A Radial Flow Characterization
Technique to Measure In Plane Permeability of Gas Diffusion Layers
Joe Feser, Ajay Prasad and Suresh
In-plane permeability of Gas Diffusion Layer (GDL) plays an important
role in convective flow of reactants and products in polymer
electrolyte fuel cells. A technique to measure in-plane
is developed which should assist with the ability to design fuel cells
with increased convective performance. Our characterization technique
uses radial permeability experiments to measure in-plane
permeability of gas diffusion layers. These experiments can use either
a wetting liquid or a gas of known viscosity as the host fluid and
reach identical conclusions. However, flow rates dependence on pressure
is different for gases and liquids and must be recognized
when large pressure differentials are present. Results on three
commonly used gas
diffusion layer materials show that the non-woven material SGL31BA and
the carbon fiber cloth material Avcarb 1071-HCB have in-plane
permeabilities substantially higher than those reported for other
materials throughout the literature.
A radial flow apparatus shown here was fabricated to test samples of
GDL for in-plane permeability at various levels of compressive strain.
The samples consisted of annuli of material 15 cm OD x 9 cm ID stacked
to a height of
approximately 1 mm with each layer of material separated by thin layer
of brass shim. Shim stock was used to control the total thickness of
the compressed stack and avoid nesting effects. The characterization
method was developed to work with a liquid or gas flow through the GDL.
For gas permeability experiments, compressed air (0-550 kPa) was forced
through the sample, passed from the outlet to a variable area rotameter
for flowrate measurements. In liquid permeability experiments, a
pressurized tank (0-200 kPa) forced water though the sample and was
collected in a graduated cylinder at the outlet. In the case of gas
permeability, pressure was measured using gauges at both the inlet and
the outlet as rotometers were found to account for a significant
pressure loss. For the case of liquid permeability, pressure was
measured by a gauge on the inlet only and assumed to be atmospheric
pressure at the outlet.
Permeability was measured on three types of gas diffusion layers: woven
carbon fiber (cloth), non-woven carbon fibers, and carbon fiber paper.
Measurements were taken at multiple levels of compression thought to
correspond with typical levels in a fuel cell. The woven carbon fiber
sample was Avcarb 1071-HCB (Ballard). The non-woven carbon fiber and
paper based samples were SGL31BA (SGL Carbon) and TGP-60-H (Toray).
Values of In-Plane Permeability Values for Various Compression Ratios.