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

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.