Research On The Heat Transfer Properties Of Self-similarity Micro/nano Porous Material

In response to the increasingly severe energy problem,the potential of micro/nano porous materials in energy utilization and management systems has received significant attention.However,the current predictions of the heat transmission properties of porous materials are mainly focused on the macro-scale,and the micro/nanoscale heat transmission research is limited to specific assumptions.In addition,there is a lack of indepth research on the heat transfer mechanism and influencing factors and an accurate experimental method to explore the thermal radiation characteristics of micro-nano porous materials.Therefore,this thesis carried out structural characterization and fractal reconstruction,formula derivation of equivalent thermal conductivity,analysis of influencing factors and mechanism of heat transfer process,prediction and prediction calculation of thermal radiation characteristic parameters.Several aspects of experimental measurement are studied,which significantly improve the efficiency and accuracy of the predicted calculation.The thermal radiation mechanism of visible light and near-infrared stripes is discussed regarding the regular metal porous structure,taking porous silver as an example.Then,for irregular micro/nano porous metal,such as porous aluminum,the fractal interscting model was established using FEM and FDTD to calculate the equivalent thermal conductivity,compared with the measured value,the error is less than 5.3%,the heat transfer mechanism and influence factors were analyzed.Taking closed-cell aluminum as an example,a third-order fractal square model was established and the Fullscale Diffusion Approximation Method(FDAM)is simplified through the Mie theory compared with the measured value,the error is less than 8.3%.The results show that increasing temperature will increase the equivalent thermal conductivity,and the influence of pore shape and volume specific surface area is more negligible.For micro/nano open-cell dielectric materials,a fractal cube model was constructed based on fractal theory to calculate the conductive thermal conductivity,and the FDAM is used to calculate the radiative thermal conductivity compared with the measured value,the error is less than 15.0%.Then the heat transfer mechanism and seven influence factors were analyzed.Taking closed-cell PMMA as an example,a third-order fractal square model was established and the FDAM is simplified through the Mie theory compared with the measured value,the error is less than 11.2%.The results show that the increase of temperature,the increase of refractive index or the decrease of extinction coefficient will increase the equivalent thermal conductivity,and the influence of pore shape and volume specific surface area is minor.The TMM and EMT were used to calculate the radiation characteristics and analyze the influencing factors for the film added porous metal materials.Taking porous aluminum added by silicon and silicon dioxide films as examples,a numerical calculation model was established and the thermal radiation characteristics were analyzed.Regarding the micro-nano porous dielectric doped composite material,taking the porous polystyrene composite material doped with CNF as an example,the radiation characteristics of the FTIR spectrometer and integrating sphere are measured.A fractal doped model was constructed based on fractal theory to calculate the conductive thermal conductivity,and the FDAM is used to calculate the radiative thermal conductivity.Then the heat transfer mechanism was analyzed.The results show that the increase of the doping proportion reduces the equivalent thermal conductivity when the cell size is greater than 1 μm and increases when the cell size is less than 60 nm.An accurate solution method for the equivalent thermal conductivity of nanoporous materials is proposed through all these researches.The heat transmission properties and the influencing factors are comprehensively analyzed and discussed.The thermal radiation characteristics of the film added porous composite material are studied.The calculation method of equivalent thermal conductivity of the CNF doped porous composite material is constructed.At the same time,the heat transmission properties are analyzed.An accurate measurement method of thermal radiation is developed based on the integrating sphere and FTIR method.The research mentioned above provides a theoretical basis and analysis methods for an in-depth understanding of the heat transfer mechanism of micro/nano porous materials and their composite materials.In particular,proposing a full-scale equivalent thermal conductivity calculation method and an integrating sphere-FTIR method guides theoretical and experimental research on micro/nano porous materials.