A simple criterion for estimating the effect of pressure gradients during hydrogen absorption in a hydride reactor

Owing to the numerous coupled phenomena which occur during the loading with hydrogen of a metal hydride reactor, numerical simulations of such systems are very time consuming. Neglecting the fluid flow can reduce significantly the computing time, but it can lead to a wrong estimation of the hydriding time. This paper suggests a criterion which allows to quantify the error which is made when the fluid flow is neglected. It assumes conductive transfer controls the local evolution of hydridation in the reactor. Based on simple equations linking hydriding velocity to pressure and temperature, this study compares the solutions obtained with and without the gas flow. Using a finite volume numerical simulation, the validity of the criterion which is derived is checked on three classical tank geometries: it is shown that the criterion is independent of the tank geometry providing typical length scales can be defined for heat and gas diffusion. Finally, this criterion is successfully applied to an experimental magnesium hydride tank and its numerical modeling.