UROP REPORT
Separation of Air-Water Mixture Through
A Micro-Channel Return Bend Apparatus; a Numerical Analysis
Aditi Sridhar
U069404Y
Engineering Science Programme, Faculty of Engineering
Abstract
This paper studies the separation of an air-water mixture in a micron-scale flow system. The
separation of the mixture occurs due to the flow of the air-water mixture through a capillary
return bend in the channel which causes centrifugal forces to act on the air and water
particles. For this project, a numerical software package Fluent is used and the flow of the
air-water mixture through the channels is studied using a two-dimensional Volume of Fluid
(Laminar fluid flow) model. Different parameters were studied, namely, the number of gaps
leading to the secondary channel, the size of gap holes, the velocity of flow and gauge
pressure applied at air and water outlets. This method is studied in order to develop an
alternative method to gas-liquid separation as opposed to the conventional method using
gravity.
Introduction
Throughout the ages, biological processes in nature have been the inspiration to many of the
modern developments in science. Man’s desire to conquer land, water and air has lead to
developments such as aero-planes, tanks and submarines. Though a lot of effort and
development has gone into the development of large-scale human transport and military
machines, equally significant developments are not well known or easily accessible for an
individual to use.
From the standpoint of an individual, particularly one involved in under-water activities, one
of the most convenient inventions that the author could foresee would be removing the need
to carry an oxygen tank into water. Hence, a machine that would separate the dissolved gases
from the sea water and then supply the gaseous form to the diver.
The idea of extraction of oxygen from sea water was born as a result of considering the
immense number of marine life-forms living on dissolved oxygen in the sea. As oxygen is
abundant in sea water, approximately 34.3% by volume of the dissolved gases
http://www.waterencyclopedia.com/Re-St/Sea-Water-Gases-in.html) can be theoretically
extracted from the sea water which makes it an attractive option.
Upon forming bubbles, the oxygen gas would then need to be separated from the sea water to
be supplied to the diver. This paper, thus, aims to optimize the separation of air and water
such that, from a mixture containing about 30% of air, almost all the water is removed from
the mixture leaving only oxygen gas for the diver to inhale. In forming this separation
apparatus, a micro-channel return bend was used that would separate the two phases using
centrifugal force.
Modeling and Simulation
Water Outlet
Sub Channel
Air Outlet
Connecting Channel
Main Channel
Air Inlet
Water Inlet
Fig 1: Schematic diagram for apparatus
For the basic design, references were made to “Separation of air from two-phase air-water
mixture flowing in a mini channel” (Kariyasaki et al. 2005). Fig. 1 shows the schematic
diagram for the return-bend channel set up with 9 connecting channels. The device is
composed of a curved main channel and a sub-channel placed outside the curvature along the
main-channel with short connecting channels (Kariyasaki et al. 2005). The effect of varying
each of the parameters was studied on the flow pattern of the fluid through the system.
To investigate the effect of velocity, the set up shown in Fig. 1(a) was used. The velocity at
which the most separation occurred and below which there was not much visible change in
the flow pattern was considered the optimum velocity. The resulting flow patterns for input
velocities of air and water are shown in Fig. 2 with the velocities of the phases at the
respective inlets.
Following this, the number of connecting channels and the size of their gaps were studied by
reducing the number of connect channels to nine channels, then increasing to thirteen
(roughly 1.5 times 9) and finally 18 channels (twice as many). The most effective number of
channels identified then had the size of its gap reduced to study the effects of reducing gap
size.
Lastly, a gauge pressure was applied to the outlets to see how it affected the flow. The least
gauge pressure at which the separation was effective was considered optimum. Surface
tension at the interface was considered to be negligible and the contact angle with connecting
channels was taken to be 45 degrees.
Results and Discussion
(a)
(b)
(c)
Fig. 2: Velocity vectors around the bend for various inlet flow velocities
(a) Air: 0.009m/s, Water: 0.03m/s; (b) Air: 0.003m/s, Water: 0.01m/s; (c) Air: 0.0018m/s, Water: 0.006m/s
Figures 2(a), 2(b) and 2(c) show examples of results obtained from the simulations. Water
and air are introduced into the main channel by separate outlets. The velocities are chosen so
that the ratio of air to water in the main channel remains at 30%. Water is colored in blue
arrows while air bubbles are multicolored depending on the particle speed.
As shown in the figures 2, part of the water from the main-channel flowed into the secondary
channel due to the centrifugal force. Most of the water flowed out of the several connectingchannels located near the exit of the main channel, this could be due to the divergence of the
mixture as it approaches the exit of the U-bend.
When the mixture enters the bend from the inlet-channel, the air bubbles drift towards the
inner curvature of the main channel (as compared to the water particles) and move along the
wall. However, high flow rates cause the bubbles to diverge when they flow into and out of
the U-bend causing them to flow into the sub-channel (Fig. 2(a)). Conversely, low flow rates
allow the bubbles more time to migrate transversely through the U-bend before exiting it and
thus resulting in the outflow of bubbles into the sub-channel (Fig. 2(c)). Hence, a balance
between the time spent in the U-tube and the centrifugal force acting on the bubbles is
derived and the best result is obtained when the air and water speeds are 0.003m/s and
0.01m/s respectively (Fig. 2(b)). The speed of mixture through the U-bend is very close to
the initial speed of the air bubbles.
(a)
(b)
(c)
Fig. 3: Velocity vectors around the bend of various connecting channel configurations but with constant
inlet air velocity of 0.003m/s and water velocity of 0.01m/s (a) 13 holes channel with 0.37mm gap size;
(b) 18 holes channel with 0.24mm gap size; (c) 13 holes channel with 0.24mm gap size.
