Enhancement of heat transfer on a cylinder by the turbulence of a slot jet: preliminary results

The present work deals with the enhancement of heat transfer due to turbulence. Experiments are carried out in order to evaluate the influence of the turbulence on the cooling of a smooth cylinder, electrically heated, impinged by a submerged slot jet of air. The increase of the turbulence can be obtained in two ways: by the natural evolution with the distance from the slot exit or by the introduction of a metallic grid. Turbulence and heat transfer measurements are presented in order to show the relation with the slot-to-cylinder distance. The metallic grid is put in two positions: on the slot exit and at constant distance in front of the cylinder. Three Reynolds numbers are investigated. The evolution of the absolute turbulence of the jet without grid is to increase with the distance because of the greater interaction with stagnant air. When a grid is present on the slot exit the absolute turbulence increases at first, then decreases, according to the degeneration law, and finally increases again, due to the interaction of the jet with stagnant air. The level of turbulence after 10 times the slot height is about the same without or with the grid. Heat transfer measurements are presented as local and mean Nusselt numbers. Local Nusselt number increases with the distance from the slot exit without the grid, while, in presence of the grid on the slot exit, increases just after, then decreases and finally increases again. If the grid is put in front of the cylinder at constant distance local Nusselt number decreases at a smaller extent than when the grid is on slot exit. The ratio of the average Nusselt number with the grid to that without is greater than 1 only for the highest Reynolds number experimented, i.e. Re=18,250.