Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 13.1.1 - Experimental Investigation and Physical Modeling of a Two-Phase Bubbly Flow in Horizontal Pipe
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00316
 Material Information
Title: 13.1.1 - Experimental Investigation and Physical Modeling of a Two-Phase Bubbly Flow in Horizontal Pipe Fluidized and Circulating Fluidized Beds
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Marchand, M.
Bottin, M.
Berlandis, J.-P.
Serre, G.
Hervieu, E.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: horizontal pipe
bubbly two-phase flow
experiment
physical modeling
 Notes
Abstract: The METERO experiment is designed by the French Atomic Energy Commission to provide exhaustive database for horizontal two-phase flow in adiabatic conditions and validate MCFD codes as well as to characterize the opponent hydrodynamic mechanisms responsible for horizontal water-air flow stratification. The test section is a 5 meter long Plexiglas pipe, 0.1 meter in diameter, equipped with various measurement techniques, including hot film anemometry, home made optical probes and fast video cameras. Different flow regimes are generated by varying the water superficial velocity from 0 to 5.3 m/s and the gas superficial velocity from 0 to 0.127 m/s. The single-phase liquid flow agreement with literature on pipe flows has been checked by hot film velocimetry and several two-phase flow conditions have been investigated for 3 axial locations and various values of the liquid and gas superficial velocities by means of fast camera video. Flow pattern maps have been plotted, according to the description of Govier & Aziz (1972) and the intermittent regime has been studied accurately. A new transition line between buoyant bubble and stratified bubble regimes has then been proposed. The features of the METERO two-phase flow revealed by videos have been also highlighted by hot film measurements carried out for the same 3 axial location, a high value of the liquid velocity (4.42 m/s) and various values of the gas superficial velocity, ranging from 0 (single phase) to 0.127 m/s. The gas influence, slight for JG< 0.06 m/s and strongly increasing beyond this value, is demonstrated by a growing bubble layer in the upper region of the pipe when the gas flow rate rises. The effect of this bubble layer is to slow down the main liquid flow and enhance turbulence in this region, as can be seen on the mean and turbulent velocity as well as kinetic energy radial profiles. These observations are in good agreement with the one depicted for similar conditions by Iskandrani & Kojasoy (2001) or Ekambara et al. (2008). Moreover, the linear behavior of the averaged kinetic energy versus the axial location suggests that bubbly induced turbulence is injected at the inlet of the test section and then transported by the main liquid flow towards the exit. These trends are attested by optical probe measurements carried out for the same conditions. The bubble layer generates increased void fraction in the upper region and when the gas injection is raised, the void fraction profiles are enhanced whatever the radial location. Supplementary data points, acquired for JL=5.3 m/s, show that the two-phase flow regime gets more dispersed and the void fraction profiles tend to flatten when the liquid flow rate is increased. All these observations are in good agreement with the literature cited previously. The interface area concentration profiles follow the same trends accounting more likely for bubble sedimentation than for coalescence. Finally, plots of the averaged interface area concentration exhibit a linear growth versus JG, already observed by authors in vertical pipe configuration, with almost the same slope for all the X locations as soon as the water flow is fully developed (20D). Finally, a new correlation is proposed for the behavior of <Ai> vs. JG.
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: VID00316
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 1311-Marchand-ICMF2010.pdf

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Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Experimental Investigation and Physical Modeling
of a Two-Phase Bubbly Flow in Horizontal Pipe



Muriel Marchand*, Manon Bottin*, Jean-Pierre Berlandis*, Guillaume Serret and Eric Hervieu*

Service d'Etudes Thermohydrauliques et Technologiques, CEA, 17 rue des Martyrs, 38054, Grenoble, France
t Service de Simulation ThermoHydraulique, CEA, 17 rue des Martyrs, 38054, Grenoble, France
Muriel.Marchand@cea.fr




Keywords: Horizontal pipe, bubbly two-phase flow, experiment, physical modeling




Abstract

The METERO experiment is designed by the French Atomic Energy Commission to provide exhaustive database for
horizontal two-phase flow in adiabatic conditions and validate MCFD codes as well as to characterize the opponent
hydrodynamic mechanisms responsible for horizontal water-air flow stratification. The test section is a 5 meter long Plexiglas
pipe, 0.1 meter in diameter, equipped with various measurement techniques, including hot film anemometry, home made
optical probes and fast video cameras. Different flow regimes are generated by varying the water superficial velocity from 0
to 5.3 m/s and the gas superficial velocity from 0 to 0.127 m/s.
The single-phase liquid flow agreement with literature on pipe flows has been checked by hot film velocimetry and several
two-phase flow conditions have been investigated for 3 axial locations and various values of the liquid and gas superficial
velocities by means of fast camera video. Flow pattern maps have been plotted, according to the description of Govier & Aziz
(1972) and the intermittent regime has been studied accurately. A new transition line between buoyant bubble and stratified
bubble regimes has then been proposed.
The features of the METERO two-phase flow revealed by videos have been also highlighted by hot film measurements
carried out for the same 3 axial location, a high value of the liquid velocity (4.42 m/s) and various values of the gas
superficial velocity, ranging from 0 (single phase) to 0.127 m/s.
The gas influence, slight for JG< 0.06 m/s and strongly increasing beyond this value, is demonstrated by a growing bubble
layer in the upper region of the pipe when the gas flow rate rises. The effect of this bubble layer is to slow down the main
liquid flow and enhance turbulence in this region, as can be seen on the mean and turbulent velocity as well as kinetic energy
radial profiles. These observations are in good agreement with the one depicted for similar conditions by Iskandrani &
Kojasoy (2001) or Ekambara et al. (21'" ). Moreover, the linear behavior of the averaged kinetic energy versus the axial
location suggests that bubbly induced turbulence is injected at the inlet of the test section and then transported by the main
liquid flow towards the exit.
These trends are attested by optical probe measurements carried out for the same conditions. The bubble layer generates
increased void fraction in the upper region and when the gas injection is raised, the void fraction profiles are enhanced
whatever the radial location. Supplementary data points, acquired for JL=5.3 m/s, show that the two-phase flow regime gets
more dispersed and the void fraction profiles tend to flatten when the liquid flow rate is increased. All these observations are
in good agreement with the literature cited previously. The interface area concentration profiles follow the same trends
accounting more likely for bubble sedimentation than for coalescence. Finally, plots of the averaged interface area
concentration exhibit a linear growth versus JG, already observed by authors in vertical pipe configuration, with almost the
same slope for all the X locations as soon as the water flow is fully developed (20D). Finally, a new correlation is proposed
for the behavior of vs. JG.


