VISUALIZATION AND PREDICTIVE MODELING OF TWO-PHASE FLOW REGIME TRANSITION WITH APPLICATION TOWARDS WATER MANAGEMENT IN THE GAS-FLOW CHANNELS OF PEM FUEL CELLS

摘要

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 in-ertial 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.