Investigation of natural convection heat transfer of self-heating packed beds

This study investigated heat transfer characteristics of natural convective flow in the packed bed geometry experimentally and numerically. The natural convection experiments for self-heating condition were performed using the copper sulfate electroplating system based on the analogy between heat and mass transfers. Also, the same conditions were simulated with Lattice-Boltzmann Method (LBM) and the results were compared. The spheres diameters were d = 4, 6 and 10 mm and the corresponding Rayleigh numbers were 5.43 x 106, 1.83 x 107 and 8.48 x 107 respectively. The packed bed height to sphere diameter ratios, H/d were 5, 10 and 20. The Nusselt numbers (Nud), velocity and shear stress distributions of both analyses were in good agreement. Both results showed the same order (10-2 m/s) of irregular velocity distributions resulting from the random arrangement of the spheres. In spite of the laminar flow condition, wake and vortex generated by the packed bed geometry increased the turbulent kinetic energies to one-or two-order higher values than those at the entrance. The temperature profiles of flow inside packed bed were approximately uniform for d = 4 mm cases but there were temperature gradients in case of d = 10 mm. This revealed that the use of smaller fuel sphere can prevent from unwanted hot spots inside the packed bed. The Nud values from LBM results and newly developed empirical correlation for natural convective flow in packed bed were consistent.