Macroscopic Modeling of Liquid Water in Channel and GDL Based on Detailed Two-Phase CFD

Abstract

In order to perform efficient simulations of full-stack PEFC systems, the behavior of liquid water in channels and GDL is modeled using detailed two-phase CFD with VOF method. The relationship between liquid water volume fraction, pressure drop and liquid water velocity are evaluated in a macroscopic control volume of well­-resolved two-phase CFD of the flow channel, resulting in the database of gas and liquid relative permeability for water saturation ratio. The exhaust velocity of liquid water from GDL to channel is estimated by another detailed CFD, where the structure of GDL is measured by X-ray CT and directly converted to computational mesh. The effect of contact angle of separator attached to GDL is also studied from the viewpoint of characteristics about water exhaust. For evaluating the models obtained, simulations of PEFC are performed. Coupled with electrochemical reactions, all relevant transport phenomena (mass, chemical species and heat) are solved on the coarse-grained mesh. 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, such as water uptake into electrolytes and effective oxygen transport resistance from gas phase to reaction sites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. The results of simulations of 25 cm2 serpentine flow cell under several operating conditions are compared with experimental data, showing that the model we propose is useful for the purpose of PEFC design.