LOCAL HEAT-TRANSFER MEASUREMENTS FROM AN ELLIPTIC JET IMPINGING ON A FLAT-PLATE USING LIQUID-CRYSTAL

Abstract

Local heat transfer measurements are reported fora turbulent submerged air jet issuing, normal to a heated flat plate, from an elliptical nozzle with an aspect ratio of 2.14. The Reynolds numbers used were 5000, 10000, and 20000 and the nozzle-to-plate distances were L/D(e) = 2,4, 6 and 10. The temperature on the heated plate surface was measured using a thermochromic liquid crystal and a digital image processing system that yielded an unbiased means for the quantitative color determination of the liquid crystal. The isothermal contour on the uniformly heated plate changed in shape from elliptic to near circle to elliptic again with an increasing nozzle-to-plate distance as the elliptical cross-section switched its orientation. For L/D(e) from 4 to 10 and for all Reynolds numbers. the local Nusselt number decreased monotonically with increasing radial distance. However, for L/D(e) = 2 and Re = 10000 and 20000, the Nusselt number distributions exhibited second and third maxima. This may be attributed by the induced toroidal vorticities that were formed at the boundary between the impingement region and the wall jet region. For L/D(e) from 2 to 6, the stagnation point Nusselt number varied according to Nu)s) is-proportional-to Re0.5. For a larger nozzle-to-plate distance of L/D(e) = 10, the Reynolds number dependence was stronger (Nu(s) is-proportional-to Re0.74) due to an increase of turbulence in the approaching jet as a result of the stronger exchange of momentum with the surrounding air. The Nusselt number for the elliptic jet in the impingement region was larger than that for a circular jet. This may be caused by the large entrainment rate and large scale coherent structure of the elliptic jet.