Heat-transfer enhancing mechanisms induced by the coherent structures of wall-bounded turbulence in channel with rib

Abstract

The detailed coherent structures and closely-related heat-transfer characteristics were investigated along the wall of channel with rib tabulator by direct numerical simulation (DNS). The three-dimensional Navier-Stokes equations were solved using the density-based algorithm and strongly-coupled with the heat-transfer equation. Two typical Reynolds numbers defined by the rib height were set at 500, 1200, and the Mach numbers was 0.2, respectively. The heat-transfer enhancing mechanisms incurred by the wall-bounded turbulence coherent structures were investigated and clarified. The DNS results suggest that the bypass transition occurs in the presence of rib structure which plays a role of strong source of disturbance. A large-scale spanwise vortical structure emerges firstly and stretches into a Λ-shaped vortex, and then evolves into a hairpin vortex. As a result, the quasi-streamwise vortice are induced downstream. The Reynolds stress and turbulent kinetic energy are much larger in the turbulent boundary layer (TBL) downstream than those in the laminar boundary layer upstream. The locations of higher Nusselts number (Nu) is consistent with the regions of higher Reynolds stress and turbulent kinetic energy where the coherent structures are active and dominant, giving rise to the phenomena of ejection and sweep. The ejections, sweeps and vortex rotations are the major forms of turbulence motions that enhance the coordination levels between temperature gradient vectors and velocity vectors, which provides the inherent mechanisms to enhance the convective heat-transfer in the rib flow structure.