| Near space hypersonic vehicles are faced with severe aerodynamic heating.Transpiration cooling thermal protection system has gradually become the focus in the hypersonic field due to the advantages of this system such as reusability and maintaining the aerodynamic shape of the vehicle.Porous medium is used as a counter flow heat exchanger and channels for coolant in this system,so the properties of porous medium have an essential effect on the cooling performance of transpiration cooling.At present,there are still many problems in this system,such as high quality and low reliability caused by the high density of transpiration-cooling materials,easy oxidation and uneven pore structures.In order to solve this problem,this paper has carried out the development of Si C based transpiration cooling materials with excellent properties such as low density,high temperature resistance,oxidation resistance,uniform pore-size distribution,superior permeability and so on.The main research work are as follows:To further understand the transpiration cooling performance of silicon carbide ceramic,this work designs and develops a set of in situ compression-permeability device applied to high-energy X-ray on the X-ray imaging platform in Shanghai Synchrotron Radiation Facility.At the same time,a transpiration cooling test platform including the control system has been designed in order to generate high temperature heat flow by using oxygen-acetylene flame.They can provide support for the transpiration cooling performance of porous ceramics and improve the performance evaluation system of transpiration cooling materials.On the basis of lightweight,aiming at uniform pore structure and excellent permeability,the design and preparation of silicon carbide based porous ceramics for transpiration cooling have been completed,and the technical approach for the preparation of porous ceramics has been optimized.The dispersion of Si C particles in xylene organic solution has been investigated.Through the introduction of polyurethane foaming process,the traditional foaming method and the impregnation method of organic template have been optimized to obtain the silicon carbide based porous ceramics with more uniform pore-size distribution and lower density.In order to further improve the uniformity of pore-size distribution of silicon carbide based porous ceramics,based on the principle that carbon fibers are not completely compacted under compression environment,a grinding-mould pressing-sintering process has been proposed and improved.The results show that:(1)Through the use of the compound dispersants(the dispersant 7096 and dispersant 6041,which account for 1% and 4% of Si C content,respectively),Si C particles in xylene organic solution can achieve good dispersion effect.(2)In the introduced polyurethane foaming process,the porous ceramics with low density and uniform pore-size distribution have been obtained by adjusting the content of polyurethane(40%),the proportion of self-skinning type and high-rebound type polyurethane(3:1)and the content of chopped carbon fibers(6.25‰).(3)The proposed and improved grinding-mould pressing-sintering process overcomes the uncontrollable size,direction and numbers of pore connections in the introduced polyurethane foaming process,and this proposed technology can accurately control the pore-size distribution inside the porous ceramics.The basic properties,compression-permeability mechanism and transpiration cooling performance of Si C based porous ceramics under high heat flow have been characterized and evaluated.The effects of different preparation processes on the density,compression strength,permeability and thermal conductivity of porous ceramics have been systematacially studied.Based on the simulation results,the characterization method of the compression-permeability mechanism of the transpiration cooling material has been established.The permeability mechanisms and the evolution laws of the pore structure inside the porous ceramics under the action of compression and permeability have been revealed by synchrotron radiation X-ray imaging tests.The influence laws of gravity of liquid water on the surface temperature distribution of transpiration cooling material and the change laws of pressure difference of liquid water as a coolant with external heat flux have also been revealed by using the developed test platform of oxygen-acetylene transpiration cooling.The results show that:(1)The densities,compressive strengths,permeabilities and thermal conductivities of porous ceramics are obviously affected by the content of carbon fibers,the molding pressure,the mass ratio of different silicon carbide powders and carbon fiber powders impregnated with polycarbosilane solution and silica sol solution,respectively.(2)The densities of the porous ceramics have been further reduced,while the wetting ability of the ceramic surface has been improved by optimizing the preparation process of grinding-mould pressing-sintering process.(3)The study of the three-dimensional reconstruction of porous ceramics has verified the uniformity of pore-size distribution in thickness direction of porous ceramic(10.7% ? 15.1%),as load increases,the permeability of porous ceramics firstly decreases,and then gradually increases,it can't accurately control flow rate and distribution of deionized water when large cracks inside porous ceramic appear.Flow velocity and distribution of the permeability simulation can reflect fluid permeability mechanism.It has a great significance for the deep understanding of permeability and microstructure evolutions of transpiration cooling material under external load.Additionally,it also will provide referential basis for the optimization of the pore structure.(4)Through the self-designed and developed oxyacetylene transpiration cooling test platform,the transpiration cooling materials prepared by different processes have been tested in the high temperature heat flow environment.The results show that the surface temperature and back-side temperature of the porous ceramics prepared by the three technologies can be kept stable in the testing environment,which can meet the requirements of transpiration cooling.Due to the influence of cooling water gravity,the temperature difference between the upper and lower parts of the ceramic sample surface is quite different.With the increase of heat flow,the temperature difference firstly increases and then decreases when the flow rate of cooling water remains unchanged.The temperature difference between the upper and lower parts of the surface of ceramic samples with higher porosity will reach the maximum at higher heat flow environment.In order to simulate flight environment more accurate,the transpiration cooling of porous ceramic are carried out by arc-heated wind tunnel,the results show that the porous ceramics can perfectly be used porous media in the transpiration cooling. |
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