LES study of contrasting behavior of pulsating plane impinging jets for heating and cooling applications

Abstract

Forced turbulent plane impinging jets at a frequency of 600 Hz, are studied using LES. The jets impinge on a curved surface for cooling and heating applications. The jets generate two free-shear layers releasing primary vortices moving with a sweeping effect above the target wall. The sweeping motion affects the convex wall boundary layer by generating secondary vortices and entraining air from both the jet and the surroundings. In both situations the fluid drawn in by the jet from the free entrainment boundary is cold, a condition considered realistic. The trajectories of the primary vortices exhibit a cyclic trend within a certain distance, in the streamwise direction. Forcing the jets leads to a decrease of the Nusselt number at the impingement region and away beyond four jet widths from the impingement point. The cooling jet is more efficient compared to the heating jet beyond four jet widths from the impingement point because it entrains cold surrounding air towards the wall which is desirable for cooling but undesirable for heating. The complex behavior of the cyclic groups of vortices cause the flow dynamics and heat transfer to be de-correlated intermittently.