Force Scaling Comparison of Transport Phenomena in Proton Exchange Membrane Fuel Cell Flow Channels

Liquid-gas two-phase flow in flow channels of proton exchange membrane (PEM) fuel cells has been investigated extensively in the literature; however, a comparison between the order of the magnitude of the forces occurring within the flow channels has not been documented. A comparison is relevant due to increased interest in practical active and passive water management strategies. The present study compares the magnitude of the forces experienced by liquid water residing in the flow channels. An analytical model of a 20-cm-long flow channel was analyzed, and key forces were compared in the stream-wise coordinate. Results clearly reinforce the dominance of the surface tension forces over other forces applied in the channel while also demonstrating how they change with key variables. For a cathode stoichiometric ratio of 1, the surface tension effects were calculated to be three orders of magnitude greater than the gravitational effects, the second largest force scale, for a droplet diameter of 0.1 mm. For larger droplets, this difference becomes smaller but the surface tension effects remain dominant. The results are useful for flow-field designers where water removal using complex geometry and hydrophobic coatings are being explored.