7" International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 June 4, 2010
EXPERIMENTAL STUDY AND FLOW VISUALIZATION OF GAS ENTRAINMENT IN
Jose M. Lopezl, Dana V. Danciu2, Marco J. Da Silva2,**, Uwe Hampel 2 and Ram Mohan3
'Mechan~ical Engineering Department, Universidad de Carabobo. Valencia, Carabobo 2005. Venezuela
2 ForschungszentrmmDresden Rossendorf, Dresden 01314. Germany
3 Mechanical Engineering Department, The University of Tulsa, Tulsa, OK 74014. U.S.A
Downward bubble flow due to the gas entminment under
a falling film was exp~erimentally investigated using a wire-
mesh sensor (WIMS) and a high speed video camera. The flow
pattern was identified using the flow visualization analysis of
the WMS. This analysis was carried out in a 2-D (x, y),
pseudo 2-D (x, t and y, t), an~d pseudo 3-D (x, y, t) dimensions.
During the experiments the observed flow pattern was bubble
flow. Good agreement was found by comparing high-speed
video images with the images obtained with the WMS.
Additionally, gas void fraction time series and bubble size
distribution are presented. Results show that images obtained
from the high speed camera, WMS and experimental
observations are in good agreement. Also, it shows that
different approaches for flow pattem recognition can be used.
Moreover, results show the infh~ence of the superficial liquid
velocity on the gas void fraction and bubble size.
Keywords: gas entrainment, flow visualization, flow pattem'
wire-mesh sensor, gas void fi-action
The gas entrainment due to a phnging jet is a common
phenomenon that can be found in nature, such as waterfalls
and breaking waves in the ocean, as well as mn many industrial
processes, i.e. nuclear engineering, cmude oil extraction, oil
separation pmcess, chemical processing, and environmental
applications. Over the years, several researchers have studied
this phenomenon trying to understand its mechanism , ,
, , . However, the measurement techniques used to
study this particular two-phase flow system, usually represent
a constraint on the investigation of such complex phenomena.
Most studies have been focused on the influence of
different parameters of the gas entrainment, such as nozzle
distance from the impact point and impact velocity , ,
, . Because of the complexity of their measurement,
parameters, such as bubble size, gas void fraction and flow
*Corresponding author. Current address: 800 SouthTucker Drive Tulsa, OK
i*uret Udres: Dp n nt -m os~etia Eninerig Ie tal University
of Technology -Parana, Brazil
visualization on the cmss section of the flow are less
accounted for in the literature. In most cases, non-intrusive
measuring teclmiques are used . A very important feature of
these devices is that they do not generate perturbations on the
flow field. Although, non-intmusive techniques are usually
unable to give detailed information of the cmss section of the
Moreover, flow image processing is of yital importance to
understand the physical mechanism of the entminment
phenomenon. Over the years, high speed video cameras have
been used to capture many fluid flow phenomena (especially
two-phase flow). This technique of measurement has the
limitation of capturing only one plane of measurements,
usually the plane that is closer to the wall. Moreover, special
attention has to be taken during data processing in older to
properly scale the images, when these processing techniques
The wire-mesh sensor (WIMS) device has been used in
several cases to study different scenarios of two -phase upward
flow , , . Even though the WMS is an intrusive
instrument, it has been proven that its intrusive effect is
negligible, especially in upward flow and at high liquid flow
rates . The data obtained from the WMS contains
information of the cross sectional area of the flow, which
makes the device a revolutionary measurement instrument in
two- phase flow systems.
In the present study, the WMS is used abong with a high-
speed video camera to study the wall jet air entrainment. The
WMS is capable of gathering information about a series of
important variables in the wall jet air entrainment pmcess,
such as gas void fraction time series, bubble size, bubble size
distribution, 2-D (x, y) flow visualization, pseudo 2-D (x, t),
(y, t) flow visualization, and a pseudo 3-D (x, y, t) flow
visualization. The images obtained with the WMS are
compared with the images obtained from the high speed
camera. Good agreement found in this comparison
demonstrates the capabilities of the WMS as a flow
visualization teclmique. The data analysis clearly shows the
effect of the superficial liquid velocity on the gas entminment
process. Furthermore, the visualization process is very useful
for flow pattern identification and to better understand the
bubble distribution in the cm~ss sectional area of the pipe. In
the case of study, downward two-phase flow, the intrusive
effect of the WMS was observed only at low superficial liquid
velocities (less than 0.6 m/s).
