A MEASUREMENT TECHNIQUE AND A NEW MODEL FOR THE WALL HEAT-TRANSFER COEFFICIENT OF A PACKED-BED OF (REACTIVE) POWDER WITHOUT GAS-FLOW

An experimental technique, designed for investigating altogether the effective thermal conductivity and the wall heat transfer coefficient in a packed bed of hydride powder, and the coupling of heat transfer with the hydriding reaction, is presented. It consists of measuring transient temperature evolution in a reactor with only geometrical symmetry and of fitting to the experimental data the numerical solution of the heat equation in the two-dimensional domain including the whole reactor. This method requires only a relatively small powder volume (30 cm3). As a first step, results on non-reactive packed beds (500 mum glass beads with argon and 20 mum iron powder with hydrogen) validate the experimental technique. A new model for the wall heat transfer coefficient is developed for packed beds without gas flow. This model attempts to unify the current differing approaches of modeling this quantity. It predicts high values for small grain size and large variations in the Knudsen transition domain: this is qualitatively confirmed by experiments, with coefficients at around 3000 W m-2 K-1 being measured for 20 mum iron powder in hydrogen. Experimental results show that the pressure-dependent thermal resistances on wall and grain surfaces are not negligible.