Effect of Channel Length on the Flow Cross-Over Through Gas Diffusion Layer and Pressure Distribution in a PEM Fuel Cell Flow Plate With Serpentine Channels

Abstract

A numerical model was developed to study the effects of channel length and bend shape on the flow cross-over through the porous gas diffusion layer (GDL) and the pressure distribution in a PEM fuel cell flow plate with a serpentine channel flow system. Usually, on the cathode side of a PEM fuel cell, air flow through a flow plate with serpentine channels with certain lengths, to supply the oxygen to the catalyst layer for the chemical reaction. There is a porous GDL between the flow plate and the catalyst layer. Flow cross-over of air through the porous GDL from one part of the channel to another can occur because of the pressure differences existing between different parts of the channel. This cross-over causes the flow rate through the channel to vary with distance along the channel, and also has an influence on the pressure distribution through the plate and, eventually, the fuel cell performance. For the conventional channel flow, the pressure drop is proportional to the channel length. To study the importance of this channel length effect on the PEM fuel cell flow field with cross-over, the pressure distribution and flow rate variation along the channels have been examined by numerically solving for the flow through the plate and porous GDL assembly. Attention has been given here to serpentine channel flow systems with single and parallel channel patterns and with different numbers of passes. A 3-D, single-phase flow has been considered. It was assumed that the flow is steady and incompressible, and the flow through the porous diffusion layer can be described using the Darcy law. The governing equations have been written in dimensionless form using the channel width as the length scale and the mean velocity at the channel inlet as the velocity scale. The resulting set of dimensionless governing equations has been solved using the commercial finite element method (FEM) software package, FIDAP. The solution was obtained by simultaneously solving the equations for the flow in the channels and for the flow through the porous GDL. The solution depends on the following parameters, (1) the Reynolds number, Re, based on the channel width and on the mean velocity at the channel inlet, (2) the dimensionless GDL permeability, (3) the dimensionless channel length, (4) the bend shape, and (5) channel configurations. The main emphasis of this study was on the effect of channel length. The numerical results obtained indicate that the channel length has a significant effect on the flow cross-over through porous GDL and the pressure distribution in the flow plate.