TWO-PHASE FLOW AND MASS TRANSFER WITHIN THE DIFFUSION LAYER OF A POLYMER ELECTROLYTE MEMBRANE FUEL CELL

摘要

The membrane of a polymer electrolyte membrane fuel cell must be hydrated with liquid water at all times in order to function effectively. At high current densities, liquid water in the pores of the diffusion layer inhibits oxygen transport to the cathode. The present paper shows the results of an analysis of two-phase flow and mass transfer in the diffusion layer of a fuel cell. A computational fluid dynamics code is adapted to perform calculations assuming Darcy's law applies, with the rate of oxygen diffusion governed by Fick's law. Both relative permeability and capillary pressure are strongly dependent on saturation. A modified version of the interphase slip algorithm is used to perform flow-field calculations. The two phases are each assigned a different pressure. Phase continuity is solved for liquid-phase saturation, from whence capillary pressure, relative permeability, and oxygen exchange coefficients are obtained. Results of numerical calculations are compared to an analytical solution with excellent agreement. Detailed calculations for a typical present-day fuel cells are presented. The results are correlated in terms of gas mass transfer driving force as a function of blowing parameter.