Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P1.32 - Profile of a thin liquid film near a solid surface discontinuity driven by air-shear
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00446
 Material Information
Title: P1.32 - Profile of a thin liquid film near a solid surface discontinuity driven by air-shear Interfacial Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Das, K.S.
Karchev, Z.
Kawaji, M.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: liquid film thickness
hydrodynamics
dry-out
critical heat flux
interfacial shear
 Notes
Abstract: Thin liquid films flowing on a solid surface driven by air shear are commonly encountered in many industrial applications, ranging from boilers and heat exchangers to coatings and semiconductor technologies (Stillwagon and Larson 1990; Mazouchi and Homsy 2001). The thickness profile of a thin liquid film near a trench or gap in the heat transfer surface is important to understand since thinning of the liquid film may induce liquid film dry-out and Critical Heat Flux, which can seriously degrade heat transfer and raise safety concerns. In this work we have measured thickness profiles of a thin water film flowing over a cylindrical rod under adiabatic conditions. The water film is driven by air shear, and the thickness profile is measured by a Laser Confocal Displacement Meter (Keyence LT-9030 M) as a function of the gap-width and air flow speed. Liquid ridge formation near the discontinuity has been detected and the effects of interfacial shear and gap size on the film thickness profile are investigated experimentally.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00446
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P132-Das-ICMF2010.pdf

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7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010





Profile of a thin liquid film near a solid surface discontinuity driven by
air-shear



Kausik S Das, Zheko Karchev and Masahiro Kawaji

Dept. of Chemical Engineering and Applied Chemistry,
University of Toronto, Toronto, ON, M5S 3E5, Canada

E-mail:* kawajiraect~utoronto.ca


Keywords: liquid film thickness, hydrodynamics, dry-out, Critical Heat Flux, interfacial shear



Abstract

Thin liquid films Hlowing on a solid surface driven by air shear are commonly encountered in many
industrial applications, ranging from boilers and heat exchangers to coatings and semiconductor
technologies (Stillwagon and Larson 1990: Mazouchi and Homsy 2001). The thickness profie of a thin
liquid film near a trench or gap in the heat transfer surface is important to understand since thinning of
the liquid film may induce liquid film dry-out and Critical Heat Flux, which can seriously degrade heat
transfer and raise safety concerns. In this work we have measured thickness profiles of a thin water film
flowing over a cylindrical rod under adiabatic conditions. The water film is driven by air shear, and the
thickness profile is measured by a Laser Confocal Displacement Meter (Keyence LT-9030 M) as a
function of the gap-width and air flow speed. Liquid ridge formation near the discontinuity has been
detected and the effects of interfacial shear and gap size on the film thickness profile are investigated
experimentally.


Introduction

The formation of thin liquid film and waves
induced by interfacial shear or gas flow, is of
interest in many industrial applications and
processes (Hewitt & Hall-Taylor 1970;
Takahama & Kato, 1980; King & Tuck 1993:
Hartely & Murgatroyd 1964; Nasr-Esfahany &
Kawaji, M., 1996: Takamasa et. al. 1997, 1998).
The examples include forced convective boiling
and two-phase flow in boiler tubes, distillation,
thermosyphons, wetting and thin film breakage.
cooling towers, thin-film heat exchangers,
painting, and cooling of nuclear fuel rods in light
water reactors (LWR) and CANDU nuclear
reactors.


In CANDU reactors, steam can be generated mn
certain fuel assemblies inside horizontal pressure
tubes. The fuel assemblies are about 50 cm in
length and when placed in the pressure tube, a
small gap exists between the neighboring fuel
assemblies. When steam is generated and flows
at a high velocity, water would form a thin film
sheared by the high-speed steam flow. It is
crucial in any heated channels to prevent the
liquid film from driving out on a heated surface.
The liquid film dry-out does not occur under
normal operating conditions, but it is important
to investigate under what steam flow rates for a
given heat flux the liquid film may dry out. Also,
the effect of a discontinuity in the heated surface
such as the gap between the fuel assemblies as





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

gap distance in-between them. The gap width
between the rods can be adjusted by turning an
outside knob.


shown in Figure 1 may disrupt the liquid film, so
the liquid film behavior along heated surfaces
and over any discontinuities needs to be
investigated
.- ~iscontinuity Region
I ;II I
I iI I I I


~ 6mm
Fuel Bundle

Fig. 1. Schematic of the discontinuity between
the fuel assemblies.

