Modeling Water-Droplet Detachment From GDL/Channel Interface in PEM Fuel Cells

The onset of water-droplet detachment from the cathode GDL (gas diffusion layer)/channel interface in PEM (proton exchange membrane or polymer electrolyte membrane) fuel cells was studied numerically and analytically in the inertia-dominating flow regime. This is the flow regime of interest in real-world PEM fuel-cell applications. Our three-dimensional (3-D) numerical model employs the VOF (volume of fluid) method to track the dynamic liquid/gas interface as water droplet is being deformed and eventually detached by flowing-air drag. Our simplified, analytical model is based on the force balance between pressure drag tending to detach the droplet and surface tension tending to hold the droplet in place. Using our numerical model and for a given initial droplet size, we compute the 3-D droplet shape as a function of air-flow velocity — this enables us to determine the critical velocity above which the droplet is detached. Analytically, we develop an explicit expression that relates the critical air-flow velocity to droplet diameter, channel height, and static contact angle. We compare computed critical velocities at various droplet diameters with experimental data available from the literature and reasonably good agreements are obtained.