Introduction way that the improvement of the simulation tools
predictions will lead to a reduction of the uncertainties
The French Atomic Energy commission (CEA) designs, linked to safety margin. In this framework, the so-called
develops and validates simulation tools to calculate the NEPTUNE software platform project, developed jointly by
behavior of multiphase flows in nuclear plants, in EDF and CEA associated with AREVA Nuclear Power
incidental and accidental situations. For pressurized water (AREVA-NP) and the French Institute for Nuclear Safety
reactors, this topic constitutes a major R&D axis, in the (IRSN), was initiated in 2001 and the experiment






Paper No


METERO, specifically designed for the study of horizontal
two-phase flows, was inaugurated in 2006.
In horizontal pipes, the evolution of a bubbly flow results
from the competition between opponent hydrodynamic
mechanisms. On the one hand, the turbulence of the main
liquid flow is responsible for bubble dispersion and also
affects bubble break up and coalescence. On the other hand,
gravity effects tend to separate the two phases: when rising
up, the bubbles can merge and coalesce. Consequently,
depending on the relative magnitude of these two
phenomena, the flow becomes stratified or not. This is the
reason why the METERO experiment was designed to
provide all the data necessary for two-phase-CFD
simulations. Within this framework, the two-phase model
developed for the CFD scale module of NEPTUNE (Morel
et al. (2" '4), Morel et al. (2005)) is devoted to the
prediction of two-phase flow in reactor components and
needs further experimental validation in adiabatic
conditions. Another major concern of METERO is the 1D
turbulence model developed by Chandesris et al. (2006),
Chandesris & Serre (2005) and Serre & Bestion (2005).
Presently, the main aim of the METERO data is to provide
a validation basis of turbulence modeling in bubbly flow.
From an experimental point of view, the features of vertical
bubbly flows have been extensively documented (one can
see for instance Wang et al. (1987), Lance & Bataille
(1991), Liu & Bankoff (1993), Suzanne et al. (1998)) but
less literature can be found on horizontal pipes, especially
concerning turbulence data collection. Even though
numerous studies were devoted to the characterization of
horizontal flow patterns (see for example Govier & Aziz
(1972), Andreussi et al. (1999), Barnea (1987), Li &
Kwauk (1994)) there is, up to now, no theoretical model
giving the local distribution of gas fraction or turbulence
field in a horizontal pipe and the need for experimental
database to build the physical modeling is still important.
The bibliographical references we rely on for this study are
the experimental works of Kocamustafaogullari & Wang
(1991), Kocamustafaogullari et al. (1994),
Kocamustafaogullari & Huang (1994), Iskandrani &
Kojasoy (2001) and the recent numerical simulation of
Ekambara et al. (2' i i").

In this context, the aim of the METERO experiment is not
solely to provide local profile measurements (velocity, void
fraction, turbulent intensity...) to describe accurately the
horizontal pipe two-phase flow mechanisms but also to
derive volume average values that will help to build the
closure relationships necessary for CFD.


Nomenclature


Mean Sauter diameter (mm)
Interface area concentration (m 1)
Turbulent kinetic energy inI- -)
Root mean square of the velocity fluctuation (m/s)
Mean velocity (m/s)
Superficial velocity (m/s)
Liquid flowrate (m3/h)
Gas flowrate (1/mn)
Axial location (mm)
Vertical location (mm)
Radial location (= y)


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

R Pipe radius (=50 mm)
D Pipe diameter (=100 mm)
Re Reynolds number based on the pipe diameter (-)

Greek letters
a Void fraction (-)
v Kinematic viscosity (m2/s)

Subsripts
L liquid
G gas



Experimental Facility


The experiment is made of stainless steel pipes linked to a
Plexiglass test section (Figure 1). Two independent air and
water supply circuits merge upstream of the test section
through an injection/tranquilization system. This system
provides a horizontal bubbly flow at the inlet of the test
section. At the test section outlet, water and air are separated
in a 1500 liters storage tank. The water temperature is kept
constant (around 18 C) by means of a heat exchanger
located inside the tank. The main characteristics of the
installation are summarized in Table 1.


