Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 10.3.3 - Measurement of Dynamic Properties of Vertical Gas-Liquid Flow
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00252
 Material Information
Title: 10.3.3 - Measurement of Dynamic Properties of Vertical Gas-Liquid Flow Experimental Methods for Multiphase Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Hunt, A.
Abdulkareem, L.A.
Azzopardi, B.J.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: electrical capacitance tomography
gas-liquid flow
huge wave
churn flow
wire mesh sensor
 Notes
Abstract: Measurements of void fraction profiles, bubble sizes and velocity distributions are required to model two-phase flows and to understand their flow regimes and physics. This paper reports on the use of electrical capacitance tomography (ECT) to measure flow characteristics in gas-liquid flows in a vertical pipe. We report measurements over a range of liquid superficial velocities from 0.05 ms-1 to 0.5 ms-1 and gas superficial velocities from 0.06 ms-1 to 6 ms-1 in a pipe 6 m long of internal diameter 0.067 m. A second complementary technique, a wire-mesh sensor (WMS), was also present in the tests and the results of the two sensors are shown to be within 2% of measurement on cross-sectional average void fraction. A previous paper using the same techniques (Azzopardi et al 2009) focused on the comparison of measurements of void fraction and bubble size between these two sensors. Here we pursue the mean values in terms of the drift flux model and potential applications of ECT as a flowmeter. We go on to use the velocity measurement capability of ECT to describe the velocity profiles in the flow and we show detailed measurements of void fraction profiles, wave and slug structures. ECT has relatively low spatial resolution in comparison to the WMS, and as shown by reference to other measurements. It is however a high speed measurement (up to 5000 frames of data per second), gives velocity information and is completely non-intrusive and suitable for use in industrial pipelines. ECT requires good physical models of the relationship of void fraction to permittivity and we publish here a general equation for that relationship allowing for the shape of the voids. Our results demonstrate that ECT measures flow structure velocity rather than gas velocity, where those structures are typically small bubbles, large 'churn' bubbles', or 'huge waves' as described by Sekoguchi and Mori (1997). Given the capabilities of ECT to measure velocity non-intrusively we are able to show detailed void fraction and velocity profile information for these flows. We observe three types of flow in these experiments: dispersed bubble, plug and huge wave. In dispersed bubble flows at higher liquid velocities and low gas flowrate the velocity profile exhibits a centre-peak, while for plug flows we a see flat velocity profile. An important transition is seen at a gas superficial velocity of about 1 ms-1 as huge waves become the dominant feature with a significant centre peak to the velocity profile. At this transition the velocity of the wave structure is about 2ms-1 and the transition is clearly measurable by the frequency of flow structures. Below the transition (in plug flow) the frequency increases with gas superficial velocity while above the transition (with huge waves dominant) the frequency is approximately constant. We believe that this transition point is associated with the moment at which gas from one plug structure 'breaks through' the liquid barrier to the higher one and a continuous gas core starts to exist in the 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: VID00252
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 1033-Hunt-ICMF2010.pdf

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


Measurement of Dynamic Properties of Vertical Gas-Liquid Flow

A. Hunt*, L.A. Abdulkareemt and B.J. Azzopardit

Atout Process Limited, 26-28 West Street, Bridport, Dorset DT6 3QP, United Kingdom
t Process and Environmental Engineering Division, Faculty of Engineering, University of Nottingham, UK
ahuntkiaatoutprocess.com

Keywords: Electrical Capacitance Tomography, gas-liquid flow, huge wave, churn flow, Wire Mesh Sensor

Abstract

Measurements of void fraction profiles, bubble sizes and velocity distributions are required to model two-phase flows and to
understand their flow regimes and physics. This paper reports on the use of electrical capacitance tomography (ECT) to
measure flow characteristics in gas-liquid flows in a vertical pipe. We report measurements over a range of liquid superficial
velocities from 0.05 ms-1 to 0.5 ms-1 and gas superficial velocities from 0.06 ms-1 to 6 ms-1 in a pipe 6 m long of internal
diameter 0.067 m.

A second complementary technique, a wire-mesh sensor (WMS), was also present in the tests and the results of the two
sensors are shown to be within 2% of measurement on cross-sectional average void fraction. A previous paper using the same
techniques (Azzopardi et al 2009) focused on the comparison of measurements of void fraction and bubble size between these
two sensors. Here we pursue the mean values in terms of the drift flux model and potential applications of ECT as a
flowmeter. We go on to use the velocity measurement capability of ECT to describe the velocity profiles in the flow and we
show detailed measurements of void fraction profiles, wave and slug structures.

