7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
Immiscible liquidliquid twophase flow in a microchannel:
flow patterns and pressure drop characteristics
Hooman Foroughi and Masahiro Kawajil,2
1. Department of Chemical Engineering and Applied Chemistry, University of Toronto,
Toronto, ON M5S 3E5, Canada
2. Departments of Mechanical and Chemical Engineering, City College of New York,
New York, NY 10031, USA
kawaji~,me.ccny.cuny.edu and hooman.foroughi~,utoronto.ca
Keywords: Microchannel: Liquidliquid flow: Flow patterns: Pressure drop
Abstract
Immiscible liquidliquid twophase flow characteristics in a circular microchannel have been investigated in connection with
the flow of heavy crude oil in porous media in the presence of water. Water and silicone oil with a dynamic viscosity of 863
mPa.s were injected into a fused silica microchannel with an inner diameter of 250 pLm. Twophase flow patterns were
determined under a wide range of oil and water flow rates. Twophase pressure drop data have also been collected and
analyzed.
Introduction
Nomenclature
Twophase flows in microchannels have a wide range of
applications in chemical and petroleum engineering.
Although gasliquid flows in microchannels are well
documented (Kawahara et al. 2002, and Chung & Kawaji
2004), liquidliquid flows in microchannels are still not
very well understood (Salim et al. 2008). Numerous
investigations have been carried out on viscous oilwater
flows in small and conventional pipes. Many of these
studies were performed in horizontal and vertical pipes as
reviewed by Joseph et al. (1997).
In small channels, Anna et al. (2003) used a microfluidic
device with flowfocusing geometry to produce droplets in
wateroil systems. In their experiments, both fluids were
forced to flow through an orifice to produce monodisperse
and polydisperse emulsions. They developed a phase
diagram illustrating drop size as a function of flow rates and
flow ratios. Dreyfus et al. (2003) studied liquidliquid
twophase flow patterns in a microchannel with crosslike
injection configuration. They showed that wetting properties
strongly control flow patterns. Kashid et al. (2007) studied
the hydrodynamics of liquidliquid slug flow in a Ytype
microchannel. Salim et al. (2008) studied the oilwater flow
patterns and pressure drops in a rectangular, micro
Tjunction. The viscosity of oil used in their study was 30.6
mPa.s.
In this work, the flow characteristics of viscous oilwater
two phase flows in a circular microchannel are studied. The
density and viscosity of silicone oil used in this study are
0.97gm/cm3 and 863 mPa.s which are comparable to that of
gassaturated heavy oil in petroleum reservoirs.
Constant
channel lenght (m)
Pressure (kPa)
superficial velocity (cm/s)
Greek symbols
AP pressure drop (kPa)
CL viscosity (mPa.s)
STwophase friction multiplier
X LockhartMartinelli parameter
Subsripts
g Gas
1Liquid
m oilwater mixture
o Oil
TP Twophase flow
w Water
Experimental Facility
The experimental apparatus used is shown in figure 1. Two
pneumatic pumps were used to inject water and silicone oil
separately. Pneumatic pumps consisted of a cylindrical
vessel filled with a liquid and pressurized with a nitrogen
gas from a cylinder. One of the pneumatic pumps
contained water and the other contained silicone oil. The
pressures in the liquid reservoirs were raised to inject
liquid into the channel. The pressure regulators on the
nitrogen gas cylinders were adjusted to cover certain
ranges of water and silicone oil flow rates.
The silicone oil was injected into a microchannel test
section through a needle with an internal diameter of 100
pLm and outer diameter of 210 pLm. As shown in figure 2,
Figure 3: Effect of optical correction: a) without optical
correction, b) with optical correction
..
II
f)
Figure 4: Flow patterns observed in viscous oilwater two
phase flow in a microchannel: a & b) bubbly flow water
bubbles in oil as the continuous phase; c & d) slug flow 
water slugs in oil; e & f) annular flow water is the core
flow surrounded by oil; and g) mixed flow water drops and
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
different flow rates while the oil flow rate was kept constant.
Figure 4 shows the flow patterns observed in this system:
bubbly, slug, annular and mixed flows.
 = 
silicone oil was injected at the centre of the microchannel
through the needle, while water was injected through an
annulus between the needle and the microchannel with an
inner diameter of 250 pLm, outer diameter of 360 lpm and
length of 72 mm.
A pressure transducer with an accuracy of 1.7 kPa (0.25
psi) was used to measure the pressure drop between the
microchannel inlet and exit which is exposed to the
atmosphere. A cross junction was used to connect the
needle, water injection line, pressure transducer, and
microchannel.