Inlet temperature air-water
Maximum outlet temperature
Maximum driving pressure
Water flow rate
Water velocity
Air mass flow rate
Air superficial velocity


18C
20C
2.8 bar
0 to 150 m3/h.
0 to 5.3 m/s
0 to 350 1/mn
0 to 0.7 m/s


Table 1: METERO main working parameters


TEST SECTION (5m)


Figure 1: Schematic diagram of the test setup

Concerning water supply, the circuit is composed by a
FinderTM pump, of driving pressure 2.8 bar, maximum
flowrate 150 m3/h (which corresponds to a maximum water
velocity of 5.3 m/s) linked to the test section by means of
two lines including a flowmeter, a 1 inch line for the scale 0
to 15 m3/h and a 4 inches line for the scale 15 to 150 m3/h.
For the small flowrates (0-15 m3/h), a YokogawaTM Coriolis
mass flowmeter is used (accuracy about 0.2 % of the
measurement point). For the higher flowrates (15 to 150
m3/h), a KrohneTM electromagnetic flowmeter is installed. It
provides a lower accuracy than the Coriolis flowmeter but
generates also reduced pressure drop. The inlet water
temperature is measured by a type K standard thermocouple






Paper No


with a +/-0.5C measurement accuracy. This accuracy
revealed inadequate for hot film anemometry, very sensitive
to flow temperature variations. The thermocouple was then
calibrated using a calibration bath and a platinum PT100
sensor.
Concerning the air supply, it is composed of a 6 bar pressure
supply line involving a depressurization/filtration system
and two lines of regulation and measurement of the mass
flowrate: one line for the range 0 to 50 1/mn and a second
line for the range 50 to 350 1/mn, corresponding to a
maximum gas velocity of 0.7 m/s. The air circuit is open:
after being separated from the water in the tank, the air is
vented to the atmosphere. The two flowmeters are Brooks
EmersonTM thermal massic flowmeters (accuracy is 0.7 %
of the measurement point). They include a PID regulation
system that provides steady inlet conditions and easy use.
The air temperature is measured by a type K thermocouple,
with a measurement accuracy of +/-0.50C.
The inlet and outlet pressures are measured by means of two
KellerTM high precision membrane sensors. They measure
the pressure relative to the atmosphere in the range 0-3 bar
with an accuracy of+/- 0.015% of the full scale, i.e. +/- 0.45
mbar.
The test section, 5.40 m in length, has an inner diameter of
0.1 m. It is composed of interchangeable and rotating
sections including instrumentation modules. The inlet
injection/tranquilization system is made of 320 tubes for the
water and 37 for the air. The number and dimensions of the
tubes have been set by iterative tests. The air bubble
injection is made uniformly in the inlet section. The void
fraction, directly depending on the number of bubbles
injected, is modified by varying the air mass flowrate. The
system also includes a series of grids designed to break
remaining vortices generated by upstream elbows and then
ensure a low turbulence level -the so-called grid turbulence-
at the inlet of the test section. Moreover, a grid located 3
diameters away from the injectors mixes the liquid and gas
phases so as to avoid the signature of injector wakes on the
velocity profiles and ensure uniform bubble distribution.
All the flow parameters can be controlled and/or acquired
directly from the control/command room, via a personal
computer and use of LabviewTM programs. These programs
also pilot the instrumentation data acquisition.

Several experimental techniques are implemented to
measure the relevant parameters of the flow.
The 3 measurement means already used on the installation
and presented herein are hot films, optical probes and fast
camera video.

Hot film velocimetry

Hot wire and hot film anemometry have been extensively
used in single phase flows over the last 50 years and also
adapted for use in two-phase flows. For our purpose, we
use DantecTM two components hot film probes or single
component conical probes composed of nickel sensors
electrically insulated from the water by quartz coating. The
probes are connected to a constant temperature
anemometer (DantecTM CTA Streamline) A filtration
system is also used downstream of the pump to retain the


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

particles of size greater than 5 vtm and ensure that the
probes could not be damaged by impacts. Moreover, the
water surface tension has been controlled by a KRUSS
K8 interfacial tensiometer and compared with the one of
several samples of water. Table 2 gives a comparison of
surface tensions (in mN/m) for METERO and several
samples.

Sample Surface tension (mN/m)
Sample Metero nl 58
Sample Metero n2 59
Non mineralized water 66
Tap water 61.5
Doubly distillated water 72.6
Table 2: METERO water surface tension
These measurements attest that the experiment surface
tension is very close to the one of tap water and that no
surfactant is spoiling the water quality.
The overall frequency response of the anemometry system
(from the sensor to the bridge) is about 7 kHz. The signal
is filtered in low pass mode at a 10 kHz frequency and
sampled at 20 kHz. For single phase flow configurations,
200.000 points are acquired, corresponding to 10 seconds.
In two phase flow configurations, this duration may be
increased to 5 minutes. The data acquisition is made using
a LabviewTM Virtual Instrument. The probe can be moved
by a Microcontr6leTM traversing system so as to measure
velocity profiles on a test section diametric (or vertical)
chord with a 1 to 5 mm step.
The water temperature exhibited a predominant influence
on the bridge voltage output. This phenomenon is linked
both to the low values used for the probe heating
coefficient in water (to avoid boiling on the films) and to
the large values of the heat transfer coefficient generated
by water convection around the probe. For that reason, a
heat exchanger installed in the water tank contributes to
keep the flow temperature around a constant value (18). A
variation of +/- 0.5 C is authorized around the set point
and before applying calibration, the voltages are corrected
from the temperature influence, using a formula proposed
by Bearman (1971).
The probe calibration is done in situ for various water
flowrates, the probe being placed at the center of the test
section where the flow velocity is maximal. The maximum
axial velocity is obtained from Pitot tube measurements.
Thanks to the extreme care taken to water quality, flow
temperature control and calibration procedure, a very
satisfactory measurement accuracy and reproducibility can
be achieved. Thus, a discrepancy of less than 0.5% can be
obtained between the flowrate reconstructed by integration
of mean velocity profiles and the one measured by the
flowmeter.
In single phase flow, the probe calibration gives access to
the axial and radial components of the water instantaneous
velocity. A statistical calculation then delivers mean
velocity, rm.s of the velocity fluctuations or kinetic energy
profiles. All these operations are realized by home made
LabviewTMprograms. In two-phase flow, a complementary
LabviewTM program specifically developed for our
application is used to discriminate the liquid and gas
phases. In practice, the occurrence of bubbles generates





