Proceedings of the Sixth International ASME Conference on Nanochannels, Microchannels and Minichannels
ICNMM2008
June 23-25, 2008, Darmstadt, Germany
DRAFT PAPER
ICNMM2008- 62200
INVESTIGATION OF WATER TRANSPORT IN AN EX-SITU EXPERIMENTAL FACILITY
MODELLED ON AN ACTUAL DOE AUTOMOTIVE TARGET COMPLIENT FUEL CELL
Z. Lu ([email protected]), A. D. White ([email protected]), J. Pelaez ([email protected]), M. Hardbarger ([email protected]), W.
Domigan ([email protected]), J. Sergi ([email protected]), S. G. Kandlikar ([email protected])
Mechanical Engineering Department, Rochester Institute of Technology, Rochester, NY
ABSTRACT
This work utilizes the channel design of a real fuel cell to
study cathode side water transport in the gas channels of a
proton exchange membrane fuel cell (PEMFC).
All
experimentation was performed under controlled water and air
flow conditions aimed to meet the DOE targets [1] for the
automotive fuel cells. The experimental facility provides
independent control for water flow along the length of the
channels to reduce the effects of channel pressure drop on the
water flow. Details of channel design and instrumentation are
described, as well as some initial results.
INTRODUCTION
Water management has been identified as one of the most
critical issues with regards to the performance and longevity of
a proton exchange membrane (PEM) fuel cell [2,3]. Sufficient
water, often controlled by externally humidified air and
hydrogen gas streams, must be present within the fuel cell to
maintain the proton conductivity of the polymer electrolyte
membrane; however, excess water must be removed from the
cell to avoid flooding. Flooding is a phenomenon in which
liquid water accumulation blocks gas transport pathways in the
catalyst layers, gas diffusion medium, and the gas channels,
inducing large mass-transport losses.
Water transport characteristics of gas diffusion media
(GDM) and gas channels greatly affect the prevalence of water
accumulation and flooding. Although water management,
particularly in terms of flooding, has previously been
investigated, there exists an urgent need for detailed
experimental studies of two-phase flow in gas channels and the
interactions between water transport and the GDM.
Experimental investigations of water transport in a PEMFC
are lagging behind numerical modeling in part because the
complexity of fuel cells limits the available experimental
techniques. Techniques that are available include neutron
imaging (or neutron radiography) and optical visualization.
Neutron imaging provides in-situ visualization of water
transport without disturbing fuel cell operations. This technique
has been utilized by a number of groups to visualize and
quantify water retention in the GDM, under the lands, and in
the gas channels [4-6], however, neutron radiography presents
several challanges that deter its wide application: (i) it is
difficult to differentiate between water on the anode and
cathode sides of the cell due to the two-dimensional nature of
this technique; (ii) the present temporal resolution of neutron
radiography (less than 30 Hz) is insufficient to resolve water
transport dynamics; (iii) the high cost and limited number of
neutron imaging facilities around the world make this
technology impractical in many cases. However, current
research efforts in this area are focused on addressing these
issues, and neutron radiography is expected to emerge as a
major diagnostic tool.
In contrast to neutron radiography, optical visualization not
only has high temporal and spatial resolution, but is a relative
low cost option making it widely practical to investigate water
transport within specially built transparent fuel cells. Tuber et
al. [7] was the first group to use this technique to study water
buildup in a cathode side gas channel at low temperature. Two
air channels with dimensions of 1.5 mm (width) by 1 mm
(depth) by 50 mm (length) were studied. Yang et al. [8,9] used
optical visualization to study water transport in gas channels
under automotive conditions, i.e., at high current density (about
0.8 A/cm2) and elevated temperature (70-80 oC). The flow field
was comprised of seven straight gas channels, each with
dimensions of 1 mm (width) by 1 mm (depth) by 100 mm
(length). They observed that, when utilizing saturated water
vapor in the gas phase, liquid water emerged at preferential
locations on the GDM surface in the form of droplets. After
emerging from the GDM surface, the water was pushed by the
air flow down the channels. Weng et al. [10] utilized optical
visualization to investigate the effects of gas concentration and
humidification at different stoichiometries on cathode channel
flooding behaviors. Ge et al. [11, 12] used optical visualization
to study liquid water and ice formation on the cathode catalyst
layer surface during startup at subzero temperatures. Most of
these investigations focused on the cathode side of the fuel cell;
although the anode side is also prone to flooding, it is rarely
studied [5, 6].
