| Aerospace technology has been highly valued by every country due to its great significance in military strategy and national economy.However,with an increase in the power and speed of aerospace vehicle,key components,such as combustor wall,nose cone of aircraft and so on,encounter increasingly severe thermal protection problems.Transpiration cooling has extremely high cooling efficiency,and thereby is regarded as the potential active cooling technology to solve the thermal protection problems of key components under extreme thermal environment.This paper carries out deeply theoretical and numerical investigations around the difficulties and deficiency in basic theory and practical application of transpiration cooling.For transpiration cooling using gaseous coolant,a simplified local thermal non-equilibrium model is widely used to describe the heat transfer process within porous matrix.However,this model ignores the thermal diffusion term of fluid in energy conservation equation,so that brings in a certain error in theoretical results.To evaluate the applicability of this simplified model,one-dimensional steady-state transpiration cooling process is simulated under typical operating conditions,and the influences of environmental conditions,coolant injection rate,and porous media properties on the error of the simplified model are discussed.Compared with transpiration cooling using gaseous coolant,transpiration cooling using liquid coolant has stronger cooling capacity due to the huge latent heat released in coolant phase change process.However,the processes of fluid flow,heat absorption,and phase change within pores are extremely complicated,and the existing theoretical models,including separate phase model(SPM)and two-phase mixing model,have obvious drawbacks when used to numerical simulation.To avoid the above issues,an improved semi-mixed model(SMM)is proposed,and validated by a transpiration cooling experiment using liquid water as coolant on a hot wind tunnel.Through the comparisons of the three models,it is found that the numerical results obtained by SPM and SMM are relatively accurate and reasonable.Based on SMM,the influences of local thermal equilibrium condition and local thermal non-equilibrium condition on the numerical results are further discussed,and the results indicate that the local thermal non-equilibrium effect cannot be ignored.Finally,the reasonability of three cold side boundary conditions and three hot side boundary conditions usually applied in numerical simulations are analyzed for the mathematical model of transpiration cooling with phase change under local thermal non-equilibrium condition.The aerodynamic heat and force loaded on aircraft during maneuver flight are always varying with time,which moment-to-moment affect the temperature response of transpiration cooling structure,the transport properties of coolant within pores,and the phase change position of liquid coolant.Therefore,it is necessary to investigate the transient problems of transpiration cooling with phase change.This paper numerically simulates one-dimensional transient transpiration cooling with phase change,and discusses the stability of two-coolant injection methods,i.e.,coolant injected with set pressure and set mass flow rate.The results show that the method of injection with set pressure is stable only when the set pressure is large enough,while the method of injection with set mass flow rate is always stable.Then the effects of initial temperature,porous media properties,and operating conditions fluctuation on the transient transpiration cooling performance are further investigated when the coolant is injected with set mass flow rate.In practical applications,the geometry of real cooling structure is very complicate,such as combustor wall and nozzle throat.In addition,the aerodynamic heat and force generated during the flight are usually unevenly distributed.Accordingly,transpiration cooling process presents obvious spatial differences,significant error will be caused when one-dimensional simplification is used in this situation.Therefore,the two-dimensional steady-state transpiration cooling with phase change on a porous plate is studied using the SMM,and the influence of non-uniform heat flux on the transpiration cooling performance and coolant transport characteristics is analyzed.The analysis indicates the local high temperature leads to serious heat transfer deterioration.To improve the heat transfer deterioration,three configuration optimization methods are designed and their effects are compared.Finally,the feasibility of applying transpiration cooling with phase change into the thermal protection of aircraft nose cone under supersonic mainstream condition is investigated using a semi-coupled numerical strategy. |
