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Experimental study on bubble absorber with multiple tangential nozzles
Santosh Kumar Panda and Mani. A*
Refrigeration and Air conditioning Laboratory, Department of Mechanical Engineering,
Indian Institute of Technology, Madras, India.
*Corresponding author. Tel.:+91 44 22574666; fax: +91 44 22570509
Email address: mania@iitm.ac.in
Keywords: Bubble absorber, bubble formation, absorption, bubble dynamics, swirl flow
Abstract
Visualization study of bubble growth with multiple tangential nozzles is investigated in a bubble absorber.
Bubble behavior is studied with different flowing condition like still, co-current and counter-current flow of water.
Effect of air flow rate, water flow rate, nozzle diameters, number of nozzles and orientation of nozzle angle with
reference to vertical plane on bubble diameter is studied. Results are compared with the available information which
is found to be in good agreement. Bubble diameter during detachment increases with increase in gas flow rate in all
the above flow conditions. Performance between single and double nozzle also compared and presented in this
paper. Based on this study non-dimensional correlations are proposed.
1. Introduction
Formation and growth of gas bubbles and their rise due to buoyancy are very important to the hydrodynamics of
gas-liquid interference study. The phenomenon of bubble formation decides the primitive bubble size in the system
whereas the rise velocity decides the characteristic contact time between the phases which govern the interfacial
transport phenomena as well as mixing. The use of bubbling devices, where in bubbles is produced by injecting gas
through submerged nozzles or orifices occurs in a large number of technical applications like bubble absorber, water
treatment, metallurgy, and chemical processing plants. Bubble absorber is an important component of vapour
absorption refrigeration system for heat and mass transfer. Absorption process is one of heat and mass transfer
process occurs in bubble absorber in which a gas bubble with the liquid to increase heat and mass transfer. Kang et
al. (2000) carried out analytical investigation of falling film and bubble type absorbers and found that absorption
rate of the bubble type absorber was found to be always higher than that of the falling film mode. Bubble type
absorber provides better heat and mass transfer coefficients, also good wettability and mixing between the liquid and
the vapor. Elperin and Fominyk (2003) studied combined heat and mass transfer mechanisms at all stages of bubble
growth and rise in a bubble absorber, which can be useful in design calculations of gas–liquid absorbers. A number
of fluid combinations used in bubble absorber for bubble dynamic as well as VAR system study, suggested by
number of investigator. Different combination used for bubble dynamic study like air-water, glycerol-air, methanol-
air, ethanol-air, etc and for VAR system ammonia–water, water-lithium bromide, R134a-DMF, etc.
A wide range of research have been reported in the literature about bubble formation from a nozzle and the effect of
orifice diameter, air flow rate and liquid properties, flowing condition on bubble formation, growth and detachment
have been considered. Bubble dynamic study started with numerical models developed for bubble formation with
different properties of gas and compared experimentally under constant flow, Ramakrishnan et al. (1969) and under
constant pressure condition, Satyanaryan et al. (1969). A simulation and experimental study conducted for single
and multi orifice to know various effect bubble dynamics for different fluid under constant gas flow conditions by Li
et al. (2000). Bari and Robinson (2013) proposed the bubble growth and pressure field adiabatically in submerged
orifice at low gas flow rate by image processing. Bubble formation from single horizontal orifices submerged in
Newtonian liquids has been investigated for chamber pressure and flow rate which are time dependent by Khurana
and Kumar (1969). Gaddis and Vogelpohil (1986) developed an equation theoretically to predict the bubble
detachment diameter in quiescent liquids under constant volumetric gas flow conditions in bubbling regime to
transition regime with viscosity Wraith (1971) proposed a two stage model for the formation of gas bubbles and
bubble coalescence between detaching bubble from a plate orifice submerged in an inviscid liquid at high gas
injection rates. Jamialahmadi et al. (2001) done experimental and theoretical investigation on bubble formation
under constant flow conditions for air and variety of solutions. A numerical simulation and experiment study done
15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014
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for bubble formation at submerged orifices under constant inflow conditions with variation of fluid properties,
Gerlach et al. (2007). Das et al. (2011) investigated experimentally and analytically on bubble dynamic as a
function of gas flow rate for three different submerged orifice sizes at various pool heights. Effect of nozzle shape
and operating parameter on bubble formation from vertically downward nozzle study was investigated, Tsuge et al.