Water Reservoir MicroChannel
Cross junction I:
Pli~ Pressure Transduer
Nitrogen Gas Oil Reservoir Ligh
Nedefoe injecting
Cylinders
cnel
Figure 1: Schematic of the experimental apparatus
Channel Wall eedle for injecting oil
into the microchannel
Figure 2: Oil and water injection section of the
microchannel
A high speed CCD camera was used to capture images of
the watersilicone oil flow at 15 frames per second. Since a
circular microchannel was used, optical correction was
necessary to capture undistorted and clear images inside the
entire cross section of the microchannel. To this end, the
microchannel was sandwiched between two glass plates
and the gap between the two plates was filled with oil to
best match the index of refraction of the microchannel.
Figure 3 shows the effect of optical correction on the images
captured by the high speed video camera.
Results and Discussion
Flow patterns of oilwater flow in microchannels strongly
depend on the nature of the first fluid injected into the
channels (Salim, 2008). In other words, different flow
patterns are observed depending on with which fluid the
channel is initially saturated, silicone oil or water. In this
work, the channel was initially always saturated with
silicone oil by injecting only the silicone oil at a constant
flow rate. Then, water was injected into the channel at
1 ;
a> L
Y
c E
b) %
E
5 1
S0.5
. n
I I II I
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
Pressure Drop Measurements and Analysis
In this section, pressure drop measurements for slug,
annular and mixed flows are presented and discussed. Salim
et al. (2008) found that the pressure drop and flow pattern
change when the first liquid injected into the channel
changes. As mentioned earlier, in this work, the channel was
always initially saturated with the silicone oil.
Figure 7 shows the pressure drop data obtained in silicone
oilwater two phase flows in the microchannel. Each curve
represents a set of data measured at a constant oil flow rate
while the flow rate of water was increased. The pressure
drop changed linearly as a function of the water flow rate
while the oil flow rate was kept constant. Also, the pressure
drop increased with an increase in the oil flow rate.
An increase and then a sharp decrease in twophase
oilwater flow pressure drops have been reported at low
water flow rates (Slim et al. 2008). Such a change in
pressure drop measurements has not been seen in this work.
The reason may be that the pressure drop data presented in
figure 7 were measured at water flow rates which were not
low enough to result in a sudden increase and decrease in
the pressure drop.
water core flow in oil. The microchannel was initially
saturated with oil and the flow direction is from right to left.
Although the silicone oil was injected at the centre of the
microchannel through a needle and water was injected
through an annular gap between the outer wall of the needle
and the inner wall of the microchannel (Figure 2), water
formed the dispersed phase or coreflow and oil was the
continuous phase and outer flow since the microchannel was
initially saturated with silicone oil.
As mentioned earlier, for each experiment, the oil flow rate
was kept constant, while increasing the water flow rate. At
low flow rates of water, the flow pattern was bubbly. With
an increase in the water flow rate, bubbly flow changed to a
slug flow. With a further increase in the water flow rate, a
transition occurred from slug flow to annular flow. Figure 5
shows the transition boundaries between the slug and
annular flow patterns.
Finally, at highest flow rates of water tested, mixed flow
patterns of water bubbles and annular flow of water in oil
were observed. Mixed flow patterns have not been observed
in lowviscosity oil and water two phase flows in larger
microchannels (Salim et al. 2008). The flow pattern map for
the current system is presented in figure 6.
Figure 5: Transition from slug to annular flow: a) slug flow,
b) transient flow: slugs merge, and c) annular flow
Water Superficial Velocity (cm/s)
Figure 7: Pressure drop data for silicone oilwater two
phase flow in the microchannel. Circles and dashed lines
represent measured data and solid lines are equation 2.
Curves are numbered from top to bottom and each curve
was obtained at a constant oil flow rate: 1) 0.051 g/min, 2)
0.045 g/min, 3) 0.031 g/min, 4) 0.027 g/min, 5) 0.022 g/min,
6) 0.018 g/min.
The singlephase pressure drop was calculated by using the
HagenPoiseuille correlation:
rrr IBubbly Flow
a so ASlug Flow
me 'I WH Annular Flow
't Mixed Flow
AP 32p1V
 =2
0 2040 6 80 where is the pressure drop over unit length of the
Water Superficial Velocity (cm/s)
microchannel,L, p1 is dynamic viscosity, Dis the channel
Figure 6: Flow pattern map for silicone oilwater twophase diameter and V is the superficial velocity of each phase.
flow in a 250 Cpm microchannel
X = = ~, (5)
where 9, is the waterphase friction multiplier Wo is the
oilphase friction multiplier, and X is LockhartMartinelli
parameter. By using equation 1 for water and oil
singlephase pressure drop, the LockhartMartinelli
parameter can be written as:
7 =J(/' (6)
By using equations 2 and 5, equation 3 can be written as:
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
with Equation 9. This figure shows excellent agreement
between Equation 9 with the present liquidliquid pressure
drop data.
