Mechanism And Characteristics Of Heat Conduction In Nanocomposites

The insulating nanocomposites are composed of aerogel substrate, fiber and opacifier. Itcan provide a theoretical basis for the thermal design and application of insulatingnanocomposites to understand the heat transfer mechanism of nanocomposites.In order to investigate the thermal conduction of nanograin, this paper simulates thephonon heat transfer by Lattice Boltzmann Method (LBM) and Monte Carlo Method (MCM).The computational results, numerical stability and computational time from both methods arecompared. Further clarify the the computational efficiency and applied scope of LBM forsimulation of phonon heat transfer. In order to understand boundary scattering effect innanomaterial, the phonon heat transfer is researched by applying the specular reflection,diffusive reflection and specular-diffusive reflection boundary condition.In order to comprehensively understand the thermal conduction of silicon dioxidenanograin, the size effect of thermal conduction in film is investigated, and the influence ofsize on temperature distribution and thermal conductivity of silicon dioxide film is obtained. Inaddition, the size in cross and parallel heat transport direction effect of thermal performance isobtained by modeling the phonon heat transfer in square.Based on the structure of the nanograin of aerogel, the3-D spherical segment model ofaerogel nanograin is built. The code of LBM is developed to model phonon heat transfer ofspherical segment. The influence of gap and axis size of spherical segment on thermalconducion is obtained. The parameter of heat transfer of silicon dioxide is calculated by thephonon kinetic theory, and the influence of temperature on thermal conduction in nanograin isobtained. Based on the computational results, the expression of thermal conductivity innanograin is obtained.Based on the results from the expression of thermal conductivity in nanograin,calculational model for solid and gas combined conduction is proposed, in which thenon-uniformity distribution of solid nano-grain is taken into account. Based on the developedmodel, the expression of thermal conductivity for solid and gas combined in aerogel isobtained. The size effect of gas heat conduction in aerogel pores is analysed based on the gaskinetic theory and builded model. Based on the obtained expression of thermal conductivity, the effects of density, side of structure, non-uniformity distribution of solid nanograin and airpressure on the coupled thermal performance of aerogel are investigated.Based on the calculated method of aerogel insulating nanomaterial, the analytical modelis builded to model thermal conductivity of aerogel composites. The thermal conductivity ofaerogel and its composites doped opacifier and fiber are measured, and the reliability ofanalytical model is validated. Further, the thermal conduction is investigated, and the effects ofvolume fraction of opacifier and fiber in aerogel composites and temperature on themacroscopical thermal performance of composites are obtained.Through the investigations, the thermal conduction characteristic of nanograin, thermalconduction for solid and gas combined in insulating nanomaterial and the thermal conductioncharacteristic of insulating nano composites are well understood. The achievements presentedin this dissertation will provide significant reference value to the system optimum design andapplication of insulating nanocomposites.