Paper No


large negative peaks on the voltage signal, as the heat
transfer between the probe and the fluid drops when the
sensor is in a gas medium. The principle of the program is
to cut out the peaks and keep the signal corresponding to
the water velocity.
A 1% accuracy can be obtained on the mean velocities and
less than 5% on the r.m.s. of the velocity fluctuations. As the
errors are added when performing the evaluation of the
turbulent kinetic energy, the final error on this quantity is
nonetheless about 10 to 15%.



Optical probes


Home made optical probes are used to measure the
temporal phase indicator function (PIF) in two-phase flows.
The principle is to emit a laser signal at the tip of an optical
fiber by means of an emission/reception box. The air
refraction index leads to a total reflection of the signal,
whereas the water refraction index induces a refraction of
the signal outside the optical fiber. A photodetector
translates this phenomenon into an electrical signal
composed of a succession of high levels (fiber tip in air,
corresponding to the transit of bubbles when the sensor is
placed into a two-phase flow) and low levels (fiber tip in
water). This electrical signal is then amplified and a
double threshold is applied. The resulting binary signal is
the PIF (phase indicator function), acquired to the PC card
at a 20 MHz sampling frequency. The temporal resolution
of the measurement is then very high (50 ns). The time
average of the PIF gives access to the local void fraction of
the flow, ca. In our optical probe configuration, a second
fiber is placed downstream of the main one, at a known
distance. The time delay between the two PIFs provides
precious information about the interface velocity or the
interface area concentration. The overall interfacial area
concentration (A,, in m 1) is defined as the total surface
area of the interface per unit volume in a finite control
volume and represents the geometrical capability of the
interfacial transfer. Finally, the mean Sauter diameter is
6a
derived from the theoretical expression D = (1)
A

and directly equals the bubble diameter in the case of
spherical bubbles. For a non-spherical bubble shape, it
gives an order of magnitude of the bubble volume to area
ratio.
The acquisition of the PIFs and the calculations of void
fraction and all other quantities are made via a LabviewTM
program named ISO and developed in the laboratory.
Concerning the spatial resolution, the typical size of our
probes is 250 vtm for the fiber diameter and several
microns for its tip. The two fibers spacing can be reduced
to 150 vtm and the axial distance between the front fiber
and the rear one is about 500 microns. The probe is
mounted in a probe support that is moved by a traverse
system in the same manner as for the hot film probes.
The acquisitions are realized on 200 000 bubbles for each
fiber in order to ensure a good statistical convergence.


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

The measurement error on the quantities measured by an
optical probe is not so easy to determine. This evaluation
was carefully studied by Cubizolles (1996). The error on
the void fraction can be estimated to be in the range 2-5%
and the error on the quantities derived from the PIF
measurement is of the order 2-10%.

Fast camera video

Two numerical Photron Fastcam SA01 and SA03 cameras
have been used for the visualizations. Their detection
matrix size is 1024*1024 pixels2. The sampling frequency
is 5400 images per second full format for the SA1 camera
and 2000 images /sec for the SA3. The shutter speed can
be set up to 1/10 000 s. Different photographic lenses have
been used (focal length ranging from 28 to 105 mm). The
scene lightning was obtained by use of two DEDOLIGHT
575 W spotlights. The software PFVTM ensures the
adjustment of the camera parameters and the acquisition of
the videos. The camera is placed so as to acquire a side
view of the pipe and a 450 mirror is clamped above the test
section to provide top and side views of the flow in the
same time.


Results and Discussion

Flow pattern

Horizontal pipe two-phase flow patterns have been
described by several authors (Govier & Aziz (1972),
Andreussi et al. (1999), Barea (1987), Li & Kwauk (1994))
and classified with respect to the values of JL and JG. An
interesting classification for our experiment is the one of Li
& Kwauk (1994). For these authors, the 2-phase horizontal
flow can be classified according to 3 dominant conditions:
Liquid dominant condition: this corresponds to the dispersed
or buoyant bubble regime. The bubble movement is
controlled by the liquid and the turbulent kinetic energy
plays a predominant role in the flow development;
Gas-liquid coordinated condition: in this regime, neither the
gas nor the liquid can rule the flow. This corresponds to the
intermittent regimes such as plug, slug or stratified flow.
These two features are encountered in the METERO
experiment as will be seen further.
The third dominant regime is the gas dominant condition
corresponding to situations where the liquid can't develop
into a continuous phase and is spread in droplets by the gas.
This corresponds to annular flows, not generated in our
experiment.
The fast camera videos realized on the METERO
experiment illustrate the various flow regimes classified for
instance by Govier & Aziz (1972). Figures 2 to 5 show
photos extracted from fast camera videos for a specific
couple of liquid and gas flowrates/velocities.
The correspondence between flowrates and phase velocities
is summarized in Table 3. The flowrates (liquid and gas) are
summarized in the left column and the right column gives
the corresponding phase velocities.





