Theoretical and Experimental Study of Linear Momentum Contours in pristine and aged Nafion 117 Membranes

Abstract

Abstract The long-time operation stability of Nafion membranes upon large current densities is important for the durability of polymer electrolyte fuel cells. It was found that under laminar regimes, medium porosities and relative low Reynolds (Graetz) numbers, the Darcy-Brinkman equation can be fully applied. Flow rate preferential paths inside the initial homogenous porous medium are formed in an aged specimen (2 years of operation at 1 A cm-2) and define the effective flow at the connecting interface with the inviscid fluid. The process is modelled by the introduction of a convective term in the Darcy-Brinkman equation which renders an analytical solution for linear velocities that is well related to experimental results. The theoretical values are compared with experimental data obtained from a thin-film prismatic tube with Nafion 117 membranes using distinct hydrodynamic conditions under the flow of an acidic water solution. The resolution of the velocity equations is also conducted for the pristine sample showing different results that can be extrapolated under the certain experimental working conditions. The friction factor is calculated for the pristine and aged specimens showing distinct values for Graetz numbers lower than 80, being 4 times higher for the brand new membrane. Furthermore, equivalent behaviours for both samples are shown for Graetz numbers higher than 200 when a porosity/Darcy number ratio of 313 is used.