The physics of unsteady jet impingement and its heat transfer performance

The influence of unsteadiness with respect to the heat transfer performance of impinging jets is studied systematically by imposing various shapes and frequencies of unsteadiness. By means of an enhancement coefficient (Nusselt number of the unsteady flow over that for the steady flow) the heat transfer performance is characterized and most efficient types of unsteadiness are identified. From PIV data details of the flow field emerge. These data help to better understand the physics of heat transfer enhancement. Finally it is shown that the influence of unsteadiness can be very different when small obstacles are added to the smooth heat transfer surface, which corresponds to a more realistic situation.