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• Investigating turbulence and heat transfer characteristics in SCO 2 flows over a vertical backward-facing step. • Revealing the influence of large thermophysical property variations on heat transfer. • Uncovering the impact of buoyancy on the flow and heat transfer characteristics of SCO 2 backward-facing step flow. • Providing valuable reference data for developing and testing turbulent heat transport models in SCO 2 flows under complex geometric conditions. This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO 2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO 2 forced convection, and SCO 2 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 SCO 2 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 SCO 2 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 SCO 2 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. [ABSTRACT FROM AUTHOR]

Highlights • DNS of LBE flow over backward-facing step is conducted to study buoyancy impact. • Statistics like velocity, temperature, Reynolds stress and heat flux are discussed. • Buoyancy has substantially altered the flow field for LBE flow at moderate Ri. • Recirculation zone decreases and heat transfer is improved in buoyancy-aided flow. • Recirculation zone largely increases and heat transfer is weakened in opposed flow. Abstract Studies of turbulent mixed convection over a vertical backward-facing step for liquid metals are indispensable to research the buoyancy effects in flow separation and reattachment scenarios that occurs in innovative nuclear and solar power facilities. Direct numerical simulations of turbulent buoyancy-aided flow at two moderate Richardson numbers (Ri = 0.1, 0.2) and buoyancy-opposed flow at Ri = −0.04 for lead–bismuth eutectic (LBE, Prandtl number Pr = 0.025) are performed. The forced convection and air mixed convection (Pr = 0.7, Ri = 0.1) are also simulated for comparison. In all the cases Reynolds number is 4805 and the expansion ratio is 1.5. Mean statistics like velocity components, temperature and heat fluxes are given. Second-order quantities such as Reynolds stresses, temperature fluctuations, and turbulent heat fluxes are also discussed. The results show that buoyancy has substantially altered the flow field for LBE flow compared to air flow at moderate Ri. For buoyancy-aided LBE flow, the recirculation zone is reduced in size and the reattachment length is shortened. With Ri increasing, the second vortex gradually prevails over the main vortex and finally pushes it detach from the wall, leading to positive skin-friction coefficient. Buoyancy weakens the turbulent quantities such as Reynolds stresses, temperature fluctuations and turbulent heat fluxes. Both skin-friction coefficient and Nusselt number increase due to the reduced reversed flow resulting from buoyancy acceleration near the wall. However, it is opposite for the case of buoyancy-opposed flow. The skin-friction coefficient and Nusselt number decrease due to the increased reversed flow though the turbulence is enhanced in buoyancy-opposed flow. These high-resolved data could promote the understanding of this scenario and improve turbulent modelling for flow separation and reattachment of low Prandtl fluids. [ABSTRACT FROM AUTHOR]