LES simulations and Nusselt number decomposition of turbulent mixed convection of liquid metals flowing in a vertical pipe

Fully developed turbulent aided mixed convection to liquid metals with Prandtl number of 0.025 and 0.005 flowing vertically in a homogeneously heated pipe is numerically investigated with Large Eddy Simulations. These are performed at Reynolds numbers, based on the radius, in the range from 2650 to 7500 and for several Richardson numbers, encompassing the heat transfer reduction and its subsequent recovery. The results are analyzed based on a novel Nusselt number decomposition for mixed convection that allows to quantitatively identify the different contributions to the Nusselt number and thus to the heat transfer mechanism. The main contributions in liquid metals mixed convection are the laminar one and those due to the laminar and turbulent buoyancy terms. The maximum heat transfer impairment, i.e. the laminarization state, corresponds to vanishing values of the Reynolds stress contribution only at low Reynolds numbers below Re=7500. For sufficiently high Reynolds numbers, the heat transfer recovery beyond laminarization occurs before the Reynolds stress becomes negative in the core of the pipe. The comparison with RANS simulations shows that these are able to reproduce qualitatively well the Nusselt number decrease and recovery at different Richardson numbers, i.e. for different buoyancy contributions.