Proceedings of ICMM2005
3rd International Conference on Microchannels and Minichannels
June 13-15, 2005, Toronto, Ontario, Canada
Paper No. ICMM2005-75118
WATER TRANSPORT VISUALIZATION AND TWO-PHASE PRESSURE DROP MEASUREMENTS
IN A SIMULATED PEMFC CATHODE MINICHANNEL
John Borrelli
Rochester Institute of Technology
Thermal Analysis and Microfluidics Laboratory
Rochester, NY 14615
[email protected]
Satish G. Kandlikar
Rochester Institute of Technology
Thermal Analysis and Microfluidics Laboratory
Rochester, NY 14615
[email protected]
Thomas Trabold
General Motors Corporation
Honeoye Falls, NY 14472
Jon Owejan
General Motors Corporation
Honeoye Falls, NY 14472
ABSTRACT
Two-phase flow and water transport in a 1.08 mm hydraulic
diameter by 25-cm long gas-transport minichannel are
investigated. High-speed side-view images are obtained of
water droplets moving through gas diffusion media (GDM) and
into a gas channel. This system simulates water transport and
the flow of air and water in a polymer electrolyte membrane
fuel cell (PEMFC) cathode gas channel.
Advancing and receding contact angles and departure
droplet diameters are measured with respect to superficial gas
velocity for two GDM samples. Pressure drop is measured and
compared to two-phase pressure drop correlations for three
different water flow and five different airflow rates, and
channel-water and GDM-water interactions are described.
INTRODUCTION
Water management is one of the most important challenges
in PEM fuel cell development. Water is a product of the fuel
cell reaction, and it forms in the GDM adjacent to the air
distribution channels of a cathode flow field plate and collects
in the air channels. While some water retention is necessary for
efficient operation, the majority must be removed from the
cathode flow field channels for optimal stack performance.
Visualization of water behavior at the air channel/GDM
interface in operating fuel cells has been employed as a tool for
qualifying and quantifying water behavior by several
investigators.
Tuber et al. (2003) performed visualization of water buildup
in the cathode of a transparent PEMFC. Their work showed
randomly distributed water droplets and formations, droplet
growth and droplet movement due to the kinetic energy of the
airflow. No departure droplet diameter or contact angle data
were presented.
Yang et al. (2004) performed visualization experiments of
water transport in an operating PEMFC. They reported water
droplets emerging from the GDM surface, droplet growth and
detachment from the GDM. They also noted that droplets
formed at preferential locations on the GDM and described the
phenomenon of active pore sites where several droplets grow
and depart from the same location over time. Some droplets
were reported to grow up to 0.8 mm in the 1 mm square
cathode minichannel, but the record rate and test section
orientation are not clearly stated. Furthermore, the bottomview images give no insight on contact angle or the mechanics
of droplet departure at the GDM /air channel interface.
Nam and Kaviany (2003) describe water transport in
hydrophobic media as consisting of two types – micro and
macro transport. The micro droplets feed into flowing macro
droplets as the water flows towards the air channel while
preferentially seeking larger pores due to the lower flow
resistance. The result of the branching micro to macro
transport is a water droplet formed in the air channel as shown
in Fig. 1.
Figure 1. Water transport model showing branching micro
to macro transport. Nam and Kaviany (2003).
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Copyright © 2005 by ASME
The goal of the present work is to simulate the operating
conditions in a single cathode flow field channel and obtain
high-speed, side view images of water emerging from a GDM
sample and into an air channel (flowing air stream). Side-view
images recorded at high frame rates are required in order to
accurately describe the droplet departure behavior.
Departure droplet diameters and contact angles are
measured and the data are presented for the two GDM samples
(A and B), and pressure drop is measured and compared to twophase flow correlations for GDM sample B.
OBJECTIVES
The objectives of the present work are to:
• Obtain side-view images of departing water droplets in
order to understand the droplet departure phenomenon.
• Measure departure droplet diameters and contact
angles to determine water removal performance of
different GDM samples.
• Investigate and detail two-phase flow patterns for the
specific flow present in these experiments.
• Measure two-phase pressure drop and compare the
experimental results to existing two-phase flow
correlations.
