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Water management studies in PEM fuel cells, part IV: Effects of
channel surface wettability, geometry and orientation on the
two-phase flow in parallel gas channels
Zijie Lu, Cody Rath, Guangsheng Zhang, Satish G. Kandlikar *
Department of Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Dr., Rochester, NY 14623-5604, USA
article info
abstract
Article history:
In this study, the effects of channel surface wettability, cross-sectional geometry and
Received 15 February 2011
orientation on the two-phase flow in parallel gas channels of proton exchange membrane
Received in revised form
fuel cells (PEMFCs) are investigated. Ex situ experiments were conducted in flow channels
28 April 2011
with three different surface wettability (hydrophilically coated, uncoated, and hydro-
Accepted 30 April 2011
phobically coated), three cross-sectional geometries (rectangular, sinusoidal and trape-
Available online 12 June 2011
zoidal), and two orientations (vertical and horizontal). Flow pattern map, individual
channel flow variation due to maldistribution, pressure drop and flow visualization images
Keywords:
were used to analyze the two-phase flow characteristics. It is found that hydrophilically
PEMFC
coated gas channels are advantageous over uncoated or slightly hydrophobic channels
Two-phase flow
regarding uniform water and gas flow distribution and favoring film flow, the most
Channel
desirable two-phase flow pattern in PEMFC gas channels. Sinusoidal channels favor film
Surface wettability
flow and have lower pressure drop than rectangular and trapezoidal channels, while the
Geometry
rectangular and trapezoidal channels behave similarly to each other. Vertical channel
Orientation
orientation is advantageous over horizontal orientation because the latter is more prone to
slug flow, nonuniform liquid water distribution and instable operation.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
Water management is one of the key challenges in the
commercialization of proton exchange membrane fuel cells
(PEMFCs) due to its association with the performance, cost and
durability issues [1e5]. The liquid water comes from two main
sources: water produced by the oxygen reduction reaction at
the cathode, and water condensing from humidified gas feeds
as reactants are consumed and the vapor pressure exceeds
saturation pressure. Liquid water accumulation in the gas flow
channels makes two-phase flow almost unavoidable for PEMFC
operation, especially at low temperature and high current
density, which has become an important concern for PEMFC
design and operation [5e8]. Researchers have investigated this
problem both experimentally and numerically [8e12]. However,
a perfect flow channel design which ensures the robust fuel cell
operation is still not available and better understanding of twophase flow in the gas channels is needed [8].
For a large (elongated) droplet or water slug in a gas flow
channel, its movement is controlled by many forces, including
gravity (FG), surface tension (Fg), and shearing force (FD) from
the gas flow (as shown in Fig. 1). The droplet (or slug) moves
only when gravity or shear forces overcome the surface
tension [13,14].
The effect of the gravity force depends on the size of the
droplet (or slug), and can be described by the Bond number,
* Corresponding author. Tel.: þ1 585 475 6728; fax: þ1 585 475 7710.
E-mail address: [email protected] (S.G. Kandlikar).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.04.226
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Fig. 1 e The schematic of the forces on an elongated droplet
or a slug. FD represents drag force applied by gas stream, FG
for gravitational force, and Fg for surface adhesion force.
Bo ¼ ðrw rair ÞgD2 =g, which is the ratio of gravity force to
surface tension. For a spherical droplet, its size in a PEMFC gas
channel is usually small due to the constraint of the small
channel dimensions (typically smaller than 1 mm), and has
a very small Bo value (w0.1) [15,16], implying that the effect of
gravity on two-phase flow is insignificant and surface tension
plays a more important role. In fact, many numerical works in
GDL and some in gas channel have neglected the gravity effect.
However, for an elongated droplet or slug, the length (>1 cm) of
liquid water can be significantly larger than the channel
dimension, resulting in a large Bo (>1). Therefore, the effect of
the gravity may not be ignored. Kimball et al. [13,14] investigated the effect of gravity by operating the PEMFC in horizontal
and vertical orientations, and found that the vertical orientation with gas inlets on top resulted in most effective liquid
water removal and most stable operation of PEMFC. The
neutron radiography study of Owejan et al. [17] also suggested
that the gravity could influence the morphology of liquid water
distribution in gas channels, with more water residing on the
lower edge of horizontal air channels. These studies indicate
that the assumption of negligible effects of gravity in PEMFC
channel is not proper for all the cases.