In general, the experimental setup can~ be divided in two
parts, the experimental facility and the wire-mesh sensor
Experimental facility. The facility consists of a
rectangular transparent aclylic pipe with a cm~ss section of
0.05 m x 0.05 m and 2.0 m long, as shown in Figure 1.a. The
fluid used is deionized water. The deionized water is stored in
a 0.40 m3 plastic tank and is pumped using an electrical
submersible pump. The falling fihn is created around a
stagnant Taylor bubble, similar to the approach explained in
. The Taylor bubble is created at the top of the vertical
pipe by injecting air though a 3.0 mm needle tip. The water in
the pipe flows downward am~und the Taylor bubble creating
the falling fihn between the bubble and the wall. The length of
the Taylor bubble was 0.50 m and it was kept constant during
all the experiments, as shown in Figure 1.b. The entminment
process occums as a result of the impact between the falling
film and the surface of the liquid at the rear of the Taylor
bubble. After the entrainment pm~cess occuxs, bubbles are
dragged downwards with the liquid crossing the metering
plane of the WMS located at 1.30 m from the top of the
vertical pipe. The fluids are returned to the water tank where
the liquid is recirculated and the gas is vented to the
atmosphere. The superficial liquid velocities used in the
experiments ranged from 0.18 m/s to 1.46 m/s.
Wire-mesh sensor (WMCS). The gas void fraction in the
pipe was measured using a square WMS. The mesh consists of
32 wires, a layer of 16 horizontal wires and a layer of 16
vertical wires. The layexs are 3 mm apart for each other. The
signal from the WMS goes to cap~acitance-measuring
electronics, as described in . During the experiments, the
sampling frequency of the WMS was kept at 1600 fps. A
schematic of the WMS is shown in Figure 2.
g g., ,, 'Transmitter
Figure 2: Wire-meshsensor
Additionally, videos are recorded using a high-speed
camera. The camera is placed perpendicular to the WMS
measuring plane and parallel to the wall of the square pipe.
The distance between the camem an~d the pipe is kept constant
at 0.5 m. The videos are recorded at a rate of 1600 fps, same
as the sampling rate of the WMS. The gas void fraction an~d
bubble characteristics are measured at one axial position.
EXPERIMEN TAL RES ULTS
In each cmssing point of the WMS (i, j), an instantaneous
gaS void fraction is measured over time (k. The WMS data is
stored in a three dimensional matrix (i, j, k). Where i is the
index used to identify the transmitter wire an~d j corresponds to
the receiver wire. This matrix represents the acquisition of the
data in the measuring plane (x, y), and the third dimension
represents the time (t). Using this matrix, different data
analysis can be perfonned, such as flow visualization and
bubble size measurements, which are very helpful for flow
2-D Flow visualization. In order to better understand the
air entrainment phenomenon, bubble break-up, bubble
COalescence and flow pattem it is necessary to have images
that show the plwsics involved in the pmcess. The flow
visualization process allows a better understanding of the
physics of such phenomenon and it helps incorporating the
real mechanisms to the mathematical models that describe the
pfOcess. Using conventional methods, such as video recording,
it is very difficult to entirely or partially visualize the cmss
section of the pipe. View obstmection of the center of the pipe
is caused by the bubble swann near the pipe wall. One of the
different analyses that can be made fmom the WMS data is the
flow visualization of the bubbles inside the pipe , , ,
. The entire cmss section of the pipe can be visualized
a. Schematic Facility b. Schematic Stagnant Taylor
Figure 1: Schematic Facility and Stagnant Taylor Bubble
using the data analysis from the WMS. This cross section
visualization allows characterizing the bubble size
distribution, the spatial bubble distribution and also gives
pictures of the flow in the cmss section area of the pipe. The
images obtained from the data processing of the WMS are
shown in Figure 3, where the red color represents 100% air
while the blue color represents 100% water. Also, images
obtained from the high-speed video camera are shown in
Figure 3. The bubbles can be easily identified in both figures,
especially where the superficial liquid velocity is low.
