
S1.20E-08 m plate at-r/z= 2.39
1.00E-08 xx A plate at-r/z= 2.395
Sx xxxxxx I x plate at-r/z= 2.36
p 8.OE-A09 L ; ; 1 I I I x x K plate at-r/z=2.16
6.00E-09 A X X
An X o plate at-rz= 2.16
6.00E-09 A xx plate at-r/z=2.16
4.00E-09 -+ plate at-r/z= 1.96
2.00E-09
0.00E+00
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-11. Glare as a function of radial and plate position for 532 nm for a downstream
location of 4 nozzle diameters and when a mixture of 20% helium and 80%
nitrogen is flowing through the nozzle.

It was observed that the glare was higher when the nozzle was closer to the plate

and gradually decreased as the nozzle is moved away from the aluminum plate. The

glare was calculated using the reference value method when there was no plate and when

the plate was at 6.083 cm, 6.08 cm, 6cm and 5.5 cm from the nozzle. When the plate was

at 5.5 cm and 5 cm from the nozzle, the glare was calculated taking into consideration

that the ratio of the glare at the two wavelengths was constant. The percentage error in

the glare calculated by two methods for the case when the plate was at 5.5 cm from the

nozzle was a maximum of 0.17%. So it was concluded that the two methods of

calculating the glare agree well with each other.










Figure 5-12 shows the glare as a function of the radial position when there was no

plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle for

355 nm.


0.000000012

0.00000001 + +
S + + + + + + + + + + + + + + plate at -r/z = 2.39
$ 0.000000008 1 'K 'K tI t plate at -r/z = 2.39
x x x x x x x A plate at -r/z = 2.395
S0.000000006 x X X X x x x x plate at -r/z = 2.36
I I a K l I : x plate at -r/z = 2.16
S0.000000004 A A A A plate at -rz = 2.16
+ plate at -r/z = 1.96
0.000000002

0
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-12. Glare as a function of radial and plate position for 355 nm for a downstream
location of 4 nozzle diameters and when a mixture of 20% helium and 80%
nitrogen is flowing through the nozzle.

As observed from Figure 5-11, it was observed that the glare was higher when the

nozzle was closer to the plate and gradually decreased as the nozzle was moved away

from the plate. The percentage error in the glare calculated by two methods for the case

when the plate was at 5.5 cm from the nozzle was a maximum of 0.25%.

Variation of Rayleigh light scattering signal as a function of radial and plate
position

Figure 5-13 shows the Rayleigh light scattering signal as a function of the radial

position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and

5.5 cm from the nozzle for 532 nm.










Reference value method was used to eliminate the glare from the raw peak voltage

shown in Figure 5-8. It was observed that the Rayleigh light scattering signal is

symmetric about the jet centerline as the glare was eliminated.


6.00E-09

5. 00E-09 -
,4 no plate
S4.00E-09 + plate at -r/z = 2.39
0.
plate at -r/z = 2.39
3.00E-09 plate at -r/z = 2.395
KI T x plate at -r/z = 2.36
I I
S2.00E-09 plate at -r/z = 2.16
1.0 0 T m + plate at -r/z = 2.16
1.00E-09 -

0.00E+00
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-13. Rayleigh light scattering signal as a function of radial and plate position for
532 nm for a downstream location of 4 nozzle diameters and when a mixture
of 20% helium and 80% nitrogen is flowing through the nozzle.

Error bars are shown for the case when the plate was at 6 cm from the nozzle. The

error bars are calculated using the formula 33, where 3 is the standard deviation in the

Rayleigh light scattering signal. This is because 99.9% of the data points fall in the range

of+ 33. The standard deviation was of the order of 0.1 for a signal of the order 1. The

ratio of the maximum to the minimum voltage was around 5 and the fall in voltage was

20% which corresponded to the ratio of scattering cross-sections of air and hydrogen

which was 4.31. The scattering cross-section of hydrogen and air are shown in Table 5-3.

Figure 5-14 shows the Rayleigh light scattering signal as a function of the radial

position when there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and

5.5 cm from the nozzle for 355 nm.











0.000000002
1.8E-09
1.6E-09
1.4E-09
1.2E-09
0.000000001
8E-10
6E-10
4E-10
2E-10
0


ST J i no plate
Si plate at -r/z = 2.39
S*T plate at -r/z = 2.39
plate at -r/z = 2.395
plate at -r/z = 2.36
+ T plate at -r/z = 2.16
ST+ plate at -r/z = 2.16
- s"


-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-14. Rayleigh light scattering signal as a function of radial and plate position for
355 nm for a downstream location of 4 nozzle diameters and when a mixture
of 20% helium and 80% nitrogen is flowing through the nozzle.

Reference value method was used to eliminate the glare from the raw peak voltage

shown in Figure 5-9. It was observed that the Rayleigh light scattering signal was

symmetric about the jet centerline as the glare was eliminated. Error bars are shown for

the case when the plate was at 6 cm from the nozzle. The error bars are calculated

according to the formula+ 33, where 3 is the standard deviation of the Rayleigh light

scattering signal.

