This study focuses on the clarification of the heat transfer characteristics of the subcooled pool boiling, the discussion on its mechanism, and finally the establishment of a boiling and condensation model for numerical simulation on the subcooled pool boiling phenomena. In this paper, the boiling and condensation model is improved by introducing the following models based on the quasi-thermal equilibrium hypothesis; (1) a modified phase-change model which consisted of the enthalpy method for the water-vapor system, (2) a relaxation time derived by considering unsteady heat conduction. Resulting from the numerical simulations on the subcooled pool boiling based on the MARS (Multi-interface Advection and Reconstruction Solver) with improved boiling and condensation model, the numerical results regarding the bubble growth process of the subcooled pool boiling show in good agreement with the experimental observation results and the existing analytical equations.

We present a closed-form classical model for the size dependence of thin film thermal conductivity. The model predictions are compared to Stillinger-Weber silicon thin film thermal conductivities (in-plane and cross-plane directions) calculated using phonon properties obtained from lattice dynamics calculations. By including the frequency dependence of the phonon-phonon relaxation times, the model is able to capture the approach to the bulk thermal conductivity better than models based on a single relaxation time.

Using lattice dynamics theory, we derive the spectral energy density and the relation between the spectral energy density and the phonon frequencies and relaxation times. We then calculate the spectral energy density and phonon frequencies and relaxation times for a test system of Lennard-Jones argon using velocities obtained from molecular dynamics simulations. The phonon properties, which can be used to calculate thermal conductivity, are compared to predictions made using (i) anharmonic lattice dynamics calculations and (ii) a technique that performs normal mode analysis on the positions and velocities obtained from molecular dynamics simulations.

Based on first-principles density-functional calculations, we have developed and tested a force-field for silicon, which can be used for molecular dynamics simulations and the calculation of its thermal properties. This force field uses the exact Taylor expansion of the total energy about the equilibrium positions up to 4th order. In this sense, it becomes systematically exact for small enough displacements, and can reproduce the thermodynamic properties of Si with high fidelity. Having the harmonic force constants, one can easily calculate the phonon spectrum of this system. The cubic force constants, on the other hand, will allow us to compute phonon lifetimes and scattering rates. Results on equilibrium Green-Kubo molecular dynamics simulations of thermal conductivity as well as an alternative calculation of the latter based on the relaxation-time approximation will be reported. The accuracy and ease of computation of the lattice thermal conductivity using these methods will be compared. This approach paves the way for the construction of accurate bulk interatomic potentials database, from which lattice dynamics and thermal properties can be calculated and used in larger scale simulation methods such as Monte Carlo.

This work develops a new model for calculating the thermal conductivity of polycrystalline silicon using an effective medium approach which discretizes the contribution to thermal conductivity into that of the grain and grain boundary regions. While the Boltzmann transport equation under the relaxation time approximation is used to model the grain thermal conductivity, a lower limit thermal conductivity model for disordered layers is applied in order to more accurately treat phonon scattering in the grain boundary regions, which simultaneously removes the need for fitting parameters frequently used in the traditional formation of grain boundary scattering times. The contributions of the grain and grain boundary regions are then combined using an effective medium approach to compute the total thermal conductivity. The model is compared to experimental data from literature for both undoped and doped polycrystalline silicon films. In both cases, the new model captures the correct temperature dependent trend and demonstrates good agreement with experimental thermal conductivity data from 20 to 300K.