Flow visualization and heat transfer experiments in fluid-superposed packed beds heated from below

Abstract

Flow visualization and heat transfer experiments are reported for buoyancy-induced flow in a cylindrical cavity filled with a fluid-superposed packed bed and heated from below. An immersion method is used to measure the refractive index of acrylic spheres, which is then matched with that of the silicon fluid. This allows visualization of interesting flow interactions between the porous bed and overlying fluid. Flow channels through large voids produce highly asymmetric and complicated flow structures, and the number of convective rolls depends on the porous layer height η. The heat transfer rate increases with the Rayleigh number Ra ∗, but the dependence on the particle size γ and the porous bed height is very complex. The average Nusselt number generally increases with the solid-bead diameter, but there may exist some values of η for which the heat transfer rate for a smaller particle size in higher than that for a larger value of γ. Also, the Nusselt number first decreases from the fluid heat transfer rates with an increase in porous matrix height η and reaches a minimum at η min. Any further increase in porous layer height beyond η min augments the heat transfer rate, and the Nusselt number curves show peaks at η min < η ≤1.