Previously, Borrelli et al. (2005) studied the fundamentals
of water droplet detachment from a GDM with an ex-situ test
setup in which liquid water was pushed through a GDM and
into a single gas channel [13]. The advancing and receding
contact angles, as well as departing droplet diameters, were
measured with respect to superficial gas velocity. A similar
investigation was conducted by Theodorakakos et al. [14] and a
numerical detachment model was built. Goodson et al. [15]
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Copyright © 2008 by ASME
generated flow and detachment regime maps using optical
imaging data obtained from a simulated microchannel.
Flooding may occur at low and high current densities on
both the anode and cathode sides of a fuel cell. Flooding may
cause performance fluctuations or, more seriously, stop the
operation of the cell all together. Because of the significant
effects of flooding in fuel cells, the task of finding effective
means of monitoring and quantifying flooding has become
increasingly urgent.
The pressure drop across the gas channels has been
proposed as a diagnostic tool for the detection of flooding [16,
17]. One method and apparatus for the detection of flooding via
the pressure drop across the flow field has been patented by
General Motors [18]. Pressure drop characteristics and the
visualization of water dynamics in both anode and cathode
parallel flow channels have been experimentally studied by Liu
et al. [19, 20]. Barbir et al. [21] designed a method to diagnose
both flooding and drying conditions inside a fuel cell by
combining pressure drop and cell resistance measurements.
Despite these studies, the relationship between pressure drop
and water accumulation, in particular the effect of random
droplet emergence events and liquid water clogging, is not
clearly understood.
The purpose of this work is to experimentally investigate
mass transport induced two-phase flow in a simulated PEM fuel
cell. Cathode flooding is investigated using air as the inlet gas
in a specially designed ex-situ test setup that is based on the
DOE targets for automotive fuel cells [1]. The ex-situ test
section is therefore specially created to remain as close to an
actual fuel cell design as possible while still allowing for the
above objectives to be met. Both the total pressure drop
between the inlet and outlet headers and optical visualization of
water transport in the channels are simultaneously collected.
The results obtained from these measurements help to better
understand the relationships between flooding, pressure drop,
and flow structure.
NOMENCLATURE
PEM
PEMFC
GDM
Va
Vw
Proton exchange membrane
Proton exchange membrane fuel cell
Gas diffusion media
Superficial Air Velocity
Superficial Water Velocity
TEST SET-UP DESIGN
Many designs used in the previously discussed works were
based on somewhat arbitrary channel geometry and dimension.
In this work, the fuel cell design was based on an actual fuel
cell flow field, as outlined by Trabold et al. [22]. Two
principles were applied to the design of the flow field geometry
and channel dimensions: optimal performance, and maximized
volumetric power density.
Eight parallel weaving channels with constant rectangular
cross sections were formed on a polycarbonate plate (Lexan)
to simulate the cathode side of the fuel cell. The gas channel
plate (i.e. the airside manifold) was vapor polished to ensure
excellent optical quality. The ex-situ experimental design
included channel widths of 0.7 mm and land widths of 0.5 mm
to mirror the optimal design of fuel cells under automotive type
conditions, as summarized by Trabold et al [22]. In order to
meet DOE requirements for volumetric power density, it is
optimal to minimize plate thickness, and thus channel depth, on
both the anode and cathode sides of the fuel cell. In this work,
the channel depth was chosen to be 0.4 mm, the minimum
depth practical when factors such as GDL intrusion and
machinability are taken into account. The total channel cross
section area is 2.24 mm2, as calculated by multiplying the
nominal (without intrusion considerations) cross-section of one
channel (0.7 mm x 0.4 mm) by the eight number of channels.