The adhesive force by surface tension is a strong function
of the surface wettability, which is usually characterized by
the contact angle (q) as well as the contact angle hysteresis
(i.e., the difference between the advancing and receding
contact angles, qAeqR) [16]. Water spreads in a hydrophilic
channel (q < 90 ), while beads up in a hydrophobic channel
(q > 90 ). When the channel surface is hydrophilic enough to
satisfy the following Concus-Finn condition [18]:
q þ a < p=2
(1)
with a denoting the half-angle of the channel cross-section
corner, the liquid water can wick into the channel corner
and is transported via film flow along the channel corner
(capillary effect). On the contrary, if the flow channel surface
is hydrophobic, the liquid water cannot wick into corner and is
forced to form large droplet or slug in channels. The hydrophobic surface tend to expel water away the surface (possibly
due to smaller contact angle hysteresis), enhancing water
removal by gas shearing.
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Owejan et al. [17] measured the liquid water retention in
PEMFC channels with or without hydrophobic PTFE coating,
and found that channel-level water accumulation was
reduced by PTFE coating. Cai et al. [19] numerically studied the
effects of channel surface wettability on the water behavior
and found that hydrophilic channel was more advantageous
than hydrophobic channel for water discharge and gas diffusion. Quan and Lai [20] also numerically studied the effects of
surface wettability on the two-phase flow behavior and pressure drop in gas flow channels and found that hydrophilic
channel surface facilitates the water transport along channel
surface or edges. These studies clearly show that there still is
some debate on choosing one over another between hydrophilic and hydrophobic channels. Hydrophilic channel can
improve capillary driven water flow, while hydrophobic
channel may enhance water removal by gas shearing.
Furthermore, both channels have drawbacks. Hydrophilic
channel may increase water retention in channels due to the
liquid water spreading, while hydrophobic channels may
prevent water droplets on GDL being wicked to the channel
surface.
Shearing force exerted on the liquid water by gas flow is
proportional to the projected area of the droplet normal to the
flow direction and it increases nonlinearly with the increase of
gas velocity [15]. Therefore, the two-phase flow characteristics
will change accordingly with the increase of gas velocity. As
reported by Lu et al. [21], who investigated the two-phase flow
patterns with ex situ experiments, slug flow is dominant twophase flow pattern at low superficial air velocity, film flow is
dominant at higher air velocity, and mist flow is obtained only
at extremely high air velocity. It was also reported that slug
flow causes severe flow maldistribution and large fluctuation
in pressure drop, mist flow is efficient in water removal but
requires too high air flow rate and thus parasitic pumping
power, and film flow is the most preferred means of liquid
water removal in PEMFC because of the relatively higher water
removal capacity than slug flow while lower pressure drop
than mist flow.
Cross-sectional geometry of flow channel was also found to
be influential to liquid water behavior and two-phase flow in
PEMFC gas channels. Owejan et al. [17] compared the liquid
water distribution in rectangular and triangular channels and
found that triangular channels retained less water and the
droplet sizes were also smaller than in rectangular channels.
Zhu et al. [22] numerically investigated the effects of channel
geometry on water droplet dynamics in several channels,
including triangle, trapezoid, rectangle, rectangle with a curved
bottom wall and upside-down trapezoid. Their results demonstrated that the channel geometry indeed affects the detachment of water droplets. Considering the mass production of
PEMFC flow fields, the rectangular cross-section is not a suitable
geometry despite being widely used for research purpose.
Instead, stamped metal plate and molded carbon composite
plate are more feasible. The cross-sections of these flow fields
can be represented by sinusoidal and trapezoidal geometry,
respectively. It is thus desirable to study the effects of channel
cross-sectional geometry on the two-phase flow from both
scientific and engineering standpoints.
Since gravity, surface tension and shearing force influence
significantly the two-phase flow in PEMFC gas channels, it is
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important to understand the effects of parameters related to
such forces, such as channel orientation, surface wettability,
channel geometry and gas velocity, etc. While the effects of
gas velocity have been studied extensively [15,21,23,24], the
studies on the effects of other parameters are still very
limited, especially experimental studies.