NOMENCLATURE
A Area of channel cross section (m2)
C Coefficient defined in Eq. (3)
Dh Hydraulic diameter (m)
DP Differential pressure (transducer)
f
Friction factor
G Mass flux (kg/m2s)
j
Superficial velocity (=Q/A) (m/s)
K Defined by Eq. (8)
m! Mass flow rate (kg/s)
P Pressure (transducer)
p Pressure (Pa)
Q Volumetric flow rate (m3/s)
Re Reynolds number
ReL Liquid Reynolds number =((1-x)GDh)/µL
ReG Gas Reynolds number =(xGDh)/µG
t
time
x Vapor mass fraction (= m! G ( m! L + m! G ) )
z Channel length axis (m)
EXPERIMENTAL SETUP
A 25-cm long minichannel was fabricated out of Lexan
plates in a manner that allowed a clear side view of the droplets
through one of the channel sidewalls. The complete test section
consisted of this air channel plate, a water chamber plate, and a
GDM sample compressed between the two plates. A schematic
of the test section is presented in Fig. 2.
Water Chamber Plate
GDM
Air Channel Plate
Figure 2. Test section assembly.
The water chamber plate has a 1-mm square minichannel
equal in length and position as the air minichannel in the air
channel plate so that they are in alignment when the two plates
are compressed together as shown in Fig. 3a.
Water enters the channel in the water chamber plate until
the channel is filled. As pressure builds up in the water
channel, water is forced through the GDM into the air channel
(see Fig. 3b). High-speed images were obtained of the water
droplets as they moved through the GDM and into the flowing
air stream. A schematic of the experimental apparatus is shown
in Fig. 4.
(a) Cross-section of compressed test section.
Greek
Pressure drop multiplier
Dynamic viscosity (N-s/m2)
Contact angle (degrees)
Density (kg/m3)
Specific volume (m3/kg)
Momentum (effective) specific volume (=1/ρe)
φ2
µ
θ
ρ
υ
υe
Subscripts
e Effective
F Friction
G Gas (air)
L Liquid (water)
(b) Schematic of just the channel walls; plates not shown.
Figure 3. Channel alignment and air and water flow paths.
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Copyright © 2005 by ASME
Water flow was provided through the use of a syringe
pump system. While one syringe infused water into the test
section water chamber, the other syringe was refilled. The
syringe-pump system provided a continuous, pulse-free flow
except for a brief period when the pumps switched directions.
The water flow rate was found to be within the pump
manufacturer specifications of 1% of pump setting for the flow
rates 0.28 ml/min, 0.56 ml/min, and 1.12 ml/min. Ultra filtered
deionized water was used throughout the experiment.
A compressed gas cylinder provided the air flow. Ultra
zero-grade air was regulated at the cylinder and adjusted to a
constant pressure of 48.3 kPa (7 psig). The superficial
velocities and their associated uncertainties are given in Table
1, where the superficial velocity is defined as the actual
volumetric flow rate divided by the channel cross section.
activity, and digital image sequences were recorded at 1000
frames per second. This frame rate allowed for recording times
of approximately 8 seconds.
PRESSURE DROP
There are several correlations available for predicting twophase flow pressure drop based on the separated flow model.
One that is widely used is the Lockhart and Martinelli (1949)
correlation. This correlation relates the pressure drop
multiplier:
φ L2 = ( dpF dz ) ( dpF dz ) L
(1)
to the Martinelli parameter X defined as:
X 2 = ( dpF dz ) L
( dpF
dz )G
(2)
where the liquid and gas pressure gradients are -(dpF /dz)L =
(2fLG2(1-x)2)/(DhρL) and -(dpF/dz)G = (2fGG2x2)/(DhρG),
respectively. The friction factors fL and fG are related to the
liquid and gas Reynolds numbers ReL and ReG.
Chisholm (1967) fit curves to the graphical representation
of the Lockhart and Martinelli correlation for pressure drop.
The relationship is:
φ L2 = 1 +
Figure 4. Schematic of experimental apparatus.
Table 1.
Gas superficial velocities and uncertainties.
