Application and research progress of transpiration cooling technology in flight vehicles
[Significance]Aerospace vehicles have undergone significant modifications in terms of aerodynamic shape,flight speed,flight environment,and flight duration compared with conventional flight vehicles.They must withstand harsh aerodynamic thermal environments for long durations and maintain a sharp leading-edge shape with a high lift-to-drag ratio,imposing extremely stringent requirements on the temperature resistance,durability,structural efficiency,and reliability of the thermal protection system.Traditional thermal protection depends largely on passive methods such as heat insulation,heat sink,and radiation heat dissipation.Although the thermal protection performance of related technologies has improved,which is restricted by several constraints,such as ensuring that the prototype is safe under harsh conditions of extremely high heat flux and ultrahigh temperature along with structural stability,long-term operation,light-weight nature,and repeatability.Thus,a new active thermal protection technology is necessary.In this context,transpiration cooling technology offers the advantage of high thermal efficiency without requiring any changes in the prototype of a vehicle.It has been widely considered a potential active thermal protection technology.However,when transpiration cooling is used for thermal protection of a flight vehicle,some challenges related to the complexity of the system,a mismatch between coolant supply and demand,unstable control of the operation,and development of a high-precision prediction model etc.,arise.[Progress]Research on transpiration cooling primarily focused on quick evaluation of performance,numerical simulation of flow and heat transfer,evaluation of cooling mechanism performance,development of optimal control algorithm for efficiency,and optimization of structure form and yielded beneficial results.However,several fundamental scientific issues needed to be urgently addressed to fully realize the engineering application of this technology in aerospace vehicles.In the context of numerical simulation,the accuracy and adaptability of the heat and mass transfer model should be improved.Most existing studies had mathematically described and solved the physical process of heat and mass transfer in porous media at the macroscale.But some parameters related to specific phase change heat and mass transfer(such as evaporation/condensation coefficient and fluid-solid convection heat transfer coefficient)that affect the model's accuracy must be modified through experiments,and the adaptation was partially successful.Most existing models assumed that the temperature of porous media,liquid phases,and gas phases were equal.Although a few models explored the nonequilibrium effect between porous media and fluids,they did not consider the nonequilibrium effect between gas and liquid phases.There were few flight experiments in the research and a large gap between the ground experimental test and practical use conditions.Furthermore,extreme effects related to high-temperature,real,and rarefied gases and shock wave/boundary layer interference during high-speed flight could not be effectively reproduced on the ground.Moreover,there was a lack of experimental data that could be used to verify the accuracy of the heat and mass transfer model.The experimental test method was relatively simple,and the flow and heat transfer process of the liquid in the porous medium could not be obtained.It was challenging to effectively obtain the boundary layer flow law of the liquid when it entered the high-speed mainstream flow from the porous medium.In terms of control strategy,the present research on transpiration cooling control systems lacked a transient simplified mathematical model that could be quickly established,particularly for liquid phase change transpiration cooling with the multiphase flow and phase change process.Simultaneously,there were few transpiration cooling control systems with practical engineering values based on modern control theory,which made it difficult to achieve optimal performance in practical engineering applications.Some adaptive and self-driven transpiration cooling systems had been proposed as new forms of transpiration cooling structures;however,they were still at the mechanism verification stage,and the engineering application effect needed to be verified.[Conclusions and Prospects]Follow-up research will focus on the micro/mesoscale fine numerical calculation model,advanced visual experimental testing methods,rapid response-precise control strategies,self-driven and adaptive structural engineering systems,and combined active and passive thermal protection.
transpiration cooling technologyactive thermal protectionflight vehicleporous medium
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维普