The channel length of 182 mm was back-calculated from the
2010 DOE technical targets for volumetric power density of 2
kW/L for an 80 kW system operating on direct hydrogen [1].
The channel dimensions and geometry utilized in this work
were specifically designed to accommodate the DOE
volumetric power density target. The flow field pattern, as seen
in Figure 1, was selected to avoid misalignment effects and
mechanical shear on the GDM associated with purely straight
channels.
A
B
Fig. 1: Section of overlapping channels from air manifold
(gray) and water manifold (black). Weaving channel
designed with a 5o weaving angle implemented to avoid
mechanical shear on the GDM associated with straight
channels of depth 0.4 mm. A: Channel width of 0.7 mm.
B: Land width of 0.5 mm. Not to scale.
Although the ex-situ experiment channel design mirrored
that of a fuel cell as closely as possible, it was necessary that
the inlet and outlet headers were modified to meet the ex-situ
experimental objectives. For example, pressure taps at the inlet
and outlet headers were included in the ex-situ design so the
total pressure drop across the gas channels could be measured.
A straight runner design was chosen due to the simplicity of its
construction and channel arrangement.
Opposite the airside channels were the four waterside
manifolds; the conceptual assembly of the two manifolds, the
GDM, and the GDM gasket are shown in Figure 2. The
waterside manifold delivered water to the GDM. After passing
through the GDM, the water appeared on the surface of the
GDM inside the gas channels. The channels on the waterside
manifold have the same geometry and dimensions as the gas
channels, i.e., weaving channels with a 5o weaving angle and
with channel width of 0.7 mm and depth of 0.4 mm. However,
the waterside channels were sectioned into four segments
corresponding to four water chambers. The four water
chambers were milled into the Lexan® plate. The water flow
rate in each chamber was controlled independently by four
individual syringe pumps, allowing each chamber to support a
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Copyright © 2008 by ASME
In order to provide uniform stress to the GDM, a spring
compression mechanism was utilized in the complete test
section assembly. Besides the gas and waterside manifold
assembly, the final assembly included a retainer plate on the
outside of the air plate, a spring force distribution plate on the
outside of the water plate, a retainer plate outside of the springs,
and two slotted, stainless-steel side plates to hold them all
together. The side plate slots were exaggerated in case the need
arose for a variety of compression forces and gasket
thicknesses. Springs with an appropriate stiffness for the
required
compression
were
chosen.
Teflon®
(polytetrafluoroethylene, PTFE) gaskets were used to seal the
water channels. The GDM gasket thickness was selected to
meet the required GDM compression and seal the assembly
under the compressive load. See Figure 3 for an exploded view
of the entire test section assembly. The entire test section, other
than the water and airside manifolds, was fabricated on site.
Assembly of the test section and all supporting systems was
also completed on site.
different water injection rate if desired. The use of four water
chambers provided an advantage over the use of a single
chamber along the entire channel length; where a single
chamber would promote water diffusion primarily towards the
outlet end of the channels (due to highest available pressure
difference between the air and water sides), the four chamber
design allows for water to emerge over a greater portion of the
GDM. To prevent an undesirable pressure drop along the length
of the gas channels, the water flow rates of the downstream
chambers were increased.
Three holes were drilled from each channel to each water
chamber, resulting in a total of 24 holes per water chamber
across the 8 channels, and 12 holes per channel across its entire
length. Each hole had a diameter of 0.7 mm, equal to the gas
channel width. Figure 2 displays the placement of these holes
relative to the water chambers. The waterside manifold, airside
manifold, and GDM contained alignment holes into which
small dowel pins were placed to ensure the components proper
placement.
Fig. 2: Exploded side-view of conceptual gas and waterside manifold assembly. Not to scale.
10
9
8
7
4
6
5
3
1
2
3
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Copyright © 2008 by ASME
Fig. 3: An exploded view of the test section assembly. The numbers in the figure represent: 1: stainless steel side plates; 2:
aluminum end plate; 3: posts to fit the inner diameter of the springs; 4: aluminum lower plate; 5: springs; 6: PTFE gasket; 7:
water chamber plate; 8: GDL; 9: parallel air channel plate; 10: aluminum block plate.