The objective of this paper is to experimentally investigate
the effects of surface wettability, channel cross-sectional
geometry and orientation (gravity) on the two-phase flow in
parallel PEMFC gas channels via ex situ experimental study to
extend fundamental understanding of the phenomena, which
can be helpful for optimization of PEMFC channel design and
operation. Two-phase flow experiments were conducted in
flow channels with three different surface wettability
(hydrophilically coated, uncoated, and hydrophobically
coated), three cross-sectional geometries (rectangular, sinusoidal and trapezoidal), and two orientations (vertical and
horizontal).
operating with the same flow field and the air stoichiometry
ratio was calculated based on the equivalent current density.
The pressure drop over the entire flow channel was
recorded with a pressure transducer (Honeywell Sensotec
FDW2AT) which has an accuracy of 0.25% or better in a range
of 0e35 kPa in all experiments. The instantaneous gas flow
rate in each channel was obtained through the measurement
of individual channel pressure drop in the entrance region.
The relation between the flow rate and the entrance pressure
drop in each channel was pre-calibrated. The theory of the
entrance region pressure drop method and calibration of the
ex situ setup can be found in Ref. [25]. The flow in the channels
was simultaneously visualized by a high-speed camera
(Fastcam 1024-PCI, Photron USA, Inc.) with a long-distance
microscopic lens. In all the experiments except the horizontal orientation study, the flow channels were placed
vertically with gas inlet on the top. All experiments were
conducted at ambient temperature and pressure.
2.2.
2.
Experimental
2.1.
Experiment conditions
The work presented here was conducted using the same ex
situ test setup and materials described earlier in another
paper [21]. Briefly, the test setup simulates the two-phase flow
in parallel PEMFC gas channels by injecting liquid water on
one side of a GDL while passing air through 8 parallel channels
on the other side of the GDL. The basic dimensions and
geometry of the flow channels (183 mm long, 0.7 mm wide
with land width of 0.5 mm, 0.4 mm deep, and with a 5
weaving angle) were taken from an actual fuel cell flow design
aimed at meeting Department of Energy targets for automotive fuel cells [5]. The air flow channels were machined into
a Lexan plate which was vapor polished allowing visualization of two-phase flow. During experiments, the test section
was maintained at a compression of 2068 kPa (300 psi), which
represents the compression in an operating fuel cell. Liquid
water was injected in precisely controlled flow rates, by using
a syringe pump (Model 11 Plus, Harvard Apparatus), to simulate the water generation in a real fuel cell. Dry air at predetermined air flow rates was supplied by a zero grade air
generator (HPZA-30000, Parker Hannifin Corp). A wide range of
water flow and air flow conditions was tested, as listed in
Table 1. The superficial air and water velocities were calculated assuming a single phase flow of air or water through the
entire gas channels. The equivalent current density was
estimated from the water flow rate assuming a fuel cell
Effect of channel surface wettability
In order to investigate the effect of channel surface energy on
the two-phase flow characteristics, specific coatings were
applied to the rectangular channels to alter the surface
wettability of the channel walls, which was characterized by
static advancing contact angle in this study. The basic channel
dimensions and geometry described in Section 2.1 were
machined into a Lexan plate which was then vapor polished.
The advancing contact angle of water on the vapor polished
channel surface was 85 , as measured using a VCA Optima
Surface Analysis System (AST Products, Inc.). This channel is
named uncoated and used as baseline surface wettability in
this work. A proprietary hydrophilic treatment with a contact
angle of 11 was provided by General Motors and studied as
hydrophilic channel surface. This treatment makes the
channel surface hydrophilic enough to meet the Concus-Finn
condition (Eq. (1)). A hydrophobic surface treatment was
applied in-house with 3M Novec Electronic Coating
EGC-1700. The vapor polished Lexan channel piece was dipped in the coating solution and slowly removed so that the
entire surface was coated. The coating was then dried in
ambient air for about 5 min. This process was repeated several
times to obtain a uniform surface coating. This fluorochemical
acrylate polymer coating provided a thin transparent film
with good anti-wetting property, producing an advancing
contact angle of 116 . The untreated channel and hydrophobic
channel apparently do not satisfy Concus-Finn condition. All
these surface treatments were transparent to allow clear
visualization of water in the channels.
Table 1 e Test matrix.
Water flow rate
(WFR, mL/min)
0.02
0.04
0.1
0.2
Superficial water
velocity UL (m/s)
1.5
3.0
7.5
1.5
104
104
104
103
Air flow rate
(AFR, sccm)
Superficial air
velocity UG (m/s)
Equivalent current
density (A/cm2)
Equivalent air
stoichiometric ratio
66e3962
132e3962
330e3302
660e3962
0.5e29.5
0.98e29.5
2.46e24.6
4.9e29.5
0.2
0.4
1
2
1e60
1e30
1e10
1e6
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2.3.
