On the effects of the free-stream turbulence on the heat transfer from a sphere

Free-stream turbulence is present in many engineering applications and is known to affect both bluff body aerodynamics and heat transfer. In this work, its effects on the heat transfer and the near wake behind a sphere are studied. To do this, direct and large-eddy simulations of the flow at the moderate Reynolds numbers of Re = 1000 and Re = 10(4),with levels of incoming turbulence up to 10% and a length scale of O(D),D being the sphere diameter, are performed. At the Reynolds numbers under consideration, significant changes are observed. Incoming turbulence delays the separation of the laminar boundary layer from the sphere, while at the same time the drag coefficient and the Nusselt number are increased. The incoming level of turbulence also increases the momentum transfer from the surrounding fluid and energises the separated shear-layer. As a consequence, there is a shrinking of the recirculation zone which intensifies the heat transfer from the sphere and thus, the Nusselt number and its fluctuations, especially in the rear zone of the sphere. It is shown that free-stream turbulence increases the turbulent heat flux in the wake, with a larger entrainment of colder fluid from the surroundings, thus producing a faster decay of the temperature at larger levels of turbulence intensity. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study demonstrates that free-stream turbulence has a significant impact on the heat transfer behind a sphere, delaying the separation of the laminar boundary layer, increasing the drag coefficient and Nusselt number, and energizing the separated shear-layer.