Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P3.26 - Measurement of Gas-liquid Two-phase flow using Ring-shaped Conductance Sensor
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 Material Information
Title: P3.26 - Measurement of Gas-liquid Two-phase flow using Ring-shaped Conductance Sensor Experimental Methods for Multiphase Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Shi, Y.
Tan, C.
Dong, F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: gas-liquid two-phase flow
conductance sensor
ring-shaped electrode
liquid fraction
velocity
calibration
 Notes
Abstract: A ring-shaped conductance sensor for measuring liquid fraction and velocity in gas-liquid horizontal flow is presented in this paper. Measurement system consists of six electrodes flushed on the internal wall of the cylindrical working section. The conductance sensor is calibrated with reference to stratified, annular and bubble phase distributions and the values are compared with available theoretical results. Experiments are carried out on multiphase flow loop and analysis is conducted to investigate the characteristics of gas-liquid two-phase flow.
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
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Bibliographic ID: UF00102023
Volume ID: VID00521
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P326-Shi-ICMF2010.pdf

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


Measurement of Gas-liquid Two-phase flow using Ring-shaped Conductance Sensor


Yanyan Shi, Chao Tan and Feng Dong

Tianjin Key Laboratory of Process Measurement and Control, School of Electrical Engineering and Automation
Tianjin University. Tianjin 300072, China
E-mail: fdlon~ -Iju iedu enI


Keywords: gas-liquid two-phase flow, conductance sensor, ring-shaped electrode, liquid fraction, velocity, calibration



Abstract

A ring-shaped conductance sensor for measuring liquid fraction and velocity in gas-liquid horizontal flow is presented in this
paper. Measurement system consists of six electrodes flushed on the internal wall of the cylindrical working section. The
conductance sensor is calibrated with reference to stratified, annular and bubble phase distributions and the values are
compared with available theoretical results. Experiments are carried out on multiphase flow loop and analysis is conducted to
investigate the characteristics of gas-liquid two-phase flow.


Introduction

Two-phase flow is of great importance in an increasing
number of applications and industries. It occurs in various
geometries, petroleum plants, chemical processes, and heat
transfer equipment. To describe and analyze the two-phase
flow properties, reliable measurements of liquid fraction and
velocity are always necessary. In literature, the liquid
fraction can be measured by several techniques, including
quick-closing valves, radiation attenuation, ultrasonic and
impedance method. However, the quick-closing method can
not measure the liquid fraction in real time, the radiation
method requires the construction of complicated, dedicated
facilities, and the ultrasonic method can not easily be
applied to a variety of flow patterns. Compared with the
above techniques, the impedance method is a real time
technique and has simple equipment requirements. A review
of electrical impedance technique for flow measurement has
been presented by Ceccio and George (1996). Impedance
method is successfully applied when the electrical
properties of mixture components are sufficiently different
and can be classified as either conductance or capacitance
sensors according to the applied a.c. excitation frequency (F
Devia and Fossa 2003). In the situation where water is the
continuous phase, conductance sensor is preferred. As to the
measurement of velocity profile, cross-correlation technique
is commonly used, which was comprehensively described
by Beck and Plaskowski (1987).
As to the conductance method, a pair of flush mounted
parallel ring-shaped electrodes was first employed by Asali
et al. (1985) to measure average liquid film thickness in
vertical annular flow. While Andreussi et al. (1988) and
Tsochatzidis et al. (1992) developed Coney s (1973) flat
electrode theory to the response of this ring-shaped
electrode configuration. Liu (1996) analyzed the electric
field behavior of a four-electrode conductance sensor and
measure the volume fraction of oil-water two-phase flow in
a production oil well. Fossa (1998) investigated and


compared the performance of the plate and ring electrodes,
showing that the ring electrode was more suitable for
practical application. Lucas et al. (2000)embedded six ring
electrodes on the surface of an insulating plastic body to
measure solid volume fraction and axial velocity of
liquid-solid flow. Jin (2008) presented an eight ring
electrodes conductance sensor to obtain volume fraction and
velocity in vertical gas-water flow based on software
measurement technique. Kim (2009) used three ring
electrodes conductance probe to obtain void fraction and
bubble speed of slug flow.
In the present work, a ring-shaped conductance sensor and
related electronic circuit are applied to investigate the liquid
fraction and velocity profile in gas-water flow. Calibration
of the sensor is performed with reference to stratified,
annular and bubble flow regimes. Static experimental points
are compared with theoretical value. Experiments are
conducted on the multiphase flow loop to characterize
two-phase flow.

