Experimental Investigation of Turbulent Supersonic Film-Cooling Flows

未来空间运输工具的现代推进系统的发动机必须承受极端的热负荷，因此必须开发新材料和更有效的冷却系统。在超音速流动时，如果冷却流体作为超音速射流切向喷射，则冷却效率显著增加。由此产生的多尺度问题组成的自由射流，尾流，边界层制度是一个明显的挑战，建模和仿真。激波与冷却膜的相互作用大大增强了冷却流的复杂性。特别是激波-冷却膜相互作用的位置，即，在层流冷却膜的潜在核心区域内和下游，以及冷却和外部流之间的密度梯度对近壁流动结构具有显著影响，因此对混合过程和冷却效率具有显著影响。先前的实验显示出两位数百分比范围内的偏差和/或有时矛盾的结果，即，由于流入数据和局部流动结构的测量不完整，冷却效率会增加而不是降低。此外，标准数值设计工具包含简化假设，例如，湍流普朗特数恒定，导致预测精度差，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，湍流外部流（空气）和注入的层流冷却流（空气和氦气）的详细实验，即，空气-空气和空气-氦气冷却配置进行了研究，将在这个项目中进行。激波-冷却膜相互作用的位置、激波强度和密度梯度是变化的。通过高速PIV测量了密度和速度的流入分布以及近壁速度分布，并且在氦气冷却时通过光片法定性地捕获浓度，以确定混合过程对激波相互作用的依赖性。为了更好地理解冷却效率的主要机制，主要感兴趣的是捕获详细的混合过程，这是由激波-冷却膜相互作用的位置和冷却膜与外部流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.