Effect of channel cross-sectional geometry
Three channel geometries, rectangular (ideal for laboratory
studies), sinusoidal (simulating stamped metal plate) and
trapezoidal (simulating molded carbon composite plate)
cross-sections, were studied. Fig. 2 shows the details of these
channel geometries. Rectangular cross-section was also the
base channel geometry design for the channel surface
wettability and orientation experiments. These channel
geometries are machined into the Lexan plates, which are
then vapor polished and have advancing contact angle of 85 .
The sinusoidal geometry is not exactly sinusoidal due to the
limitation of available tooling and machining capability of
Lexan.
To reduce the size effect as much as possible, the varying
geometries were designed to maintain a similar hydraulic
diameter (DH). Fig. 2 lists the hydraulic diameter and crosssectional area of the three channels. It can be seen that the
hydraulic diameter of rectangular, sinusoidal and trapezoidal
channels are 0.51 mm, 0.47 mm and 0.53 mm, respectively.
The sinusoidal channel has a slightly smaller cross-sectional
area (0.274 mm2), and the trapezoidal has a slightly larger
area (0.292 mm2) in comparison with the rectangular channel
(0.280 mm2).
2.4.
Effect of flow channel orientation
To study the effects of orientation (or gravity) on two-phase
flow in the channels, the test section was rotated counterclockwise 90 to have the flow channels in horizontal direction
and the air flow from left to right. With the change of channel
orientation, the synergy of gravity and shearing force from gas
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flow will be changed and different behaviors of gas-liquid twophase flow in the channels are expected. The hydrophilic
treated channel was used in this case because it was found
that hydrophilic treatment of the channels is more favorable
for liquid water removal than untreated channels, which is
described in details in the results and discussion section on
the effects of surface wettability.
3.
Results and discussion
The two-phase flow experiments were conducted at various
air and water superficial velocities and the liquid water
behavior in channels was recorded by the high-speed camera.
The characteristics of the two-phase flow, including the flow
pattern, pressure drop, and flow maldistribution, in each case
were studied and the effects of channel wettability, channel
geometry and orientation were determined. The detailed
results are reported in the following.
3.1.
Effects of channel surface wettability
In almost all experiments, a transition of flow pattern from air
inlet to outlet region was observed. Generally little water was
observed at the inlet region, no matter what superficial water
(UL) and superficial air velocity (UG) were used. This is attributable to the use of dry inlet air, which is able to carry water
vapor until the vapor pressure reaches saturation. It is thus
expected there exists a single phase to two-phase transition
line in the upstream region. Similar phenomena have been
reported in fuel cell operation. On the contrary, the flow
pattern in the outlet region is remarkably different and
Fig. 2 e Schematics of channel geometry (rectangular, sinusoidal and trapezoidal) cross-sections, dimensions (in mm) and
the respective hydraulic diameters (in mm).
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considerable variation was observed with UL and UG. Three
types of flow pattern, slug flow, film flow and mist flow [21], can
be readily identified in this region, depending on flow conditions. The abrupt change of flow structure along the channel
causes difficulty in the determination of flow pattern in parallel
gas channels. In order to better compare the effects of channel
design and surface treatment (which is the focus of this work),
the flow patterns in the outlet region were selected. Fig. 3
displays the two-phase flow pattern maps for the uncoated,
hydrophilically and hydrophobically coated channels.
Mist flow provides an efficient way of water removal. But it
only occurs at very high air flow rates, where no obvious liquid
water is observed in the gas channel. Water can be carried
away by air flow via water vapor. The maximum water that
can be removed as water vapor by dry air stream can be
calculated by the following equation (see Appendix):
PV_ air Psat ðTÞMw
V_ w ¼
RTr½P Psat ðTÞ
(2)
where V_ w and V_ air are volumetric flow rate of water and air,
respectively; T and P are temperature and pressure in channel,
Psat is saturation water vapor pressure, Mw is water molecular
weight, r is density of water, and R is universal gas constant.
