A comparison between frictional and accelerational components of two-phase flow pressure drop in PEM fuel cell flow channels

During the operation of proton exchange membrane (PEM) fuel cells, liquid water is produced in the cathode side. The produced water passes through the porous structure of the gas diffusion layer and emerges from its surface within the flow channel. This forms liquid-gas two-phase flow within the flow channels of PEM fuel cells. The liquid-gas two-phase flow in PEM fuel cell flow channels has been extensively studied in literature. One of the objectives of these studies is to propose models that can accurately predict the pressure drop. The two-phase flow pressure drop, in general, includes frictional, accelerational, and gravitational pressure drop terms. In all of the models proposed for the application of PEM fuel cells, the accelerational pressure drop term is ignored, as it is negligible compared to the frictional term of the two-phase flow pressure drop. However, a comparison between these two terms has not been documented in literature. The current study compares the frictional term to the accelerational term of the two-phase flow pressure drop in PEM fuel cell flow channels. The accelerational pressure gradient is calculated based on the required air flow rate and the water production rate for a current density of 1 A/cm ² during the operation of a PEM fuel cell with a flow channel with 1 mm × 1 mm cross-sectional dimensions. Results indicate that for the cathode stoichiometric ratio of 1 the frictional pressure gradient is around 80 Pa/m while the accelerational pressure gradient is around 0.045 Pa/m. This indicates that the accelerational pressure gradient is less than 0.1% of the frictional pressure gradient in PEM fuel cell flow channels. For higher stoichiometric ratios, the difference between these two components of two-phase flow pressure drop is even larger.