Development of an experimental technique for high temperature impingement studies
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Modern gas turbine engines commonly operate at temperatures above the melting point of the turbine's blades and vanes. Internal and external cooling of the blades is required for sustained operation and prolonged engine life. Jet impingement, an aggressive form of cooling, is typically used in the airfoil leading edge which is exposed to extreme heat loads. Temperature differences between the cooling jets and blade walls can exceed 1000F (556C) in gas turbine engines. Frequently, jet impingement experimentation is performed at significantly lower temperature differences on the order of 60F (33.3C). The current experimental investigation attempts to bridge the gap between low temperature impingement studies and realistic turbine conditions. Using a transient technique, the lumped capacitance analysis, average Nusselt numbers are obtained from a row of round jets impinging on a leading edge model. Aluminum plates imbedded in a cylindrical target surface act as the lumped capacitance masses from which average Nusselt numbers can be obtained. The geometry of the impingement configuration is fixed at l/d = 4, s/d = 4, and D/d = 5.5. The jet Reynolds number and jet-to-target surface temperature difference range from 5,000 - 20,000 and 60F - 400F (33.3C - 222C), respectively. Heat transfer results are compared to existing leading edge impingement correlations. Over the range of temperatures considered in this investigation, the measured Nusselt numbers compare favorably to those predicted by existing correlations derived from low temperature experiments. Although lower than actual gas turbine engines, the temperature differences investigated in this study represent an advance in impingement cooling research.
