Stagnation region heat transfer of a turbulent axisymmetric jet impingement

Abstract

The turbulent heat transfer characteristics in a stagnation region were investigated experimentally for an axisymmetric submerged air jet impinging normal to a heated flat plate. The temperature distribution on the heated flat surface was measured by a thermochromic liquid crystal (TLC) with digital image processing technique. To get precise heat transfer data inside the stagnation region, a fully developed straight pipe nozzle was used in this study. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate spacing as Nu(0) proportional to Re-0.565(L/D)(0.0384). For the nozzle-to-plate spacing of L/D = 2, the stagnation Nusselt number varies according to Nu(0) proportional to Re-0.50 and the local heat transfer rates exhibit two maxima. The primary and secondary maxima are attributed to the accelerated radial flow and the transition from a laminar to a turbulent boundary layer, respectively. For larger nozzle-to-plate spacings (L/D > 6), the local heat transfer decreases monotonically with radial distance. The Reynolds number dependence is enhanced as L/D increases (Nu(0) proportional to Re-0.58 for L/D = 6 and Nu(0) proportional to Re-0.65 for L/D = 10). This results from the increase of the centerline turbulent intensity of approaching jet due to strong momentum exchange with ambient fluids.