Nozzle-geometry effects in liquid jet impingement heat transfer

Experiments were conducted to determine the effect of nozzle geometry (diameter and aspect ratio) on the local heat transfer coefficients from a small heat source to a normally impinging, axisymmetric, submerged and confined liquid jet of FC-77. A single jet with nozzle diameters in the range of 0.79-6.35 mm and up to seven different nozzle aspect ratios in the range of 0.25-12 were tested, at turbulent jet Reynolds numbers from 4000 to 23 000 and nozzle to heat source spacings of 1-14 jet diameters. The results indicate that at very small nozzle aspect ratios (l/d < 1), the heat transfer coefficients are the highest. As the aspect ratio is increased to values of 1-4, the heat transfer coefficients drop sharply, but with further increases in l/d of up to 8-12, the heat transfer coefficients gradually increase. This effect is less pronounced as the nozzle to target spacing is increased. Possible explanations for these trends are provided in terms of flow separation at the nozzle entrance and its effect on the exit velocity profiles. The nozzle diameter also has a definite effect on the heat transfer coefficients. Copyright (C) 1996 Elsevier Science Ltd.