Application of transpiration cooling on hypersonic vehicles to mitigate material oxidation
Keywords: Transpiration Cooling
Pressure Sensitive Paint
Stagnation Point
Oxidation Protection
Hypersonics

This thesis investigates the application of transpiration cooling to mitigate material oxidation of hypersonic vehicles. This was motivated by the early onset of surface oxidation of many high-temperature aerospace materials and the resulting limitation of the flight envelope. For example, Ultra-High-Temperature Ceramics have melting points exceeding 3500 K, thus could balance the incoming heat flux through re-radiation alone, but start oxidising at 1000 K, which limits the passive cooling capability severely. The aim of this thesis is to increase this oxidation limit by shielding the surface from freestream oxygen through boundary layer mass injection.
A novel pressure sensitive paint diagnostic is developed in this work to measure the concentration of molecular oxygen on a transpiration cooled surface. This is a fundamental tool to quantify the ability of the coolant film to act as a barrier against freestream species that would lead to oxidation. The paint consists of [Ru(dpp)3]2+ luminophores, which are dissolved in a dichloromethane solution and then sprayed onto porous alumina. This method is validated experimentally on a transpiration cooled flat plate in a hypersonic cross-flow in the Oxford High Density Tunnel. Tests were conducted with no coolant injection, nitrogen injection and air injection at increasing blowing ratios. Oxygen partial pressure maps on the transpiration cooled surface were obtained for several conditions at unit Reynolds numbers between 2.58−5.0e7/m and blowing ratios between 0.016%−0.078%. It is found that the oxygen pressure decreases as the unit Reynolds number decreases and the blowing ratio increases. The direct measurements provided by this technique will aid the development of empirical correlations for the oxygen concentration on a porous surface with mass injection.
Before applying this diagnostic to a transpiration cooled stagnation point, a theoretical prediction of the species concentration at the wall is derived. This is achieved by combining the self-similar boundary layer equations with thin film theory. The resulting semi-analytical correlation explicitly expresses the concentration of freestream species on a hypersonic stagnation point as a function of freestream conditions, surface geometry and coolant properties. The concentration depends on the boundary layer edge pressure, temperature and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas. The method is compared against the numerically obtained self-similar solutions of a wide range of flow conditions and showed an accuracy of ±4%.
The correlation was compared to experiments on a transpiration cooled stagnation probe model tested in the Oxford High Density Tunnel. A porous Al2O3 material, developed in collaboration with Imperial College London, features a similar pore size and outflow homogeneity as ZrB2, with the additional capability of bonding PSP. Experiments were conducted at Mach 6.9 at three different Pitot pressures: 10 kPa, 20 kPa and 30 kPa. Nitrogen, Argon and Krypton are used as injection gases at mass flow rates ranging from 0.01 - 0.55 kg/m2s, in order to displace up to 99% of the freestream gas at the surface. The experimental data shows that transpiration cooling is more effective in displacing freestream gas than predicted by analytical models and numerical solutions. The microheterogeneous surface with recessed pores means there is an additional pressure gradient within the first layer of pores, which increases the displacement effectiveness. A test campaign in the plasma wind tunnel at the Institute of Space Systems in Stuttgart provided the first qualitative data of the oxidation behaviour of transpiration cooled ZrB2 in a continuous, steady state high-enthalpy environment. A 42% porous ZrB2 sample is exposed to a cold-wall, fully catalytic stagnation point heat flux of 3.95 MW/m2 at 3.22 kPa Pitot pressure. While the uncooled sample fully oxidised at a surface temperature of 2150 K, 20.25 g/m2s of helium and 620.11 g/m2s of nitrogen injection mitigated oxidation of the transpiration cooled samples.
Combining theoretical, numerical and experimental findings, the oxidation response of the stagnation point in a representative hypersonic flight scenario is predicted. An existing oxidation model for ZrB2 is combined with low order correlations for the heat transfer and oxygen concentration at the surface, to yield the surface recession and oxide scale thickness. A 500 s steady state trajectory at 44 km altitude and 3.6 km/s velocity is found to lead to a 2.2 mm recession of the 3 mm nose radius. A constant mass injection at a blowing parameter of 0.6 reduces the recession to just 0.21 mm. The displacement of freestream oxygen by transpiration cooling has a significant effect on oxidation. Not accounting for the displacement of oxygen at the surface would increase the recession by up to 197%. The recession along the transient trajectory of an envisioned hypersonic vehicle with a 3 mm nose radius is found to exceed 0.94 mm with no mass injection. It is shown that nitrogen and helium injection at a blowing parameter of 0.6 can reduce the recession to 0.13 mm and 0.075 mm, respectively.