Experimental Investigation of Turbulent Supersonic Film-Cooling Flows

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Engines of modern propulsion systems of future space transportation vehicles have to stand extreme thermal loads such that novel materials and more efficient cooling systems have to be developed. At supersonic flow the cooling efficiency is dramatically increased if the cooling fluid is tangentially injected as a supersonic jet. The resulting multiscale problem consisting of free jet, wake, and boundary layer regimes represents a pronounced challenge with respect to modeling and simulation. This intricacy of the cooling flow is massively enhanced by shocks interacting with the cooling film. Especially the location of the shock-cooling-film interaction, i.e., within and downstream of the potential core region of the laminar cooling film, and the density gradient between cooling and outer flow possess a significant impact on the near-wall flow structures and as such on the mixing process and the cooling efficiency. Previous experiments show deviations in the double-digit percentage range and/or sometimes contradictory results, i.e., increasing instead of decreasing cooling efficiency, due to an incomplete measurement of the inflow data and the local flow structures. Furthermore, standard numerical design tools contain simplifying assumptions, e.g., a constant turbulent Prandtl number, that result in a poor prediction accuracy.This means the existing experimental database has to be improved to develop higher-quality prediction methods. Therefore, detailed experiments of a turbulent outer flow (air) and an injected laminar cooling flow (air and helium), i.e., air-air and air-helium cooling configurations are investigated, are to be performed in this project. The location of the shock-cooling-film interaction, the shock strength, and the density gradient are varied. The inflow distribution of the density and the velocity and the near-wall velocity distribution are measured by high-speed PIV and at helium cooling the concentration is qualitatively captured by light-sheet methods, to determine the dependence of the mixing process from the shock interaction. To better understand the main mechanisms of the cooling efficiency, it is of primary interest to capture in detail the mixing process, which is determined by the location of the shock-cooling-film interaction and the density gradient between cooling film and outer flow, and the distribution of the turbulent Prandtl number.