Application of Transpiration Cooling in Heat Shields of Hypersonic Vehicles to Mitigate Material Oxidation

发汗冷却是一种很有前途的主动热防护系统，它将冷却剂气体送入多孔材料中。气体通过内部对流冷却材料，并在离开多孔介质时形成保护膜。这种薄膜减少了空气加热，还可以起到阻挡质量扩散的作用。该项目将研究发汗冷却是否可以防止高超声速飞行器隔热罩上的材料氧化。限制隔热罩冷却性能的因素之一是氧化，氧化会导致烧蚀和表面退缩，不利于隔热罩的性能。对于许多超高温陶瓷(UHTCs)来说，氧化温度是材料的极限边界。这降低了辐射造成的被动冷却性能，并限制了飞行包线。因此，氧化的缓解将对高超声速飞行器的隔热设计做出重大而实质性的贡献。2018年9月在斯图加特美国国税局的PWK 1等离子风洞进行的测试活动标志着这项调查的第一个里程碑。一个4 2\%的多孔$ZrB2$圆盘将暴露在3$mW/m^2$和3.5$mW/m^2$的驻点热流密度下。结合氧化理论模型的数值模拟预测，未冷却的样品将在这些热流密度下开始氧化。预计地表温度为1800摄氏度和2100摄氏度。冷却剂的质量流量将增加，直到Echelle光谱仪检测到氧化产物的特征谱线强度降低。这一结果将为发汗冷却引起的氧化还原提供初步的实验评估。第二个里程碑将是研究如果在冷却剂中加入一部分氨($NH_3)，是否可以进一步减少氧化。$NH3$与氧发生反应，可能会在表面催化之前捕获边界层中的氧分子和原子。这可以减少表面热流，防止表面损伤。第三个里程碑将是在高密度隧道进行的实验，它可以复制真实飞行的气动再入条件，如雷诺数和马赫数。将在多孔表面使用压敏涂料来标记氧气扩散的变化。本项目涉及EPSRC的工程主题。它与EPSRC的战略一致，因为它专注于一项技术，在蒸腾冷却研究计划的帮助下，英国将成为世界领先的技术。参与的合作者包括为UHTCS提供材料的伦敦帝国理工学院和曼彻斯特大学，后者将在最后一次实验中分享他们在压敏涂料方面的专业知识。

Transpiration cooling is a promising active thermal protection system (TPS), in which a coolant gas is fed through a porous material. The gas cools the material through internal convection and forms a protective film upon exiting the porous medium. The film reduces aerothermal heating and can also act as a barrier against mass diffusion. This project will investigate whether transpiration cooling can prevent material oxidation on the heat shields of hypersonic vehicles. One of the limiting factors for the cooling performance of heat shields is oxidation, which leads to ablation and surface recession and is detrimental for the heat shield performance. For many Ultra-High-Temperature-Ceramics (UHTCs), the oxidation temperature is the limiting boundary of the material. This reduces the passive cooling performance due to radiation and confines the flight envelope. The mitigation of oxidation would hence make a significant and substantial contribution to the heat shield design of hypersonic vehicles. A test campaign in the PWK 1 plasma wind tunnel at the IRS in Stuttgart in September 2018 marks the first milestone of this investigation. A 42\% porous $ZrB_2$ disk will be exposed to stagnation point heat fluxes of 3 $MW/m^2$ and 3.5 $MW/m^2$. A numerical simulation coupled with a theoretical model for $ZrB_2$ oxidation predicts that the uncooled sample will start oxidising at these heat fluxes. Surface temperatures of 1800 $\degree$C and 2100 $\degree$C are to be expected. The coolant mass flux will be increased until the Echelle spectrometer detects a reduction in the intensity of the characteristic spectral lines of the oxidation products. The results will provide an initial experimental assessment of the oxidation reduction due to transpiration cooling. The second milestone will be an investigation of whether oxidation can be further reduced if a fraction of Ammonia ($NH_3$) is added to the coolant. $NH_3$ reacts with oxygen and could potentially capture the oxygen molecules and atoms in the boundary layer before they undergo surface catalysis. This could reduce the surface heat flux and prevent surface damage. The third milestone will be an experiment in the High Density Tunnel, which can replicate the real-flight aerodynamic re-entry conditions such as Reynolds and Mach number. Pressure sensitive paint will be employed on the porous surface to mark the changes in oxygen diffusion.This projects relates to the Engineering theme of EPSRC. It aligns with the EPSRC strategy, since it focuses on a technology in which the UK is set to become world-leading, thanks to the Transpiration Cooling Research Programme. The collaborators involved include Imperial College London, who are supplying the UHTCs and the University of Manchester, who will share their expertise on Pressure Sensitive Paint for the last experiment.