heat transfer rate and uniformity in multichannel swirling impinging jets

the influence of the swirl number on the wall heat transfer distribution on a flat plate with a swirling air jet impinging on it is experimentally analyzed. this investigation is concerned with a swirling nozzle based on helical inserts, which is easily applicable in an industrial environment. measurements are performed at a fixed value of the reynolds number re (28,000), for five values of the swirl number s (0, 0.2, 0.4, 0.6 and 0.8) and for five values of nozzle-to-plate distance z (2, 4, 6, 8 and 10 diameters), with infrared thermography and with the heated thin foil sensor. results obtained with swirling impinging jets are compared with those obtained with a circular impinging jet in the same testing conditions. the obtained experimental data supply information on the behaviour of circular jets, multichannel jets, weak swirl jets and strong swirl jets. their performance in terms of heat transfer uniformity and heat transfer rate is accounted for through evaluation of mean value and standard deviation percentage of the nusselt number on the impinged area. the dependence of heat transfer rate and uniformity on the swirl number is also explained. in particular, this study shows two aspects: the multichannel jet induces a general enhancement in heat transfer with respect to the circular impinging jet, the swirl motion decreases the rate and increases the uniformity of heat transfer.