Heat transfer due to unsteadily impinging jets

In a systematic experimental study the heat transfer under three different periodically unsteady jets is compared to that of the corresponding steady inline jet. Unsteadiness of the jets is created by vortex shedding (Karman jet nozzle), alternating flow passages (flip-flop nozzle) as well as by a precessing motion of the whole jet (precessing jet nozzle). The unsteadiness of the jets is self-sustained, i.e., there are no external (mechanical) means and no external energy input to create unsteadiness. Heat transfer data in terms of Nusselt numbers are determined on a specially designed heat transfer plate in the plane of jet impingement. Whilst there is a slight increase in heat transfer performance when vortex shedding occurs within the jet (Karman jet nozzle), in both other cases there is a considerable decrease in heat transfer. (C) 2004 Elsevier SAS. All rights reserved.