Fig. 4: Schematic of ex-situ experimental test set-up. Digital flow meter and illumination for camera not shown.
The ex-situ experimental test setup provided the means to
control and monitor air flow and water injection rates,
measure the total pressure drop across the channels, and
record images of two-phase flow structures in the channels. A
number of design elements were necessary to encompass each
of the above requirements; an overview of these elements is
shown in Figure 4.
All components except the air generator were mounted on
a vibration isolation table (Newport RS4000). Clean dry air,
provided by a zero-grade air generator (Parker HPZA-30000,
Haverhill, MA), was used in all the ex-situ experiments. The
air was delivered to the gas inlet manifold through an air
regulator and flow control system composed of a bank of
rotameters set up in parallel for medium to high air flow rates
and a digital flow meter for lower air flow rates. The digital
flow meter (Omega FMA-1620A), with an accuracy of ±3% in
the operating range of 0 – 1200 mL/min (corresponding to
superficial air velocity of 0-8.9 m/s in the gas channel), was
used to control the lower flow rates. The mid-range rotameter
(Omega FL-3804G) with an accuracy of ±2% was used for
flow rates in the range of 1200 to 2200 mL/min
(corresponding to air velocity range of 8.9 – 16.4 m/s). Flow
rates above 2200 mL/min (16.4 m/s) were controlled by the
largest rotameter (Omega FL-3805ST), which has an accuracy
of ±2% in the flow rate range up to 7600 mL/min . Deionized
water (18.2 MΩ, Direct-Q 3, Millipore) was supplied to the
test section water chambers. Water was delivered to the test
section at water injection rates between 0.02 mL/min to 0.2
mL/min (corresponding to 1.5x10-4 m/s to 1.5x10-3 superficial
water velocities) via syringe pumps (Havard Apparatus
702211) with an accuracy of ±0.5%. Each syringe pump had
the capability of delivering water into the chambers at
different independently controlled flow rates.
The total pressure drop across the flow field was
measured with a differential pressure sensor (Honeywell
Sensotec FDW2AT) with an accuracy of 0.25% or better in a
range of 0-34.5 kPa (0-5 psi). The individual water pressures
in each water chamber were also monitored. All data was
collected with a LabView program through a DAQ system
(National Instruments, Austin, TX).
A high-speed camera with an Infinity model K2/S™ longdistance microscope lens was used to capture water formations
and associated two-phase flow inside the gas channels.
Recorded videos had a resolution of 1024x1024 and a frame
rate range of 60-2,000 fps. For this work, the test section
remained stationary while the camera was positioned to
capture the desired area of the test section. The high speed
camera was mounted on screw-type elevating stages for
translation in the x, y and z directions. A dual light guide fiber
optic light was used to light the test section under observation.
All optical equipment was mounted on a vibration isolation
table.
EXPERIMENTAL
Two types of GDM samples were studied in this work: a
SGL-25BC GDM sample and a plain (non-PTFE treated)
Toray carbon paper sample. The SGL-25BC GDM is a PTFE
treated carbon paper with a microporous layer and have a
thickness of 235 µm at free standing. The PTFE coating
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Copyright © 2008 by ASME
provides a hydrophobic medium to the GDM, aiding in water
removal through the channels. The plain Toray paper sample
has a thickness of 190 µm (uncompressed) and exhibited
hydrophilic properties. The GDM, gasket materials, and a die
to cut each GDM sample and gasket were provided by General
Motors.
