DNS of heat transfer in a plane channel flow with spatial transition

Abstract

In the present work Direct Numerical Simulation of heat transfer in a spatial laminar-turbulent transitional channel flow is carried out to investigate the effects of the transitional phenomenon in the temperature field. The configuration is a spatially developing channel flow with two thermal boundary conditions: Uniform Wall Temperature (UWT) and Uniform Heat Flux (UHF). The set of simulations are carried out for Re=4200 and Pr=0.1,0.2,0.5,0.71 and 1. The spatial evolution of the Nusselt number (Nu) along the transition is computed to show five distinctive qualitative zones: a laminar development, the spike zone, where the quantities evolve to reach the peak transitional zone, the post-transitional zone and finally, the fully turbulent zone. This data is employed to develop a correlation that captures the space evolution of Nu along the transition. Moreover, the analysis of flow structures shows that near the wall and downstream the onset of the transition, streaks of velocity and temperature fluctuations have strong similarity. Different turbulent statistics are calculated along the transition and compared with fully turbulent reference data. It is shown that upstream the peak transitional zone, the existence of coherent hairpin vortices yields a high-intensity region for these quantities. Additionally, it is found that the thermal boundary condition exerts influence on the profile of the second-order parameters near the wall and in the logarithmic region all along the transition.