Comparison between these images (WIMS and high-speed
video camera) are showing good agreement.
aWMS unage t-062b Photo t-062 msc WMS unage t=278d Photo t-278s
ms Vsl=0 18 m/s Vsl=0 18 m/s s Vsl=0 28 m/s Vsl=0 28 m/s
Figure 3: High-s peed camera and WMCS images.
Moreover, image analysis of the flow, as explained in '
is carried out over the mid-plane of the pipe (central
electmdes) of the WMS, as shown in Figure 4. For this type of
data analysis two electmdes of the WMS are chosen,
horizontal an~d vertical (middle electmdes). Fmm these two
electmdes two different images are obtained in a pseudo 2-D
plane (x, t). The axial slice images of the flow are presented in
Figure 4: Horizontal and vertical WMCS electrodes.
These images have the advantage of showing the gas-
liquid flow inside the mid-plan~e of the pipe, and can~ be used
to identify the flow pattern map in a more objectively. These
images are used as a complement of the cmss section images
to identify the location of bubbles in the pipe.
e WMS unage t=134f Photo t=134 msg WMS unage t=35 6f Photo t-35 6 ms
ms Vsl=0 66 m/s Vsl=0 66 m/s ms Vsl=0 76m/s Vsl=0 76m/s
c Vsl=0 76 m/s, t=580 ms
d Vsl=0O83 m/s, t=685 ms
f Vsl=1 13 m/s, t-1 64 s g Vsl=1 27 m/s, t=39 5 ms
Figure 5: Axial slide images.
e Vsl= 1 00 m/s, t-1 89 s
h 1 47 m/s, t-1 13 s
3-D Flow visualization. In addition to the 2-D cm~ss
sectional flow visualization, a three-dimensional bubble
reconstruction of the two-phase flow can be made. This
reconstruction has the advantage of showing the bubble
structure inside the pipe in a 3-D fashion. The 3-D flow
reconstruction method is discussed widely in . As can be
observed in Figure 6, the 3-D views are in agreement with the
2-D images shown in Figure 3 and Figure 5. For the pseudo 3-
D image reconstmection, the z coordinate is replaced by the
time scale. This reconstmection of the bubbles allows a better
understanding of the intemal stmecture of the flow in the pipe
and can be used to recognize the flow pattern. Figure 6 shows
that at low superficial liquid velocities, there is a high bubble
accumulation. This is due to the fact that at low superficial
velocities the turb~ulence forces are not high enough to break
the bubbles and the drag forces are not high enough to carry
the bubbles down. Then bubbles tend to accumulate an~d
recirculate at some distance fmom the impingement point. This
observation can also be made by analyzing Figure 6.a and
f Vsl=1 13 m/s g Vsl=1 27 m/s
Figure 6: 3-D image processing
e Vsl= 1 00m/s,
h Vsl=1 47 m/s
Time series. Spatial average of the gas void fraction
obtained at each measuring point of the WMS grid can~ be
made. This spatial average leads to a matrix of gas void
fraction as fiction of time. This method is discussed in ,
, , , , . Figure 7 shows plots of the spatial
average gas void fraction as function oftime.
The peaks on the gas void fraction time series shown in Figure
7, can be interpreted as abig bubble passing through the WMS
or as a cluster of smaller bublles crossing the WMS. The
combination of the time series gmaphs and the flow images
presented in Figure 3, Figure 5 and Figure 6 help to identify
the cause of the peaks. In this particualr study, it is observed
that peaks on the time series are caused by big bubbles passing
though the WMS. Aditionally, Figure 7 shows that at higher
superficial liquid velocities the amplitude and the length of the
peakcs are smaller than at lower superficial liquid velocities.
Flow pattern. The bubble flow is characterized by small
bubbles in a liquid continuum. This is the flow pattem
observed for superficial liquid velocities higher than 0.6 m/s,
as shown in Figure 4 to Figure 6. Even though the flow in
Figure 6.a seems to be a very cloudy gas pocket similar to
chum flow, bubble flow was the flow pattem observed for all
For superficial liquid velocities lower than~ 0.6 m/s, big air
pockets were formed at the lower wire layer of the WMS. This
occurs because the drag and turb~ulent forces are not strong
enough to break the bubbles and canry them downwards with
the liquid. So, the bubbles move ran~domly in the lower wire
layer of the WMS and coalesce, increasing their diameters.