To this end, the present study is performed to
investigate the liquid film behavior in the
neighborhood of a discontinuity in the surface
occurring at the transition from one fuel rod to
another. An air-water flow loop has been used to
conduct measurements of the thickness profile of
a thin water film flowing along the surface of a
horizontal cylindrical rod and sheared by a
high-speed aidr flow.

The objectives of the current project can be
summarized as follows:

Design and construction of an
experimental set-up allowing
measurement of the thickness of thin film
of water on metal rods sheared by air.

Experimental investigation of the system
parameters affecting the liquid film
formation.

Investigation of thin film profile near a
discontinuity in the rod surface.
Thin film break-up and dry-out
phenomena.

Theoretical modelling and comparisons of
analytical or numerical results with experimental
results will also be performed in the future.

Experimental Facility

The experimental set-up is designed with a metal
rod of 12.7 mm outer diameter is inserted into the
centre of a square glass channel with a cross
section of 25 x 25 mm as shown in Fig. 2. The rod
is composed of two separate sections and variable


Metal rod


Variable gap


Water Inlet


Air Inlet


Fig. 2. Schematic of the apparatus used to
measure thin liquid fim promie. The inner rod
is placed inside a square glass channel.

Water is pumped inside the central rod and
brought to the surface of the rod through small
perforation on the rod surface. Air is injected into
the entrance of the square glass channel and
flows along the rod and over the water film
which forms on the rod surface. Air and water
flow rates are adjusted to obtain a long
continuous film of water which runs along the
rod and over the gap between the ahigned rods.
The flow rates are measured by the respective
flow meters. The thickness of the film is
measured by using a high precision laser
confocal displacement meter (LT 9030 M,
Keyence) as shown in Fig. 3.

The laser focus displacement meter consists of a
semiconductor laser and a position-sensitive
detector. The position of the target surface can be
determined by the displacement of an objective
lens moved by a tuning fork. The intensity of the
reflected light becomes highest in the
light-receiving element when the laser beam is
focused on the target surface. The objective lens
is vibrated continually in the range of 0.3 mm.
The laser beam reflected from the target is
focused on a position-sensitive detector, forming
a beam spot. Liquid film thickness is obtained
from the detector signals. The resolution of the
present laser focus displacement meter is 0. 1 Cpm,
the laser spot diameter is 2 Cpm and the response
time is 640 ps.








































Fig. 5. Schematic of the thin film profile
measurement near the gap.

The measurements were performed at Eixed air
and water Hlow rates and the film thickness was
measured continuously before the ridge, at the
peak of the ridge and after the gap. The collected
data was subsequently processed to obtain the
time average film thickness at each measurement
location and the results are presented in Figure 6.







Lmm


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


3.1. Ridge Size Measurements

An axial thickness profie of the water film on
the metal rods was measured by the displacement
meter which was mounted on an X-Y stage.
Measurements were done at different axial
positions along the rod. The experimental
investigation was performed by measuring the
liquid film thickness under constant superficial
liquid velocity kept at 20.9 m/s and air velocity
equal to 49.8 m/s.


AirVelonty Meter Omm _3mm


Semicondue
laser



Ob active


ULight-receiving
element
Pinhole

Tuning fork

Sensor


Fig. 3. Schematic of the laser confocal
displacement meter:

Measured liquid film thickness is transformed to
DC voltage signal in the range of +10 V. Output
signal was sent to a PC through a data
acquisition module and recorded in real time.
One particular advantage in this technique is that
it involves high spatial resolution, but since it is
non-intrusive, it does not disturb the Hlow.

3. Experimental Observation

The inside width of the square channel used in
the experiments was 25 mm, the diameter of the
central rod being 12.7 mm. Water is pumped at a
constant rate and sheared by constant air Hlow. A
thin film of water is observed to form on the
metal rod surface. Once the liquid forms a
continuous film covering the outer surface of the
rod, a ridge of liquid can be observed
downstream just before the discontinuity of the
surface. On the contrary no ridge can be
observed near the edge of the second rod after
crossing the gap.
Ridge formation


Lq~udFilm
water now Meter


Fig. 6. Thickness film profile
discontinuity. Formation of a
observed.


near the
ridge is


Fig. 4. Circular ridge formation near the
discontinuity.