Paper No


QL (m3/h)= 125 JL (m/s)= 4.42
QL (m3/h)= 150 JL (m/s)= 5.30
QG (1/min) = 4 JG (m/s)= 0.008
QG (1/min) = 12 JG (m/s)= 0.025
QG (1/min)= 30 JG (m/s)= 0.063
QG (1/min) = 60 JG (m/s)= 0.127


Table 3: Correspondence
velocities


between flowrates and phase


In each photo, the mixing is flowing from the right to the
left. The bottom photo shows a side view of the test section
and the top one gives a view from above thanks to the 450
mirror clamped above the test section.


Figure 2: Buoyant bubble flow regime (JL=5.30 m/s;
JG=0.025 m/s)

It has to be pointed out that for this experiment, the bubbly
regime is more likely related to the so-called buoyant
bubbly regime than to true dispersed regime: due to
buoyancy effects, the bubbles are observed to move from
the bottom to the top of the pipe leading to an asymmetrical
flow. This regime has been investigated by several
researchers (for example Holmes & Russel (1975),
Kocamustafaogullari & Huang (1994), Beattie (1996),
Andreussi et al. (1999) and Iskandrani & Kojasoy (2001).
Figure 2 highlights the buoyant bubbly regime generated by
high liquid and low gas superficial velocities. Even though
the flow is not quite axisymmetric and the bubbles tend to
migrate towards the upper wall, it can nonetheless be
classified as a water dominant regime. Indeed, the
determination of the bubble velocities from the videos
shows that these velocities are very close from the one of
the liquid phase, measured but thermal velocimetry.
When the water flowrate is decreased, the two phase flow
enters an intermittent gas-liquid coordinated regime: the top
bubbles coalesce to form plugs as can be seen in Figure 3
for JL=2.4 m/s and JG=0.03 m/s. For smaller values of JL, the
flow regime completely changes: a free surface is created
but under the effects of gas injection, instabilities (described
as Kelvin-Helmoltz instabilities) cause the liquid to reach
periodically the upper wall, generating a high velocity slug


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

(Figure 4). This behavior matches well the Govier & Aziz
(1972) description.
Then, for very low values of the liquid flowrate, the waves
cannot reach the upper wall and the flow becomes
completely stratified whatever the gas flowrate (as
mentioned previously, the gas flowrates are not sufficient to
generate wavy stratified or annular flow in the present
experiment). This is illustrated by Figure 5.
A precise classification of more than 150 videos for various
values of JL and JG allowed the construction of the
METERO flow pattern map for two axial locations: X=20
and 40 diameters downstream the injectors.
Figure 6 shows this result for X=40D. Each cross
corresponds to an acquisition for a couple of gas and liquid
superficial velocities. The solid lines materialize the
transition lines between one regime and another. They were
determined by visual inspection of the videos.
One can see the transition lines between some of the
regimes cited above:
-transition from slug to stratified flow (TSS): pink line
-transition from plug to slug flow (TPS): orange line
Anyway, concerning the high liquid velocities, our results
show that the intermittent regime corresponding to the
evolution from bubble to plugs may be described in a more
precise way. Indeed, as mentioned previously, the buoyant
regime is characterized by the fact that the bubbles are
transported by the liquid phase and therefore their velocity
is very close to the liquid phase one (Figure 2). On the
opposite, the plug regime is characterized by the existence
of big plugs of gas resulting from bubble coalescence and
having their own velocity.
In between, the observation of the videos shows that for
each value of the gas flow rate, there exist a liquid flowrate
for which the bubbles form a uniform bubble layer at the top
of the test section. What characterizes this regime is that the
bubble layer has its own behavior and is less governed by
the liquid phase Therefore, its velocity is significantly lower
than the one of the bubbles located in the lower part of the
test section. Anyway, in this regime, no coalescence can be
pointed out and no plugs are formed. This is illustrated by
Figure 7: one can see a "mattress" of bubbles in the upper
region but no plug is formed. The values of JL and JG for
which the flow exhibit a significant difference in velocities
between the upper bubbles (bubble layer) and the lower
bubbles (free bubbles) have been used to build a transition
line that could be called iri.ii-nl .: between buoyant bubble
regime and stratified bubbles regime" (TBBSB).
It is important to stress that this transition has a physical
meaning as it materializes the boundary between the liquid
dominant condition (buoyant bubble flow) and the
gas-liquid coordinated condition (stratified bubbles).
The map of Figure 6 can then be completed by the 2
following lines:
-transition from buoyant bubble flow to stratified bubble
flow (TBBSB): green line
-transition from stratified bubbles regime to plug flow
(TSBP): purple line


----






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


X=40D


IX x


S*x Experimental points
2 -TBBSB
-TSBP
1 -TPS
-TSS

0 01 02 03 04 05 06 07
JG (m/s)
Figure 6: METERO flow pattern for X/D=40


Figure 3: Plug flow regime (JL=2.4 m/s; JG=0.03 m/s)