ECT has relatively low spatial resolution in comparison to the WMS, and as shown by reference to other measurements. It is
however a high speed measurement (up to 5000 frames of data per second), gives velocity information and is completely
non-intrusive and suitable for use in industrial pipelines. ECT requires good physical models of the relationship of void
fraction to permittivity and we publish here a general equation for that relationship allowing for the shape of the voids. Our
results demonstrate that ECT measures flow structure velocity rather than gas velocity, where those structures are typically
small bubbles, large 'churn' bubbles', or 'huge waves' as described by Sekoguchi and Mori (1997). Given the capabilities of
ECT to measure velocity non-intrusively we are able to show detailed void fraction and velocity profile information for these
flows.

We observe three types of flow in these experiments: dispersed bubble, plug and huge wave. In dispersed bubble flows at
higher liquid velocities and low gas flowrate the velocity profile exhibits a centre-peak, while for plug flows we a see flat
velocity profile. An important transition is seen at a gas superficial velocity of about 1 ms-1 as huge waves become the
dominant feature with a significant centre peak to the velocity profile. At this transition the velocity of the wave structure is
about 2ms-1 and the transition is clearly measurable by the frequency of flow structures. Below the transition (in plug flow)
the frequency increases with gas superficial velocity while above the transition (with huge waves dominant) the frequency is
approximately constant. We believe that this transition point is associated with the moment at which gas from one plug
structure 'breaks through' the liquid barrier to the higher one and a continuous gas core starts to exist in the flow.


Introduction

This paper reports on the use of two complementary
techniques for making measurements of flow parameters in
two-phase gas-liquid flow: electrical capacitance
tomography (ECT) and a wire-mesh sensor (WMS). Both
sensors were mounted on the same pipe and concurrent
measurements were made over a period of 1 minute at each
flow condition.

Comparisons between measurements using various
combinations of imaging and other sensors in two-phase
flows have been made in the past including: ECT and
gamma-ray densitometers (Hunt, Pendleton and Ladam


2004); WMS and ECT (Azzopardi et al 2009); WMS and
x-ray tomography (Prasser et al 2005); ECT and weigh
scales (Hunt, Pendleton and Byars 2004). These
comparisons have shown that for measuring dynamic
properties of local flow ECT is fast, accurate, non-intrusive
but with low spatial-resolution; while WMS is fast, accurate,
of high spatial resolution but intrusive and disruptive of the
flow.

In this paper we focus on the use of ECT to give detailed
information about gas-liquid flows while using WMS as a
check on the void measurement accuracy in this application.









Nomenclature


c Constant in drift flux model
D Pipe internal diameter
g Acceleration due to gravity
n Shape factor in permittivity equation
U Velocity
v Void fraction or concentration of dispersed phase

Greek letters
E Electrical permittivity

Subsripts
1 Continuous phase (in electrical equations)
2 Discontinuous phase (in electrical equations)
g Gas
L Liquid
m Mixture
s Superficial (flowrate divided by pipe area)


Electrical Capacitance Tomography

Electrical capacitance tomography (ECT) is a non-intrusive
technique which can be used for imaging and velocity
measurement in flows of mixtures of 2 non-conducting
materials. Developments over the last 15 years have made
fast, accurate measurement systems available for laboratory
research. Using ECT can offer measurements
unobtainable with other measurement technologies, but the
interpretation of quantitative flow data requires a good
physical model of the interaction of the materials with the
electric field in the sensor and appropriate reconstruction
and analysis algorithms. Hunt, Pendleton and Ladam
(2004) reported suitable algorithms for flows of dry solids
in air, here we study the requirements and results in
gas-liquid flows.

An array of electrodes was arranged around the outside of
the non-conducting pipe wall (see Figure 1) and all unique
capacitance pairs were measured using a Tomoflow R5000
flow imaging and analysis system. The instrument contains
16 identical measurement channels and 16 identical driven
guard circuits and in the tests reported here was operated
with a twin-plane sensor.