Each experiment was carried out at specific air flow and
water injection rate combinations at ambient temperature and
pressure. As a first ex-situ experiment, the GDM was placed
in a vertical down orientation. However, please note that the
orientation (or gravity) may play an important role in the
water transport in the channel, especially when the film flow
and slug flow are the dominant transport mechanisms, though
the effect of gravity on the formation and detachment of water
droplets on the GDM surface is insignificant [9]. The effect of
different orientation on the water transport is a subject of our
ongoing research. Table 1 displays air flow and water
injection rates correlated to current densities. The current
density is not actually produced during ex-situ
experimentation, rather it provides an idea of what a real fuel
cell would produce at the given water injection rates. Table 1
displays air flow rates associated with a stoichiometric ratio
(stoic number) of 1 for each water flow rate, however,
experiments at each water flow rate were also performed with
stoic numbers of 2, 3, 5, 8, 10, 15, 20, 25, 30, 35, 40, and 45.
The air surperficial velocities are varied accordingly.
Table 1. Air and water flow rates used in testing and corresponding operating current
density. Superficial air and water velocities are also shown which were derived using a total
cross-sectional area of 2.24 mm2. The Reynolds number of air is shown for each air velocity.
*Current density values found by back calculating what the associated water and air flow
rates would produce in a real fuel cell with same channel number and dimensions (with an
active area of 18.4 cm2).
Water
Injection Rate
(mL/min)
Air Flow Rate at
Stoic Number = 1
(mL/min)
Current
Density*
(A/cm2)
Superficial
Water
Velocity, Vw
(m/s)
Superficial
Air
Velocity, Va
(m/s)
Reynolds
Number
Rea
0.00
0.02
0.04
0.10
0.20
0
66
132
330
660
0.0
0.2
0.4
1.0
2.0
0.0
1.5×10-4
3.0×10-4
7.4×10-4
1.5×10-3
0.0
0.5
1.0
2.5
4.9
0.0
16.2
32.3
80.8
161.7
PRELIMINARY RESULTS
Visualization of Two-Phase Flow Structure
The two-phase flow structure (flow patterns) in the gas
channel with SGL-25 BC and plain Toray paper was studied
by using the high speed imaging technique. It was observed
both the hydrophobic properties of the SGL-25BC sample and
the hydrophilic properties of the plain Toray paper sample
aided in the removal of water from the gas channels, however,
the two-phase flow structures that appeared for sample
differed significantly.
Droplet Formation
At lower superficial air velocities, less than 15 m/s, water
was observed to emerge on the SGL-25BC GDM surface in
the form of droplets. The water droplets grew in diameter until
a critical size was attained, at which point the droplet would
either wick to a channel wall or be swept down the GDM by
the air flow. The critical droplet size was dependant on both
air flow and water injection rates. When the droplets detached
from the GDM surface and began to slide down the channel,
they would inevitably come in contact with the gas channel
wall due to the wavy channel design. Once contact had been
made, the droplets would wick to the channel wall and
continue down the channel by either slug flow, GDM film
flow, corner flow or sidewall droplet flow as described
subsequently. Figure 5 shows the growth of a droplet and its
interaction with the channel wall.
Droplet formation was not observed on the plain Toray
paper sample due to its hydrophilic nature. Water pushed
through the SGL-25BC sample would soon bead up on the
GDM surface because of the high surface tension of the
material. In contrast, the plain Toray paper’s high wettability
caused water pushed through its surface to quickly spread
instead of forming distinct droplets.
Slug Flow
Slug flow, characterized by complete channel obstruction
by a column of water, was commonly observed for water
transport in the gas channel at lower air flow rates. Two small
slugs of water can be seen in Figure 6. Slug flow restricts air
flow through the channel it occupies, thus increasing the
likelihood of channel flooding and decreasing the water
removal efficiency of the channel. Flooding, and therefore
slug flow, is detrimental to fuel cell performance since it
restricts the amount of oxygen which can pass through the
GDM to the cathode catalyst layer, especially if the slug
occurs towards the top of a gas channel.
Slug flow was observed in both the SGL-25BC and plain
Toray paper GDM samples. At the low superficial air
velocities at which slug flow was observed (less than 10 m/s),
the hydrophobic and hydrophilic properties of the two GDM
samples did not make a large difference in two-phase flow
patterns because the air velocity was too low to remove
droplets from the GDM surface.
