Study On Heat Transfer Characteristics Of Porous Materials Formed By Silica Nano-powder

In the face of energy crisis,insulation materials can effectively prevent the energy loss of construction projects,heat transmission pipes and high temperature equipment,so as to achieve energy conservation and emission reduction.Silica nanoparticles have become one of the most popular raw materials for thermal insulation due to their high specific surface area,controllable geometric structure,good thermal stability and low manufacturing cost,and have wide application prospects in many fields such as energy,construction,medical treatment,chemical industry and so on.Powder-formed porous materials made from silica nanoparticles have the advantages of high porosity,high specific surface area,low density,extremely low thermal conductivity and good thermal stability.In recent years,they have been widely concerned as a new type of thermal insulation materials.However,the pore structure and heat transfer characteristics at high temperature are not clear,which seriously affects the development and utilization of these materials in the direction of micro-scale materials and high temperature applications.It is of great significance to study the heat transfer mechanism and influencing factors of porous materials formed by silica powder.Based on this,this thesis systematically studied the thermal conductivity characteristics of silica nano powder forming materials.First of all,on the basis of reading and summarizing the thermal conductivity measurement methods of adiabatic materials,the transient tropical method of high temperature thermal conductivity measurement experimental equipment was independently built to prepare for the subsequent research on high temperature thermal conductivity characteristics.The experimental device is mainly composed of high temperature control system,tropical heating system and data acquisition system,which can realize the measurement of thermal conductivity of materials at high ambient temperature.At the same time,the reliability verification and error analysis of the experimental device are carried out through theoretical analysis and experiment.The results show that the experimental device meets the requirements of this experiment.Then,the porous materials with different structural parameters were prepared by cold pressing method.The influences of particle diameter and preparation pressure on sample volume,porosity and thermal conductivity were investigated.The thermal conductivity of powder forming materials prepared by 10 nm silica could be as low as0.0413W/(m·K).The existence form of agglomeration structure in materials was observed by scanning electron microscopy and the relationship between particle diameter and agglomeration degree was studied.Combined with the literature and experimental data,a hierarchical stacking heat conduction model for nanoparticle forming porous materials was analyzed and derived.The predicted results of the model were co MPared with the experimental results and the predicted results of others’ models.The results show that the model established in this thesis has good accuracy and applicability.Thirdly,a high temperature thermal conductivity measuring device was used to measure and analyze the high temperature thermal conductivity of porous materials formed by silica powder.Combined with radiation parameters such as spectral transmittance,extinction coefficient,Rosseland mean extinction coefficient,the effects of preparation pressure and particle size on the radiant thermal conductivity were studied.On the basis of the experimental results and considering the effect of radiation,the calculation model of gas-solid effective thermal conductivity of materials at high temperature is improved,and the calculation formula of gas-solid effective thermal conductivity of porous materials formed by 10 nm silica powder is given.Due to the existence of radiation heat transfer,the high temperature thermal conductivity of porous material formed by pure silica powder is as high as 0.15W/(m·K).By adding silicon carbide as shading agent,this thesis further prepared composite nano-powder molding material,and studied the influence of the particle size and content of silicon carbide on the high temperature thermal conductivity of the material,to provide a reference for the preparation of high temperature superadiabatic material.Finally,considering that porous materials are easy to absorb water in the air,and adsorbed water has a great influence on the thermal conductivity of materials,this thesis studied and analyzed the relationship between environmental humidity and moisture absorption rate and saturated moisture content of powder molding materials,as well as the influence of environmental humidity and sample parameters on the thermal conductivity of saturated moisture samples.The curves of the thermal conductivity of the samples in different humidity environments were obtained when the temperature rose,which provided basic data for the application of micro-nano powder molding materials in building energy conservation and other fields.