Heat transfer characteristics of circular impinging jet arrays in an annular section with cross flow effects

Abstract

Jet impingement has been shown to be an effective method for enhancing convective heat transfer. There are a variety of applications of impinging jets in industry, including tempering and shaping of glass, annealing of metal and plastic sheets, cooling of electronic components and drying of paper, veneer, etc. Another application is the use of impinging jets in internal cooling channels in modern gas turbine blades. High velocity jets provide an effective way to augment the rate of heat removal from the blades to the cooling air, maintaining the blade temperatures within tolerable and metallurgical limits. This experimental research examines, for turbulent parallel and counter flows of air through an annular channel with holes for jet impingement along the inner tube, the effects of varying the geometries of the channel and the hole array, and the air flow rate, on the heat transfer distribution on the inner surfaces of the outer tube. Air enters one end of the inner tube, whose other end is closed, passes through the holes on the inner tube and exits through one end of the annular passage between the inner and outer tubes. Each hole array has inline or staggered circular holes with a given diameter and has a fixed number of holes around the circumference of the inner tube at any cross section of the annular channel. Heat transfer experiments are performed for Reynolds numbers of 5,000, 12,250 and 30,000 to determine the distribution of the regionally average Nusselt numbers along the outer tube, as a result of the impinging jets along the inner tube. Pressure measurement experiments give the pressure distributions in the airflows in the inner tube and between the inner and outer tubes. The pressure data is needed to determine the mass flow rates of the impinging jets along the inner tube. The results of the experiments are presented and discussed, the Nusselt numbers for the various cases studied are compared and major findings are reported. The results of this investigation should be particularly useful to engineers in the design of the leading edges of airfoils of modern gas turbine blades.