EXPERIMENTAL INVESTIGATION OF LOCAL DISTRIBUTION OF HEAT TRANSFER AND FLUID FLOW CHARACTERISTICS OF IMPINGING SLOT AIR JET

Abstract

Jet impingement cooling has received considerable attention due to its inherent characteristics of high heat transfer rate. Impinging flow devices allow for short flow paths and relatively high rate of cooling from comparatively small surface area. Few industrial processes that employ impinging jets are drying of food products, textiles, films and papers, processing of some metals and glass, cooling of gas turbine blades and outer wall of the combustion chamber, cooling of electronic equipment, etc.” Air will continue to be used as the working fluid due to its low cost, easy availability and reliability. “The requirement for increased gas turbine engine performance has led to the use of much higher turbine entry temperature. The higher gas temperatures require active cooling of the turbine blade using compressor bleed air. Heat transfer enhancement by jet impingement has significant potential to increase the local heat transfer coefficient. Arrays of impinging jets are one of the methodscurrently used to reduce the blade temperature on the mid-chord and leading-edge regions. The present study focuses on one of the thermal issues of uniformity of distribution of heat transfer.” The influence of various geometric and flow parameters of the impingement system on the local distribution of heat transfer coefficients are investigated. The local heat transfer characteristics are studied experimentally using a thin foil heater and an infrared thermal imaging technique. “The local heat transfer and fluid flow distribution on smooth flat surface impinged by a single slot air jet is studied for different jet-to-plate distances (0.5Dh-10 Dh) and Reynolds numbers (2500-20000).” Semi-empirical correlations for local heat transfer coefficients in the stagnation region are obtained. Further, local heat transfer enhancements on rough flat surface (detached ribs) impinged by a single slot air jet is examined for different geometric parameters of ribs at various jet-to-plate distances. The influence of confinement and inclination angle of the jet with reference to impingement plate on the fluid and thermal characteristics of the single slot air jet are studied experimentally. Finally, the combined effect of confinement and surface roughness is evaluated for the local distribution of the heat transfer and the pressure on the impingement plate.