The critical vapor transport boundary is calculated with
Eq. (2) (T ¼ 293 K) and also plotted in Fig. 3 as a solid line. It can
be seen that the mist flow boundary calculated from Eq. (2) is
close to that obtained from experimental observations in the
uncoated channel (Fig. 3(a)). This implies that water is
transported via water vapor at very high air velocity (or flow
rate). Similar results are also observed in the hydrophobically
coated channels. This is because these two channels have
close surface wettability. However, no mist flow pattern was
experimentally observed in the hydrophilically coated
channel, despite the calculations that show it should exist at
high air and low water superficial velocities. The reasons for
this are still not clearly understood.
At lower air superficial velocity, the air stream is not
enough to evaporate all the liquid water and two-phase flow
patterns develop in channels. Two major two-phase flow
patterns, slug and film, are mostly observed in fuel cell gas
channels, unlike the variety of flow patterns often found in cocurrent two-phase flow in mini and microchannels [26]. This
difference may be attributable to several important distinctions of PEMFC channels: 1) continuous addition/removal of
liquid and gas in PEMFC channel along its length; 2) gas cross
over between adjacent channels through the porous GDL; and
3) heterogeneous channel surface properties.
A direct consequence of liquid water holding in channel is
the flow maldistribution. Fig. 4 shows the flow distribution in
the hydrophilically coated channel at condition of UL ¼
7.5 104 m/s (WFR of 0.1 mL/min, equivalent to current density
of 1 A/cm2) and UG ¼ 2.46 m/s (AFR of 330 sccm), as a typical
example. For clarity, only the channels undergoing a significant
flow variation are plotted in this figure. The abrupt changes in
channel flow rates are obviously seen from this figure, some
channels increasing and some channels decreasing but the
Fig. 3 e Flow pattern map for (a) uncoated, (b) hydrophilic, and (c) hydrophobic channel. The dot lines, which indicate the
boundary between different flow patterns, are just guide to eye. The solid line in (a) and (c) is the vapor boundary calculated
from Eq. (2).
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Fig. 4 e Flow distribution in the hydrophilic channel at
condition of UL [ 7.5 3 10L4 m/s (WFR of 0.1 mL/min) and
UG [ 2.46 m/s (AFR of 330 sccm). For clarity only those
channels with great flow fluctuation are plotted. In other
channels the flow rates almost maintain constant during
the measurement period.
total flow rate maintaining constant. In our previous study [28],
we have found that the flow distribution even in the single gas
phase (air in this work) is not uniform due to the uneven
intrusion of GDL into the channels and probably to the flow field
design as well. However, in the two-phase study in this work,
the flow redistribution induced by the water accumulation in
channel is of most interests. The degree of the flow redistribution in one channel due to presence of water can be determined
by the induced flow fluctuation, which is defined as the ratio of
the largest flow variation (¼ maximum flow rate e minimum
flow rate) to the average flow rate. The mean flow fluctuation of
all eight channels is then calculated and plotted as a function of
superficial air velocity, as shown in Fig. 5. As expected, the flow
fluctuation decreases as superficial air velocity increases. This
is in agreement with the transition of flow pattern from slug to
film. Slug flow pattern induces significant flow redistribution, as
high as 190% for the uncoated channel (Fig. 5(a)), due to its entire
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or partial blockage of a channel. Film flow normally induces less
flow redistribution.
Another outstanding result from Fig. 5 is that the hydrophilically coated channel induces less flow redistribution
compared to the uncoated channel. This effect is more
profound at lower air velocities. This difference may be
explained by the difference in the flow structure in these
channels. Fig. 6 shows the images of the two-phase flow at UL
of 7.5 104 m/s and UG of 5.0 m/s for different channel
wettability. It can be seen that water is present in the hydrophilically coated channel as very small droplets on the GDL
surface or as thin film along the side wall, while the water
builds up as elongated water droplets or water slugs in the
uncoated and hydrophobic channels. Water is also more
uniformly distributed in the hydrophilic channels. These
results (Figs. 4, 5 and 6) demonstrate that even the hydrophilically coated channels can induce certain water accumulation and flow fluctuation, but it is the preferable channel
surface treatment in terms of flow distribution compared to
uncoated and hydrophobically coated channels.
Pressure drop is another important two-phase parameter.
In order to compare the effect of channel surface wettability,
the mean pressure drop at each superficial water velocity is
normalized to the respective dry air pressure drop and
a pressure drop factor, F2g , is commonly applied in two-phase
studies:
F2g ¼
DP2B
DPg
(3)
where DP2B and DPg are the pressure drop with two-phase flow
and with only single phase gas flow in the channels, respectively. This pressure drop factor has shown to be able to reflect
the overall water holding over the entire flow channels [21,27].
