Technical NoteTurbulent heat transfer from a convex hemispherical surface to a round impinging jet

Abstract

As an effective method to enhance heat and mass transfer, impinging jets have been widely used in a variety of engineering applications such as cooling of hot steel plates, tempering of glass, drying of papers and textiles, cooling of turbine blades and electronic components, and manufacturing of TFT-LCD plate. Numerous studies of the heat transfer and flow characteristics for the jet impingement on surfaces have been reported. These studies have dealt with the effects of Reynolds number, nozzle-to-surface distance, nozzle geometry, jet orientation, multiple jets, cross flow, and impinging surface shape on the resulting flow and heat transfer. Several review and summary papers on the impinging jet heat transfer have been published by Martin [1], Jambunathan et al. [2], and Viskanta [3]. Although many different jet characteristics have been considered, a great majority of impinging jet studies in the past were on the flat surface. However, many engineering applications of jet impingement cooling on curved surfaces may be encountered. A few papers have studied the impinging jet heat transfer from the curved surface. Chupp et al. [4]studied the heat transfer characteristics with an array of round jets impinging on a concave surface. Thomann [5]and Hrycak [6]reported the total heat transfer on the concave surface is higher than on the flat surface due to larger surface area especially for small nozzle-to-surface distances. For a highly concave surface, the optimum distance is less than for the equivalent flat surface case. With decreasing surface curvature and increasing nozzle-to-surface distance, the heat transfer characteristics approach those for the flat surface. Gau and Chung [7]investigatged the effects of surface curvature on the slot jet impingement heat transfer along semi-cylindrical concave and convex surfaces. They observed a series of three-dimensional counterrotating vortices near the stagnation point region on the convex surface. And they also found Taylor–Görtler vortices initiated along the concave surface. Yang et al. [8]investigated jet impingement cooling on the semi-cylindrical concave surface with two different nozzles (round edged nozzle and rectangular edged nozzle) . In the present study, the local Nusselt numbers are measured for an air jet issuing from a long straight pipe and impinging perpendicular on the convex hemispherical surface. The experiments are made for Re = 11 000–87 000, L⧹ d = 2–10, and D⧹ d = 10.6. An electrically heated gold film Intrex (a very thin gold-coated polyester substrate sheet) is used to create a uniform heat flux on the convex surface. The temperature on the surface is measured using a thermochromic liquid crystal and a digital color image processing system.