Superficial Velocity Uncertainty
jG (m/s)
(±)
0.7
0.07 m/s
1.25
0.07 m/s
2.5
0.07 m/s
5.0
0.07 m/s
10.0
0.70 m/s
The pressure transducers were calibrated with an Omega
DPI 610 pressure calibrator. A voltage output was recorded for
16 pressure settings, and a linear regression was performed in
order to obtain an equation for pressure as a function of
voltage. This process resulted in a pressure reading that was
within ± 138 Pa (± 0.02 psi) of the calibrator setting.
After the test section was assembled, the system was run
near the highest water flow rate and at the desired airflow rate
for two hours before the start of data collection. After this
time, active locations were identified by eye and then examined
more closely with the high-speed camera and high-intensity
halogen lighting. The channel length was scanned for droplet
C
1
+ 2
X X
(3)
where C is a dimensionless parameter related to the nature of
the two phase flows (laminar or turbulent). For flows where
the gas and liquid are both laminar, the parameter C is
recommended to be 5, and C is recommended to be 12 for
flows where the gas phase is turbulent and the liquid phase is
laminar (Collier (1972)).
Mishima and Hibiki (1996) modified the parameter C,
given by Chisholm (1973) as C = 21 for two-phase turbulent
flow, by relating it to the channel inner diameter. A new
equation for C was developed for vertical and horizontal round
tubes and rectangular ducts:
(
C = 21 1 − e −319 Dh
)
(4)
where Dh is the hydraulic diameter in meters. In the present
case Dh = 1.08 × 10-3 m which makes the value of C = 6.12.
Measured pressure drop for horizontal two-phase flow can
be described by:
∆pmeasured = ∆p friction + ∆pcontraction + ∆pmomentum
(5)
where ∆pcontraction term represents the pressure drop resulting
from contraction and expansion at the channel inlet and outlet,
respectively. The ∆pmomentum term is given by Chisholm (1983)
as:
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Copyright © 2005 by ASME
(6)
where G is taken to be the total mass flux and υG and υL are the
specific volumes of the air and water, respectively. The ratio of
υe/υL is given by:
  1− x 
υe   υG 
=  x   + K (1 − x )   x +
(7)
K 
υ L   υ L 
 
where x is the vapor mass fraction and K is defined as:
K = (υG υ L )

1/ 4


0.28
(8)
The contraction and momentum terms are subtracted from the
total measured pressure drop in order to isolate the frictional
pressure drop and compare it with theoretical predictions. A
comparison of theoretical prediction and experimental data
follows.
A plot of pressure drop as a function of superficial gas
velocity for each water flow rate is presented in the Results and
Discussion section that follows. Since the two phases are not
mixed before entering the test section minichannel, half of the
water flow rate is used to calculate the average vapor mass
fraction x in the test section for the frictional pressure drop
calculations. However, Chisholm specifies the exit vapor mass
fraction for use in Eq. (7), so the full water flow rate is used in
calculating the vapor mass fraction for the pressure drop due to
momentum flux.
RESULTS AND DISCUSSION
DEPARTURE DROPLET DIAMETER
A quantitative and qualitative analysis of droplet behavior
was undertaken. The quantitative analysis consisted of contact
angle measurement and of measuring the departure diameter of
the water droplets as they were sheared off from the GDM
surface. High-speed video was used to analyze droplet
behavior and obtain the droplet diameters. The uncertainty
associated with the droplet diameter measurement scheme was
± 0.05 mm for the GDM sample A data and ± 0.03 mm for the
GDM sample B data due to different measurement techniques.
Figure 5 shows high-speed images of pre and post departure
droplets, and Fig. 6 shows a schematic of where the diameter is
measured on the pre-departure droplet.
Figure 6. Pre-departure droplet form.
The departure droplet diameter is measured in the highspeed video frame just before the droplet tail (shown in Fig. 6)
breaks away. Shortly after the droplet leaves the GDM surface,
droplet instability makes it difficult to measure the droplet
diameter as can be seen in Fig. 5 with two frames from a highspeed video that are just 0.002 of a second apart.
There appears to be a strong relationship between
departure droplet diameter and superficial gas velocity as can
be seen by the plots in Figs. 7 and 8. Figure 7 relates to the
GDM sample A, and Fig. 8 corresponds to the GDM sample B.
There is clearly a decreasing trend in departure diameter with
increasing superficial gas velocity.
