Sponsors: Honda, NSF
Collaborators: John Eaton Group (Stanford ME), Honda R&D
Proton exchange membrane (PEM) fuel cells show promise as CO2-free energy-conversion devices. Reducing the size of the gas delivery channels could improve the efficiency and power density of PEM fuel cells , but the formation of liquid-phase water can impair the flow of reactant gases to catalyst sites and dramatically reduce cathodic reaction rates. Channels with small dimensions exhibit an increased propensity toward flooding as surface tension forces become significant when compared with viscous, inertial, and pressure forces.
We have studied the two phase flow regimes in fuel cell delivery channels using a variety of microscopy and MEMS-based measurement tools, as well as modeling and simulations. We investigate fundamental water-gas interactions in silicon channels using a high-contrast fluorescent imaging technique. Flow regime mapping corroborates previously published boundaries between intermittent and separated flow regimes and extends the inquiry to include the effect of side-wall injection conditions which lead to the formation of stratified flow in small square channels and high aspect ratio channels. These stratified flows predominate in high aspect ratio channels and the formation of intermittent plug/slug flow regimes is suppressed. An analytical, stratified flow model is validated using film thickness measurements, and then used to explore the effects of channel aspect ratio and film configuration on parameters such as the pressure losses and liquid coverage of a channel wall. We have also studied the evolution of two-phase flow structures in a microchannel geometry that mimics conditions in a fuel cell channel. Intermittent flow regimes transition to stable films as more water is introduced into the channel downstream. We develop simple relationships relevant for the optimization of channel design for the evacuation of water.