CONVECTIVE HEAT-TRANSFER TO A CONFINED IMPINGING ARRAY OF AIR-JETS WITH SPENT AIR EXITS

This investigation has examined the influence of spent air exits locat ed between the jets on the magnitude and uniformity of the local heat transfer coefficient for a confined 3 X 3 square array of axisymmetric airjets impinging normally to a heated surface. The heat transfer coe fficient was measured using a 0.025-mm-thick stainless steel impingeme nt surface coated with liquid crystals. The temperature distribution a long the surface was determined by measuring the reflected wavelength of light from the liquid crystal with the use of bandpass filters and an electronic digitizer board. The effect of small nozzle-to-plate spa cings (0.25 and 1.0 diameters) commonly used in material processing ap plications was also considered. Average Nusselt numbers are presented for a Reynolds number range of 3500 to 20,400 along with radial distri butions of the local Nusselt number. The local Nusselt number distribu tions illustrate the uniformity of the convective heat transfer coeffi cient and contribute to understanding the variations in the magnitude of the average Nusselt number. Results have shown that the addition of spent air exits increased the convective heat transfer coefficient an d changed the location of the optimal separation distance. In addition , significant enhancement of the uniformity and magnitude of the heat transfer coefficient was observed at the 0.25 and 1.0 jet diameter noz zle-to-plate spacings when compared to a 6.0 diameter spacing.