(2006). This study shows the bubble formation and bubble size are influenced by the edge angle of nozzle, inner and
outer diameters of nozzle and gas flow rate. Experimental and numerical investigations have been carried out by
Suresh and Mani (2010) by visualing bubble behavior and studying the effect of gas flow rate and liquid
concentration on bubble characteristics of R134a– DMF solution in a glass absorber. Bubble behavior was studied in
still and flowing solution. Different measuring techniques were used in many literatures to measure the bubble
shape, bubble diameter and bubble frequency. Akita and Yoshida (1974) measured bubble size by using
photographic method for fluid pairs. Single and two phase heat transfer in a vertical flow with tangential injection
nozzle with heat transfer study done by Guo and Dhir (1989).
Air-water out
(co-current flow)
AIR OUT
(COUNTER-CURRENT FLOW)
WATER IN
-PRESSURE TRANSDUCER
-THERMOCOUPLE
-FLOW CONTROL VALVE
ABSORBER TUBE
w
o
l
f
t
n
e
r
r
u
c
–
o
C
P2
GAS FLOW METER
P3 T2
AIR
BYPASS
V3
V2
P1
T1
V1
CAMERA
ROTAMETER
WATER PUMP
NON RETURN VALVE
DISTRIBUTOR
AIR IN
WATER OUT
COMPRESSOR
Co-current
Counter-current
WATER TANK
Figure 1 Schematic diagram for bubble absorber set up.
2. Experimental study
A Schematic diagram of bubble absorber experimental set up is shown in Fig.1 and a photograph of the experiment
set up is shown in Fig. 2. The set up consists of bubble absorber, angle measurement device, water tank, water
pump, air compressor, nozzles with polymer bearing, flow distributor, pressure, temperature and flow measuring
instruments, and various control valves. Bubble absorber and angle measurement device are made up of acrylic
(Polymethyl methacrylate). Acrylic tube is used for observing of bubble phenomena in the absorber to enable
visualization and capture photographs while release from nozzles. A centrifugal pump to supply water to the water
circuit is connected at bottom and top of the tube to realise co-current and counter-current flow configurations
respectively, while other end is connected back to tank. A glass tube rotameter, flow control valve and bypass valve
in water circuit are used to control the flow rate of water. At room temperature water flow rate was varied between 1
lpm to 3.72 lpm. A two stage reciprocating compressor with storage tank capacity of 220 liters and maximum
15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014
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Figure 2 Photograph of bubble dynamic experimental set up
working pressure of 12.5 bar is used to supply air. Experiments were conducted under constant inlet pressure; which
was achieved by bypassing a portion of compressor discharge to surrounding. A thermal mass flow controller based
gas flow meter is used to measure air flow rate before it is admitted to distributer. Distributer is assumed to
distribute flow equally to nozzles; a non-return valve is fitted to ensure that water does not flow back to gas flow
meter when no air supply. Bubbles grow at the bottom of absorber tube where air is injected through copper nozzles.
Standard copper tubes of inner diameter 1.7 mm, 3.2 mm, 4.8 mm are fitted on the periphery of absorber tube to
inject air. The nozzles are fitted using spherical bearing so that it can be tilted in vertical and horizontal plane to
study effect of inclination on bubble detachment diameter. Flexible plastic tubes connect distributor outlets to
nozzles which also allow free movement of nozzles in bearings. Air, after bubbling through the tube passes along
with water stream in case of co-current flow and is vented out through air vent at the top of absorber tube in case of
counter-current flow and still water. Pressure and temperature of air are measured at inlet and outlet of the bubble
absorber by sensors. A camera placed adjacent to transparent absorber tube was used to take still photographs. All
the measuring instruments are pre-calibrated. Three numbers of copper-constantan thermocouple are used with an
uncertainty up to ±0.5°C. Four numbers of piezo-electric type pressure transducers and a pressure gauge are used as
pressure sensors with a measurement uncertainty up to ±1%. Glass rotameter used to measure the flow rate of water
with uncertainty up to ±3%. Mass flow controller unit is used to measure the volume flow rate of air with a
measurement uncertainty of ±2%.
Experiments are started by positioning the nozzles at 0° with reference to horizontal using the nozzle holder. The
compressor is turned on and discharge valve is kept closed until pressure of 5 bar is reached in the storage tank; then
air bypass valve and flow control valve is opened to allow air flow thorough the non-return valve to the absorber
tube. Typical air pressure at the inlet of nozzle was recorded in the range of 1.1 to 1.5 bar atmospheric temperature.
