Investigation On Heat Transfer Characteristics Of Nanoscale Fractal Structure By Molecular Dynamics Simulation

Nanoscale heat transfer has been widely applied in micro/nano electromechanical systems, microelectronics cooling, thermoelectric refrigeration, etc. In addition, it is of great significance to fully explore and understand the microscopic motion and transportation. With the miniaturization of heating component size, heating component size can reach the magnitude order of the mean free path of the heating particles, causing the size effects more important on heat transfer behaviors of heating components, which receives more and more attention. Nowadays, the research of nanoscale heat transfer has become an advanced issue in the field of international micro/nanoscale heat and mass transfer.In nanoscale systems, the heat transfer occurs in a confined nanospace, which results in significant solid-solid and fluid-solid interface effects and hence leads to a special micro transport mechanism in nanoscale heat conduction and phase change heat transfer, such as the phonon heat conduction of nanocomposite and the liquid film evaporation on the rough surface. Up to now, the mechanism and inherent law of interface effects on nanoscale heat transfer are not completely understood. Especially, the thermal conductivity of nanocomposite and the liquid film evaporation on the rough surface are less understood. In addition, the structure regulation of the thermal conductivity of nanocomposite and a quantitative description of rough surface are still waiting to be explored. In this context, the fractal geometry is introduced to characterize the optimization design for the nanoscopic structure of nanocomposite and to describe the rough surface morphology. The theoretical models of phonon heat conduction of nanocomposite and phase change on rough surfaces are developed to study the heat transfer mechanism of the thermal conduction of Si/Ge nanocomposite and the liquid film evaporation on rough surface by molecular dynamics simulation. In summary, the main research contents and research conclusions are as follows:(1) In order to improve the thermoelectric figure of merit for thermoelectric materials, the Sierpinski carpet fractal is introduced into the construction of Si/Ge nanocomposite in this paper, and the molecular dynamics simulation is applied to investigate the thermal conductivity of the Si/Ge nanocomposite with Sierpinski carpet fractal. The results indicate that:(a) With the same Si atomic percentage, the thermal conductivity of Sierpinski carpet fractal nanocomposite is significantly lower than the embedded rectangular structure of nanocomposite owing to stronger phonon interface scattering; (b) With the increasing of Si atoms percent, the thermal conductivity which is affected by Si/Ge interface scattering firstly decreases to a minimum value and then increases; (c) Increases in axial length and cross-sectional size of nanocomposite lead to the improvement of thermal conduction.(2) In the context of the natural merit of fractal tree in increasing interface density, the tree-shaped architectures is introduced into the construction of nanostructure and propose a construction of Si/Ge nanocomposite embedded with tree-shaped network to maximize the interface-area-to-volume ratio and hence to enhance the phonon scattering at the interface with a focus on the reduction of thermal conductivity. The results indicate that:(a) Compared with conventional rectangular core, the tree-shaped network structure is superior in the reduction of thermal conductivity of nanocomposite owing to stronger phonon interface scattering for larger interface density and is beneficial to the improvement of thermoelectric figure of merit; (b) For the embedded-tree nanocomposite, the larger length and width fractal dimensions as well as more branch level are preferred in the construction of Si/Ge nanocomposite with fractal tree-shaped network. It is certified that the application of fractal tree-shaped structure is a useful way for the optimal design of nanocomposite.(3) A molecular dynamics simulation is also carried out to investigate the liquid film evaporation on the rough surface. The results indicate that:(a) The non-evaporative liquid layer exists on the solid surface. The binding capacities forced by the solid surface are enhanced by the existence of rough, thickening the non-evaporation liquid layer with the increase of the rough height; (b) The surface fractal dimension (the irregular degree of rough profile) is an important factor on liquid-solid interface energy transfer under the same rough height. With the increase of fractal dimension, the rough height undergoes more frequently variation, thickening the non-evaporation liquid layer; (c) Evaporation rate increases with the increase of the rough height and the fractal dimension.