Experimental investigation on mixed convection heat transfer in a packed bed varying sphere diameter and bed height

•We performed mixed convection experiments in packed beds by mass transfer system.•Sphere diameter, bed height, Redh and flow direction were varied to cover laminar and turbulent.•Measured velocities and turbulent productions by the PIV were presented and analyzed.•Influence of vortex generation on mixed convection due to packed bed geometry is analyzed. We performed mixed convection heat transfer experiments in a packed bed, which has rarely been investigated but has practical applications in thermal energy storage (TES) systems using highly buoyant fluid operating at low velocities with additional complexity by packed bed geometry. To achieve high buoyancy, mass transfer experiments using a copper sulfate-sulfuric acid (CuSO4-H2SO4) electroplating system were conducted based on the analogy between heat and mass transfers. The sphere diameters (d) ranged from 0.004 m to 0.01 m, which correspond to Rad of 5.43 × 106 and 8.48 × 107. The cases for bed height-to-sphere diameter ratio (H/d) were 5 to 10. The Redh varied from 1 to 170. The velocity and shear stress were measured to explore the local flow behaviors using Particle Image Velocimetry (PIV). Characteristics of both laminar and turbulent mixed convection were observed depending on the parameter changes. For small bed height (H/d = 5) with large sphere diameter, the Nusselt number (Nudh) of buoyancy aided flow increased by up to 12.7 % compared to that of buoyancy aided flow, which is the same as the laminar mixed convection behavior. However, for large bed height (H/d = 10) with small sphere diameter, the Nudh value of buoyancy opposed flow was higher by up to 17.8 % than that of buoyancy aided flow as the Reynolds number (Redh) increased beyond 50. Despite laminar conditions of this experiment, turbulent mixed convection behavior was observed in the packed bed due to the influence of vortex and turbulent production. This study revealed the geometry influence of packed beds on the vortex generation, which behaves like turbulence. An empirical correlation was developed for mixed convection heat transfer in the packed bed.