Influence of Microstructure on CFD Simulation of Water Removal in a PEM FC Channel

Water removal from Proton Exchange Membrane (PEM) Fuel Cell (FC) mainly involves two phenomena: some of the emerging droplets will roll on the Gas Diffusion Layer (GDL), others may impact channel walls and start sliding along the airflow direction. This different behaviour is linked to the hydrophobic/hydrophilic nature of the surface the water is moving on. In this paper, the walls of the channel of a FC were characterized by applying optical techniques. The deposition of droplets on the channel wall led to an evaluation of the proper range for Contact Angle Hysteresis (CAH = 55° - 45°), and due to the high wettability of the surface, droplets dimension was defined with a dimensionless parameter B/H. Under high crossflow condition (15 m/s) a sliding behaviour was observed. The channel features determined through image processing were used as boundary conditions for a 2D CFD two phase simulation employing the Volume of Fluid (VOF) model to keep track of the fluids interface. A droplet was initialized on the wall and its behaviour was observed under 15 m/s airflow. Starting from the values observed experimentally, three different contact angle strategies were adopted to manage the liquid-solid-gas interface: static contact angle, quasi-dynamic contact angle and dynamic contact angle. The calculated sliding velocity was validated against experimental data. The static contact angle led to an overestimation of water removal capability. The quasi-static approach improved the accuracy of the simulation, but only the dynamic contact angle consistently falls inside the error bars of experimental measurements. Moreover, bigger droplets show higher sensitivity to contact angle boundary conditions.