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This paper presents the performance of pressure sensitive paint (PSP) for the direct investigation of a transpiration cooled surface. This technique allows the quantification of the pressure distribution on a porous surface with blowing. Additionally, it can be used to detect where the coolant is on the surface, thus measuring the transport of molecular oxygen to a transpiration cooled surface. For highly turbulent flows, it can also be used to evaluate the film effectiveness. A porous aluminium sample was anodised and dip coated in a PSP luminophore solution. It was fitted into a flat plate model and exposed to a Mach 5 cross-flow in the Oxford High Density Tunnel. Tests were conducted with no coolant injection, air injection and with nitrogen injection at increasing blowing ratios. The film effectiveness of the transpiration cooled surface was obtained for several conditions at Re = 15.5 - 31.5 10^6/m and F = 0.001 - 0.002. The film effectiveness increases as the Reynolds number decreases and the blowing ratio increases, which is in good qualitative agreement with the literature. Furthermore, it shows the same features as a velocity map of the outflow.

This paper presents transpiration cooling experiments conducted in the Oxford High Density Tunnel. The flow structure and mixing phenomena between the injected coolant gas and the hypersonic free stream are investigated. A flat plate model is used which is equipped with a gas injection system, four surface mounted heat flux gauges and three surface pressure sensors located downstream of the injection point. Mass injection is realized through a porous ceramic material, and alternatively through a series of slots. Spatially and temporally resolved film effectiveness downstream of the injection point is measured with fast response pressure sensitive paint. The flowfield is visualised by a Z-type Schlieren imaging system. Results show a significant influence of injection on the boundary layer thickness through the additional coolant gas. High blowing ratios can lead to early transition which increases coolant-external flow mixing and leads to a lower film effectiveness. Transpiration cooling requires approximately two orders of magnitude lower blowing ratios to achieve a similar film coverage as in the case of the injection through slots. The film effectiveness increases with an increasing blowing ratio for locations close to the injection point. However, a lower blowing ratio results in a more stable film for locations further downstream.