CFD simulation of two-phase flow patterns in the gas channel of a PEM fuel cell

Water management is one of the key issues in the commercialization of low temperature polymer electrolyte membrane fuel cells (PEMFCs). An adequate hydration of the proton exchange membrane is absolutely necessary to maintain the proton conductivity of the polymer electrolyte membrane; on the other hand, an effective evacuation of the excess liquid water avoids flooding. Moreover, the liquid water emerging from the gas diffusion layer (GDL) into the gas flow channel of a PEM fuel cell gives rise to liquid-phase flow patterns that may affect the fuel cell performance. Slugs are dominant at low superficial air velocity, leading to severe flow maldistribution and large fluctuations in the pressure drop. As the air velocity is increased, a water film is formed on the channel walls if they are hydrophilic. As the superficial air velocity increases further, mist flow is obtained. By enabling the volume of fraction (VOF) model, the water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. In particular, typical operational conditions are assigned and the numerical mesh was constructed to represent the real fuel cell configuration, with several liquid water inlets characterizing the porous GDL structure. Therefore, with many water inlet pores, the effects of water emerging from one pore on the formation and subsequent behavior of water emerging from other pores as well as the effects of the hydrophobicity of the channel walls, and the air inlet and water inlet velocities on the water behavior are carefully addressed. Results show that the channel inclination aids the water removal. Indeed, the water drains out more quickly as the channel inclination is increased. In particular, the tapered channel shows to favor the slugs formation, thus assisting the water management. On the contrary, the presence of slugs increases the pressure drop along the channel, besides causing the blockage of large pores and thus a decrease in the reactant access to the catalyst sites. A channel with hydrophobic walls shows higher water removal frequency and higher water. Moreover, as the hydrophobicity is decreased the presence of film is more evident even for less tapered channels.