Momentum and Buoyancy Repartition in Turbulent Mixed Convection

Abstract

Turbulent mixed convection is a common phenomenon in applications such as heat exchangers and climatisation in transport vehicles and buildings and was often studied in the past. Metais and Eckert [6] experimentally analyse the heat transport in a vertical pipe heated from the outside. The suppression of turbulence in vertical pipes is analysed by Bae et al. [1] by means of direct numerical simulation (DNS). Mixed convection in a vertical channel is studied in DNS by Kasagi and Nishimura [4], who report profiles of one-point statistics and budgets of the turbulent kinetic energy. They also compare asymmetrically heated turbulent channel flow with channel flow subjected to uniform wall mass injection and suction, as well as with liquid metal channel flow under the influence of a transversal magnetic field. All three flow types exhibit similar effects regarding the suppression and enhancement of turbulence near the walls and Kasagi and Nishimura state that the buoyancy does not affect the velocity fluctuations directly. In a more recent work, He, He and Seddighi [3] study the effects of generic body forces on vertical pipe flow and relate the relaminarization to a lowering of the apparent friction velocity. Wall-normal profiles of the mean streamwise velocity and the turbulent kinetic energy are presented in this paper for different Grashof numbers and demonstrate the buoyancy-induced flow asymmetry. Furthermore, budgets of the Reynolds stress tensor are analysed with regard to the repartition of turbulent stresses between the velocity components. Finally, the heat transport characteristics of the flow are examined with the help of a quadrant analysis of the Reynolds shear stress and the turbulent heat fluxes in order to understand how the coherent structures transport heat.