Two-Phase Flow in Small Channels and the Implications for PEM Fuel Cell Operation

Abstract

A significant challenge in designing robust PEM fuel cells is managing the flux of water from the humidified reactant streams and the electrochemical reaction. The problem of water management is especially challenging under cold conditions because of the reduced water carrying capacity of the reactant streams as well as the potential for water freezing within the fuel cell assembly which can lead to degradation of the bulk materials and surface treatments of the various fuel cell components. Effective water management begins in the gas flow channels where the product water is removed from the cell. Because of the small length scales associated with the gas flow channels, capillary forces are important to the behavior of liquid surfaces in the channel. This implies that the cross- sectional geometry and surface chemistry of the gas flow channels are important to the behavior of liquid surfaces in the channel and, subsequently, to the behavior of two-phase flow. Recent research efforts have focused on investigating the role of capillarity on the behavior of two-phase flow in systems where capillary forces are important The results of this research are dramatic. The presence of corners in a small channel can result in a completely different pressure drop and flow morphology from that found in circular cross sections. In addition, a change in the channel wall wetting (hydrophilic or hydrophobic) can also result in unique pressure drops and flow morphologies.