Influence of turbulence on heat transfer upon a cylinder impinged by a slot jet of air

Abstract

The present work deals with the enhancement of heat transfer on a cylinder due to the turbulence of the impinging jet. Experiments are carried out to cool a smooth cylinder, electrically heated, with a submerged slot jet of air at Reynolds numbers equal to Re = 4180 and Re = 7630. The increase of turbulence is obtained by the introduction of a metallic grid and by the natural evolution of the jet with the distance from the slot exit. Turbulence, velocity and heat transfer measurements are presented in order to show the relation with the slot-to-cylinder distance. The metallic grid is set in two positions: just on the slot exit or at a constant distance in front of the cylinder. In the natural evolution of the free jet the turbulence increases with the distance because of the interaction with stagnant air, reaches a maximum and then decreases. If the grid is on the slot exit the turbulence increases at first, then decreases according to the degeneration law, and finally increases again due to the interaction with the stagnant air. Turbulence at a distance of 10 times the slot height is about the same whether is present or not the grid. Heat transfer measurements are presented as local and mean Nusselt numbers. Without grid local and mean Nusselt numbers increase with the distance from the slot exit reaching the maximum at a distance of about 8 time the slot height. With the grid on the slot exit the local Nusselt number has a maximum immediately after the grid and a minimum at 4–5 time the slot height. With the grid in front of the cylinder at the distance of the slot height the local Nusselt number has the maximum immediately after the grid and then is about constant up to 10 times the slot height. The mean Nusselt number with the grid in front of the cylinder is greater than without the grid only at the higher Reynolds number experimented.