700
A slug Flow
600
o Annular Flow ,
5001 4~ Mixed Flow
& 400 Equation 9
300
200
100
00 100 200 300 400 500
X2
Figure 8: Twophase friction multiplier variation with
LockhartMartinelli parameter
Pressure drop data presented in figure 7 show that the
twophase pressure drop in this system varies as a function
of the singlephase oil and water pressure drops:
I = 60* + + 1.284 I
\L TP \LW \L/o
where ( T)~is the twophase pressure drop. and )and
o) are the water and oil singlephase pressure drops
calculated by using equation 1 with superficial velocities of
water and oil. The results show that equation 2 predicts
pressure drops for this system with a maximum error
oft*10%.
Kawahara et al. (2002) and Chung and Kawaji (211114) used
LockhartMartinelli model to describe pressure drops for
gasliquid twophase flow in microchannels. In this work
LockhartMartinelli model is used for the system of
oilwater two phase flows by using a gas correlation in
LockhartMartinelli for water and liquid correlation for oil:
l ) TP
4" =P
Equation 9 which is equivalent to equation 2 shows the
correlation between waterphase friction multiplier and the
( LockhartMartinelli parameter. Equation 9 is also
comparable to Chisholm's (1967) correlation used for
gasliquid twophase flows in pipes which gives the
gasphase friction multiplier 4, as a function of
,,LockhartMartinelli parameter as follows:
4~2 .. C 2
(10)
where C is a constant and depends on the laminar or
turbulent nature of the flow and pipe size.
Equation 9 can also be compared to the correlation
developed by Salim et al. (2008) for low viscosityoil and
water twophase flow in a larger microchannel made of
quartz and with an inner diameter of 793 plm.
49 2 = X+P2 (
Constants a and fi were found to be 0.26 and 0.8,
respectively, by Salim et al. (2008) for oilwater twophase
flow in a microchannel.
Flow of Water in a Stagnant Oil Annulus
'l 60 I L w + 1.284 r L o ~
In this section, the flow of water with zero oil flow rate in a
microchannel initially saturated with oil is investigated. To
initially saturate the channel with oil, the oil is injected into
the channel first and the channel is filled completely with oil.
Then the oil injection is stopped and water is injected into
the microchannel at a constant flow rate. The flow patterns
observed in this case are shown in Figure 9.
Under these conditions, the initially injected oil sticks to the
inner wall of the microchannel and forms a stagnant
continuous layer. After the oil injection is stopped and water
is injected into the channel, water flows as a core in a wavy
form through a stagnant annulus of oil.
This can be written as,
TL )
49w = /60 + 1.284~ ( 7
4~2 = 60 + 1.284 72 (9)
The twophase friction multiplier, 9, and square of the
LockhartMartinelli parameter, X were calculated from the
present experimental data and are plotted in Figure 8 along
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
Microchannels", Phys. Rev. Lett. Vol. 90, No. 14, 144505
(2003).
Joseph, D.D., Bai R., Chen, K. P. & Y. Y. Renardy,
"CoreAnnular Flows", Annu. Rev. Fluid Mech., Vol. 29,
6590 (1997).
Kashid, M.N. & D.W. Agar, "Hydrodynamics of
liquidliquid slug flow capillary microreactor: Flow
regimes, slug size and pressure drop" Chemical
Engineering Joumnal Vol. 131, 113 (2007).
Kawahara, A., Chung, P.M. Y., & M. Kawaji,
"Investigation of twophase flow pattern, void fraction and
pressure drop in a microchannel", Intemnational Joumnal of
Multiphase Flow Vol.28, 14111435 (2002).
Salim, A., Fourar, M., Pironon, J. & J. Sausse, "OilWater
TwoPhase Flow in Microchannels: Flow Pattemns and
Pressure Drop Measurements", Canadian J. of Chem. Eng.,
Vol. 86, 978988 (2008)
a)
Figure 9a & b: Flow of water in a microchannel initially
saturated with oil. Water flows as a core in a wavy fonn
while a stagnant layer of oil sticks to the side wall of the
microchannel.
To investigate whether water would be able to fully wash
away the oil, in one experiment, water at a constant flow
rate of 0.5 gm/min was injected into the microchannel
which had been saturated with oil for more than 3.5 hours. It
was found that the water could not completely wash away
the oil laver from the inner channel wall. Instead, the water
flowed as a core through a stagnant wavy annulus of oil
during the whole experiment.
Conclusion
An experimental study of waterviscous oil twophase flow
in a microchannel of 250 Cpm diameter initially saturated
with oil has been perfonned. Different flow patterns were
observed over a wide range of oil and water flow rates. It
was found that the twophase friction pressure drop is a
linear function of singlephase water and oil flow rates. A
LockhartMartinelli model could also be used to predict
liquidliquid pressure drop data obtained in this system.
Since the channel was initially saturated with oil, a stagnant
laver of oil would remain on the inner wall of the
microchannel which could not be washed away by a
continuous flow of water even with the oil flow stopped.
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