Integrated Analysis for the Design of Reusable TPS based on Variable Transpiration Cooling for Hypersonic Cruise Vehicles (Invited Paper)

摘要

The thermal management of hypersonic air-breathing vehicles presents formidable challenges. Reusable thermal protection systems (TPS) are one of the key technologies that have to be improved in order to use hypersonic vehicles as practical, long-range transportation systems. Both the aerodynamic and the material performances are strongly related to the near-wall effects. The viscous dissipation within the hypersonic boundary layer, coupled with the high dynamic pressure flight trajectories, generates surface temperatures for which the strength and the environmental durability of the material can be widely exceeded. In this type of environment, active cooling systems have to be considered in order to afford long duration flights in hypersonic regime. Transpiration cooling represents a promising technique in terms of temperature reduction and coolant mass saving. In order to explore the potential of this technique, it is important to understand the physics that characterize the boundary layer and its interaction with the vehicle's surface. The integrated analysis of the hypersonic boundary layer coupled with the thermal response of a porous medium is performed here for a flat plate and a 2-D blunt body configuration. A constant value of the transversal wall velocity is used to simulate uniform transpiration. A saw-tooth wall velocity distribution is used to simulate the variable transpiration strategy. An equal amount of coolant usage has been imposed in order to compare the cooling effectiveness in the two cases. The uniform transpiration allows a reduction of 49% on the stagnation point heat flux in comparison with the case without transpiration. The variable transpiration reduces the stagnation point heat flux by an additional 7% with respect to the uniform transpiration case. The heat fluxes derived from the solution of the hypersonic boundary layer as well as the imposed wall temperature are used to perform an integrated analysis that includes the porous material. The test cases analyzed emphasize the importance of evaluating the influence of the material's thermo-physical properties at the initial design stage. For the flight conditions considered in this analysis a combination of low material porosity and high thermal conductivity are necessary to generate the required injection strategy. The integrated analysis is essential for the purpose of establishing the optimum transpiration strategy needed to maintain the surface temperatures in the required range. The change in the transpiration distribution along the vehicle surfaces (variable transpiration) allows to selectively cool down the structure in the regions where the higher heat fluxes are located (i.e. nose, leading edges) and diminishes the amount of required coolant fluid. The transpiration for the blunt body can be limited to the regions where the local wall heat flux is greater than or equal to approximately 20% of the stagnation point heat flux. This strategy allows the reduction of the total amount of coolant by 62% for the uniform transpiration and by 58% for the variable transpiration.