Optimum Jet-to-Plate Spacing of Inline Impingement Heat Transfer for Different Crossflow Schemes

A nine-by-nine jet array impinging on a flat plate at Reynolds numbers from 15,000 to 35,000 has been studied by the transient liquid crystal method. The spacing between the impingement plate and target plate is adjusted to be 1, 2, 3, 4, and 5 jet diameters. The effect of jet-to-plate spacing has been investigated for three jet-induced crossflow schemes, referred as minimum, medium, and maximum crossflow, correspondingly. The local air jet temperature is measured at several positions on the impingement plate to account for an appropriate reference temperature of the heat transfer coefficient. The jet-to-plate spacing, H/d - 3, is found to be better than the others for all the crossflow schemes. Jet-to-plate spacings H/d = 1 and H/d = 2 result in a sudden decrease in the stagnation zone. The large jet-to-plate spacings H/d = 4 and H/d = 5 could not provide higher heat transfer performance with higher crossflow.