Macroscopic Modeling of Liquid Water Based on Detailed Two-Phase CFD and Application on Full-Cell Scale Multiphysics Simulation
Authors: Tanaka Shoichi , Takayama Tsutomu , Motegi Haruki , Tsukamoto Takayuki , Takayama Ryo , Yoneda Masakazu 
Published: February 2020

Overview
In this work, full-cell scale simulations of PEFC are performed. The simulations deal with the behavior of liquid water in channels and porous structures in GDL and MPL, where the behavior is largely affected by geometry of channels and surface wettability. In order to execute efficient simulations in full-cell scale, all relevant macroscopic transport phenomena in PEFC (mass, chemical species and heat) are considered, where the phenomena are coupled with electrochemical reactions. Computational models for full-cell scale simulations are constructed based on the macroscopic transport models. In order to take into account liquid water flow depending on configurations of channels and porous structures, physical characteristics of liquid flow in channels are extracted from detailed two-phase CFD with resolved single channel with the fine computational mesh whose resolution is order of 10 micrometers. With these simulation techniques, we can estimate effects of liquid water on performances of PEFC.
Numerical Modeling and Results
A hybrid simulation method of the full-cell scale engineering model simulation and the detailed two-phase CFD is introduced. In order to estimate performance of PEFC cell taking all relevant transport phenomena (mass, chemical species and heat) is solved on the coarse-grained mesh, coupled with electrochemical reactions. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from gas phase to reaction cites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models.
In order to apply this method for conditions in which liquid water is important, two-phase fluid models should be constructed carefully. In our approach, the behavior of liquid water in channels is simulated in advance by the conventional volume-of-fluid model on the fine mesh whose resolution is order of 10 micrometers. Characteristics of gas and liquid transport under the presence of liquid water is extracted as two-phase permeability depending on volume fraction of liquid water and gas flow velocity. Those permeability are imported to the full-cell scale multiphysics simulation on the coarse-grained mesh. Results of calculations show dependence of cell performance on behavior of liquid water, which is effected by geometry of channels and wettability. This method turns out to be efficient to estimate cell performance and all relevant transport phenomena taking liquid water into account.