Figure 5-15 shows the signal-to-glare ratio as a function of the radial position when

there was no plate and when the plate was at 6.083 cm, 6.08 cm, 6 cm and 5.5 cm from

the nozzle for 532 nm and 355 nm.







56



signal to glare as a function of nozzle and plate position
plate position
1.20E+00
1.00E+0 4 plate at -r/z= 2.39; 355 nm
1.00E+00 xxxx
8 x E m plate at -r/z= 2.39; 532 nm
I 8.00E-01 X A plate at -r/z = 2.395; 355 nm
x x xxxx
.t 4 4 x x xplateat-r/z=2.395;532nm
6.00E-01 x *
S* plate at -r/z= 2.36; 355 nm
l.00 1 M ++++
1' 4.00E-01 + + plate at-r/z= 2.36; 532 nm
S.00E-01 + + plate at -r/z= 2.16; 355 nm
2.00E-01 *
+ + + -plateat-r/z=2.16;532nm
O.OOE+00 I plate at -r/z= 1.96; 355 nm
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 plate at-r/z= 1.96; 532 nm
r/z

Figure 5-15. Signal-to-glare as a function of radial and plate position for 532 nm and 355
nm for a downstream location of 4 nozzle diameters and when a mixture of
20% helium and 80% nitrogen is flowing through the nozzle

It was observed that signal-to-glare ratio was higher outside the shear layer in the

ambient and minimum at the jet center line. It was also seen that the signal-to-glare ratio

is not symmetric about the jet center line. This was because the glare is higher when the

nozzle was closer to the plate and gradually decreased as the nozzle was moved away

from the nozzle. The lowest signal-to-glare ratio was observed when the plate was at 5

cm. It was seen that the hydrogen leak can be detected when the signal-to-glare ratio is

as low as 0.166.

Detection limits

Figure 5-16 shows the raw peak voltage as a function of the radial position when

there was no plate for 532 nm and 355 nm. These measurements were taken at nozzle

position of 8 nozzle diameters downstream and when 100% helium was flowing through

the nozzle. The Reynolds number was 100.







57



0.6

0.5 .

| 0.4 .
355 nm
S0.3
S0 532 nm
0.2 -* *

0.1 -

0
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
r/z

Figure 5-16. Raw photomultiplier-tube voltages as a function of radial and plate position
for 532 nm and 355 nm at 8 nozzle diameters downstream and when 100%
helium is flowing through the nozzle.

It was observed that the fall in voltage occurred over a wider range as compared to

the nozzle position of 4 nozzle diameters downstream. This was because when the

nozzle was at 8 nozzle diameters downstream, the jet spreads out and the variation in

voltage due to the presence of helium occurred over a wider radius. The drop in voltage

was around 15%.

To test for the detection limits, measurements were taken when the plate was

placed as close as possible to the nozzle. Figure 5-17 shows the Rayleigh light scattering

signal as a function of the radial position when the plate was at 5.5 cm from the nozzle

for 532 nm. The measurements were taken at a nozzle position of 8 nozzle diameters

downstream and when 100% helium was flowing through the nozzle. The Reynolds

number was 100. To attenuate the intensity of the scattered beam a neutral density filter

of optical density 0.6 was used.







58



532 nm


*. *****


* plate at 5.5 cm


. *.


-2 -1.5 -1 -0.5


0 0.5 1 1.5 2 2.5


Figure 5-17. Rayleigh light scattering signal as a function of radial and plate position for
532 nm for a downstream location of 8 nozzle diameters and when 100%
helium is flowing through the nozzle.

Figure 5-18 shows the Rayleigh light scattering signal as a function of the radial

position when the plate was at 5.5 cm from the nozzle for 355 nm.


355 nm


6.60E-09
6.50E-09
6.40E-09
6.30E-09
6.20E-09
6.10E-09
6.00E-09
5.90E-09
5.80E-09


* **


* plate at 5.5 cm


*. *


-2 -1.5 -1 -0.5 0 0.5


1 1.5 2 2.5 3


Figure 5-18. Rayleigh light scattering signal as a function of radial and plate position for
355 nm for a downstream location of 8 nozzle diameters and when 100%
helium is flowing through the nozzle.

The glare here was eliminated by considering that the ratio of the glare at the two

wavelengths was constant. From Figures 5-17 and 5-18, it was observed that there was a


1.90E-08
1.88E-08
1.86E-08
1.84E-08
1.82E-08
1.80E-08
1.78E-08
1.76E-08
1.74E-08
1.72E-08
1.70E-08
1.68E-08










fall in voltage at the jet centerline but the fall in voltage does not correspond to the ratio

of the scattering cross-section of helium and air. This was because at 8 nozzle diameters,

the jet spreads out and there will be greater entrainment of air. This caused the signal to

be greater than expected since the scattering cross-section of air was greater than that of

helium. The scattering cross-sections of air and helium are shown in Table 5-3.