Fig. 7 compares the normalized pressure drop for different
channel surface energies at UL ¼ 3.0 104 m/s (0.04 mL/min,
equivalent to a current density of 0.4 A/cm2) and
UL ¼ 7.5 104 m/s (0.1 mL/min, equivalent to a current
density of 1 A/cm2). F2g decreases abruptly as UG increases, due
to the transition of flow pattern from slug to film and mist, as
expected. For mist flow, F2g ¼ 1 is obtained.
As shown in Fig. 7, hydrophilic channels have a lower F2g
than uncoated channels at lower UG with lower water flow rate
Fig. 5 e Comparison of flow fluctuations as a function of superficial air velocity in channels with different surface wettability
at (a) UL [ 3.0 3 10L4 m/s and (b) UL [ 7.5 3 10L4 m/s.
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Fig. 6 e Images of film flow in (a) hydrophilic, (b) uncoated, and (c) hydrophobic channels at UL [ 7.5 3 10L4 m/s and
UG [ 5.0 m/s.
(UL ¼ 3.0 104 m/s). This is in agreement with the lower degree
of flow redistribution (Fig. 5) and can be explained by the more
uniform distribution of water over the entire channel. Another
interesting result from Fig. 7 is that, at higher water flow
(UL ¼ 7.5 104 m/s), the hydrophilic channel shows higher F2g
at intermediate air velocity than the uncoated channel. This
flow range is corresponding to water film flow. For the
hydrophilically coated channel, the surface contact angle is
small enough so that the Concus-Finn condition (Eq. (1)) is met
and water is wicked into the corner to form a continuous film
along the top corner. Water in this channel is removed through
the continuous film flow more than driven by the air flow. At
the same time, due to the high surface tension, water is prone
to adhere to channel, rather than being removed by air
Fig. 7 e Comparison of pressure drop factor as a function of superficial air velocity in flow channels with different surface
wettability at (a) UL [ 3.0 3 10L4 m/and (b) UL [ 7.5 3 10L4 m/s.
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Fig. 8 e Flow pattern map for (a) sinusoidal and (b) trapezoidal channels.
shearing. Therefore, water holding can be higher in hydrophilic
channel and it results in a slightly higher pressure drop. This
effect is larger at higher water flow rates.
Although the hydrophilic channel may hold slightly more
water in films, its advantages of more uniform water distribution in whole channels and the consequent less flow redistribution and lower pressure drop (especially at lower air
velocities, which is the normal fuel cell operation range) make
it a preferable channel surface treatment. From Fig. 3, the
hydrophilic channel shifts the slug pattern zone to a lower air
velocity, which is also desirable. The hydrophobically coated
channel in this work behaves similarly to the uncoated
channel, which is because of the close surface wettability
values. Although super hydrophobic treatment (with contact
angle greater than 165 ) is possible to remove water quickly and
to prevent the formation of large water slug [16], its application
to PEMFC channel has not been reported in literature.
3.2.
Effects of channel cross-sectional geometry
Fig. 8 shows the two-phase flow pattern maps for flow channels with sinusoidal and trapezoidal channel cross-section
geometry, respectively, which can be compared with that of
the rectangular channels, shown in Fig. 3(a). It can be seen
that the flow pattern maps for the trapezoidal and rectangular
channel geometries are similar to each other. This is expected
given the close channel shape and dimensions of these two
geometries. In comparison, the flow pattern map for the
sinusoidal channel shows some difference. It has a slightly
larger film flow zone compared the other two. This may be
explained by the easier tendency for the formation of water
films in the sinusoidal geometry. This point will be further
discussed in a later section.
Fig. 9 shows the pressure drops in the three channel
geometries. Similar to the analysis in Section 3.1, the pressure
drops are normalized with respect to the respective dry
pressure drops in order to reveal the effect of water holding in
the channels. Fig. 9(a) and (b) display the pressure drop factor
at two water flow conditions, UL ¼ 3.0 104 m/s (0.04 mL/
min) and UL ¼ 1.5 103 m/s (0.2 mL/min), respectively. It is
found that the sinusoidal channel shows a lower pressure
drop factor than the rectangular and trapezoidal channels for
all water velocities studied in this experiment. On the other
hand, there is no clear trend between rectangular and trapezoidal channels. This is in agreement with the flow pattern
maps (Fig. 8).
