A comparison of numerical methods on the two-phase flow in PEMFCs

Abstract

Numerical simulation of two-phase flow is important for the performance evaluation of proton exchange membrane fuel cells (PEMFCs). Exchange of water between vapor and liquid phases in the gas diffusion layer (GDL) was considered by Nguyen and co-workers using the evaporation and condensation rate constants. Such an approach has been widely employed by many followers on the two-phase modeling of PEMFCs. However, the effect of water exchange rate parameters on the modeling results has never been evaluated. Another two-phase flow approach was proposed by Hsuen and Yin, in which a pseudo-phase-equilibrium function is used to approximate the vapor/liquid phase equilibrium. This method gives reasonable species profiles in the GDL and the expected polarization curves at various degrees of inlet humidification. The present study compares the numerical results of these two approaches in detail. Both 1-D and 2-D results reveal that these two methods provide accurate predictions of the overall cathode performance as long as the rate constants for water evaporation/condensation in the Nguyen's method are large enough. It was shown that profiles of liquid water saturation, water evaporation rate, water vapor flux and liquid water flux evaluated by these two methods exhibit quite different characteristics particularly in the region nearby the water front. However, the effects of such variations are averaged out within the GDL such that the polarization curves calculated using these two methods are almost identical. Although 2-D numerical results cannot be validated by any analytical means, a simplified 1-D exact solution is feasible to justify the accuracy of the proposed numerical approaches.