Measurement Principles

Application of flush mounted sensors immersed in
two-phase flow is based on the theory of electrical potential
field. As the inner diameter of pipe is much smaller than the
wavelength of electric field, the electric field can be
modeled as time invariant. This problem is described by
Laplace equation of static field:


V2u = 0


Depending on the geometry of the sensor and the interface
between phases, this equation can be solved either
analytically or numerically.
For flat electrodes geometry, Coney (1973) provided
theoretical solution of electrical behavior of a liquid layer
with thickness h and electrical conductivity 7. If I is the






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


length of electrodes, s their width and De their spacing, the
medium conductance Ge is then given by:


Ge = Ge xlx y
k(m)
Gre =
k (1 m)

k(ml)=~lj (1min o)SdO
sinh (miP/ 2h)
m=sinh [Fr(s + De) / 2h)


Figure 1: Cross correlation technique

The cross-correlation velocity U,, used to predict the axial
velocity of the mixture is given as:


where k represents the complete elliptic integral of the first
kind and m is a function of geometrical parameters.
Andreussi et al. (1988) showed that the Coney's solution can
also be used in ring-shaped electrodes geometry. For annular
and stratified flow, the above equation is still valid if the
length I is replaced by the wetted length of the electrodes p
and the film height h by hl, defined as:


hi = 4*HI(3)


where 4 is the cross section area of the pipe and HI is the
liquid fraction.
Ring electrodes have also been considered by Tsochatzidis et
al. (1992), who theoretically solved Laplace equation in
cylindrical co-ordinates for ring probe response to a
conducting annulus of thickness h.
In order to eliminate the effect of liquid conductivity,
normalized conductance Ge* is proposed as the ratio of
conductance between medium at a given liquid fraction and
liquid-only.
When ring-shaped electrodes are employed under uniformly
dispersed flow condition (bubble flow), the normalized
conductance Ge is given by the models of Maxwell (1982):


L
max


where r,,, is the maximum of Rx,(r) corresponding to time
delay of the mixture flowing from sensor A to sensor B.

Experimental apparatus

Much previous study in to the development of conductance
sensor has been performed for measuring volume fraction
and very little attention has been put on the velocity. This
paper proposes a conductance sensor with six ring-shaped
electrodes to measure both the liquid fraction and axial
velocity in two-phase flow. Electric circuit along with the
conductance sensor is shown in Figure 2. As can be seen
from the figure, it mainly consists of four parts: current
generating unit, six ring-shaped conductance sensor, signal
processing unit and signal acquisition unit.


Sensor A

seosor c


S2HI
G~e=
3 H


Sensor B ~ SL43 Signal Processing Unit





Figure 2: Ring-shaped conductance measuring system

The six ring-shaped electrodes are axially separated and
mounted on the inner wall of a pipe with diameter D. El
and E2 are exciting electrodes with distance Ll: M1 and M2
together with M3 and M4 are two pairs of upstream and
downstream correlation electrodes denoted as sensor A and
sensor B separately with distance L2: M2 and M3 are
volume fraction electrodes denoted as sensor C with
distance L3. The electrode width is S. An optimum
geometry of the electrodes is obtained based on the
numerical study of electric field and sensitivity to allow
sensor C optimized for liquid fraction measurement while
sensors A and B optimized for cross correlation velocity
measurement. A simple analysis of electrical field shows
that the distribution of voltage is very peak and
approximately equal to the supplied voltage in the region
adjacent to El and E2 while in the region far from these two


As reported in the literature, relationship between calculated
value of Ge* and the value of liquid fraction HI is dependent
on the separation distance De between electrodes for the case
of stratified and annular flow, with Ge very close to HI when
De is larger than the diameter of the tube. However, for
bubble flow, Ge is predicted to be independent of De and
Maxwell equation can be used to obtain HI.
Apart from the liquid fraction measurement, axial velocity of
two-phase flow is commonly measured by cross-correlation
technique with its basic principle shown in Figure 1.
When gas-liquid two-phase flow passes the pipe, the similar
signals from upstream sensor A and downstream sensor B
can be correlated. Suppose x(t) and v(tiare signals
respectively from sensor A and sensor B, L is the separation
between two sensors, the cross-correlation function R,,(r)
can be calculated bv:


