Visualization and Predictive Modeling of Two-Phase Flow Regime Transition With Application Towards Water Management in the Gas-Flow Channels of PEM Fuel Cells Available to Purchase

Understanding the behavior of gas and water vapor flow through the microchannel gas flow passages of a proton-exchange membrane (PEM) fuel cells is critical to reliable fuel cell operation. Recent research efforts have illustrated the importance of capillarity on the behavior of two-phase flow (gas-liquid) in low Bond number systems; that is, systems where capillary forces are important relative to gravitational forces. Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. The key characteristic scaling factors for two-phase flow in capillaries have been determined. The choice of length scales and velocity scales in dimensionless groups used to characterize two-phase flow is critical to correctly delineating phase distribution. Traditional scaling for these types of flows have considered the interaction between gas and liquid phases to be primarily inertial in nature. The role of liquid film stability where the phase interaction is a combination of viscous and capillary effects is shown to be a more appropriate scaling for low-Bond number, low-Suratman number two-phase flows. Microscopic visualization at high frame rates has been used to identify the flow regime under various gas-liquid mass ratios, channel geometries and surface energies. The observations collected via high speed microscopy and corresponding pressure measurements are reported for square and circular cross-sectional microchannels with contact angles of 20 degrees (hydrophilic) and 70 degrees (hydrophobic). The effect of geometry and contact angle on the phase distribution and the pressure drop are dramatic.