Abstract
When reactant gases flow in a serpentine flow field in a proton exchange membrane (PEM) fuel cell, a pressure difference occurs between the neighboring channels and it induces under-land convection (cross-flow) from the higher pressure channel to the lower one through the gas diffusion layer (GDL). Although this cross-flow is believed to enhance fuel cell performances, up to now, no direct experiments have been conducted to measure the amount of cross-flow and its true effects on fuel cell performances. In this work, a unique experimental fixture is developed and the effective permeability and the cross-flow are measured directly in an actual fuel cell. The non-Darcy effect is also investigated and effects of the land width and the operational condition are evaluated. The cross-flow is measured with varying the valve closure and then, the cross-flowrate is obtained as a function of the pressure difference at different operational conditions. The cross-flowrate is expressed as a function of two dimensionless parameters by dimensional analysis and this correlation agrees well with the experimental data for different operational conditions and different inlet flowrates. The current density and the power density are evaluated with varying the pressure difference between the adjacent channels. It is revealed that a small increase in the pressure difference between the adjacent channels is more efficient to enhance the current density than a large pressure increment. The net power density is introduced in order to evaluate the pumping power effect and the results show that the fuel cell performance increases significantly from the zero to low closure conditions. Finally, it is found that an additional small increase in the pressure difference can increase the net power output of a fuel cell and excessive stoichiometry decreases the net power output of a fuel cell. A three-dimensional PEM fuel cell model is also developed and the flow fields, the oxygen distribution and the local current density distribution around the land area are studied. The results show that the cross-flow gradually decreases along the channel direction and it exists mainly under the land area. The oxygen concentration in the channel is affected by the secondary flow which is induced by the U-bend if the channel length is short. The modeling results also show that the cross-flow has a significant effect on the local current density distribution under the land area: the local current density under the land area decreases along the channel and the difference between the upstream and the downstream increases with the decrease of the cell voltage.