It can be observed that the thin film profile is
highly nonlinear near the discontinuity and there





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

size of the gap was varied from 0 to 20 mm with
1.0 mm increments in-between the experiments.
The time-averaged film thickness as a function
of the gap size is presented in Fig. 9.


is a sharp reduction in the thickness just after the
edge of the rod.

3.2 Liquid Film Breakage

The measurements were performed at fixed air
and water flow rates and the film thickness was
measured at three locations: before the ridge, at
the peak of the ridge and after the gap.

As it can be seen from Fig. 7, the instantaneous
film thickness after the gap shown by a yellow
curve can often reach zero thickness indicating
the breakage of the 11quid film. However, the
dry-out condition exists temporarily and the rod
surface is again rewetted and covered by the
liquid film. If the dry-out condition lasts for a
longer time, the temperature of the fuel rod
surface could start to increase.


Laser


at 20 nin


Variable Gap Distance


Thin film breakage and
formation of a dry-out region


600
5 500
E
S400
ui300
f~200
E 100


Fig. 8. Schematic of the thin fim promie
measurement away from the discontinuity.















Fig. 9. Liquid fim thickness as a function of
the gap width.

The experimental results show that with an
inCTreSe in the gap size the time-averaged
thickness of the liquid film decreases. This
results from the increased intensity of the liquid
film breakage. At relatively small gap widths the
liquid film breakage is not observed to occur due
to the formation of a liquid bridge between the
adjacent edges of the consecutive metal rods.

3.4 Liquid Bridge Breakage and Formation of
Dry-out Region

Thin liquid films driven by air shear form a
liquid bridge in the gap between the adjacent
cylindrical rods. The stability of the liquid bridge
depends on the gap width between the adjacent
rods. In Fig. 10 the effect of the gap width on the
stability of the liquid bridge is shown for three


1 5
Time, s


2 2 5


Fig. 7. Experimental Results Thin fim
break-up. The blue line indicates fim
thickness before the discontinuity, the pink
line indicates the peak height of the ridge and
the yellow line shows the fim thickness
downstream of the gap.

3.3 Liquid Film Breakage Effect of Gap
Distance

This part of the experimental study was
performed by measuring the liquid film thickness
20 mm downstream of the gap as shown in
Figure 8 at a superficial liquid velocity of 20.9
m/s and superficial air velocity of 49.8 m/s. The





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


gap distances.


400
350
300
250
pLm 200
150


Formation of /a) Gap distance 2mm
liquid bridge.


O 5 10 15 20 25
Distance away from the edge of the cylinder (mm).


Fig. 11. Liquid film thickness after the
discontinuity.

As it can be seen from the above figure, the
liquid film thickness increases with the distance
from the leading edge which indicates the
re-deposition of the liquid and restoration of the
liquid film. This can be attributed to the highly
turbulent air Hlow which deposits entrained liquid
droplets back onto the surface of the rod.

4. Discussion and Concluding Remarks

In this paper we have investigated the behaviour
of a liquid film flowing over a metal rod surface
and presented the film thickness profie data to
explain the effect of air shear on the liquid film
profile and its stability near the surface
discontinuity. A shear driven liquid ridge is
formed at the trailing edge of the first rod just
before the gap. Although capillary ridge
formation is reported in gravity driven thin fi1m
flows over topography (Kalliadasis et al., 2000)
or on inclined planes (Kriegsmann et al., 1998),
formation of capillary ridge in shear driven flow
has not been reported previously. For the
problem of a contact line driven by gravity down
an inclined plane, Bertozzi and Brenner (1997)
pointed out that when the gravitational
component normal to the substrate is retained,
the height of the capillary ridge becomes smaller,
and in fact disappears at small inclination angles
as the hydrostatic head associated with the
normal gravitational component dominates the
capillary pressure. Our future goal is to model
the problem theoretically through lubrication
approximation where air flow velocity would be
a control parameter to investigate whether there


Breakdown of
liquid bridge\ b) Gap distance 4 mm


c) Gap distance 6 mm


Fig. 10. Stability of liquid bridges as a
function of the gap width.

As it can be seen from the above photographs,
with an increase in the gap distance the liquid
bridge gets thinner and thinner and eventually
comes to a point of breakage. Under these
conditions, the liquid film thickness after the gap
oscillates in time, periodically between zero and
re-deposited finite thickness. Within the
investigated range of system parameters it was
observed that that the gap size above which the
breakage starts to occur was approximately 5
mm.

