Study On Heat Transfer Mechanism Of Alumina Aerogel-Alumina Fiber-Opacifier Composites

Aerogels are materials with the lowest density and the lowest thermal conductivity.Fibers are added to enhance strength,and opacifiers are added to inhibit high-temperature radiation to form aerogel-fiber-opacifier composite.However,due to the complex structure of composites,this paper improved theoretical model for calculating the heat transfer of alumina aerogel-alumina fiber-opacifier composite,which provides a theoretical basis for material design.In this paper,the thermal conductivity of alumina aerogel-alumina fiber-opacifier composite is studied by theoretical analysis methods.The main contents include four parts:Based on the near-real structure of secondary particle packing of alumina aerogel,a theoretical model of thermal conduction is established,which not only considers the nanoscale effect of alumina aerogel particles,but also considers the effect of temperature and internal structure of the particles.A set of theoretical solution models for obtaining optical properties of fibers,opacifiers and aerogels were selected.According to the complex refractive coefficient,size,shape and other parameters of the material to be studied,the radiation characteristics are calculated by using this model.Based on the heat transfer mechanism model of thermal conductivity of alumina aerogel composite,including the heat transfer model of nano-scale alumina aerogel,micro-scale opacifiers,alumina fiber,and gas,the structure-activity relationship model of the overall thermal insulation properties of alumina aerogel-alumina fiber-opacifier composite are established.It is analyzed that the radiation characteristics and overall thermal conductivity of the composite are affected by the material volume content,inclinnation angle and size of the fiber and the opacifier,thus guiding the design of the material.To sum up,this paper conducts theoretical research on heat transfer mechanism and thermal insulation performance verification for the research object,and establishes an analytical model to provide effective theoretical support for material design.The main conclusions are as follows:The gas-phase thermal conductivity of the aerogel theory predicted by the improved gas-solid coupling model is in good agreement with the experimental results.At the same time,the theoretical prediction values of the overall thermal conductivity of alumina aerogel-alumina fiber-opacifier composite is consistent with the experimental data with temperature and pressure changes,which also indicates the effectiveness of the current theoretical model in predicting the heat transfer of alumina aerogel composite.With the increase of shading agent and fiber volume fraction,radiation heat transfer is inhibited,but heat transfer is increased.The total thermal conductivity first decreases and then increases.The optimum doping volume fraction corresponding to the lowest total thermal conductivity is obtained.For the alumina aerogel-alumina fiber-opacifier composite,the optimal doping volume fraction of the opacifier is approximately 5%,and the optimum doping volume fraction of the alumina fiber is approximately 20%.The diameter of alumina fiber and the particle size of opacifier significantly affect thermal insulation performance.The smaller the diameter of the alumina fiber and the particle size of the opacifier,the better the insulation performance of the material.Considering the strength of the material,the diameter should not be too small.The average extinction coefficient obtained by the preparation method in which the opacifier is spread on the surface of the fiber insulation to form a composite material is higher than the random distribution of the fiber and opacifier,which can limit the heat transfer of the high temperature radiation,thereby making the heat insulation performance of the material better.The preparation of composite materials can be tried using this method.