Effect of Hole Spacing on Jet Array Impingement Heat Transfer

Data which illustrate the effects of hole spacing on the heat transfer from an array of jets impinging on a flat plate are presented. Considered are Reynolds numbers ranging from 8200, to 30500, and Mach numbers from 0.1 to 0.2. The spacing of the holes used to produce the impinging jets is either 8D or 12D in both the streamwise and spanwise directions. Local and spatially-averaged Nusselt numbers show strong dependence on the impingement jet Reynolds number for both situations. Experimental results show that local Nusselt numbers show some dependence on the Mach number for the smaller jet hole spacing, with negligible dependence for the larger jet hole spacing. This is partially a result of the accumulating cross-flows produced by the jets, as well as the interactions of the vortex structures which initially form around the jets, and then impact and interact as they advect away from stagnation points along the impingement target surface. Spatially-averaged Nusselt numbers generally decrease as x/D increases when hole spacing is 8D, whereas Nusselt numbers are generally about constant as x/D increases when hole spacing is 12D. This is partially due to cross-flow effects, as well as behavior of each jet in the array, which is similar to that of a single, isolated jet for the larger hole spacing. Spatially-averaged Nusselt numbers for 8D jet hole spacing are also often higher than values for the 12D jet hole spacing when compared at the same x/D location.