Flow regimes for liquid water transport in a tapered flow channel of proton exchange membrane fuel cells (PEMFCs)

The chemical species distribution inside a fuel cell, particularly the water management, is crucial to determining the cell performance. Reactant maldistribution can induce efficiency loss and needs to be avoided. The use of tapered flow channels is recommended to improve the reactant distribution since it increases the pressure at the end of the channel, where the reactant concentration is reduced. However, the effect of the channel inclination on the water management has still not been sufficiently investigated. In this study, a two-dimensional dynamic and isothermal model was used to numerically investigate the liquid water transport inside a tapered flow channel. In order to capture the dynamical evolution of the interface between the liquid and gas phases, the volume of fluid method was adopted. Typical operational conditions were considered and the numerical mesh was constructed to represent the real fuel cell configuration, with several liquid water inlets characterizing the porous gas diffusion layer (GDL) structure. Different flow regimes, such as slug flow and film flow, appear simultaneously in different channel regions. It was found that the liquid water distribution and transport inside the channel are strongly dependent on the air velocity and thus different behaviors have been found along the channel. In particular, near the channel outlet, the formation of a liquid film on the GDL surface is observed, while, at the same time, in the central part and near the channel inlet the main form of liquid water transport is as slugs. Although responsible for a significant increase in the pressure drop, the slugs act as the main mechanism of water removal, removing attached droplets as they move toward the channel outlet and thus helping to reduce the water saturation inside the channel.