Data can be captured at rates up to 5000 image frames per
second with typical measurement noise level at 500 fps of
0.02fF rms. The typical average value between two
opposite electrodes is 10fF. In the experiments reported
here the frame rate was 1000 fps.

Measurements were made between all pairs of electrodes
within each plane around the sensor using a
charge/discharge capacitance technique. An excitation
signal was used in the form of a 15V peak to peak square
wave with a frequency of 5 MHz.

The sensor included a full set of driven guard electrodes
running axially before, between and after the measurement
planes giving a total of 5 axial sets of 8 azimuthal electrodes
ensuring that an axially-uniform electric field was
maintained over the capacitance sensor cross-section and


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

the two sensor 'planes' (actually short cylindrical sections).

Inversion of the 28 capacitance pairs to a 812 pixel image
on a 32x32 square grid was undertaken as described below,
and component information (void fraction etc.) was
extracted from these images.


Figure 1: ECT sensor mounted on transparent plastic pipe
with electrical guard removed for clarity.

Cross-correlation between the image planes gives the
velocity distribution across the flow. The resolution of
ECT images is limited (see example in Figure 2), so
cross-correlation is not carried out for all pixels, but for a set
of larger 'zones' containing the average of a number of
pixels.

















Figure 2: Typical 812 pixel image from ECT. A false
colour scale runs from red (100% liquid) to blue (100%
gas).

Most ECT sensors are non-linear, both in the relationship
between the measured capacitances and the permittivity of
the sensor contents and also in the relationship between the
concentration of a 2-phase mixture and its effective
permittivity. All images shown and used in this work were









reconstructed using linear back-projection and a Tikhonov
regularisation factor of 10, see Byars (2001) for more
details of this technique.

Figure 2 shows a typical image from ECT with the
cross-section of a large bubble in the lower left part of the
pipe, with mostly liquid in the upper right. The
intermediate permittivity (shown as green) indicates that the
liquid has significant amounts of gas bubbles distributed in
it that are below the resolution of the simple linear back
projection reconstruction.


Experimental Facility

The experiments were carried out near the top of a 6m long
pipe of internal diameter 0.067m. The pipe is mounted on
a frame that can be inclined at any angle from from vertical
to 200 above horizontal, though the tests reported here were
only measured in vertical flows. Figure 3 shows a
photograph of the test rig in vertical position.


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


The ECT sensor was mounted approximately Im from the
upper end of the test pipe with the WSM about 0.5m above
it. It was not possible to mount the WSM below the ECT
sensor as visual observation showed that the intrusive wire
mesh of the WSM changed the nature of the flow
completely by breaking up large bubbles and generally
homogenising the flow.


Wire Mesh Sensor

In a wire-mesh sensor electrodes are stretched across the
flow cross-section within two axially-separated planes. A
fast electronic measurement system connected to the wires
measures the relative permittivity E at each crossing point.
Scanning a complete cross-section of the flow can be done
at up to 10 000 frames per second. The sensor employed in
our experiments has 2 x 24 wires which have 2.8 mm
separation and is the same as described by Azzopardi et al
(2009).


Estimation of void fraction from permittivity

For the WMS the choice of permittivity model is not critical
as the gap between the wires is small and is essentially either
filled with liquid or with gas.

For ECT however, the choice of physical model is critical.
The capacitance measurement in ECT is converted to
electrical permittivity using a look-up table linearisation
from calibration at various permittivities. To move from
this electrical measurement to a fluid-mechanically useful
measure of concentration or void fraction (terms used here
interchangeably) involves the use of a physical model
linking the two. The expression used by Hunt, Pendleton
and Byars (2004) as the 'Maxwell' model applies to
non-conducting spheres distributed uniformly in a
non-conducting medium.


Em = 81 [1 + 3.v.( 82 81)/( 2 + 2.81 V.( 2 81))


Figure 3: Overall view of flow rig, the pipe test section is
mounted on the yellow frame.

Silicone oil of density 910 kg.m-3 and viscosity 5 cSt was
circulated through the pipe by a pump at a range of
flowrates, while air was injected at the lowest part of the
pipe before being vented off after leaving the test section.

Given the development length/pipe diameter ratio of
approximately 75 it is believed that the gas distribution and
bubble size at the working section were unaffected by the
injection method.


where Em is the effective mixture permittivity of a
distribution of spherical particles, e1 is the material
permittivity of the continuous medium, 82 is the material
permittivity of the spherical particles, and v is the
volumetric fraction of space occupied by the spheres.