The spread in the data is the result of variable channel
cross-section and/or airflow pattern variation due to water slug
formation upstream and downstream of the droplet location. It
is impossible to accurately monitor the entire channel length
simultaneously as conditions in the air channel and droplet
behavior change rapidly and in many cases occur faster than the
human eye can see. However, water droplets were observed
occurring at preferred locations, often repeatedly emerging at
the same location over time as described by Nam and Kaviany
(2003) and as seen in the work of Tuber et al. (2003) and Yang
et al. (2004). An examination of the GDM structure at these
preferred-location sites is necessary in order to obtain more
information about the nature of these sites.
Diameter (mm)
υ
υ 
∆pm = G 2υ L  G − e 
 υL υL 
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
5
j G (m /s)
10
15
Figure 7. Departure droplet diameter versus superficial
velocity for GDM sample A.
Figure 5. Pre-departure and post-departure droplets. Time
between images is ∆t = 0.002 sec; jG = 10 m/s; Water flow
rate = 1.12 ml/min.
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Copyright © 2005 by ASME
1.00
0.90
Diameter (mm)
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
5
j G (m /s)
10
15
Figure 8. Departure droplet diameter versus superficial
velocity for GDM sample B.
The advancing contact angle appears to increase slightly as
the departure droplet diameter increases for GDM sample A,
but over the larger range of departure droplet diameters in the
GDM sample B experiment (Fig. 11), the advancing contact
angle appears to remain constant. The receding contact angle
shows the opposite behavior, as it appears to decrease with
increasing diameter for both GDM samples for diameters below
0.75 mm. The larger diameters in the GDM sample B data are
due to the much lower airflow rates that the experiment for
GDM sample B covered which were not covered in the GDM
sample A experiment.
The spread in the contact angle data is a result of the
spread in the departure droplet diameters due to local air
velocity changes in the channel. Departure droplets with the
same diameters can have greatly different receding contact
angles due to the difference in air flow rate, but the advancing
contact angles seem to be less affected by air flow rate.
160
Static advancing and receding contact angles were also
measured from the high-speed videos just before a drop was
sheared off. The error in this measurement is estimated to be ±
2 degrees. Figure 9 shows the advancing contact angle
orientation (receding is similar but opposite).
140
Contact Angle (deg.)
CONTACT ANGLES
120
100
80
60
40
20
0
0.25
Figure 9. Schematic of contact angle measurement.
Contact Angle (deg.)
160
140
120
100
80
60
40
A dvancing
20
Receding
0.50
0.75
0.50
0.75
1.00
Departure Droplet Diameter (m m)
The following figures are plots of contact angle as a
function of departure droplet diameter for the two GDM
samples. Figure 10 shows the GDM sample A contact angle
data as a function of departure droplet diameter. The data points
have been sized to the height of the error bars in order to reduce
clutter.
0
0.25
Advancing
Receding
Figure 11. Contact angle versus departure droplet diameter
for GDM sample B.
FLOW PATTERNS
At low superficial gas velocities departure droplet
diameters grow large and most often interact with the air
channel sidewalls or water slugs and other water formations in
the air channel. Generally, flow patterns for low superficial gas
velocities lead to discrete lumps of water, stationary or flowing
through the air channel, or stratified flow with air flowing over
a layer of water. The discrete water lumps were observed
sliding on the channel bottom and the channel sidewalls. In the
high-speed video images that follow, the airflow is from left to
right and gravity is in the plane of the page pointing downward.
The time between frames is indicated in the figure labels.
a
c
1.00
Departure Droplet Diameter (mm)
Figure 10. Contact angle versus departure droplet diameter
for GDM sample A.
b
d
Figure 12. Large droplet diameter due to low superficial gas
velocity: jG = 0.7 m/s; ∆t = 0.001 s; Water = 1.12 ml/min.
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Copyright © 2005 by ASME
The relatively large droplet (Fig. 12) is removed from the
GDM surface through contact with water flowing in the far
bottom corner of channel. Figure 13 shows slug droplet
interaction as the large droplet is pulled into a water slug that
formed on the channel bottom.
a
a
e
b
f
c
g
d
h
d
b
e
c
Figure 13. Large droplet diameter due to low superficial gas
velocity: jG = 0.7 m/s; ∆t = 0.002 s; Water = 1.12 ml/min.
