Numerical investigation on flow characteristics and heat transfer of mixed convection in a packed bed

Flow characteristic and heat transfer of mixed convection in the packed bed were investigated numerically comparing with the previous experimental studies. For the fixed packed bed and sphere diameters of 0.06 m and 0.01 m, the ratios of bed height to sphere diameter and Reynolds numbers were varied from 5 to 20, and from 1 to 300, respectively for both buoyance aided and opposed flows. The temperatures difference between fluid and sphere was 78.85 K. The packed bed structure was modeled by generating location coordinates through the random number with an in-house code and computational analyses were performed using ANSYS FLUENT software. Laminar mixed convection behaviors were observed: heat transfer enhancement for buoyancy aided flow and impairment for buoyancy opposed flow. For larger bed heights, even under a laminar flow condition, turbulent behaviors were observed: heat transfer impairment for buoyance aided flow and enhancement for buoyance aided flow. The same trend was observed in the analysis of turbulence intensity. Local flow analysis within the packed bed confirmed that this is due to the vortices and the wakes resulting from the interaction between mixed convection flows and the randomly stacked spheres in the packed bed. This study unveiled mixed convection phenomena accompanied by the geometric effects of the packed bed on vortices and wakes, which exhibit turbulence-like behavior.