Molecular Dynamic Studies On Momentum And Energy Accommodation Coefficients In Microchannels And Nanochannels

With the rapid developments of micro- and nanoscale technology, fluid flow and heat transfer in micro/nano electro mechanical systems have drawn more attention. For gas flows in microchannels and nanochannels, the rarefaction and wall effects are evident to induce insufficient momentum and energy exchange between the gas and wall. Accommodation coefficients characterize the transport in gas-solid interfaces quantitatively that the values closer to unity represent more complete momentum and energy exchange. In the present dissertation, the tangential momentum, normal momentum and energy accommodation coefficients (TMAC, NMAC and EAC) are studied using molecular dynamics (MD) method to investigate the effects of temperature, wall lattice configuration and wall roughness on accommodation coefficients as well as gas-wall interactions.According to the definitions of accommodation coefficients, the statistical algorithm is set up based on the incident and reflected gas molecules determined by cutoff radius in MD method.The simulation results show that the normal momentum of reflected gas molecules is the key factor affecting the adsorption time and molecular trapping-desorption behaviors near the wall. In smooth channels, lower wall temperature and stronger gas-solid interaction extend the adsorption time so that the TMAC, NMAC and EAC approach unity; with larger Knudsen numbers (Kn) in gas flows, the gas-gas interactions are weakened by strengthen rarefaction so that the TMAC and EAC decrease with less adsorption time.The lattice configurations of smooth surfaces will induce atomic roughness, and different lattice configurations result in different gas-solid potential energy distributions near the wall. Larger gradient of normal potential leads to larger TMAC and better accommodation of tangential momentum in gas-wall interactions, while larger gradient of tangential potential enhances energy exchange in gas-solid interface and results in a larger EAC. Furthermore, the nanoscale rough cells on the walls alter the distribution of gas-solid potential significantly, in which the gradients of normal and tangential potentials increase and the average normal momentum of reflected molecules decreases bringing longer adsorption time. When the height of rough cells increases, both of the TMAC and EAC increase while the NMAC decreases.Due to the huge computational costs of three-dimensional (3D) MD method, the accommodation coefficients are also calculated by two-dimensional (2D) MD method in this dissertation. The TMAC and EAC in 2D simulations are smaller than the 3D results since the different EAC expression and wall structures in 3D method. Based on the comparison and analysis of 2D and 3D results, the relations between 2D MD and 3D MD accomodation coefficients are presented. From the relations, the 3D accommodation coefficients can be predicted using 2D ones.The direct simulation Monte Carlo (DSMC) and MD methods are both employed in the simulations of isothermal flows in the Kn range of 0.01 to 0.3. The first- and second-order slip coefficients in linear and nonlinear slip models are modified by comparing velocity profiles from the DSMC results and the theoretical solutions. With the TMAC and velocity profiles calculated in MD simulations, the modified expression of first-order slip coefficient is presented.