For comparison purposes we will also refer to simple
models where the measurement represents simple
arrangements of material between the plates of a parallel
capacitor. These simple systems are effectively the upper
and lower bounds for all permittivity-concentration models

The 'parallel' model applies when the dielectric material is
distributed as parallel plates normal to the capacitor plates:


and the 'series' model is the model for capacitances in series
when the dielectric material is distributed as plates parallel
to the electrodes:


Fm = F1 .0 -V) + F2V






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


Cm = (1i. e2)/ (1i. V + F2.(l v))


See PTL Application Note 4 (1999) for a more complete
description of these simple expressions.

Wagner (1914) gave the original derivation of the electric
field through an array of distributed spheres, based on
equations proposed by Maxwell in 1873. Van Beek (1960)
quotes from Sillars (1937) giving a more general form of
Wagner's derivation:


Em = 81 [1 + n.V.(82 81)/( 2 + (n-1). 81)]


where n is a function of particle shape. For spheres n = 3
and then equation 4 reduces to equation 1. For oblate
spheroids n 1 and for prolate spheroids n > 3. We refer to
equation 4 as the Maxwell-Wagner-Sillars (MWS) equation.

Hunt (2007) introduced the same factor n into equation 1 by
analogy to derive a more generalised form:

m = 81 [1 + n.v.( 82 1)/( 2 + (n -1). 81 V.(82 1)] (5)

This extended expression is we believe novel and has the
significant advantage that it allows for the particles to be
non-spherical and reduces to the other forms as follows:

n = 1 : series model
n = 3 : Maxwell-Rayleigh
n = o : parallel model.

Previous experiments with gas-liquid flows led us to choose
n=100 for the results presented here, this choice being
justified by the comparisons shown in the next section.


Cross-sectional average values

The mean void fraction was calculated from each sensor
using the average of all ECT pixel values and the average of
all WMS nodes over the 60 seconds of data taken at each
flow condition. Figure 4 shows these results plotted
against each other.

It can be seen from Figure 4 that the two sensors agree on
average void fraction to within 2% of reading for most of
the flows measured. The liquid superficial velocity varies
from 0.05 ms-1 to 0.524 ms-1 and the gas superficial velocity
from 0.06 ms-1 to 6.05 ms-1.

The flows measured in this set of experiments visually
exhibited three basic patterns: bubbly flows, through plug to
huge wave structures. The terminology and flow pattern
conditions are consistent with those described in Sekoguchi
et al (1997) who first described the 'huge waves'.

The cross-correlation process used in ECT estimation of
velocity emphasizes the largest scale changes in
concentration, which are associated with the passage of gas
bubbles, clusters of bubbles, slugs, chur structures or liquid
waves and films, as the gas velocity increases.


0 1
Void fraction(WMS)
Figure 4: Average void fraction from each sensor.
Velocity figures for each data set refer to different liquid
superficial velocities.

Velocity measurements were taken from the twin-plane
ECT system by cross-correlating the concentration time
signal in the same zone of the two axially-separated
measurement planes. For this purpose each plane was
divided into 13 roughly equally-sized zones as in described
by Hunt, Pendleton and Ladam (2004). The resulting
velocities were averaged to give the 'average ECT velocity',
while the void-fraction-weighted average of the velocities is
equivalent to a calculation of superficial velocity.


(A
E

U
24





fa
u
o
m 3

, 2

b1
UJ


00 X
o


0
o


o 0.05 m/s
a 0.16 m/s
A 0.26 m/s
x 0.31 m/s
X 0.42 m/s
o 0.524 m/s
-y=x


0 1
0 2 4 6 8
In-situ gas velocity m/s
Figure 5: Average ECT velocity plotted against in-situ gas
velocity calculated from reference gas flowrate and void
fraction measured by WMS. Velocity figures for each data
set refer to different liquid superficial velocities.

The in-situ gas velocity, Ug, is related to the superficial
velocity Ugs in the normal manner:


Ug = gs /v


where v is the void fraction. Figure 5 shows that for gas
in-situ velocities below about 2 ms-' the average ECT
velocity represents the in-situ gas velocity, while at higher
flowrates the average ECT velocity becomes less and less
dependent on gas velocity.

