| Because of its very low apparent density and quite amount of nano and micro scale pores,the fiber aerogel composite material has the effective thermal conductivity far below the air in the vacuum environment.Besides,it has good mechanical properties,fire resistance and long service life,so it is a kind of excellent material can be used to make vacuum insulation panel（VIP）core.In order to obtain the structural characteristics of the fiber-SiO2 aerogel composite material,its physical feature,fiber size,pore size and distribution and other parameters are tested and measured.Based on these characteristics,a 3D random model of three phrases including fibers,aerogels and macron pores is generated.Based on this,the calculation model of effective thermal conductivity of composite material is established by using LBM method and ADT radiation theory.To improve the computing speed,the computational domain is minified and parallel computation method is adopted.Comparsion with the experimental data shows thatthe effective thermal conductivity calculated by the model has high accuracy.The relationships between thermal conductivity and several key parameters,which are involved in the VIPs production process,and their mechanisms are studied.The results and conclusions are as follows:The effective thermal conductivity of composite material firstly drops in sharply with its environment pressure decreases.The reason is that,decreases due to the air molecules mean free path（AMMFP）increasing and their free motions gradually bounded by the solid skeleton.However,when the environmental pressure is lower than 100 Pa,the AMMFP would be larger than the almost all pores size,so lower pressure would not furtherly enhance the solid skeleton constraints on free movement of gas molecules,and the effective thermal conductivity remains nearly the same.Meanwhile,the heat transfer through the material will be mainly conducted by the conduction through the fibers and aerogel skeletons and also the radiation.In vacuum,the effective thermal conductivity of composite material increases with the increase of fiber volume fraction,but the increase trend is a little slow compared to that in atmosphere.This is because that the radiation plays a more important role in vacuum compared with that in atmosphere.Therefore,with the increase of fiber volume fraction,the decreasing of radiant conductivity offsets part of the increasing caused by the thermal conduction in solid,which results in the overall effective thermal conductivity increasing relatively slower.The study also indicates that,with the increase of the density of aerogel,the effective thermal conductivity of the composite material firstly decreases and then goes up.This is because when the density of aerogel is extremely low,the effective thermal conductivity is mainly affected by the decreasing of radiant conductivity,which decreases with the aerogel density increases.But when the density of aerogel is larger than a certain value,the thermal conductivity of solid in aerogel will play a leading influential role,which makes the overall effective thermal conductivity increases with the aerogel density increasing. |
