Investigation On Coupled Convection And Radiation Heat Transfer Within Foam And Gap Structure At High Temperature

In recent years,millimeter-sized pore materials represented by foam structure have begun to show important application prospects in the fields of energy and power,chemical engineering and aerospace.In these technical areas,the physical processes within the foam porous material are high strength energy conversion and transport.In addition,the hypersonic vehicle thermal seal structure,which consists of millimetre-sized gaps and fibrous porous material(micron-sized pores)fillings,undergoes a similar high temperature process.The co-existence of fluid domain,solid domain and porous domain is a common feature of such processes in both high and low speed flows.Because this kind of multi-region problem involves the complex coupling of heat conduction,convection and radiation,the existing research still lacks the necessary understanding of momentum and heat transport rules,which restricts the development of relevant technologies.In this paper,the high temperature heat exchanger enhancement,the drag and aero-heating reduction of aircraft as well as the thermal seal structure of aircraft are taken as the application background,the coupled convection-radiation high temperature heat transfer mechanism,numerical computational method and process transport characteristics regarding foam and gap structures are investigated.Based on the structured grid system and Monte Carlo method(MCM),a hybrid strategy is introduced to solve the radiation heat source term in irregular region.This strategy is used to calculate the discrete scale radiative heat transfer between the surfaces of the foam struts and combined with CFD simulation of fluid-solid coupled heat transfer to establish a method for simulating coupled convection-radiation heat transfer in foam structure at discrete scale.The reliability of the computational method is verified in two aspects,including the simulation of discrete scale convective heat transfer and solving the radiative heat transfer.For the rectangular channel filled with nickel foam,the flow and high temperature coupled heat transfer characteristics in the foam structure at discrete scale under constant wall temperature heating are analyzed,and the inapplicability of continuum-scale simulation to large gradient heat transfer region is clarified.In view of the limitation of continuum-scale simulation in large gradient heat transfer region and the huge computational burden of discrete-scale simulation,based on the concept of multi-region coupling,the continuum-scale simulation and the discrete-scale simulation are combined to establish a scale-integrated analysis method of coupled convection-radiation heat transfer in the foam structure.An investigation regarding forced convection and high temperature coupled heat transfer within nickel foam layer subjected to external incident radiation is carried out.The reliability of scale-integrated simulation is tested and analyzed by referring to the results of discrete-scale simulation and the calculation results of the existing literature correlation.Moreover,the computational efficiencies of the discrete-scale simulation and the scale-integrated simulation are compared.Regarding the coupled convection-radiation heat transfer problem of foam structure in high speed flow,the continuum-scale simulation is carried out by using the density based high speed flow computational method and considering the multi-region coupling heat transfer effect.The coupled heat transfer and aerodynamic drag characteristics of the leading cylindrical foam structure under the impact of high speed airflow are analyzed.The transient characteristics of the flow field in the porous domain,the heat transfer state between the internal airflow and the strut,and the influence law of the length variation of the porous domain on the aerodynamic effect are obatined.On this basis,the analysis regarding the blunt configuration structure with leading foam spike is performed.By referring to the solid spiked counterpart,the flow characteristics around the foam spike are analyzed,and its reduction on aero-heating effect on the frontal wall of main body and the overall wave drag is tested.The mechanisms of drag and aero-heating reduction are revealed.For the high temperature coupled heat transfer problem of thermal sealing structure of the aircraft,the transient heat invasion process of local gap structure in the high speed flow field is numerically simulated by using the density based all-speed computational method and considering the multi-region coupling effect.The one-way transport characteristic of external high speed flow field exerting on the aerodynamic heat flux of upper wall and the inlet flow field parameters of the slot is obtained.Furthermore,a fast analysis method is proposed to decouple the external high speed flow field and the low speed flow field within the slot-cavity,and the correlations of decoupling parameters of the external wall are summarized.The reliablity of the decoupling algorithm is tested by the all-speed coupled simulation results and the experimental data of the arc wind tunnel.Finally,this decoupling algorithm is utilized to analyze the long time transient thermal invasion characteristics of local slot-cavity structures with different characteristics.Through this investigation,the computational method of coupled convection-radiation high temperature heat transfer analysis applicable for foam and gap structures is established,which provides analytical measure for predicting the characteristics of foam structures in the fields of high temperature heat transfer enhancement,drag and aero-heating reduction of high speed aircraft,and the thermal seal structures.The obtained analysis results provide the cognitive basis and reference for the relevant application investigations and technology development.