DNS investigation of flow and heat transfer characteristics of supercritical carbon dioxide over a backward-facing step

Abstract

This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO2 forced convection, and SCO2 mixed convection. All cases were conducted with an inlet Reynolds number of 4805, and the expansion rate of BFS is set at 1.5. Mean statistics like velocity, temperature, Reynolds stress, turbulent heat flux, and thermophysical properties are given and discussed. The results show that the turbulent and heat transfer characteristics of SCO2 over a backward-facing step are significantly different from those of conventional fluids due to the dramatic changes in its thermophysical properties near the pseudo-critical temperature. In the case of SCO2 forced convection, though the large thermophysical property variations do not have an obvious impact on the mean flow field, the near-wall mean density decreases largely and thus Reynolds stress and turbulent heat flux near the wall are reduced significantly, which in turn leads to a substantial reduction in the local skin friction coefficient and Nusselt number. In the SCO2 mixed convection, buoyancy substantially distorts the flow, resulting in a significant reduction in the size of the recirculation zone. Simultaneously, buoyancy acts as an additional driving force leading to the formation of a wall jet. These combined effects induce intensified turbulence near the heated wall, which enhances the turbulent heat flux and facilitates efficient heat transfer. The instantaneous vortical structures based on Q criterion also clearly show the intensified turbulence in the near-wall region.