Experimental investigation on seeping gas film effectiveness in supersonic flow downstream of a porous injector

Abstract

Based on seepage flow through porous media, the seeping gas film cooling method is an effective means to protect large areas of optical windows and other hot components of hypersonic vehicles. Here, the pressure-sensitive paint (PSP) technique was applied and experiments were conducted on seeping gas film effectiveness downstream of a porous injector. A Mach 3 wind tunnel was used to explore the influence of different cooling gas blowing ratios, gas types, and incoming boundary-layer conditions on film effectiveness. Results show film effectiveness decreases monotonically along the downstream wall; however, with blowing ratio increases the film effectiveness increases linearly. Once the blowing ratio exceeds 0.4% film effectiveness ascends nonlinearly, indicating that the contribution to film effectiveness per unit mass flow-rate promotion gradually decreases. In contrast, the non-uniformity of the film coverage on the downstream wall in the spanwise direction becomes more significant at a higher blowing ratio. For the same blowing ratio, helium, with a low molecular weight has a higher film effectiveness compared to nitrogen and carbon dioxide. With blowing ratio less than 0.4 %, the film effectiveness downstream of the porous injector under laminar flow conditions is almost twice that of turbulent flow. However, with blowing ratio above 0.5 %, the growth rate of the film effectiveness decreases dramatically to 50 %. Preliminary analysis suggests that this is caused by the complete transition of the laminar boundary layer to turbulent flow after passing through the porous wall under high blowing ratios, where the mixing effect of turbulence is fully manifest.