3.5. Liquid Film Formation after the Gap

Of particular interest in the current study was to
investigate how the liquid film re-deposits after
the discontinuity. In order to perform this
experiment, the liquid film thickness was
measured at different locations after the gap
under constant superficial liquid velocity of 20.9
m/s, superficial air velocity Eixed at 49.8 m/s and
gap distance kept at 6 mm. The time-averaged
fi1m thickness is plotted as a function of the
distance from the leading edge of the second rod.





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

Kalliadasis S., Bielarz C., and Homsy GM., 2000.
Steady free-surface thin film flows over
topography. Phys. Fluids, 12, 1889.

King, A. C. and Tuck, E. O., 1993. Thin liquid
layers supported by steady air-flow surface
traction. J. Fluid Mech. 251, 709.

Mazouchi A and Homsy GM., 2001. Free surface
Stokes flow over topography, Phys. Fluids, 13,
2751.


exists any power law behaviour in Reynolds
number-Capillary number plane.

The stability of the thin fi1m after the gap has
been investigated as a function of gap width. It is
observed that an increase in gap width makes the
formation of thin film unstable, i.e., fi1m breaks
down as the gap width is increased.

Formation of a liquid bridge between two
cylindrical rods is also reported in the
experiments. Similar to the liquid films liquid
bridges also become unstable and break down as
the gap width increases. It is hypothesized that
through the liquid bridge the thin fi1m is
re-deposited in the downstream region of the
cylindrical rod after the gap.

One interesting observation in the investigation
of the liquid film formation after the gap is that
the thickness of the re-deposited film increases
as we go further away from the gap downstream.
One hypothesis is that the air re-circulations
created by the gap around the inner metal rod
prevents gravity drainage and hence the
thickness of the thin fi1m increases.

Acknowledgements

The authors would like to thank Ontario Power
Generation, Bruce Power Inc. and Atomic
Energy of Canada Limited for financially
supporting this work through a grant from the
University Network of Excellence in Nuclear
Engineering (UNENE) of Canada in conjunction
with a CRD grant from the Natural Sciences and
Engineering Research Council of Canada.

References

Bertozzi AL. and Brenner MP., 1997. Linear
stability and transient growth in driven contact
lines, Phys. Fluids 9, 530.

Hewitt, GF., Hall-Taylor, N.S., 1970. Annular
Two-Phase Flow. Pergamon Press, Oxford.

Hartely, D. E. and Murgatroyd, W., 1964.
Criteria for the break-up of thin liquid layers
flowing isothermally over solid surface. Intl J.
Heat Mass Transfer 7, 1003.


Nasr-Esfahany, M.,
Turbulence structure
induced wave at
Proceedings of AIChE
203-210.


Kawaji, M., 1996.
under a typical shear
a liquid/gas interface.
Symposium Series 310-92,


Poltalski, S., Clegg, A.J., 1972. An experimental
study of wave inception on falling liquid fim.
Chemical Engineering Science 27, 1257-1265.

Serizawa, A., Kamei, T., Kataoka, I., Kawara, Z.,
Ebisu, T., Torikoshi, K., 1995. Measurement of
dynamic behavior of a liquid film flow with
liquid droplets in a horizontal channel.
Proceedings of 2nd International Conference on
Multiphase Flow 2, 27-34.

Stillwagon LE. and Larson RG, 1990. Leveling
of thin fi1ms over uneven substrates during spin
coating, J. Appl. Phys. 2, 1937.

Takahama, H., Kato, S., 1980. Longitudinal flow
characteristics of vertical falling liquid film
without concurrent gas flow. International
Journal of Multiphase Flow 6, 203-215.

Takamasa, T., Hazuku, T., and Kobayashi, K.,
1997. Measurements of the interfacial waves on
a film flowing down a vertical wall using laser
focus displacement meters. Proceedings of Fifth
Triennial Intemnational Symposium on Fluid
Control, Measurement and Visualization 1,
189-194.

Takamasa, T., Tamura, S., and Kobayashi, K.,
1998. Interfacial waves on a film flowing down
plate wall in an entry region measured with laser
focus displacement meters. Proceedings of 3rd
International Conference on Multiphase Flow,
CD-Rom, #453.




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