Influence of buoyancy on turbulence and heat transfer of supercritical CO2 in vertical backward-facing step flows

This paper investigates the influence of buoyancy on the flow and heat transfer characteristics of supercritical carbon dioxide (SCO2) in a vertical backward-facing step (BFS) configuration. Utilizing direct numerical simulations (DNS), the study systematically examines how buoyancy affects turbulence behavior and heat transfer performance. Two approaches are employed: adjusting the total channel height to modify the Grashof number while maintaining a constant nondimensional wall heat flux and varying the wall heat flux with the same geometry. The results show that buoyancy enhances heat transfer by intensifying turbulence through ejection and sweep processes. Unlike conventional fluids, strong buoyancy in SCO2 flows can lead to a slight expansion of the recirculation zone, attributed to the entrainment effect of the wall jet, which accelerates fluid in the attachment zone and shifts the shear layer downstream. Additionally, high wall heat flux reduces the recirculation zone and enhances near-wall turbulence but does not fully compensate for the adverse effects of fluid property deterioration, resulting in decreased heat transfer efficiency. These findings provide valuable insight for optimizing SCO2 systems, contributing significantly to advancements in heat transfer and energy technologies.