Theoretical And Numerical Investigations On Transpiration Cooling Of Hypersonic Vehicles

The aerospace industry has a significant improvement effect on the society development and human lifestyle,and therefore every developed country has attached importance to aerospace industry.In recent years,widely focused hypersonic gradually may promote convenient civil transport and advanced military strategy in the future.However,thermal management technologies always restrict the development of hypersonic vehicles.Among these,transpiration cooling benefiting from the massive specific surface area formed by pores inside the porous media and the uniform film coverage reveals the outstanding capability of active thermal protection and intelligent control.Therefore,it has the potential to overcome an extremely heat environment confronted by hypersonic vehicles.For detailed investigating the characteristics of transpiration cooling,this thesis carries out relatively comprehensive research on theoretical models and numerical methods.In the aspect of transpiration cooling with gaseous coolant,this thesis analyses the aerodynamic heat and force characteristics with hypersonic fly,as well as low-speed flow and heat absorption of coolant inside the pores of porous matrix.The mathematical model with the corresponding solution of the density-based coupled internal flow and external flow has been improved including single-phase flow and heat absorption inside the porous matrix,Favre-average Navier-Stokes equations for compressible turbulence and two models describing real gas effect at the high-temperature environment i.e.,chemical non-equilibrium model and chemical equilibrium model.The applicability of the two models has been discussed,and the results show that the duration of the non-equilibrium effect is shorter than 0.15 ms at 1 atm,so the chemical equilibrium model can be used.However,as the altitude of fly increases,the ambient pressure decreases,the chemical reaction rates will slow down and the chemical non-equilibrium effects cannot be ignored.Using a wedge leading edge of hypersonic vehicles as specimen,transpiration cooling is applied by gaseous coolant.The characteristics of the cooling system are numerically researched by the coupled model of the pressure and temperature fields inside and outside porous matrix at different Mach numbers.In simulations,the real gas properties obtained from the chemical equilibrium model and approximate gas properties are both used.The numerical results predicted by the real gas properties and approximate gas properties indicated that using the real gas properties,the cooled wall temperature of transpiration cooling is lower than that of approximate gas properties,but the nonuniformity of the cooled wall temperature is enhanced,and these two effects become more significant with the increase of Mach number.Transpiration cooling with liquid coolant phase change owns a more obvious capability of thermal protection than gaseous coolant,because of the huge latent heat.However,the mathematical model describing liquid phase change inside the porous matrix is quite complicated and its solution is also very difficult.The main reason is that a two-phase region of vapor and liquid may appear in the porous matrix during coolant flow and heat absorption,the relevant mathematical model includes a series of nonlinear coupled equations,and the location of the two-phase region needs to be obtained by tracking the moving boundary of phase change.So,the numerical solution is extremely difficult.To overcome these problems,a new and modified separated phase model is established in this thesis,which includes a pressure modification equation to solv non-linear coupled equations,and a phase equation to capture the unstable interface of phase change.Based on this modified model,a solver is coded using the open-source computational fluid dynamics framework OpenFOAM.In the past numerical researches of transpiration cooling with liquid phase change,the constant property model of water are mostly used,which means that the temperature of phase change is considered as a constant,and the errors caused by the assumption has been not discussed.In this thesis,the real property of water and vapor obtained by a published model(IAPWS-IF97)is used,and the numerical results are compared with that of the constant property model.The comparison reveals that the temperature distribution of the fluid in the two-phase region,if the numerical results using the constant assumption are significantly different from that using the real property.The reason is that the phase change is not only dependent on local temperature,but also dependent on local pressure,and the significant increase of pressure caused by the viscosity of coolant from the real property model changes the performances of the liquid coolant phase change.Since the transpiration cooling system of hypersonic vehicles will experience quickly changed external environment during maneuvering flight,it is necessary to study its transient cooling characteristics.Using the modified separated phase model,this thesis simulates the transient transpiration cooling with liquid phase change of a flat plate,and analyzes the responses of coolant flow and heat transfer under the suddenly changed heat flow and external pressure.From the numerical results,some unique and important phenomena are identified,including the counter flow and condensation of coolant during vaporization,and the transient temperature fluctuation due to the pressure change at the hot side.These phenomena can provide important references for the designers of the transpiration cooling systems of hypersonic vehicles.Four innovative points of the research work are summered follows:1)The densitybased coupled model of internal and external flow is proposed,and the applicability of chemical equilibrium and non-equilibrium model of gas properties under hypersonic conditions is discussed.2)To overcome the problems of complex mathematical model and solve the phase change process,a pressure modified equation and a phase equation are introduced into the original separated phase model,and a solver is coded using the open-source computational fluid dynamics framework OpenFOAM.3)In the most previous researches of two-phase flow on porous media,the temperature of phase change is considered as a constant,but the errors caused by this assumption have been not discussed.This thesis analyzes the errors introduced by the constant boiling temperature through the comparison with the results obtained by the real properties of water and vapor.4)To accommodate the transient changes of aerodynamic heat and force during the maneuvering cruise of hypersonic flight,the responses of the velocity,pressure and temperature fields of transpiration cooling within the porous matrix are investigated in this thesis.