Fig. 9 e Comparison of pressure drop factor in different channel geometries at (a) UL [ 3.0 3 10L4 m/s and (b)
UL [ 1.5 3 10L3 m/s.
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Fig. 10 e Images of slug flow in (a) rectangular, (b) sinusoidal and (c) trapezoidal channels at UL [ 3.0 3 10L4 m/s and
UG [ 3.0 m/s.
Fig. 10 shows the comparison of flow images in the three
channel geometries. Once again, no significant difference is
seen in the flow structure between the rectangular and trapezoidal channels. However, the flow structure of the sinusoidal channel is remarkably different. A number of small
slugs (circled area) or films are observed, rather than a few
long slugs as observed in rectangular and trapezoidal channels. In addition, water is more uniformly distributed among
all the channels. This would be beneficial to fuel cell operation
due to the uniform flow distribution and lower pressure drop
as shown in Fig. 9.
The sinusoidal channel shows noticeable distinction on the
flow pattern map (Fig. 8), pressure drop (Fig. 9) and flow structure (Fig. 10). This can be attributed to its special channel
geometry. According to Fig. 2, the sinusoidal channel has
continuous circular profile, unlike rectangular or trapezoidal
channel with corners, which makes the liquid water spread
more easily over the entire channel surface and transport along
the channel as film flow, reducing water holdup in channel and
Fig. 11 e Flow pattern map for hydrophilic channels
operating at horizontal orientation.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 9 8 6 4 e9 8 7 5
9873
Fig. 12 e Images of two-phase flow in (a) horizontal and (b) vertical hydrophilic channels at UL [ 3.0 3 10L4 m/s, UG [ 2.0 m/s.
decreasing pressure drop as well as flow maldistribution.
Because the channel surface is slightly hydrophilic, liquid water
is likely to reach the back wall in the sinusoidal channel along
the circular profile, which was observed by the high-speed
video. In comparison, in rectangular and trapezoidal channels, water can only build-up along channel side wall, forming
large slugs and severe flow maldistribution. In addition to the
continuous circular profile of the sinusoidal channel, the much
smaller angle at the corner between GDL and the channel side
wall (40 as designed, even smaller considering GDL intrusion in
to the channel [28]) than that in rectangular channel (90 ) and
trapezoidal channel (80 ) could also contribute to the special
characteristics of two-phase flow in sinusoidal channel [29].
3.3.
Effects of channel orientation
According to Fig. 1, as the orientation of the flow channels is
changed from vertical to horizontal, the synergy between the
air shearing force and the gravity on liquid water movement
in the flow channels is changed. Therefore, the two-phase
flow characteristics would also change accordingly.
Fig. 11 shows the two-phase flow pattern map for the
hydrophilic channels with horizontal orientation. In comparison with the flow pattern map for the same channel with
vertical orientation, shown in Fig. 3(b), it can be seen that the
slug/film flow boundary is shifted to a higher air velocity as the
orientation changes from vertical to horizontal. This implies
that water is more easily built up in horizontal channels than in
vertical channels and thus slug is more readily formed. This
can be attributed to the synergy effect of the shearing force and
the gravity. In the vertical orientation, the direction of gravity is
the same as the gas flow direction (as shown in Fig. 1), accelerating the movement of liquid water. However, in the horizontal case the direction of gravity is perpendicular to the flow
direction. The gravity would keep the water as slugs (also as
droplets and films) attached to the downside channel wall,
increasing the resistance for water to be moved by the air flow.
Therefore, water slugs could be formed at a relatively higher
gas velocity in horizontal channel than in vertical channel.
This is in agreement with the flow pattern map (Fig. 11).
The effect of orientation (or gravity) can be further seen
from Fig. 12, which shows the images of two-phase flow in
horizontal and vertical channels at the same superficial water
and gas velocity. It can be seen that very long water slug,
blocking the whole channel, can be formed in horizontal
channels, which could result in severe flow maldistribution
and high pressure drop, while in the vertical channels, water
flow as very thin film along the channel side wall with small
water droplets on the GDL surface. The different two-phase
flow patterns in horizontal channel and vertical channel
agree well with the flow pattern map (Fig. 11) and suggest that
horizontal operation is more prone to slug flow formation in
the parallel gas channels. It can be also seen from Fig. 12(a)
that film flow exists along the lower side channel wall, which
is also the evidence of the effect of gravity.