These results suggest that below 2 ms-' in-situ gas velocity
the flow is dominated by dispersed gas bubbles so that the
cross-correlation velocity is equal to the average gas
velocity, at the highest gas flowrates the ECT average
velocity represents the speed of passage of complex liquid
structures moving slower than the gas.

Figure 6 shows the void-fraction weighted ECT average


L 0.05 m/s
o 0.16 m/s
A 0.26 m/s
x 0.31 m/s
x 0.42 m/s
o 0.524 m/s
-y=x
y=x-2%
y=x+2%









velocity plotted against the input gas superficial velocity.
In conventional flowmetering this would be equivalent to a
calibration curve of estimated flowrate against reference
flowrate.


S/ 4
E
a3


S2 0 o 0.05 m/s
SaL 0.16 m/s
-I: / A 0.26 m/s
x 0.31 m/s
x 0.42 m/s
0 0.524 m/s
S-- Us=Ugs

0 1 2 3 4 5 6 7
Input gas superficial velocity m/s
Figure 6: Average void-weighted ECT velocity plotted
against reference superficial gas velocity. Velocity figures
for each data set refer to different liquid superficial
velocities.

It can be see from the figure that at low gas flowrates the
average void-weighted ECT velocity tends to overestimate
the gas superficial velocity, but above a value of about 2.5
ms-' the difference becomes negative and of growing
magnitude.

Use of ECT as a flowmeter in these conditions would
require a calibration curve fitted to the graph in Figure 6.
The effect of varying liquid superficial velocity appear to be
small, but how general this would be is impossible to say.


Ul,
E
S4
0
o
a 3


m l
Lu


0 2 4 6 8
Total superficial velocity m/s
Figure 7: Average void-weighted ECT velocity plotted
against reference superficial gas velocity. Velocity figures
for each data set refer to different liquid superficial
velocities.

We now compare our results to the drift-flux modelling
approach. Nicklin et al (1962) first generalised the
modelling of the rise velocity of single Taylor bubbles in the
form:

Ug = c.(Ug+ ULs)+0.35(g.D)05 (8)

where c took a value between 0.9 and 1.85 depending on the
liquid velocity, but above a liquid velocity of 0.3 m/s took
the value 1.2.

Figure 7 shows that while the slope of this line looks


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

reasonable, the intercept should be much higher to fit the
data in the range 0 to 1 ms-' where we know from Figure 4
that the ECT average velocity is representative of the in-situ
gas velocity.

We explain this discrepancy by remarking that the Nicklin
model was established for smooth-fronted Taylor bubbles in
a 25.4 mm diameter pipe, where the limiting factor to
velocity is the passage of the liquid in the annular film
around the bubble and the centreline velocity of the liquid in
between the bubbles. In our work in a pipe of over 3 times
the cross sectional area we observe that the irregular 'churn'
bubbles seem visually to 'tunnel' up the pipe in the
fast-moving wake of the preceding one. This, associated
with the larger flow area available around the bubble for
liquid backflow could explain such a difference. We
return to this observation of tunnellingg' below.

Void fraction profiles

One advantage of tomographic measurements is that
measurements at particular points of interest can be defined
after the measurement has taken place simply by defining
the image zone of interest. To study the profile of void
fraction across the pipe, a set of zones was defined as shown
in Figure 8.


03 02





>Vfl^^^^HHOL


Figure 8: Zone map for void and velocity profiles. 16
zones of 2x2 pixels distributed equally along the horizontal
diameter. Zone 7 highlighted in white, other colours are
simply to distinguish the zones.

Taking the average of the 60 seconds of data for each set of
experimental conditions for each zone enables to calculate
the void fraction profiles across the flow. Examples are
shown in Figures 9 and 10 for liquid superficial velocity of
0.06 ms-' and 0.524 ms-' respectively.

We can see from these figures that the void fraction profile
is peaked at the centre of the pipe for all of the flows
measured. As the gas flowrate increases at a given liquid
flowrate the void fraction increases fairly uniformly until at
high gas rates the centre of the pipe is almost completely
empty of liquid.

For a given gas flowrate an increase in liquid flowrate gives
a lower void fraction level so that the point at which the
centre becomes gas-only is at a higher rate.