The number of connecting channels is also perceived as affecting the flow of the mixture
through the U-bend. As the numbers of connecting-channels were increased in the setup, the
flow of water into the sub-channel had visibly increased. This can be observed when
comparing Fig. 2(b), 3(a) and 3(c) where more holes allowed greater flow of water into the
sub-channel. This phenomenon was attributed to a capillary effect on the mixture flowing
through the return bend which caused the mixture to get sucked into the sub-channel.
However, separation is only effective once the maximum amount of water flows out of the
main-channel while the maximum amount of water is retained in the main-channel. Thus, it
is crucial to obtain a balance between the numbers of connecting-channels as an increase in
the number of connecting-channels leads to a decrease in the size of channel gaps.
As per the simulation done, this is confirmed, as the 18 channel return bend (Fig. 3(b))
allowed more air into the sub-channel as compared to the 9 channel return bend (Fig. 2(b))
and 13 channel return bend (Fig. 3(a)). The 13 channel return bend, however, reduced the
amount of air flowing into the sub-channel as compared to the 9 channel return bend with the
exception of allowing air in at the last connecting-channel. A possible reason for this could
be the fact that allowing more water into the sub-channel earlier on as the mixture passed
through the U-bend reduced the pressure gradient between the two channels and prevented
the air from flowing into the sub-channel. Hence, in the 18 channel return bend, a gap size of
0.24mm caused the capillary forces to play a more predominant flow through the channel
whereas in the 13 channel return bend, a higher gap size of (approx.) 0.37mm allowed the
pressure gradient to play a more predominant role in the separation. The theory of capillary
forces was confirmed by reducing the gap size to 0.24mm in a 13 channel return bend which
displayed a greater amount of air flowing into the sub-channel (Fig. 3(c)). Also, the
divergence of air bubbles towards the end of the U-bend might have caused the air particles
to flow into the sub-channel. An effort was made, therefore, to reduce the amount of airflowing into the sub-channel through the connecting-channels nearing the end of the U-bend.
To reduce the amount of air diverging into the sub-channel, the effects of applying a pressure
at the water-outlet and then applying a suction force at the air-outlet was observed.
(a)
(b)
(c)
(d)
(e)
Fig. 4: Velocity vectors around the bend for various gauge pressure applied at outlets but with constant inlet air
velocity of 0.003m/s and water velocity of 0.01m/s (a) air: 0Pa, water: 5Pa; (b) air: –5Pa, water: 0Pa;
(c) air: –2Pa, water: 0Pa; (d) air: –1Pa, water: 6Pa (e) air: –0.5Pa, water: 6 Pa.
From fig 4(a) it was deduced that an applied positive gauge pressure at the outlet created a
force against the flow of water. Comparing fig 4(a), 4(b) and 4(c), it is observed that the best
way to regulate the air flow into the air outlet channel is to apply a positive pressure at the
water outlet. This could be due to an increase in speed in the main-channel when under
influence of a suction force. This increase in speed would cause the air to migrate out
towards the connecting-channels and therefore leading it into the sub-channel. Fig 4(a) also
shows that having only a pressure at the water outlet would cause the flow of water in the
sub-channel to decrease in speed. This would then reduce the air water separation as water
would tend to reduce its flow into the sub-channel therefore effectively reducing any
separation.
Thus, an optimum setup would use a combination of a small suction force at the air outlet
and a larger pressure force at the water outlet. The most optimum setup was identified by
comparing fig4(d) and fig4(e). In both figures, the flow through the last gap was effectively
reduced. However, fig4(e) presented a more optimal case as the speed of water into and
through the sub-channel was compromised less for getting the separation.
Conclusion
Through this study, there is significant proof to suggest that the speed of mixture flow, the
applied gauge pressure at the outlets and particularly the number of holes and gap separation
values play a key role in the separation of air-water mixture in a U-bend. From the obtained
results there were two main observations;
(1) The centrifugal force causes the water particles to flow into the sub-channel and keeps
the bubbles away from the connecting-channels.
(2) Pressure difference between the main and sub-channel compels the water to flow into
the sub-channel while excessive force applied causes air to flow into the sub-channel as
well.
Therefore, in order to optimize the setup, values need to be chosen for the parameters that
would allow almost all the water out of the main-channel curvature while retaining the air
bubbles so as to obtain separation. Though with the present apparatus, it would seem
inconceivable to achieve complete separation, with more investigation into the various
parameters affecting the mixture flow and a look into factors such as surface tension and
dimensions of the apparatus, complete separation might eventually be accomplished.
Possible Future Efforts
Future efforts could involve the study of effects of surface tension and apparatus dimension
for optimal separation. An insight into how this micro-scaled apparatus can be magnified into
one that can be used to separate large volumes of air-water mixtures is also a possible avenue
for study.
Acknowledgements
This research was supported by the National University of Singapore Engineering Faculty.
Special thanks for his irreplaceable guidance to Dr. Pang Sze Dai and to Dr. Erik Birgersson
for guiding the author till the successful completion of this project.
References
Kariyasaki, A., Yamasaki, Y., Fukano, T., Ousaka, A. and Morooka, S., 2005, "Separation of
Air From Two-Phase Air-Water Mixture Flowing in a Mini Channel", Proc. of 3rd
International Conference on Microchannels and Minichannels, pp. 585-589.
Mark Ayre (2004), “Biomimicry – A Review”. Issue 2, European Space Agency Work
Package Report
“Gases in Sea Water” obtained from http://www.waterencyclopedia.com/Re-St/Sea-WaterGases-in.html on 03 Feb 2008