Effect of cathode-side gas flow channels section geometry on the removal process of liquid slug in a proton exchange membrane fuel cell

Abstract

In low operating temperatures, provided water of Proton-Exchange Membrane Fuel Cells (PEMFCs) is discharged in the state of tiny/large droplets, slugs, and semi-slugs, which strongly depends on the flow channels' characteristics. In this study, the discharge capabilities of channels with different height-to-width aspect ratios are inspected using a transient two-phase numerical simulation. The numerical model includes a segment of the gas channel on the cathode side, and the operating conditions are those related to an actual fuel cell application. Results showed that channels with minimum height could lift the initial slug to the upper part 2.16 times faster and immediately form film flow at the corners. For a fixed width, decreasing channel height from 1.5 mm to 0.5 mm, results in 38.3% faster discharge time and a 62.3% increment in the clearance rate of the gas-diffusion layer. For higher channels, the combined effect of shear stress, gravity, and the adhesive forces of hydrophilic walls cannot lift the initial slug continuously and causes it to rupture, distort, and partially spread over the Gas-diffusion layer (GDL) surface, which leads to mass transport limitation for the cell. Furthermore, when the channel width decreases from 1.5 mm to 0.5 mm for a fixed height, the initial slug leaves the computational domain 51.1% faster. Overall, it was found that the smaller dimensions for height and width show superior cell performance regarding faster water removal, clearer GDL surface, and more uniform flow distribution over the entire electrochemical active area. A channel with a cross-section dimension of 0.5 mm × 0.5 mm is suggested as the best channel design among the analyzed cases for maximum efficiency for the cathode-side flow fields of a PEMFC.