*5,541
0 3,242
A 2,690
x 2,180
* 1,638
+ 0,829
- 0,640
00,412
0 0,349
a 0,077
x 0,060


Non-dimensional radius O=centre


Figure 9: Void fraction profiles from ECT using zones as
shown in Figure 8 at a liquid superficial velocity of 0.052
ms-1. Numbers in the legend are gas superficial velocity in
ms1


o I



0* .
SU a .
-i a U f


S0 O=centre
Non-dimensional radius, 0=centre


Figure 10: Void fraction profiles from ECT using zones as
shown in Figure 8 at a liquid superficial velocity of 0.524
ms 1. Numbers in the legend are gas superficial velocity in
ms1

Void fraction profiles can also be extracted from the WMS
data and a comparison is shown between these and the ECT
profiles in Figure 11. To eliminate some of the extraneous
non-symmetric features from the profiles, Figure 11 shows
half-pipe profiles where the two sides of the profile have
been averaged.


Non-dimensional radius O=centre
Figure 11: Void fraction profiles from ECT and WMS at a
liquid superficial velocity of 0.052 ms 1. Numbers in the
legend are gas superficial velocity in ms'1.


* 6,065
0 3,446
A 2,838
) 2,255
* 1,691
* 1,135
- 0,664
0 0,426
o 0,359
A 0,078
x 0,061


# 1 : # I m *

0 +
*)S -t I-



9 +
+ 8 4 a
AA o s +


Velocity profiles using ECT


0
*
A A A






N1 0 O
Non-dimensional radius, 0=centre


* 5,541
0 3,242
A 2,690
X 2,180
* 1,638
+ 0,829
- 0,640
00,412
E 0,349
A 0,077
x 0,060


Figure 12: Velocity profiles from ECT using zones as
shown in Figure 8 at a liquid superficial velocity of 0.052
ms-1. Numbers in the legend are gas superficial velocity in
ms1


*
A AO *
0 .%>.. A0

S + + + + _+ + + + +

0 X

0
Non-dimensional radius, O=centre


* 6,065
0 3,446
A 2,838
X 2,255
* 1,691
+ 0,858
- 0,664
0 0,426
E 0,359
A 0,078
x 0,061


Figure 13: Velocity profiles from ECT using zones as
shown in Figure 8 at a liquid superficial velocity of 0.524
ms1. Numbers in the legend are gas superficial velocity in
ms1.

Whereas both ECT and WMS can measure the void fraction
profiles, of the two only ECT can measure velocity. Using
the same zone map as shown in Figure 8, we have
calculated the transit velocity between the two ECT


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

It can be seen from Figure 11 that the overall values of void
fraction are similar (as shown also by the mean data in
Figure 4) but that there are differences in form the ECT
data shows a consistent tendency to a shallow horizontal 'S'
shape while the WMS data shows a steady decline towards
the wall.

Previously published data from similar flows has shown
profiles with simple central peaks rather as the WMS data
(eg Couet et al 1990) and also profiles with wall peaking (eg
Prasser et al 2005) so both forms shown here are possible
within the range expected. While ECT can exhibit
artefacts in reconstruction which might distort the profiles,
WMS undoubtedly breaks up the bubble structure in the
flow and so may distort the distribution. Further work is
required to establish the cause of the differences shown
here.


WMS0.06m/s ED E 0
*ECT 0.06m/s *
AWMS 0.412m/s E
AECTO 0.412m/s
EWMS 5.541m/s
SECT 5.541m/s *0



oA A A

^ o 0,^ ^ ^~






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


measurement planes by cross-correlating the time-varying
concentration in each zone with the same zone in the second
plane. Typical results are shown in Figures 12 and 13 for
the same conditions as Figures 9 and 10.

Considering these velocity profiles we can see that, unlike
the void profiles which all exhibited essentially the same
form, there are three distinct types present. For high liquid
flowrate and low gas flowrate the velocity profiles show
centre peaks, for moderate gas rates at all liquid flowrates
the profiles are flat, while at high gas flowrates for all liquid
flowrates the velocity profiles are centre-peaked.

The transition between flat velocity profiles and
centre-peaked is striking and occurs always at a gas
superficial velocity of about 1 ms-1, the same value as that
at which the ECT-measured flow structure velocity starts to
deviate from the in-situ gas velocity (see Figure 5).


