Confined, milliscale unsteady laminar impinging slot jets and surface Nusselt numbers

Different modes of unsteadiness which develop within confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements and numerical predictions of different flow characteristics, including spatially-resolved distributions of local Nusselt numbers measured on a constant heat flux surface. The effects of Reynolds number, and nozzle-to-plate distance on the local Nusselt number are investigated for a slot nozzle width B of 1.0 mm, Reynolds numbers Re from 120 to 200, nozzle-to-plate distances H/B from 0.75 to 12.5, and a nozzle aspect ratio y/B of 50. Observed are several different types of unsteady slot jet behavior, including: (i) a flow fluctuations/flapping motion mode of unsteadiness which is present for B = 1.0 mm, 4.75 ⩽ H/B ⩽ 5.5, and 120 ⩽ Re ⩽ 140, (ii) an intermittent flapping motion of the jet column which is present for B = 1.0 mm, 9 ⩽ H/B ⩽ 11.25, and 120 ⩽ Re ⩽ 200, and (iii) a continuous sinusoidal oscillation state, which is observed to be present for Re = 160 and H/B = 10. The flow fluctuations/flapping motion mode of unsteadiness, and the intermittent flapping motion of the jet column are both associated with local maxima in local, stagnation point Nusselt number distributions. The variations of these stagnation point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/B. Also described is the lateral variation of local Nusselt numbers for five nozzle-to-plate distances H/B of 2, 6, 8, 10, and 12, and Reynolds numbers from 120 to 200.