Numerical Analysis on the Dynamic Behavoir of Plug Flow in PEMFC Cathode Channel Return Bends
Water management in a PEMFC is crucial to its performance and the pressure drop in the cathode channel is the one of the key indicator to interpret the non-accessible PEMFC internal status. The purpose of this paper is to understand the liquid water dynamic behavior in 180o bend PEMFC cathode channels. A parametric study has been carried out using computational fluid dynamics and the effect of the channel shape, liquid water initial amount, operating temperature, liquid water initial location, liquid water shape, gas velocity profile, gas velocity magnitude, gravity force direction, and gas diffusion layer contact angle are investigated. The volume of fraction model was used to trace the gas-liquid interface. The transient behavior of the pressure drop in the channel and the liquid water residual was interpreted by coupling the corresponding two-phase flow pattern in the channel. One of the most common two phase flow patterns, plug flow, was investigated because it leads to flow mal-distribution and deteriorates PEMFC performance. The plug flow creates the analogous situation to the water hammer in return bends and generates the sudden pressure drop in the single channel. It is found that the gas passage blockage is the most important parameter to describe the pressure drop behavior in the channel. However, the pressure drop itself cannot provide the appropriate liquid water residual amount in the channel. The channel shape and the initial liquid water location influence both the dynamic behavior of liquid water residual time and the pressure drop. The smooth curved return bends attenuates the pressure drop but prolongs the liquid water residual time in the channel. Single spherical liquid water droplet in the smooth curved return bend does not increase the pressure drop noticeably as long as it does not form the plug flow and gas velocity is high enough to distort the liquid droplet shape. The pressure drop cannot be explained with only the single phase concept. It is also found that the velocity profile affects the pressure drop but has minor impact on the liquid water residual. The gravitational force direction and the GDL contact angle do not show significant effect on the two-phase flow pattern in the considered range.
Keywords: Plug flow, two-phase flow, PEMFC, return bend, VOF, Cathode channel, CFD