Figure 14 shows a water drop being pulled into a water
slug with a small portion of the drop being splashed back onto
the GDM surface. This behavior was observed sporadically
throughout the experiment.
a
d
b
e
c
Figure 14. Water splashing back onto GDM surface.
jG = 0.7 m/s; ∆t = 0.001 s; Water = 1.12 ml/min.
f
The effect of gravity on a sheared-off droplet can be seen
in Fig. 15. The droplet falls toward the channel bottom as it
moves in the direction of the air flow.
a
b ∆t = 0.01 s
Figure 16. Water slug growing into a wave then sweeping
through channel. jG = 1.25 m/s; ∆t = 0.001 s; Water = 1.12
ml/min.
Figure 17 shows water flowing on the far channel sidewall
in Frame a. Frame b shows the sidewall slug connecting with a
film of water that is in the top channel corners and on the GDM
surface. Subsequent frames show the sidewall water slug
flowing into the water film as a portion of the film thickness
can be seen to increase (center, near channel top corner, Frame
f).
a
d
b
e
c
f
Figure 17. Water flowing on channel sidewall in Frame a
merges with water in the top channel corner and continues
to flow along the GDM surface. jG = 10 m/s; ∆t = 0.04 s;
Water = 0.28 ml/min.
The following figures (Figs. 18 – 22) indicate typical flow
patterns observed throughout the experiment. Different flow
patterns often occurred simultaneously in different regions of
the channel not too far from each other. In many of the flow
patterns water poses an air flow obstruction and channel crosssection reduction which leads to an increase in pressure drop.
c ∆t = 0.003 s
d ∆t = 0.001 s
Figure 15. Effect of gravity seen as droplet falls towards
the channel bottom. jG = 5 m/s; Water = 1.12 ml/min.
In Figure 16, the water slug that had been growing on the
left side of the water droplet (Frame a) nearly chokes the air
channel. Before the air flow can be blocked by the water slug
formation, the air flow pushes the slug through the channel. A
wave formation can be seen in Frame c, which sweeps through
the channel knocking off the water droplet and the water slug
on the near channel sidewall.
Figure 18. Large departure droplet diameter and water
slug. Superficial gas velocity: jG = 0.7 m/s; Water = 1.12
ml/min.
Figure 19. GDM location continuously feeding water slug.
Superficial gas velocity: jG = 1.25 m/s; Water = 0.56 ml/min.
Figure 20. Stratified flow: water below air with droplets on
GDM. Superficial gas velocity: jG = 1.25 m/s; Water = 1.12
ml/min.
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Copyright © 2005 by ASME
Figure 21. Wave formation. Superficial gas velocity: jG =
2.5 m/s; Water = 0.56 ml/min.
Figure 22. Water draining down sidewalls. Superficial gas
velocity: jG = 10 m/s; Water = 1.12 ml/min.
The effect of gravity on the sheared off droplets is seen for
all superficial gas velocities in this experiment meaning that
these air velocities are not high enough to overcome the
gravitational force. Stratified flow still occurred at higher gas
velocities, and it was seen to form wave structures as seen in
Fig. 21 and Fig. 16-c. Water was also observed flowing along
the top corners of the air channel for significant lengths as seen
in Fig. 17. The flow patterns observed throughout the
experiment are summarized in Fig. 23.
a) Droplets removed by channel wall or water slug interactions.
b) Discrete droplets, sheared off by airflow.
f) Water draining down channel sidewall. The neck-like structure
usually starts as a droplet growing very near to the sidewall.
g) Water flowing in top channel corners and on GDM surface.
This behavior was seen at high superficial gas velocities and low
water flow rates.
Water
Air
Figure 23. Two-phase flow patterns for the simulated
horizontal PEMFC minichannel.