According to this study, positioning the parallel PEMFC gas
channels vertically would result in less water build-up and
flow maldistribution in channels. This is in agreement with
the findings by Kimball et al. [13,14] that the fuel cell performance in a vertical orientation is more stable than that in
horizontal operation.
4.
Conclusion
The effects of channel surface wettability, channel crosssectional geometry and channel orientation on the twophase flow in parallel gas channels were investigated in an
ex situ setup in this study. The flow pattern map, channel flow
distribution, pressure drop and flow visualization were
analyzed to obtain the flow characteristics in each case.
Hydrophilically coated gas channels show more uniform
water distribution, less degree of flow maldistribution, and an
increased tendency to film flow than uncoated and slightly
hydrophobic channels. The result suggests that the PEMFC
with hydrophilic channels surface would have higher and
more stable performance than non-treated channels.
Sinusoidal channel geometry is more likely to form film
flow compared to rectangular and trapezoidal geometries.
Lower pressure drop is also found in the sinusoidal channel.
The rectangular and trapezoidal channels behave similarly to
each other. The special two-phase characteristics in sinusoidal channel can be attributed to its continuous channel
profile and a small angle with the GDL.
9874
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 9 8 6 4 e9 8 7 5
Vertical channel orientation with downflow of reactants is
found to be more advantageous over horizontal channel
orientation regarding the liquid water distribution, film flow
formation and stability of two-phase flow, which can be
attributed to the synergistic effects of gravity and shearing
force on liquid water removal in the gas channels.
The results obtained in this work can be closely tied to the
optimization of parallel gas channel design and in situ fuel cell
performance, and help improve understanding of water
management in PEMFC gas channels.
Converting molar flow rate to volumetric flow rate, the
maximum liquid water flow rate that can be removed by the air
stream (i.e., critical mist flow condition) can be obtained
PV_ air Psat ðTÞMw
V_ w ¼
RTr½P Psat ðTÞ
(A6)
where, V_ w and V_ air are volumetric flow rate of water and air,
respectively; Mw is water molecular weight, r is density of
water, and R is universal gas constant.
Acknowledgments
This work is conducted in the Thermal Analysis, Microfluidics
and Fuel Cell Laboratory in the Mechanical Engineering
Department at Rochester Institute of Technology and is supported by the US Department of Energy under contract No.
DE-FG36-07G017018 and No. DE-EE0000470. The support
provided by Jon Owejan and Thomas Trabold of General
Motors is sincerely acknowledged.
Appendix: A
The maximum water carrying capability by air stream. Water
vapor that can be carried by air flow is determined by the
relative humidity of the air stream, which is defined as
RH ¼
n_ v
Pv ðTÞ
yv P
P
¼
¼
Psat ðTÞ Psat ðTÞ n_ v þ n_ air Psat ðTÞ
(A1)
where Pv, Psat and P are water vapor partial pressure, saturation vapor pressure and total pressure, respectively; n_ v and n_ air
are mole flow rates of vapor and air. Thus, the water vapor
flow rate is determined by
n_ v ¼ n_ air
RH$Psat ðTÞ
P RH$Psat ðTÞ
(A2)
Generally in fuel cell application, the inlet air stream is
humidified to a certain RHin, and outlet RHout is different from
inlet due to addition of water in the channel (see Fig. A1). The
net water vapor carried by the air stream is decided by the
difference of RHin and RHout, derived as
n_ v;net ¼ n_ v;out n_ v;in ¼ n_ air
RHout $Psat ðTÞ
RHin $Psat ðTÞ
PRHout $Psat ðTÞ PRHin $Psat ðTÞ
(A3)
Here the consumption of oxygen is not considered, which is
in agreement with this experiment, while in real fuel cell
operation the oxygen consumption has to be taken into
account. Since dry air is used in this experiment, the above
equation is simplified as
n_ v;net ¼ n_ air
RHout $Psat ðTÞ
P RHout $Psat ðTÞ
(A4)
It is normally interesting to calculate the maximum water
vapor that can be taken away by an air flow. This implies that
RHout ¼ 1. Equation (A4) is further reduced to
n_ v;net ¼ n_ air
Psat ðTÞ
P Psat ðTÞ
Fig. A1 e Schematic of flow in a gas channel.
(A5)
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