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Copyright © 2008 by ASME
Fig. 7: GDM film flow covering GDM surface. Image
taken using plain Toray paper sample at Va = 19.7 m/s and
Vw = 3.0x10-4 m/s. The air velocity corresponds to
stoichiometric ratio of 20.
Fig. 5: Droplet formation and interaction with channel
side. SGL-25BC sample used at Va = 24.6 m/sec and Vw =
7.4x10-4 m/s. The air velocity corresponds to stoichiometric
ratio of 10.
Fig. 6: Slugs of water developing in the gas channels. SGL25BC sample used at Va = 2.0 m/s and Vw= 3.0x10-4 m/s.
The air velocity corresponds to stoichiometric ratio of 2.
GDM Film Flow
A water transport mode unique to the plain Toray paper
was GDM film flow. GDM film flow involved a film of water
that wholly covered the GDM surface without blocking the
channel, as happened in slug flow. Unlike slug flow, whose
prevalence was determined by air flow and water injection
rates, GDM properties such as wettability or hydrophobocity
dictate whether or not GDM film flow will occur. The high
hydrophilic nature of the plain Toray paper caused water to
spread over its surface instead of forming droplets as with the
SGL-25BC sample. After the Toray sample was saturated
with water, the GDM film flow steadily moved water down
the GDM surface. Figure 7 shows the channel ends during
GDM film flow. The clear areas directly at the land ends
indicate where water has built up to touch both the GDM and
the top of the airside manifold. The darker, slightly blurred
areas are where GDM film flow exists.
Although GDM film flow is highly effective at removing
water from the gas channels and results in a very low total
pressure drop, it would result in poor performance in a real
fuel cell. Since the water film completely covers the GDM
surface, it prevents the passage of oxygen to pass through the
GDM to the cathode catalyst layer and a drop in cell
performance is expected to occur.
Corner Flow
Upon wicking to the gas channel wall, a droplet may form
what is known as corner flow. Corner flow is characterized by
water collection as a film in a corner of the air channel
opposite the GDM, as seen in Figure 8. After forming the
corner film, capillary forces pulled the water down the
channel. Corner flow was observed during SGL-25BC
experiments at superficial air velocities less than about 15 m/s;
at superficial air velocities above 15 m/s water droplets would
not form, therefore corner flow could not form. Corner flow
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Copyright © 2008 by ASME
would not occur during plain Toray paper sample experiments
because water droplets could not form and wick to the channel
corners.
Trailing
Face
Sidewall Droplet Flow
Sidewall droplet flow consisted of a droplet that touched
both the channel walls and the GDM surface. During sidewall
droplet flow, the leading face of the droplet retained a round
shape, whereas the trailing face was dragged into a tail like
corner flow as it moved down the channel. Figure 9 shows an
example of sidewall droplet flow with the characteristic
leading and trailing faces. Like corner flow, sidewall droplet
flow occurs only on the SGL-25BC GDM sample at
superficial air flow velocities lower than 15 m/s.
Mist Flow
At flow rates above 15 m/s, water emerging through
either GDM surface was removed by what is referred to as
mist flow. Mist flow, seen in Figure 10, is comprised of very
small droplets moving at very high velocities. The size of the
droplets in mist flow and the speed at which they are moving
prevent them from being observed visually. Mist flow may be
the assumed method of water transport when a known water
flow rate is passing through the channels and water is
collected at the air and water outlet, but no water formations
are visible.
Fig. 8: Corner flow on airside manifold. Image taken
using SGL-25BC sample at Va = 12.3 m/s and
Vw = 7.4x10-4 m/s. The air velocity corresponds to
stoichiometric ratio of 5.
Leading
Face
Fig. 9: Sidewall droplet flow down channel. Image taken
using SGL-25BC sample, Va = 4.9 m/s and Vw = 7.4x10-4
m/s. The air velocity corresponds to stoichiometric ratio of
2.
Fig. 10: High air velocities often resulted in mist flow.
Image taken using SGL-25BC sample at Va = 19.7 m/s and
Vw = 3.0x10-4 m/s. The air velocity corresponds to
stoichiometric ratio of 20.