PRESSURE DROP
The experimental pressure drop data was compared with
the two-phase pressure drop correlations given by Lockhart –
Martinelli (1949), Chisholm (1967) and Mishima and Hibiki
(1996). The pressure drop multiplier given by Eq. (3) was used
with the recommended parameter values in order to obtain
theoretical pressure drop values at a given superficial gas
velocity. Figures 24 through 26 show the relationship between
superficial gas velocity and pressure drop for each of the water
flow rates. The experimental data is presented as solid data
points, and the theoretical predictions for two-phase pressure
drop are presented as hollow points connected by line
segments. The theoretical predictions are labeled by the
pressure drop multiplier equation used and its distinguishing
constant value. For the case of both phases being laminar, the
value of C is 5, and the value of C is 12 assuming that the gas
phase is turbulent and the liquid phase is laminar. With Dh =
1.08 × 10-3 m, the Mishima and Hibiki (1996) equation (Eq. (4))
reduces to C = 6.12.
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Exp. Data
c) Active location continually feeding a water slug.
Eq. 3 (C = 5)
Pressure Drop (kPa)
5
d) Stratified flow with water below air. Discrete droplets attached
to the GDM at selective locations are also present.
Eq. 3 (C = 6.12)
4
Eq. 3 (C = 12)
3
2
1
0
e) Wave formations that sweep through the channel. Oblong
formation represents water slug flowing on channel sidewall.
0
5
j G (m/s)
10
15
Figure 24. Pressure drop versus superficial gas velocity for
water flow rate of 0.28 ml/min: GDM sample B.
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Copyright © 2005 by ASME
6
Pressure Drop (kPa)
Exp. Data
5
Eq. 3 (C = 5)
4
Eq. 3 (C = 6.12)
Eq. 3 (C = 12)
3
2
1
0
0
5
10
15
j G (m/s)
Figure 25. Pressure drop versus superficial gas velocity for
water flow rate of 0.56 ml/min: GDM sample B.
6
Pressure Drop (kPa)
Exp. Data
5
Eq. 3 (C = 5)
4
Eq. 3 (C = 6.12)
Eq. 3 (C = 12)
3
2
1
0
0
5
10
15
j G (m /s)
Figure 26. Pressure drop versus superficial gas velocity for
water flow rate of 1.12 ml/min: GDM sample B.
The theoretical pressure drop equation (Eq. (1)), using the
pressure drop multiplier given by Eq. (3), does not do a good
job of predicting the experimental pressure drop data with C =
5, or 6.12, and in the higher air flow rate cases Eq. (1) severely
underpredicts the experimental pressure drop data. This could
be the result of stopped water slugs forming restrictions in the
channel and causing the pressure drop to increase - a condition
not present in two-phase flow conditions where both phases are
mixed together before entering the channel, each having an
initial momentum. The percent difference between the
theoretical and experimental pressure drop is presented in Table
2.
Table 2.
Theoretical and experimental pressure drop difference.
Water Flow Rate = 0.28 ml/min
% Difference = (Exp. - Theo.)/(Theo.) * 100%
jG (m/s)
C=5
C = 6.12
C = 12
0.7
102.4
77.4
256.5
1.25
70.9
51.5
163.2
2.5
150.4
125.4
223.0
5
89.8
73.7
98.7
10
85.8
73.0
49.1
Water Flow Rate = 0.56 ml/min
jG (m/s)
C=5
C = 6.12
C = 12
0.7
23.6
7.3
98.0
1.25
84.5
61.5
156.5
2.5
90.8
69.2
121.9
5
68.9
52.0
60.1
10
83.7
68.3
35.3
Water Flow Rate = 1.12 ml/min
jG (m/s)
C=5
C = 6.12
C = 12
0.7
-9.5
-21.9
33.8
1.25
15.8
0.4
48.2
2.5
43.7
25.8
53.1
5
52.3
35.1
31.9
10
61.1
45.2
9.4
Furthermore, water droplets must grow through the GDM
fibers making it difficult for small droplets to flow on the
surface as they might for the case of steam condensing on the
inner wall of a tube. The extra force necessary to shear off the
droplets and the extra force needed to overcome the flow past
the water droplets may not be adequately accounted for in the
pressure drop equations related to momentum or acceleration.
Assuming that the gas phase is turbulent (C = 12) slightly
improves the agreement with the experimental data for the
highest water flow rate case, but overall this still results in an
underprediction of the experimental observations. The manner
in which the water enters the channel through the GDM is
believed to be the main cause for a higher pressure drop as
compared to a conventional air-water flow with both phases
entering a flow channel simultaneously.
