Local heat transfer for jet impingement onto a concave surface including injection nozzle length to diameter and curvature ratio effects

Abstract

The effect of jet nozzle length to diameter ratio on local heat transfer coefficient measurements for a row of circular jets impinging onto a concave surface for varying target spacing are reported here. The nozzle length to diameter ratio (L/d) and the nozzle to target spacing (H/d) were varied from 0.2 to 6 and 0.67 to 8 respectively. Three curvature ratios, defined as the ratio of jet diameter to target surface diameter (d/D), equal to 0.1, 0.2 and 0.3 were studied and the jet to jet pitch to diameter ratio (P/d) was kept constant at 4.0. The Reynolds number was varied between 10,000 and 50,000 and wall static pressures for some cases were measured for obtaining a better understanding of the heat transfer coefficient variations. Higher stagnation zone Nusselt numbers were observed for the jet nozzles with small L/d at small H/d values whereas at larger H/d values nozzle length was observed to affect the data only marginally. The difference between Nusselt number values, between H/d = 2 and H/d =8, was observed to progressively reduce with increasing L/d ratio and this was true for both the local stagnation point as well as the spanwise averaged stagnation line values. The maximum local Nusselt number values were observed to shift from the geometric impingement location to nearby locations for very small nozzle diameter to target distance ratios (H/d≤ 1) for larger curvature ratios. A constant heat transfer impingement area with constant pumping power criterion is presented to compare the performance of the different configurations studied. The longer nozzle lengths and smaller curvature ratios are observed to perform better based on this criterion. A correlation is presented for the overall averaged Nusselt number, with a validity within the parameter range studied, with a±10% error band