This investigation has examined the influence of spent air exits located between the jets on the magnitude and uniformity of the local heat transfer coefficient for a confined 3×3 square array of axisymmetric air jets impinging normally to a heated surface. The heat transfer coefficient was measured using a 0.025-mm-thick stainless steel impingement surface coated with liquid crystals. The temperature distribution along 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 spacings (0.25 and 1.0 diameters) commonly used in material processing applications was also considered. Average Nusselt numbers are presented for a Reynolds number range of 3500 to 20,400 along with radial distributions of the local Nusselt number. The local Nusselt number distributions illustrate the uniformity of the convective heat transfer coefficient 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 and 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 nozzle-to-plate spacings when compared to a 6.0 diameter spacing.

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