Enhancement of heat and mass transfer in metal hydride beds with the addition of Al plates

A numerical study is made of transient heat and mass transfer in metal hydride beds in the hydriding process with the addition of internal aluminum plates. The two-dimensional equations governing the hydration kinetics, hydrogen ̄ow and heat transfer are solved by using the iterative method based on the ®nite-volume technique. It is found that the heat transfer is enhanced by the installation of aluminum plates, and the ratio of the gap distance between the aluminum plates …H† and the thickness of the bed …L† emerges to be an important parameter. The reaction process is also strongly in ̄uenced by H=L. An optimal value of H=L exists to yield the fastest reaction rate, which is also shown to depend on other relevant parameters, such as the thickness of hydride bed and the inlet pressure. List of symbols C hydrogen concentration in metal [molHmol ÿ1 r ] _ C concentration reaction rate Cp speci®c heat capacity [J kg ÿ1Kÿ1] D diffusion parameter dp average diameter of particle [m] Ea activation energy [J mol ÿ1] H gap distance between aluminum plates [m] DH heat of formation of hydride [J molÿ1] ka thermal conductivity of aluminum [W m ÿ1 Kÿ1] keff effective thermal conductivity of the bed [W mÿ1 Kÿ1] KD Darcy permeability of powder [m ] Keff effective Darcy permeability of powder [m ] Kn Knudsen number L bed thickness [m] _ m mass reaction rate of hydrogen [kg mÿ3 sÿ1] M molecular weight [kg molÿ1] P pressure [Pa] Peq equilibrium pressure [Pa] R universal gas constant [J molÿ1 Kÿ1] DS reaction entropy [J molÿ1 Kÿ1] t time [s] T temperature [K] u horizontal velocity component [m sÿ1] v velocity vector [m sÿ1] x horizontal coordinate [m] y vertical coordinate [m] Greek symbols b plateau hysteresis factor porosity of powder d thickness of Al plate [m] l dynamic viscosity [kg mÿ1 sÿ1] x hydrogen concentration ratio, ˆ C=Cmax / plateau ̄atness factor q density [kg mÿ3] h dimensionless temperature