Eventually, the buoyancy forces increase to a level where they
are able to overcome the drag forces and the bubbles rise up to
the surface of the liquid. In this particular case, the WMS
helps holding the bubbles in the lower wire layer and its
intrusive effect is observed.
The image obtained from the WMS at a superficial liquid
VelOcity of 0.18 m/s, shown in Figure 3.a, corresponds to pure
liquid. For this particular case the bubble stays at the lower
wire layer of the WMS an~d never crosses the mesh. The WMS
can~ sense the gas phase only when it goes though the mesh of
Bubble size distribution. In older to construct the
bubble size distribution, a series of algorithms for bubble size
measurement such as, identification of bubbles and
quantification of bubble characteristics, have to be executed
first. Such procedures and details are explained in . In
general, bubble size distributions are constmeted by
summarizing the contribution of the bubbles of a given range
Of diameters to the integral volumetric gas void fraction.
These partial gas void fractions doc/dDbub are plotted
against the equivalent bubble diameter Dbub, where oc
represents the gas void fraction. In the results shown in Figure
8, logarithmic distribution of the bubbles class is presented,
where bubble class width increases logarithmically starting
from a diameter of 3 mm. For smaller bubbles, a linear class
Width of 0.1 mm is used.
The general trend observed in Figure 8 is that the gas void
fraction increases as the superficial liquid velocity increases
and that the bubble size decreases as the superficial liquid
8 ',,, ,,
-200 -100 t=tD +100
200 -100 b t~DVsl=0 671r/s
, 0 e
-200 -100 t=tO +100
. L / ',
-200 -100 t=t0 +100
-200 -100 t=t0
on .1_'. ,I
+100 +200 frames
f Vsl= 11
-200 -100 t=tD +100 +200 frames
g Vsl=1 27 m/s
-200 -100 t-t0 +100 +200 knaes
h 1 47 m /s
Figure 7: Time series
found by comparing high-speed video images with the images
obtained by the WMS.
Gas void fraction time series analysis was presented. This
allowed monitoring of the gas void fraction at a desired
measuring crossing point. The time series analysis permitted
inferring about the bubble size an~d bubbling frequency. It is
showed that this analysis along with the flow visualization is
very usefu~lto identify flow patterns.
furthennore, bubble size distribution was presented. The
bubble size distribution shows that the bubbles are smaller as
the superficial liquid velocity increases. Also, it showed that
the gas void fraction increases as the superficial liquid velocity
In general, the capability of the wire-mesh sensor to
investigate gas-liquid two-phase downward flow due to wall
jet impingement and also the capability for flow pattem
recognition has been demonstrated.
This paper is a result of the collaboration between the
University of Tulsa (TU) and Forschungszentmum Dresden-
Rossendorf (FZD), Germany. The Authors want to
aclalowledge both institutions for their financial and technical
Jose M. Impez would like to thank the graduate school of
The University of Tulsa, Tulsa University Separation
Teclmology Pmojects (TUSTP), Tulsa University Center of
Research (TU-CoRE), and Forschungszentmum Dresden-
Ross endorf (FZD), Germany, for their fmancial support.
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Figure 8: Gas void fraction distribution by bubble
In Figure 8 the bubble size distribution is shifted to the
left, which means that the bubbles size decreases as the
superficial liquid velocity increases. As mentioned above, the
turb~ulent forces increases as the superficial liquid velocity
increases, consequently the break-up rate increases and
smaller bubbles are generated. Also, the drag force increases
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in the gas void fraction in the measuring section.
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(< 15 mm) increases as the superficial liquid velocity
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Downward bubble flow due to gas entminment under a
wall jet was exp~erimentally investigated using the wire-mesh
sensor (WIMS) and high speed video observations. Valuable
qualitative and quantitative information for flow pattem
identification and wall jet phenomenon was obtained in this
study. Quantitative information, such as time series of cmss-
sectional gas void fraction and bubble size distributions, was
extracted by special pmcessing algorithms. In additionally to
this, qualitative information such as 2-D and pseudo 3-D
images was also gathered by special image-pmcessing
WMS data and high-speed observations are in good
agreement with the bubble flow pattem (superficial liquid
velocities higher than 0.6 m/s). For superficial liquid velocities
lower than 0.6 m/s the flow pattem was in a transition to
bubble flow and WMS data was also in agreement with
experimental observations. Moreover, good agreement was
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