CONCLUSIONS
An experimental model of an operating PEMFC cathode
channel was created to study the water flow behavior in a
PEMFC gas channel.
• The qualitative behavior of the water droplets is
similar to the actual droplet behavior in operating
PEM fuel cells as observed by Tuber et al. (2003) and
Yang et al. (2004).
• Discrete droplets were seen to form at preferred
locations as described by Nam and Kaviany (2003).
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Copyright © 2005 by ASME
•
•
•
•
•
•
The mechanism of droplet departure due to airflow can
be described as a symmetrical water drop permeating
the GDM at a preferred location. The droplet grows
due to water production, and it becomes deformed (see
Fig. 5) by the force of the air. The droplet continues to
grow to a critical size until the “tail” breaks free
sending it in the direction of the airflow. The effect of
gravity cannot be neglected even in such a small
channel with relatively high gas velocities.
Contact angles were visible in the side-view images
and it was possible to measure them as well as the
departure droplet diameters. There appears to be a
difference in the water removal characteristics of the
two GDM samples, however, there is significant
spread in the data due local changes in the air velocity
around a droplet.
Flow pattern diagrams were created (Fig. 23) for the
flow ranges used in this experiment. For the single
channel PEM fuel cell model, full annular flow was
not observed, although water was seen, at different
times, flowing on all four channel surfaces. In fact,
the flow patterns presented in Fig. 23 represent
specific forms, which often occur simultaneously.
Two-phase pressure drop was measured, and the
experimental data is presented as a function of
superficial gas velocity for each water flow rate. The
pressure drop multiplier given by Eq. (3) leads to a
severe underprediction of the experimental pressure
drop data for superficial gas velocities of 2.5 m/s and
above for each experimental water flow rate.
Water enters through the GDM fibers making it
difficult for small droplets to slide on the GDM
surface. The extra force necessary to shear off these
droplets and overcome their obstruction to air flow
may not be adequately accounted for in the pressure
drop equations.
Generally, the air and water in two-phase flow
experiments are mixed before entering the test section.
In the present experiment, water enters at discrete
locations along the test section, and the liquid mass
fraction varies along the channel length. Furthermore,
the water droplets growing through the GDM surface
have zero initial velocity in the air flow direction.
Collier, J.G. 1972. Convective Boiling and Condensation.
2nd ed. McGraw-Hill, New York.
Lockhart, R.W. and Martinelli, R.C. 1949. Proposed
Correlation of Data for Isothermal Two-Phase, TwoComponent Flow in Pipes. Chem. Eng. Prog. 45: 39-48.
Mishima, K. and Hibiki, T. 1996. Some Characteristics of
Air-Water Two-Phase Flow in Small Diameter Vertical Tubes.
Int. J. Multiphase Flow. 22: 703-712.
Nam, J.H. and Kaviany, M. 2003. Effective Diffusivity and
Water Saturation Distribution in Single and Two-Layer
PEMFC Diffusion Medium. Int. J. Heat Mass Transfer. 46:
4595-4611.
Tuber, K., Pocza, D., Hebling, C. 2003. Visualization of
Water Buildup in the Cathode of a Transparent PEM Fuel Cell.
J. Power Sources. 124: 403-414.
Yang, X.G., Zhang, F.Y., Lubawy, A.L., Wang, C.Y. 2004.
Visualization of Liquid Water Transport in a PEFC.
Electrochemical and Solid-State Letters. 7: A408-A411.
ACKNOWLEDGMENTS
This research was made possible through a sponsored
project from General Motors Corporation.
REFERENCES
Chisholm, D. 1967. A Theoretical Basis for the LockhartMartinelli Correlation for Two-Phase Flow. Int. J. Heat Mass
Transfer. 10: 1767-1778.
Chisolm, D. 1973. Pressure Gradients Due to Friction
During the Flow of Evaporating Two-Phase Mixtures in
Smooth Tubes and Channels. Int. J. Heat Mass Transfer. 16:
347-358.
Chisholm, D. 1983. Two-Phase Flow in Pipelines and
Heat Exchangers. Godwin, New York.
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