Rp.=~l(r) =li (tz) yt+rf ta


S00


O


























ssoSensorA
.o Sensor I x i
I 25 x Sensors
B 20

4 soO

1 0 2 4 6 0 1 4 1

Condu:ctivity 1
Figure 4: Six-ring electrode conductance sensor

The change in the electrical property of measured fluid has
great effect on measurements. As a result of this, the
changes of voltage on the measured electrodes under fluid
with different conductivity are firstly investigated in the
static experiment. The result is shown in Figure 4. It can be
seen that there exists a good relationship between reciprocal
of measured voltage and conductivity of the fluid measured
on the liquid fraction electrodes while the linearity is not
very well on the cross correlation velocity measuring
electrodes. However, changes are consistent with each other.
Figure 5 shows the normalized conductance Ge as a
function of liquid fraction HI under the stratified gas-liquid
flow regime which is simulated by introducing known
volumes of liquid into a horizontally positioned test pipe.
The experimental result shows that Ge is very close to HI
which is in agreement with theory.












Llquid fraction HI

Figure 5: Static calibration for stratified flow

Figure 6 illustrates the normalized conductance Ge against
liquid fraction HI for annular flow. Annular liquid film is
established by inserting plastic rods of known diameter into
the pipe. The experimental points coincide with the
theoretical deduction. It can be noted that, for the proposed
conductance sensor geometry, good linearity is attained


electrodes, a linear dependence exists. Radical current
density is equal to zero and axial current density tends to be
uniform in the region between electrodes El and E2. It is a
well known fact that the irregular variations of voltage
distribution and current density in the vicinity of the
exciting electrodes will introduce nonlinear components into
the output of the measuring sensors, resulting in great
problem in the sensor design. Some research presents that
the measuring electrodes should be mounted in the linear
dependence region (Liu 1996, Hu 2008). However, further
study is required to get an optimum sensor configuration
based on the investigation of sensitivity (Shi and Dong
2008). The optimal parameters of the ring-shaped
conductance sensor are shown in Table 1.

Table 1 Parameters for the conductance sensor geometry
Prameters D/mm L1/mm L,2/mm L,3/mm S/mm
Value 50 200 77 60 5

Bi-directional current signal is employed as exciting source
in this paper which is comprised of a voltage generating and
a voltage controlled current circuit Apart from the general
advantages of traditional alternative current source such as
minimizing corrosion of electrodes, having a much higher
output impedance to maintain a constant output amplitude
over a wide range of load impedances encountered in
different process applications, the presented bi-directional
current source help the system consume less time and have
higher efficiency because filter and demodulation parts are
eliminated. The frequency of the exciting current generated
in this system is adjusted between 5 and 120 kHz, while the
amplitude can be regulated in the range between 0 mA and
10 mA. Fluctuation of the current is less than 1% which
meets the precision requirement well.
The output of sensors A, B, C is fed to identical signal
processing circuit. Each of these circuits has a high input
impedance voltage follower to ensure negligible current is
drawn from the electrodes. The output of the voltage
followers is connected to operational amplifiers with low
voltage offset.
The voltage output from the signal processing umit is
obtained by data acquisition system based on the software of
Labview, as is shown in Figure 3.


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


Sensor Calibration

In this work, a preliminary set of experiments is carried out
in static conditions to relate measured conductance value to
volume fraction. In order to eliminate the measuring error of
liquid conductivity, the normalized conductance Ge is used
and can be written~as:


Ge. w
G V
where G and G, are the conductance
fraction and liquid-only condition,
corresponding voltage value.


(7)

at a given liquid
V and V, are


Figure 3: Signal acquisition program




















































i.


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

measured at the inlet and the result is shown in Figure 9.


between Ge* and HI for the case of stratified and annular
flow.


* Calibratzon data
Linearfit


DD 02 04 06
Liquid fraction HI


08 1


Figure 6: Static calibration for annular flow

Figure 7 depicts the relationship between normalized
conductance Ge and liquid fraction HI of bubble flow
condition which is achieved by a number of plastic spheres
of known diameter which are suspended at fixed positions.
The measurements almost coincide with theoretical value.
Static calibration of the ring-shaped conductance sensor
indicates that the proposed system can be employed for
accurate measurements of liquid fraction under different
flow conditions. Apart from the above three flow
distributions, intermittent flow is also encountered for a
wide range of flow conditions. However, intermittent flow
in horizontal pipes can be described as the flow of liquid
regions where the liquid bridges the whole pipe separated by
stratified regions.