Correlation studies

Figure 5-19 shows the correlation of Rayleigh light scattering signal between the

two wavelengths: 355 nm and 532 nm when there was no plate, when the plate was at

6.083 cm, 6.08 cm, 6 cm and 5.5 cm from the nozzle.


5.00E-09
4.50E-09 m x
4.00E-09- X
3.50E-09 no plate
E 3.00E-09 m plate at -r/z = 2.39
S2.50E-09 X / plate at -r/z = 2.395
O 2.00E-09 x plate at -r/z = 2.36
1.50E-09- plate at -r/z =2.16
1.00E-09 -
5.00E-10 -
0.OOE+00
O.OOE+0 2.00E-10 4.00E-10 6.00E-10 8.00E-10 1.00E-09 1.20E-09 1.40E-09 1.60E-09
0
355 nm

Figure 5-19. Correlation of Rayleigh light scattering signal between two wavelengths.

The correlation of the Rayleigh light scattering signals between the two

wavelengths was drawn so that there are two sets of data to analyze and detect the

hydrogen leak present in the jet. Figure 5-20 shows the R2 value for the correlation of the

Rayleigh light scattering signal at the two wavelengths as a function of the plate position.

It was observed that the value of the correlation coefficient, R2 is a minimum of 0.9567

when the plate is at 5.5 cm from the nozzle. Hence it was concluded that a decrease in










voltage on one line will imply a decrease in voltage on the second line as well. Hence

there are two sets of data available and by analyzing the two sets of data simultaneously;

it is possible to predict the presence of hydrogen leak in the jet.


1 -

0.99

0.98

S0.97

0c 0.96

0.95

0.94

0.93
no plate 6.083 cm 6.08 cm 6 cm 5.5 cm
plate position

Figure 5-20. Variation in R2 value as a function of plate position.

It was observed that the R2 value for the correlation was highest and is equal to

0.9944 when there was no plate and gradually decreased as the plate was moved closer to

the nozzle. So it was concluded that as the plate was moved closer to the nozzle, the

uncertainty in the Rayleigh light scattering signal increased.














CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS

A Nd:YAG pulsed laser induced dual line-detection Rayleigh light scattering

technique was used to detect the presence of hydrogen in the jet in the presence of high

amounts of glare. Also, a mathematical code in MATLAB has been written to compute

the glare numerically, design the experimental system and study the variation in the glare

as a function of the plate position.

The MATLAB code computed only the glare from the aluminum plate and did not

take into account the glare from the background surfaces. The results from the

mathematical code are compared with experimental results by normalizing the glare and

the results corresponded well. The percentage error between the numerical and

experimental results was a maximum of 5%.

Two methods were used to eliminate the glare from the signal. When multiple

surfaces are considered, glare from the signal is eliminated taking reference values at

each point in space and when the glare from one surface is dominant compared to the

glare from the other surfaces; the data analysis to eliminate the glare was simplified. The

ratio of the glare at the two wavelengths is proved to be constant and the glare is

eliminated by solving a set of simultaneous equations

Dual line-detection technique was used successfully to eliminate the glare from the

signal and get accurate results even when the glare to signal ratio was as high as 6:1.

When a mixture of 20% helium and 80% nitrogen was used for the measurements at 4

nozzle diameters, the voltage at the centerline of the nozzle was around 0.2 times that of









voltage outside the shear layer. This indicated a reduction in the scattering cross-section

of the gas in the jet centerline by an amount of 0.2 as compared to the scattering cross-

section of the gas outside the shear layers. This reduction in the scattering cross-section

corresponded well to the ratio of the scattering cross-sections of hydrogen and air. The

scattering cross-section of hydrogen is equal to 0.23 times that of air.

When pure helium was used for measurements at a downstream location of 8

nozzle diameters, a fall in voltage was observed in the centerline of the jet. The voltage

at the jet centerline was observed to be 0.14 times that of the voltage outside the shear

layers. The scattering cross-section of helium is equal to 0.015 times that of air. The

decrease in the voltage at the jet centerline does not correspond well the ratio of the

scattering cross-sections of helium and air. This is because at a downstream location of 8

nozzle diameters, the jet spreads out and there will be greater entrainment due to air.

Future work:

* A study should be done to eliminate the glare from the signal at downstream
distances greater than 8 nozzle diameters and establish the detection limits.

* Also, a study should be done to detect the hydrogen leaks in the presence of cross
currents.

* The feasibility of the technique should be determined in the back scatter mode to
enable remote detection.
















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BIOGRAPHICAL SKETCH

Raghuram Vempati completed his undergraduate degree in mechanical engineering

from Jawaharlal Nehru Technological University, India, in April 2002. He is pursuing

his master's degree in mechanical engineering at the University of Florida. He has been a

research assistant under Dr. Jill Peterson since August 2002.