Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization

We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh-Benard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number Ra = 10(8) and fixed Prandtl number Pr = 5.3, while the wall-shear Reynolds number (Re-w) is in the range 60 = Re-w = 6000. Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of Re-w. However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger Reiv, the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.

Automated summary

We studied the flow organization and heat transfer properties in different types of Rayleigh-Benard cells with external wall shear. The heat transfer efficiency and global flow strength were enhanced with increasing wall shear Reynolds number (Re-w). Interestingly, increasing wall-shear strength led to the laminarization of thermal turbulence, and the heat transfer efficiency in the laminar state was found to be higher than that in the turbulent state. We proposed that the reduction of thermal plumes was responsible for the laminarization of thermal turbulence, and the enhanced heat transfer efficiency in the laminar regime was attributed to the formation of more stable and stronger convection channels. The efficiency of facilitating heat transport via external shearing increased with larger Re-w, but came at the cost of a larger expenditure of mechanical energy.