Water pump is turned on then and a desired co-current, counter-current flow or still water as the case may be; is
maintained using rotameter and flow control valve. Air flow is always started before water flow to avoid back flow
of water in air circuit and subsequent damage to air flow meter. For a set nozzle angle; air flow rate is accurately
15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014
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measured and varied between 400-900 ccm, water flow rate varied between 1-3.72 lpm for both co-current and
counter-current configurations. Then nozzle angle is increased in step of 10° up to 30° and all above variations are
repeated. All the parameters viz., water inlet and outlet pressure, temperature, water flow rate, gas flow rate,
pressure and temperature, are monitored using a data acquisition logger unit. Photographs are taken for every
different set of variables with shutter speed varying from 1/8-1/2500 seconds. A number of photographs are taken to
capture detachment stage of bubble. Bubble photographs are taken along with reference objects (measured by a
calibrated scale) inside and beside the bubble absorber, which are in parallel with the absorber. These photos are
uploaded and zoomed in Adobe Photoshop software version 7.0. Bubble shape either spherical or hemispherical or
elliptical is divided into number of segments with different radii. Circles are drawn corresponding to respective radii
and superimposed on the zoomed-in profile of bubble image uploaded in Photoshop software Suresh and Mani
(2010).
(
p
6
=
V
b
3
d
1
+
3
d
2
+
3
d
3
+
…
d
3
i
)
i
D
b
=
6 b
V
p
1
3
Bubble diameter
(2)
The method of estimating bubble diameter is based on the assumption that the bubble is a sphere and the measured
diameter is volumetric bubble diameter, based on equivalent volume of a sphere. Also the diameter of nozzle in the
photograph was measured using Photoshop software and compared with its actual bubble diameter. This method
used as calibration scale to measure the bubble diameter in glass absorber. An uncertainty of 2-3% found in the
bubble diameter during number of repeated measurements. Errors due to distortion of the bubble surface and
location are taken into account to estimate the bubble diameter. Image processing techniques have been used to read
the color value of pixels in the image, to adjust color balance, brightness, contrast and image sharpness of the
available quality of the bubble images taken account for the accuracy of measurement.
Equivalent volume of sphere
(1)
Figure 3 Stages of bubble growth at different air flow rate with still water
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Figure 4 Stages of bubble growth at different air flow rate with co-current water
Figure 5 Stages of bubble growth at different air flow rate with counter-current water
15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014
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single
two
4.8 mm
3.2 mm
1.7 mm
3. Results and discussion
6
5
4
3
2
1
0
*
b
D
)
m
m
(
r
e
t
e
m
a
i
d
e
l
b
b
u
B
18
16
14
12
10
8
6
4
2
0
0
50
100
100
150
250
300
350
400
200
Rea
Figure 6 Effect of gas flow rate on bubble diameter in still water
Effect of gas flow rate on bubble diameter in still water for single and two nozzle
and two nozzles
Experiments with visualization study were carried out
ters, viz. air flow rate from 300 to 1000 ccm, water flow rate from 1 to 3.72 lpm
visualization study were carried out on a bubble absorber system by varying the
on a bubble absorber system by varying the
low rate from 1 to 3.72 lpm, three nozzle of
operating parameters, viz. air flow rate from 300
°, three types water flowing condition
inner diameters (4.8 mm, 3.2 mm and 1.7 mm), vertical nozzle angle 0° to 30°, three types
inner diameters (4.8 mm, 3.2 mm and 1.7 mm)
(still, co-current and counter-current), one and two number of nozzle used for bubble diameter during detachment at
current), one and two number of nozzle used for bubble diameter during detachment at
current), one and two number of nozzle used for bubble diameter during detachment at
growth at different stage in
Figures 3-5 show the visualization of air-water bubble
normal pressure and temperature. Fig
still, co-current and counter-current
with different air flow rate at two different nozzle angles 0° and 30°.
current water flow with different air flow rate at two different
Based upon the observations recorded of image at low and continuous gas flow rates, bubble dynamics takes place,
Based upon the observations recorded of image at low and continuous gas flow rates, bubble dynamics takes place
Based upon the observations recorded of image at low and continuous gas flow rates, bubble dynamics takes place
bubble growth at the tip of nozzle and detachment from nozzle, then travel to the top due to buoyancy force. Figures
bubble growth at the tip of nozzle and detachment from nozzle, then travel to the top due to buoyancy force.
bubble growth at the tip of nozzle and detachment from nozzle, then travel to the top due to buoyancy force.