Figure 4: Slug flow regime (JL=0.53 m/s; JG=0.062 m/s)


Figure 5: Stratified flow regime (JL=0.38 m/s; JG=0.094
m/s)


Figure 7: Stratified bubbles flow regime (JL=4.55 m/s;
Jo=0.094 m/s)

Velocity and turbulent kinetic energy

A first series of instantaneous velocity measurements was
carried out for various water flowrates and different
locations inside the test section to make sure that the
behavior of the single phase liquid flow is in good
agreement with literature on pipe flows.
Figure 8 shows a profile, measured 40 diameters away from
the pipe inlet, of the liquid axial velocity root mean square
divided by the local mean velocity and compared with data
collected from literature (Laufer (1953), Ljus et al. i1 I2),
Lewis et al. (" 111)) for various values of the Reynolds
number based on the pipe diameter. The experiment
METERO (Re=410 000) is in good agreement with the
literature data even if the turbulence level is slightly higher
at the center of the test section.
As already mentioned, the integration of the mean velocity
profiles provides an experimental value of the liquid
flowrate. The difference with the flowrate measured by the
test section flowmeter is about 0.5%.


Paper No






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


X/D=40,JL=4.42 m/s

SLaufer Re=430000
Laufer 40 000
- Lews Re 70 000
-- Ljus Re 80 000
+- present study Re=410 000


0.20


0.15


n in -


-1.0 -0.5 0.0
Position, r/R


Figure 8: Single-phase radial rm.s velocity profiles

Two-phase flow velocity measurements were then carried
out for 3 axial locations (5, 20 and 40D), a value of the
liquid flowrate (125 m3/h, JL=4.42 m/s) and various values
of the air mass flowrate ranging from 0 (single phase) to 60
1/mn (JG=0.127 m/s).


X=40D JL=4.42 m/s


-0.4 -0.2 0
r/R


Figure 9: Two-phase radial mean v


X=40D, JL=4.42 m/s


-1 -0.8 -0.6 -0.4 -0.2 0 0.2
r/R
Figure 10: Two-phase radial rm.s v


Figure 11: Two-phase radial rm.s
Iskandrani & kojasoy (2001).


0.5 1.0


velocity profiles from


In a first attempt, 2 components of the liquid instantaneous
velocity were measured but for liquid flowrates greater than
100 m3/h, air pockets trapped in the wake of the hot films
spoiled the voltage signal. The solution was to replace the 2
component probe by a conical one not sensitive to this
phenomenon.
As a result, only the axial velocity component is measured
and the turbulent kinetic energy is derived from:
3
k = 12 (2)
2


A specific methodology had then to be developed to ensure
a correct estimation of the turbulent kinetic energy. This
methodology relies on a comparison with the data of Laufer
(1953) and provides corrective coefficients for the values of
k averaged over the section for a given X location.
In two-phase flow configuration, the liquid velocity and
energy profiles exhibit a strong influence of the bubbles for
0.2 0.4 0.6 0.8 1 gas flowrates greater than 30 1/mn (JG=0.063 m/s).
Concerning the mean liquid velocity, the effect of gas
injection can be seen in Figure 9 presenting profiles,
eociy proxies measured 40 diameters away from the pipe inlet, of the axial
mean velocity versus r/R for QL=125 m3/h (JL= 4.42 m/s)
and various values of QG, ranging from 0 to 60 1/mn (i.e. JG
ranging from 0 to 0.127 m/s). For small values of the gas
flowrates (0 to 12 1/mn) the profiles merge satisfactorily, but
for Q, 2 30-l/mn, the velocity profile drops in the upper
part of the test section (r/R 1) and consequently raises in
the lower region (r/R -1). It has to be noticed that the
value of r/R for which the velocity is maximal, is shifted
from 0 to -0.2 for the highest value of the gas flowrate.
Concerning turbulence, the gas injection influence is
characterized by increased fluctuating velocities and energy
in the upper wall region (r/R 1) associated with a
decrease in the lowest region of the pipe (r/R -1) as can
be seen in Figure 10 showing profiles, measured 40
diameters away from the pipe inlet, of the axial velocity root
mean square divided by the local mean velocity versus r/R
0.4 0.6 0.8 1 for the same values of QL and QG. This main feature has
been observed by other authors for two-phase horizontal
velocity profiles pipe flows. An example is given in Figure 11 collected from
Iskandrani & Kojasoy (2001).


Paper No


o
a = 0.50 m/s
V

-JG=0
-JG=0.008 m/s
rJG=0.025m/s
-JG=0.063m/s
+JG=0.127m/s


-1 -0.8 -0.6






Paper No


-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
r/R


Figure 12:
profiles


Two-phase radial turbulent kinetic energy


X=40D, JL=4.42 m/s


A
v
^
y


v


-e-JG=0
0.2 JG=0.008 m/s -
JG=0.025 m/s ..
JG=0.063 m/s "- ...-
-.5 -JG=0.127m/s ,-" .

-r
0.1 , .