Measured Total Pressure Drop
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Copyright © 2008 by ASME
As seen in previous experiments from Ma et al [17] and
Lui et al. [19], total pressure drop is a function of the amount
of liquid water in the gas channels; the total pressure drop
across the channels increases with the amount of liquid water,
resulting in channel flooding and poorer cell performance.
Figure 11 shows the total differential pressure at a low and a
high air superfacial velocity for both the SGL-25BC and plain
Toray paper samples.
(a)
slugs. By comparing the total differential pressure data with
video at high and low air velocities, the conclusion was drawn
that the total pressure drop can be used to indicate when slug
flow has developed within the gas channels.
Water Transport Dynamics
High speed video of two-phase flow within the gas
channels was examined alongside pressure drop data to
develop deeper understanding of water transport dynamics in a
PEM fuel cell. Because they block the flow of oxygen
through the GDM, both slug flow and GDM film flow are
considered undesirable modes of water transport. Despite the
similar effects of slug flow and GDM film flow, the pressure
drop patterns associated with each mode are very different.
Slug flow creates a large pressure drop because it blocks the
gas channels completely, whereas the pressure drop generated
by GDM film flow is very low because it results in only a
small difference in the effective channel dimensions.
Boundary or transition regions exist between the different
modes of water transport, for example, around superficial air
velocities of 10 m/s, some water slugs may form in the
channels, but corner and sidewall droplet flow may also be
seen. Further experimentation at or around these transition
flow rates will further refine the conditions at which each
mode of water transport may occur. The flow patterns are
strongly dependent on the surface energies of the GDM and
channel surfaces. The present experimental setup is capable of
evaluating the two-phase performance of the gas channels
under different GDM and wall surface conditions.
CONCLUSION
(b)
Fig. 11: Total pressure drop at different superficial air
velocities for (a) SGL-25BC GDM sample and (b) plain
Toray paper GDM sample. Superficial water velocities
fixed at 7.4x10-4 m/s for both samples.
For both samples, the pressure drop maintains an almost
constant value at the higher superficial air velocity, indicating
that mist flow doesn’t significantly affect the pressure drop.
The fluctuations present in the pressure differential data are
likely due to water build up at the outlet header. The more
significant fluctuations at the lower superficial air velocities
are caused by the formation and flow of water columns or
An experimental facility is designed and fabricated to
provide a basis for not only predicting the two-phase flow
patterns in arbitrarily designed gas channels, but a basis for
predicting two-phase flow patterns within an actual PEMFC
that meets DOE targets. A number of unique design features
were implemented to make the ex-situ setup both more
effective and more flexible. The wavy flow channel design
reduces intrusion effects of the GDM into the gas channels,
while also reducing the shearing effects on the GDM
associated with straight channels. By using four water
chambers instead of one, a more even water distribution is
provided along the length of the GDM. The experimental
facility also provides a means to visualize flow patterns in
individual channels and correlate the visual data with the total
channel pressure drop. Preliminary testing was done and the
aforementioned two-phase flow patterns were identified.
The data collected from these experiments will be used to
better understand two-phase flow within a PEMFC and, more
importantly, be used to improve the over all performance of
working fuel cells. This ex-situ experimental facility is
designed with specific features to meet the DOE fuel cell
targets, but also with enough flexibility to enable more
exhaustive testing in the future.
ACKNOWLEDGMENTS
This work was supported by the US Department of
Energy under contract No. DE-FG36-07G017018. Technical
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Copyright © 2008 by ASME
13. Borrelli, J., Kandlikar, S.G., Trabold, T., and
Owejan, J., Water transport visualization and twophase pressure drop measurements in a simulated
PEMFC cathode minichannel, Proceedings of ICMM
2005, 3rd International Conference on Microchannels
and Minichannels, June 13-15, 2005, Toronto,
Canada.
14. Theodorakakos, A., Ous, T., Gavaises, M., Nouri,
J.M., Nilolopoulos, N., and Yanagihara, H.,
Dynamics of water droplets detached from porous
surfaces of relevance to PEM fuel cells, J. Colloid
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