3-5 show three different stages of bubble growth at three different flow rates (500
of bubble growth at three different flow rates (500, 700, 900 c
, 700, 900 ccm).
100
200
300
300
700
800
900
400
500
Air flow rate (ccm)
600
Figure 7 Effect of gas flow rate and nozzle diameter on bubble detachment diameter
Effect of gas flow rate and nozzle diameter on bubble detachment diameter in still water
in still water (Panda and Mani,
2014)
increases with increasing the air flow rate for different
dimensional bubble diameter (Db*) increases with increasing the air flow rate for different
Figure 6 shows the non-dimensional
and single nozzle produces more or less same diameter bubble compare to two
nozzle diameters in still water and single nozzle produces more or less same diameter bubble compare to two
and single nozzle produces more or less same diameter bubble compare to two
15th International Refrigeration and Air Conditioning
Refrigeration and Air Conditioning Conference at Purdue, July 1
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2207, Page 7
nozzles. Due to frictional loss the pressure of injecting air decrease so it affects t
he bubble diameter. Figure 7 shows
Due to frictional loss the pressure of injecting air decrease so it affects the bubble diameter. Figure 7 shows
the bubble diameter comparison between different nozzle diameter for air mass flow rate (Panda and Mani, 2014). It
the bubble diameter comparison between different nozzle diameter for air mass flow rate (Panda and Mani, 2014).
the bubble diameter comparison between different nozzle diameter for air mass flow rate (Panda and Mani, 2014).
bble diameter for the same Reynolds number of air (Rea) is
is seen from Figs. 8 and 9 that the
tends to increase in co-current flow and
with increase in water flow rate. This
current flow and decrease in counter-current flow with increase in
decrease in bubble diameter within the tested range of liquid flow rate can be attributed to increased upward force
decrease in bubble diameter within the tested range of liquid flow rate can be attributed to increased upward force
decrease in bubble diameter within the tested range of liquid flow rate can be attributed to increased upward force
Liquid flowing in counter-
acting on bubble surface caused by liquid
delays the detachment which results in decreased
current flow exerts force on bubble downwards and
diameter of bubble.
face caused by liquid; which tends to detach the bubble earlier. Liquid flowing in
on bubble downwards and hence delays the detachment which results in
non-dimensional bubble diameter for the same Reynolds number of air (R
single nozzle 1 lpm
single nozzle 1 lpm
single nozzle 2 lpm
single nozzle 2 lpm
two nozzle 1 lpm
two nozzle 1 lpm
two nozzle 2 lpm
two nozzle 2 lpm
Nozzle Φ = 3.2 mm
0
50
100
100
150
200
250
300
350
400
450
Figure 8 Comparison between one and two nozzle bubble diameter
Comparison between one and two nozzle bubble diameter with different air flow rate for
with different air flow rate for co-current water
Rea
Flow
Rea
water flow
single nozzle 1 lpm
single nozzle 2 lpm
two nozzle 1 lpm
two nozzle 2 lpm
*
b
D
3.5
5.5
4.5
5
4
3
2
2.5
1.5
*
b
D
4.5
3.5
2.5
4
3
2
1
1.5
50
100
150
200
250
300
350
400
Figure 9 Comparison between one and two nozzle bubble diameter
Comparison between one and two nozzle bubble diameter with different air flow rate for
with different air flow rate for counter-current
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*
b
D
7
6
5
4
3
2
1
0
)
n
d
/
b
d
(
t
l
u
s
e
r
n
o
i
t
a
e
r
r
o
C
l
9
8
7
6
5
4
3
2
1
0
)
n
d
/
b
d
(
t
l
u
s
e
r
n
o
i
t
a
e
r
r
o
C
l
9
8
7
6
5
4
3
2
1
0
0
50
100
100
150
250
300
350
350
400
450
200
Rea
Figure 10 Comparison between one and two n
Comparison between one and two nozzle bubble diameter with on 0° and 30° orientation
with on 0° and 30° orientation in still water
2207, Page 8
single nozzle 0
single nozzle 30
two nozzle 0
two nozzle 30
+20%
-20%
Still water
6
7
+20%
+20%
-20%
current water
Co-current water
0
1
2
2
3
5
4
Experimental result (db/dn)
8
d correlation
Figure 11 Comparison between experimental and correlation
Figure 11
2
2.5
3
3.5
4
4.5
5
5.5
5.5
6
6.5
Experimental result (db/dn)
correlation
Figure 12 Comparison between experimental and correlation
Figure 1
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