0.05 -


0 5 10 15 20 25 30 35 40 45
Axial distance X/D
Figure 13: Mean turbulent kinetic energy evolution

The same behavior can be observed between Figure 10 and
Figure 11 plots for high values of the gas superficial
velocities even if it is less pronounced for the METERO
experiment, probably because the gas superficial velocities
are lower (the maximum gas superficial velocity
investigated in METERO is 0.127 m/s whereas it reaches
0.8 m/s in the Iskandrani & Kojasoy (2001) experiment).
Like these authors, we can point out the increase of the
bubble layer in the upper wall region and a slight inflexion
in the curve for 60 1/mn (JL=0.127 m/s) and r/R- 1.
Anyway, the drastic decrease of u'/Umoy, local when r/R 1
pointed out by Figure 11 for =0.5 and 0.8 m/s is not
clearly observed in our experiment, probably due to the
important size of the thermal sensor that prevents
measurements closer than 2.5 mm from the wall and also
because, as said previously, the METERO phase velocities
are lower than that of Iskandrani & Kojasoy (2001). Another
feature related to gas injection is that the minimum of rm.s
fluctuation is shifted towards the lower part of the pipe
when the gas velocity increases. This behavior is in
agreement with Figure 11: the u'/Umoy local curve minimum is
shifted from 0 to -0.1 for =0.8 m/s.
A physical explanation has been given by
Kocamustafaogullari & Wang (1991) for this behavior: the
effect of the bubble layer is to slow down the main flow in
the upper region. As a consequence, the flow speed is
increased in the lower region to ensure the continuity


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

condition. Let us stress that this behavior is totally coherent
with the visualizations presented before and the flow pattern
maps derived from them. The thickness of the bubble layer
exhibited by the videos is the same as the one highlighted
by the velocity measurements for a given couple of
water/gas flowrates.
Another consequence of gas injection is the increase of the
turbulent kinetic energy near the upper wall by the
additional effect of bubble induced turbulence (see the
radial profile of k in Figure 12 for X/D=40, JL=4.42 m/s and
JG=O to 0.127 m/s).
Concerning the evolution of the mean turbulent energy
(averaged on each radial profile) with the axial distance X, it
can be pointed out that it linearly increases from 5D to 40D
(Figure 13). We observe a good merging of the plots for
low values of the air superficial velocities (0 to 0.063 m/s)
and a deviation from the cloud for JG=0.127 m/s. This
accounts for the different behavior already pointed out for
high gas flowrates.
The increase of k with X could be explained by the fact that,
as the water flows downstream, an increasing number of
bubbles rise towards the upper wall, due to gravity effects,
and feed the bubble layer, increasing turbulence Anyway, it
has to be pointed out that no significant difference in the
slope is observed between the single phase flow and the two
phase ones. This could suggest that this additional
turbulence is not increased by the bubbles segregation along
the pipe but injected at the test section entrance and then
transported towards the exit.

Void fraction, interfacial area and mean Sauter
diameter

Two-phase flow optical probe measurements have been
carried out on vertical profiles for the same 3 axial locations
(5, 20 and 40D), a value of the liquid flowrate (125 m3/h,
JL=4.42 m/s) and the same various values of the air mass
flowrate (ranging from 4 to 60 1/mn, JG from 0.008 to 0.127
m/s). Complementary measurements were performed at X=
40 D for QL=150 m3/h (JL=5.3 m/s) and the same values of
the gas flowrate. Local void fraction, interface area
concentration and mean Sauter diameter profiles are derived
from the PIFs. Bubble frequency and velocities, not
presented here, are also of great interest.


Axial distance 40D
0 01 0
o'


>ff

i .
u *


2 03 04 05 06 07


JL=4 42 m/s, JG= 008 m/s
JL=4 42 m/s, JG= 025 m/s
JL=4 42 m/s, JG= 063 m/s
SJL=4 42 m/s, JG=0127 m/s
-+- JL=5 3m/s, JG=0127m/s


Void fraction
Figure 14: Radial void fraction profiles for X/D=40 and
various JL and JG.






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


The results show that the local void fraction a increases
with JG whatever the vertical location. Figure 14, showing
the void fraction profiles versus y/D, gives an example of
this feature: the JL=4.42 m/s void fraction profiles raise
when JG is increased and especially in the region y/D -- 0
(upper wall). This plot has to be compared to the velocity
fluctuations behavior highlighted by Figure 10. It should be
emphasized that the value of r/R (or y/D) for which the
curves are strongly inflected, is the same for the two figures.
This confirms that the bubble layer, responsible for the void
fraction huge increase, has a direct effect on the liquid
kinetics. It is also coherent with the visualizations of the
flow, as mentioned previously.
On the contrary, the void fraction decreases in the upper part
of the test section (and increases in the lower part) when the
liquid flowrate is increased from 125 to 150 m3/h. This
illustrates the fact that for high water flowrates, the bubbles
are more dispersed in the flow and the void fraction profiles
tend to flatten. Moreover, the same profiles acquired for 150
m3/h (JL=5.3 m/s) exhibit a noticeable change of slope near
the upper wall (see plots for JG=0.025 and 0.063 m/s in
Figure 15, giving the radial a profiles versus y/D for JL=5.3
m/s and JG=0.008, 0.025, 0.063 and 0.127 m/s). This
behavior has already been observed by Iskandrani &
Kojasoy (2001) and Ekambara et al. (21 I") for a value of
the liquid superficial velocity equal to 5.1 m/s. It has to be
noticed from these publications, that for lower values of JL
(JL=3.8 m/s), this peak is observed also, but for values of JG
greater than 0.5 m/s. As the gas superficial velocities are
lower on METERO, this could be the reason why it was not
observed for JL=4.42 m/s.
To end this discussion, it has to be recalled that some
attempts were made to compare the void fraction measured
by optical probes to the one derived from the hot film
voltage signal in two-phase configuration. Indeed, the
discrimination gives access to the fractions of voltage signal
corresponding to bubble occurrence and then the possibility
to infer a local void fraction. Anyway, the comparison
revealed a very bad agreement between the void fractions
calculated from the two methods (the one measured by
conical probe was far too high compared to the one acquired
by the optical probe). This is indeed not surprising, as the
size of the conical sensor (0.2 mm*1.4 mm) is very large
compared with the one of the optical probe tip and also with
the bubble size which is quite small (about 1 mm in
diameter). The velocity probe dimensions are just too high
to ensure an accurate local measurement of the void
fraction.

