Research on Combined Transpiration and Supersonic Film Cooling for Hypersonic Optical Windows

Transpiration cooling, supersonic film cooling, and combined cooling are effective means for thermal protection of critical regions on hypersonic vehicles. Under Mach 6 wind tunnel conditions, the effects of transpiration location, transpiration area, and coolant type on the wall heat flux of a hypersonic optical dome are investigated, and the interaction mechanism of upstream transpiration cooling on downstream supersonic film cooling is numerically explored. Experimental results indicate that transpiration cooling is jointly influenced by transpiration location and area. Transpiration location plays a dominant role at low coolant mass flow rates, while area becomes controlling at high coolant mass flow rates. Coolant physical properties significantly affect transpiration cooling, with high specific heat capacity and low density exhibiting superior cooling performance. Compared with transpiration cooling, supersonic film cooling provides superior thermal protection for the downstream wall. However, transpiration cooling reduces the effective cooling length of the supersonic film by 25.8%. Further analysis indicates that transpiration cooling significantly diminishes the velocity, momentum, and kinetic energy of the downstream mainstream boundary layer. This intensifies the momentum exchange between the supersonic film and the mainstream, thereby increasing the growth rate of the interacting shear layer. Consequently, the mixed gas impinges on the wall prematurely, ultimately degrading the film cooling performance.