Figure 8: Multiphase flow loop


a s

a -


Soa

300


* Calibration data
*Maxwell equation


oio obs 090 obs 1o
Liquid fraction HI


0 2 0.4 0 6
Normalized conductance Ge'


0.8 1.0


Figure 9: Liquid fraction at inlet vs normalized
conductance


Figure 7: Static calibration for bubble flow


0.o .0 0.2 0.'s 0.6 0.8 i. o
Lhqurd cractlon measured at tise minet

Figure 10: Predictive liquid fraction vs liquid fraction at
inlet

As is shown in the figure, the trend line moves towards a
new direction when Ge is higher than 0.4, expressed by:


SHI=Ge
~H = 1.47G e


0.08 Ge < 0.4
-0,23 G, > 0.4


Figure 10 compares the measured liquid fraction at the inlet


Experiment

Measurements of the distribution of liquid fraction and cross
correlation velocity are conducted on oil-gas-water
three-phase flow loop established at Tianjin University as
shown in Figure 8. Tap water and oil can be used as liquid
phase. The working section consists of a horizontal Perspex
tube with diameter of 50 mm. Air coming from a
compressor is mixed with tap water in this paper. After the
test section, air is released and tap water flows back to the
fluid reservoir. Flow rate is controlled in the range of
3~11m3/h for water and 0~362m3/h for gas. The average
temperature in the experiment is 28"C. Pictures from the
high-speed camera allow for better visual flow regime
determination. Sensor signals are captured using data
acquisition system.
Wavelet de-noising treatment method is applied to reduce
the high frequency interference. The obtained normalized
conductance Ge is compared with liquid fraction HI






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


According to Figure 12, the relationship between
cross-correlation velocity Uce and mixture velocity Uh can
be represented as:


to the values that are calculated from conductance
measurements with the above equation. It can be seen that
the measurement result has excellent linearity and average
relative error is 10%.
The relationship between Uce and Uh can be written by drift
flux model:


u. =1 :1% +0.07 Fr~= <3.5

llcc = 0.96uh +0.5 Frm.= h> 35


U,=COUh + V


When two-phase flow passes the horizontal working section,
fluctuation of signal is caused. The cross correlation velocity
U,, of two-phase flow, calculated by using the fluctuating
signals from sensor A and B, is compared with the mixture
velocity Uh measured at the inlet and plotted in Figure 11.


It is obvious that Co is smaller while Vd iS higher with a
higher Froude number, which is coincided with the result
given by Al-Lababidi (2006).


Conclusions


. ,W


A ring-shaped conductance sensor and related electronics
are developed in this work for measuring the liquid fraction
and velocity in gas-liquid mixtures. The conductance sensor
and the signal processing circuit assure a cost-effective
system able to characterize two-phase flow. Static
calibration is performed under simulated stratified, annular
and bubble flow regimes. Medium conductance is
normalized with respect to the conductance of the pipe full
of liquid so as to avoid monitor the electrical conductivity of
liquid during the measurements. The performance of the
ring electrodes flushed to the pipe wall is described by
theoretical solutions for different two-phase distributions.
The static calibration results are in close agreement with the
theory in literature. Experiments are conducted on the
multiphase flow loop. Liquid fraction predicted by the
proposed conductance sensor is compared with the
measured liquid fraction at the inlet and average relative
error is +10%. Relationship between cross correlation
velocity and mixture velocity is obtained according to
different range of Froude number. The proposed six
ring-shaped electrode conductance sensor performs well in
the investigation of liquid fraction and velocity and could be
further used to study other characteristics of two-phase low.

Acknowledgements

This paper is supported by Supported by National Natural
Science Foundation of China (No.50776063) and Natural
Science Foundation of Tianjin (No. 08JCZDJC17700).

References

Al-Lababidi, S. Multiphase Flow Measurement in the Slug
Regime Using Ultrasonic Measurements Techniques and
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Andreussi, P., Donfrancesco, A. D. and Messia, M. An
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Asali, J. C., Hanratty, T. J. and Andreussi, P. Interfacial
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.............. .s .,
;.?
*


.


Figure 11: Comparison of Uce and U

It can be seen from Figure 11 that trend of the data is
somewhat different when Frm<3.5 and Frm>3.5, which is in
accordance with observation of Bendikse (1984). Figure 12
refers to U,, against Uh under different range of Froude
number.


- Linear Fit of Adaptive CC
- -


ur mis)
Frm<3.5


- Linear Fit of Adaptive C C


i-
',*


Frm>3.5

Figure 12: Uce vs Uh under different range of Froude
number






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

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