Measurement of flow structures using ECT

To consider the flow structures further we undertook simple
power-spectral analysis of the ECT concentration
time-series data, taking the value of the low-frequency peak
as a measure of structure frequency of passage. Visual
analysis of the time-series (as shown in Figure 16 and 17)
confirms this frequency as being typical.


N 3

U
a2
0-


A
A
A

A A A A
A
AA A A A A
A
A
A 0.052 m/s
A 0.524 m/s

0 2 4 6 8
Average velocity m/s


Figure 14: Structure frequency against velocity at
non-dimensional radius of 0.1875 (zone 7). Numbers in
the legend are gas superficial velocity in ms1.

From Figure 14 it can be seen that below an average
velocity (that is the average ECT velocity zone 7) of about 2
ms-' the passing frequency increases with velocity, while
above point the frequency remains fairly constant, though
lower than at the transition.

Dividing the average velocity by the passing frequency
gives us a 'wavelength' typical of the structure. Figure 15
shows that this seems to increase fairly linearly with
velocity.


4


E3
c-
c2



l1


0


0 2


4 6 8


Average velocity m/s

Figure 15: Structure wavelength against velocity at
non-dimensional radius of 0.1875. Numbers in the legend
are liquid superficial velocity in ms1.

We now consider 2 typical flow conditions, one below the
transition point on the passing frequency graph, and the
other well above the transition. Figure 16 shows
concentration time-series for the lower point clearly
showing large scale 'bubbles' separated by liquid zones
containing smaller quantities of (presumably) dispersed gas.

The 'bump' on the curve at the back of each bubble is taken
to be the swelling zone described by Sekoguchi et al and is
typical of many of the flow conditions. 3-D images in
Azzopardi et al (Figure 14 of that paper) show the bubbles
in these conditions as being irregular in shape at both nose
and tail.


22 23 24 25
Time, seconds
Figure 16: Example time series data for liquid superficial
velocity of 0.052 ms-1 and gas superficial velocity of 0.077
ms.

Considering now flows above the transition shown in Figure
17, it is clear that the flow is essentially a gas core with
occasional liquid structures passing at high velocity. (The
velocity is Figures 16 and 17 is of course indicated by the
time delay between the time-series in each of the image
planes).


A
A A
A


A

a aAA A 0.052 m/s
A 0.524 m/s









1


Q)
0
N
C-
0

4-
Lower plane
:> -Upper plane

0 1
22 23 24 25
Time, seconds
Figure 17: Example time series data for liquid superficial
velocity of 0.524 ms-1 and gas superficial velocity of 3.446
ms.

This wave structure is shown in cross-section in Figure 18,
and we believe is entirely consistent with the pictures of
huge waves shown by Sekoguchi et al.













Figure 18: Example images of wave structure. Left shows
wave passing through plane with zone 7 approximately 70%
void, right shows the 'lull' between waves with zone 7 100%
void.


Conclusions

Our results demonstrate that ECT measures flow structure
velocity rather than gas velocity, where those structures are
typically small bubbles, large 'chum' bubbles', and 'huge
waves' as described by Sekoguchi and Mori (1997). Given
the capabilities of ECT to measure velocity non-intrusively
we have been able to show detailed void fraction and
velocity profile information for these flows.

We observe three types of flow in these experiments:
dispersed bubble, plug and huge wave. In dispersed
bubble flows at higher liquid velocities and low gas flowrate
the velocity profile exhibits a centre-peak, while for plug
flows we a see flat velocity profile. An important
transition is seen at a gas superficial velocity of about 1 ms-1
as huge waves become the dominant feature with a
significant centre peak to the velocity profile. At this
transition the velocity of the wave structure is about 2ms-1
and the transition is clearly measurable by the frequency of
flow structures. Below the transition (in plug flow) the
frequency increases with gas superficial velocity while


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

above the transition (with huge waves dominant) the
frequency is approximately constant.

Below the transition point the bubble are large, irregular
'chum' bubbles which seem to 'tunnel' up the pipe into the
fast-moving wake of the bubble in front. We believe that
the transition point is associated with the moment at which
gas from one plug structure 'breaks through' the liquid
barrier to the higher one and a continuous gas core starts to
exist in the flow.


Acknowledgements

The authors would like to thank John Pendleton of
Tomoflow Ltd for software support and Bill Lionheart of
Manchester University for contributing to lively discussions
on permittivity modelling.

References

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7th International Conference on Multiphase Flow
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