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

蒸腾冷却是一种很有前途的主动热保护系统（TPS），其中冷却剂气体通过多孔材料进入。所述气体通过内部对流冷却所述材料，并在离开所述多孔介质时形成保护膜。薄膜减少了空气热加热，也可以作为一个屏障，防止质量扩散。该项目将研究蒸腾冷却是否可以防止高超音速飞行器隔热板上的材料氧化。影响隔热板冷却性能的因素之一是氧化，氧化会导致烧蚀和表面衰退，不利于隔热板的冷却性能。对于许多超高温陶瓷（UHTCs），氧化温度是材料的极限边界。这减少了由于辐射和限制飞行包线的被动冷却性能。因此，减缓氧化将对高超音速飞行器的隔热板设计作出重大和实质性的贡献。2018年9月，在斯图加特IRS的PWK 1等离子体风洞中进行的测试活动标志着这项调查的第一个里程碑。多孔率为42%的ZrB_2盘，其滞止点热通量分别为3 $MW/m^2$和3.5 $MW/m^2$。结合ZrB_2氧化理论模型的数值模拟表明，未冷却的样品将在这些热通量下开始氧化。预计表面温度为1800 $\度$C和2100 $\度$C。冷却剂的质量通量将增加，直到梯队光谱仪检测到氧化产物的特征谱线的强度降低。结果将提供由于蒸腾冷却氧化还原的初步实验评估。第二个里程碑将是研究在冷却剂中加入少量氨（NH_3）是否能进一步减少氧化。NH_3与氧发生反应，在氧分子和氧原子进行表面催化之前，可能会捕获它们。这可以减少表面热通量，防止表面损坏。第三个里程碑将是高密度隧道的实验，它可以复制真实飞行的空气动力学再入条件，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。本项目与EPSRC的工程主题相关。它与EPSRC的战略一致，因为它专注于一项技术，由于蒸腾冷却研究计划，英国将成为世界领先的技术。参与的合作者包括提供uhtc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.