Experimental Investigation of Turbulent Supersonic Film-Cooling Flows

未来空间运输飞行器的现代推进系统的发动机必须承受极端的热负荷，因此必须开发新的材料和更高效的冷却系统。在超音速流动条件下，当冷却液以超音速射流的形式切向喷射时，冷却效率显着提高。由此产生的由自由喷流、尾迹和边界层组成的多尺度问题对建模和模拟提出了明显的挑战。这种错综复杂的冷却流动由于激波与冷却膜的相互作用而大大增强。尤其是激波-冷却-气膜相互作用的位置，即在层流冷却液膜的势核区内和下游，以及冷却和外流之间的密度梯度对近壁流动结构，从而对混合过程和冷却效率有显著的影响。以前的实验表明，由于对流入数据和局部流动结构的测量不完整，导致了两位数百分比范围的偏差和/或有时相互矛盾的结果，即冷却效率增加而不是降低。此外，标准的数值设计工具包含简化的假设，例如一个恒定的湍流普朗特数，这导致预测精度较低，这意味着必须改进现有的实验数据库以开发更高质量的预测方法。因此，本项目将对湍流外部流动(空气)和注入层流冷却流动(空气和氦)，即空气-空气和空气-氦冷却结构进行详细的实验研究。激波-冷却-气膜相互作用的位置、激波强度和密度梯度是不同的。用高速PIV测量了入流密度和速度的分布以及近壁速度分布，并用光片方法定性地捕捉了氦冷却时的浓度，以确定激波相互作用对混合过程的依赖关系。为了更好地了解冷却效率的主要机理，有必要详细地捕捉混合过程，这是由激波-冷却-气膜相互作用的位置、冷却气膜与外部流动之间的密度梯度以及湍流普朗特数的分布决定的。

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.