Concerning the averaged values, their evolution with the
distance X is the following:
slightly increases, decreases and increases
with X for the smallest values of the gas flowrate (up to 30
1/mn, JG=0.063 m/s). This accounts at least for bubble
sedimentation and could also be explained by bubble
coalescence. Nevertheless, this assumption was not
confirmed by fast camera videos. For JG=0.127 m/s, the
behavior is quite different, the trends are amplified for

and but is greater for 40 D than for 20 D.


Axial distance 40D, JL=5.3 m/s
0 004


008 012 016 02


0 1 i
-I, & X .. -
S2 1 .. JG=0.008 m/s
03 -6 -A-JG=0.025m/s
0 -x-JG=0.063m/s
0 ,J2"
/7 -i-JG=0.127m/s

I,;


l,
.11'___________________


Figure 15:
JL=5.3 m/s.


250


200


S150
E
A
<100

5
50


Void fraction
Radial void fraction profiles for X/D=40 and


-U-X=5D JL=4.42 m/s
-A-X=20D JL=4.42 m/s
-4-X=40D JL=4.42 m/s
- X=40D JL=5.3 m/s


0 0.02 0.04 0.06 0.08
JG (m/s)


y = 751.58x +11.732
R: = 0.9989


0.1 0.12 0.14


Figure 16: Averaged interface area concentration versus JG

Figure 16 gives the evolution of versus JG for various
X locations. One can notice a linear evolution of with
JG whatever the axial location and the liquid flowrate.
Moreover, the slope is almost the same for all the plots as
soon as the liquid flow gets developed (20 D). For the
entrance region (5 D), the slope is quite different, but this
could be related to several causes: injectors wake effects,
lack of integration accuracy....
From these plots, one can infer the following correlation
valid for a fully developed two phase bubbly dispersed
horizontal flow (corresponding to the case X=40 D and
JL=5.3 m/s):

= 751.58-JG +11.732 (3)
Let us end this analysis by remarking that this linear
behavior was already observed in the past but for a vertical
configuration by Delhaye (1994).

Conclusions

Measurements have been carried out on the METERO
experiment in a horizontal two-phase water/air flow
configuration, for various values of the liquid and gas
flowrates and different axial locations along the pipe, using
fast camera video, hot film anemometry and home made
optical probes. The database generated includes more than


Paper No






Paper No


150 videos, U, u, k (for the liquid phase) and ac A, or D,m
profiles for JL=4.42 m/s, JG ranging from zero (single-phase
flow) to 0.127 m/s and X/D=5, 20 and 40.
Supplementary points have been measured by means of
optical probes at the X=40 D location for a higher liquid
superficial velocity (JL=5.3 m/s) and the same values of the
gas velocity. A special care has been devoted to the accuracy
and reproducibility by controlling the water quality and
temperature and by improving calibration and control
procedures.
The first comparisons showed that the single phase flow is
in very good agreement with literature on pipe flows (Laufer
(1953) and for instance Ljus et al. 2. i"2) or Lewis et al.
G2' -'2)).
Concerning the two-phase flow, the comparisons of the
velocity, energy or void fraction profiles show a good
agreement with Iskandrani & Kojasoy (2001) or Ekambara
et al. ((2 1,). Especially, the coherence between the 3
different measurement means (video, hot films and optical
probes) has to be emphasized.
The influence of the bubble layer generated at the upper
wall of the test section when JG rises for a fixed value of JL
can be seen by an increase in u' and k at the upper wall and,
on the contrary, a decrease near the bottom of the test
section. Consequently, the mean liquid velocity drops at the
upper wall and rises in the lower region. These phenomena
are well described in the literature. The videos highlight
these features and the void fraction and interface area
concentration profiles confirm these trends.
Flow pattern maps derived from the videos exhibit the
regime transitions described by Govier & Aziz (1972). From
these, one can propose a supplementary transition according
to the description of Li & Kwauk (1994) that depicts the
transition from a water dominant regime (bubbly buoyant
flow) to a water-gas coordinated regime (stratified bubbles).
This regime has to be distinguished from the plug regime
occurring for lower values of the liquid velocity.
Then, the A, and k profiles have been averaged for each
axial location to infer new correlations for the modeling. In
that framework, an original correlation has been proposed
for the evolution of versus JG.

Acknowledgements

This work has been achieved in the framework of the
NEPTUNE project, financially supported by CEA, EDF,
IRSN (Institute for Radioprotection and Nuclear Safety)
and AREVA-NP.


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7th International Conference on Multiphase Flow
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