Experimental And Numerical Investigations On The Key Problems Of Transpiration Cooling

With the development of hypersonic flight technology,the design of efficient and active thermal protection systems has become a great challenge for key structures suffered increasing severe thermal environment,such as the leading-edges and combustion chamber of scramjet engines.Transpiration cooling,carried by the porous material,has a great application prospect in the thermal protection of key structures of hypersonic vehicles due to the advantages of excellent cooling efficiency and small coolant consumption,thus it has become the focus of scientific researchers.At present,there are still many difficulties and unkonwns in the research of transpiration cooling,for example,the spatial distributions of aerodynamic thermal/force are non-uniform over the leading edges,the phenomenon of insufficient cooling at stagnation point and overcooling downstream occurs when adopting the uniform porous matrix or uniform injection pressure.Based on the non-uniform pressure and heat flux distributions,how to supply coolant on demand by means of innovative structure design or manipulating material parameters,and then optimize transpiration cooling system with the purpose of "locally enhancing the cooling efficiency and getting uniform temperature distribution using a small coolant consumption"?Besides,the transpiration cooling process with liquid water phase change is hard to control,and the phenomena of ice formation and temperature oscillation may seriously affect the stability of cooling systems,thus it's of vital importance to study the transient cooling process of liquid water in depth and to modify the coolant properties or explore an alternative liquid coolant.In this dissertation,experimental and numerical methods are adopted to explore the key problems about the aerodynamic thermal/pressure environment and optimization design of transpiration cooling system.The main work is summarized as follows:(1)The optimization of transpiration cooling through gradient porosity design is limited in theory aspects,and there are few experimental data sets available.The transpiration cooling characteristics with gradient porosity layout under low-speed and high-temperature flow condition are investigated experimentally and numerically.Two wedge-shaped porous nose cones made of porous material with gradient porosity are used as test specimens.At first,using air as coolant,the surface temperature of test specimen is captured by IR-camera,the influence of porosity layout on the single-phase gaseous transpiration cooling effect is investigated with the help of numerical simulations.Then,using pure water as the coolant medium,the influence of porosity layout on the transpiration cooling effect with coolant phase change is investigated,and the phase change effect on the cooling performances of gradient porosity layout is discussed.The results demonstrate the feasibility of gradient porosity design by locally enhanced cooling effect in stagnation regions,and depict the cooling behavior differences with the presence and absence of coolant phase change.The experimental data sets provide us comprehensive supports for the optimization of transpiration cooling TPS through gradient porosity design.(2)Considering the existence of shock interaction in real flow field,the mechanism of shock interaction effect on the supersonic aerodynamic thermal/pressure characteristics and transpiration cooling of leading edge is numerically investigated.In view of the typical characteristics of aerodynamic thermal/pressure at the location of inlet cowl clip,a simplified physical model is designed.The detailed flow fields and aerodynamic thermal/pressure characteristics are obtained under the given arc-heated wind tunnel environment.The numerical results show that the aerodynamic thermal and pressure data vary dramatically in mainstream and extension directions.The flow field and heat transfer characteristics of different transpiration cooling methods with nitrogen as coolant are investigated,which can provide good guidance for the further design of transpiration cooling systems in the ground test.(3)To solve the severe problem of the lowest cooling effectiveness in the stagnation point of leading-edge structures,as well as the difficulty of capturing high heat-flux region due to changing operating conditions and shock interactions,an adaptive locally activated transpiration cooling method is proposed,i.e.a combined cooling method of sublimation and transpiration.The porous matrix is covered with an impermeable coating layer with a lower melting point,and the combined cooling method is achieved by the sublimation of coating layer.Using nickel-based superalloy sintered porous plate as the substrate,and Teflon as the sublimation coating material,the experiment is carried out under the high-temperature wind tunnel,the feasibility and high efficiency of the new combined cooling method is verified.(4)According to the unstable cooling phenomenon and ice formation phenomenon in the phase change transpiration cooling process of liquid water,a possible way to modify the coolant properties by propylene glycol(PG)additive is proposed.Using a porous flat plate as test specimen and four propylene glycol aqueous solutions(PG-water)with different PG concentrations as coolants,a series of experiments are carried out to investigate the transient cooling performances and the transient data variations of surface temperatures and driving forces are recorded.Through the analysis and comparison of experimental results,the effect of PG concentration and coolant injection ratio on the phase change transpiration cooling characteristics is revealed.