Convective heat transfer for multiple rows of impinging air jets with small jet-to-jet spacing in a semi-confined channel

The effects of jet-to-jet pitch ratios in both streamwise and spanwise directions (x_n/d and y_n/d, ranged from 2 to 5), jet-to-target spacing ratio (z_n/d, ranged from 2 to 4) and jet array arrangement (inline and staggered) on the jet impingement heat transfer behaviors are experimentally investigated under the jet Reynolds number ranged from 5000 to 25,000. Special attention is paid to examine the dense-array jet impingement behaviors inside a semi-confined channel with one-directional spent air outflow where the jet-induced crossflow is more serious. The detailed surface heat transfer distribution under the densest-array jet impingement inside the semi-confined channel is behaved as the character of a channel-like flow due to strong adjacent jet interference. The strongest jet impingement heat transfer is achieved for the case with x_n/d = y_n/d = 3 among the presented jet-to-jet pitch ratios under the same impingement Reynolds number. While evaluating on the base of the same mass flow rate of coolant per unit area of cooled surface, the heat transfer performance of multiple-jets with x_n/d = y_n/d = 5 is obviously superior to that of denser multiple-jets array. The uniformity coefficient is not significantly affected by the jet-to-jet pitch investigated in the present study. Besides, the heat transfer performance under z_n/d = 2 is confirmed to be superior to the other jet-to-target spacing ratios for the dense array of multiple jets. For the multiple-jets with x_n/d = y_n/d = 3, the local area-averaged row Nusselt number corresponding to the front rows under staggered array is somewhat higher than that under the inline array. From the fourth row on, the jet-induced crossflow effect behaves more obviously for the staggered array. While for the multiple-jets with x_n/d = y_n/d = 5, the heat transfer for the staggered array seems to be somewhat superior to that of inline array.