Theoretic Investigation On Basic Problems Of Transpiration Cooling

With the development of the aero space technology,a highly efficient cooling technology has been gradually become a hot topic in the research area of engineering thermalphysics and astronavigation.In order to meet the requirements of the improving aero space vechicle performance and increasing the gas turbine efficiency of fighter aircraft,enhancing the capacity of anti-high temperature and antioxidant of hot components is an essential problem.Although the transpiration cooling with porous media has been more and more attented and studied due to its high cooling effectiveness,there are many problems in the investigations of experimental methods, theoretical models and numerical approaches.Except the traditional heat and mass transfer problems,there are the contentions of mathematical models and boundary conditions,limitations of phase-change problem of liquid coolant within porous media,and so on.In addition,the most past investigations on transpiration cooling were limited within the effect of individual parameter on cooling efficiency,and not integrated with the acual application together.With the increasing requirement of the cooling efficiency,how to achieve the highest cooling efficient using the least coolant consumption? how to meet the requirement of these thermal protections using a reasonable combination of two porous material layers? Such as basic scientific issues are that the researchers in developed countries have to face,and at the same time; these issues provide to our scientific researchers with new opportunities and challenges.Based on the theoretical analysis and numerical simulation,this dissertation will present some explorative investigations on the basic problems of transpiration cooling.Boundary conditions of local thermal non-equilibrium model:Firstly the thermal boundary conditions of porous media can be divided into two types:at solid-solid interface and at solid-fluid interface.The solid-fluid boundary conditions can be also divided into two categories:parallel and vertical relations between internal and external fluid flow fields.The boundary conditions of the transpiration cooling process are usally in solid-fluid-vertical type;such boundary conditions have been controversial in the academic community for a long time.To compare and discusse five kinds of different boundary conditions(BCs),an analytical solution of a steady and one-dimensional problem of transpiration cooling described by a local thermal non-equilibrium(LTNE) model is presented in this work.The influence of the five BCs on temperature field and cooling effectiveness is discussed using the analytical solution.Two physical criteria:1) if the analytical solution of coolant temperature may be higher than hot gas temperature in a steady state,2) if the variation trend of thermal effectiveness with coolant mass flow rate at hot surface is reasonable,are used to estimate the five BCs.Through the discussions,it is confirmed which BCs in all cases are usable,which BCs under certain conditions are usable,and which BCs are thoroughly unreasonable.Coupled effect of cooland injection and external hot flow in transpiration cooling process:In the past studieds,the investigations on transpiration cooling were generally focused on the internal coolant flow filed and hot surface temperature,but an important effect of the coolant flow on the external hot flow field was neglected.In this work,heat transfer characteristics in the laminar boundary layer with transpiration cooling function are numerically analyzed by an integral method of coolant injection and thermal boundary layer.The effects of coolant injection ratio,the Re and Pr numbers of the exterior hot flow on the temperature at porous plate surface are discussed.Phase change problem in transpiration cooling with liquid coolant:Because of the limitation of heat-absorbing capacity of gaseous coolant and the requirement of a higher cooling efficiency,liquid coolant as a potential candidate has a higher value in thermal protection application,due to its large latent heat of phase transition, smaller storage space,lower power comsumption of transport.This dissertation will present a numerical method to simulate the transpiration cooling processes with coolant phase change within porous matrix.This method is based on coupling Local Thermal Non-Equilibrium(LTNE) with Two-Phase Mixture Model(TPMM),using the LTNE to describe the heat exchange process,and using the TPMM to describe the liquid coolant phase change within the matrix.The effects of thermal conductivity, porosity,and sphere diameter of the porous matrix on the temperature and saturation distributions within the matrix are analyzed using the LTNE-TPMM.Through this work,an inverse phenomenon with the foregone investigations on the transpiration cooling without coolant phase change is discovered,namely in two-phase region, coolant temperature may be higher than solid temperature.It is clear,this inversion can be captured only by the LTNE-TPMM.Although requiring verification by an experiment or explanation from a view point of micro-scale in the future,this inversion phenomenon leads to an interesting research topic.Minimum coolant injection rate and optimized structure design of two layered porous media:This dissertation presents some groping investigations on the optimized design of transpiration cooling systems.The optimization includes two parts:1) A numerical investigation on transient process of transpiration cooling,and the control parameters of the lowest coolant mass flow rate are carried out using a compressible,unsteady and local thermal non-equilibrium model.The numerical investigation shows that it is important to study transient cooling process,because the porous matrix could be ablated before the cooling process achieves to a steady state, though the temperature in steady stat may be lower than the melting point.2) An optimization structure design of diffusion cooling with two-layer porous matrixes is presented.Coolant is injected into the porous structure under a certain differential pressure over the porous matrix with a total thickness.With the variation of the porous material,porosity and thickness ratio of two layers,the lowest temperature at the hot surface is the ultimate target of the optimization which satisfying the global constraint conditions of weight and cost,etc.The genetic algorithm is used to find the feasible optimization design with the analytical solution of one-dimensional LNTE model.The results indicate that this method of optimization is effective,and several similar optimal designs are found under the different constraints.Inverse problem of transpiration cooling:In this work,an inverse problem of transient transpiration cooling is investigated in detail.The heat flux to hot surface, which is dependent on time or space,is estimated according to the temperatures measured by thermal sensors.The conjugate gradient method(CGM) is applied to solve this inverse problem.Through the CGM,satisfactory results can be obtained. This investigation expands the application area of inverse problem.Because the characteristics of transpiration cooling are differect from the tranditional inverse problems:1) Local thermal non-equilibrium model requires fluid-solid couple solving; 2) The compressibility and variable thermal properties must be considered under a large temperature gradient and high pressure.Therefore,this work provides a useful reference for the future research on inverse problem.