Direct numerical simulation of turbulent heat transfer in liquid metals in buoyancy-affected vertical channel

• Direct numerical simulation (DNS) of mixed convection turbulent heat transfer in a vertical channel case for low Prandtl number fluid. • The influence of buoyancy on aiding-flow and opposing-flow side is analyzed. • Extensive results of flow and temperature statistics are analyzed and presented. • The budgets are investigated and related to the variations of the quantities in mean fields. This article investigates the impact of buoyancy force in a vertical channel with combined natural and forced convection liquid metal flow using direct numerical simulation (DNS). In particular, a liquid metal with Pr = 0.025 and Re = 4667 is considered in the combined natural and forced convection regime (Ri = 0, 0.25, 0.50 and 1.00). The liquid metal is driven upward while buoyancy aids the flow close to the hot wall and opposes the flow close to the cold wall. The influence of buoyancy on turbulent heat transfer and mean flow are analyzed. Turbulent transport is enhanced in aiding flow and weakened in opposing flow with increasing Richardson number for low-Prandtl-number fluids. The enhancement of normalized Reynolds stresses is more significant in the opposing flow compared to the aiding flow. Turbulent heat flux also increases and, consequently, enhances heat transfer. Besides, the budgets of streamwise and wall-normal turbulent heat flux, temperature variance, Reynolds shear stress and turbulent kinetic energy are investigated and related to the variations of the quantities in mean fields. The budgets of temperature variance and turbulent kinetic energy are nearly balanced by shear production and dissipation. As for the budget of Reynolds shear stress, streamwise and wall-normal turbulent heat flux, temperature pressure-gradient correlation contributes to the budget significantly in addition to production and dissipation. The DNS results can not only shed light on the heat transfer mechanism of turbulence in liquid metal flows, but also be used for validating and improving turbulent models for low-Prandtl-number fluids, which could be useful for developing Gen IV liquid-metal-cooled fast reactors.