The effect of nozzle aspect ratio on stagnation region heat transfer characteristics of elliptic impinging jet

The local heat transfer characteristics of an elliptic impinging jet on a heated flat plate were experimentally investigated for various nozzle aspect ratios. Five elliptic nozzles of aspect ratio (AR) of 1, 1.5, 2, 3 and 4 having the same equivalent diameter (D-e) were tested at jet Reynolds number of 10,000 to study the effect of nozzle aspect ratio on the local heat transfer enhancement in the stagnation region. The temperature distributions on the heated flat plate were measured using a thermochromic liquid crystal thermometry with a digital image processing system. The flow structure was visualized using a smoke-wire technique to get a better understanding on the impinging elliptic jet. With varying the nozzle-to-plate spacing, the isothermal contours on the heated flat plate showed the axis-crossover in heat transfer due to the self-induced curvature variation of a three-dimensional jet structure. For smaller nozzle-to-plate spacing (L/D-e < 4), as the nozzle aspect ratio AR of elliptic jet increases, the heat transfer rate was increased larger than that for the axisymmetric jet (AR = 1) in the stagnation region. This was mainly attributed to the engulfing large entrainment and enhanced large-scale mixing of the elliptic jet. The stagnation point Nusselt number was correlated for the nozzle aspect ratio and the nozzle-to-plate spacing as Nu(o) proportional to (AR)(-0.082) (L/D-e)(-0.077). (C) 1999 Elsevier Science Ltd. All rights reserved.