Numerical simulations of high Knudsen number gas flows and microchannel electrokinetic liquid flows.

摘要

Low pressure and microchannel gas flows are characterized by high Knudsen numbers. Liquid flows in microchannels are characterized by non-conventional driving potentials like electrokinetic forces. The main thrust of the dissertation is to investigate these two different kinds of flows in gases and liquids respectively.; High Knudsen number (Kn) gas flows were characterized by ‘rarified’ or ‘microscale’ behavior. Because of significant non-continuum effect, traditional CFD techniques are often inaccurate for analyzing high Kn number gas flows. The direct simulation Monte Carlo (DSMC) method offers an alternative to traditional CFD which retains its validity in slip and transition flow regimes. To validate the DSMC code, comparisons of simulation results with theoretical analysis and experimental data are made. The DSMC method was first applied to compute low pressure, high Kn flow fields in partially heated two dimensional channels. The effects of varying pressure, inlet flow and gas transport properties (Kn, Reynolds number, Re and the Prandtl number, Pr respectively) on the wall heat transfer (Nusselt number, Nu) were examined.; The DSMC method was employed to explore mixing gas flows in two dimensional microchannels. Mixing of two gas streams (H₂ and O₂) was considered within a microchannel. The effect of the inlet-outlet pressure difference, the pressure ratio of the incoming streams and the accommodation coefficient of the solid wall on mixing length were all examined.; Parallelization of a three-dimensional DSMC code was implemented using OpenMP procedure on a shared memory multi-processor computer. The parallel code was used to simulate 3D high Kn number Couette flow and the flow characteristics are found to be very different from their continuum counterparts.; A mathematical model describing electrokinetically driven mass transport phenomena in microfabricated chip devices will also be presented. The model accounts for the principal physical phenomena affecting sample mass transport in microchip channels. Numerical simulations were performed to study the capillary electrophoresis flow in microchannels. Flow fields and species distribution are simulated for both the loading and the dispensing steps in a two dimensional cross channel device for species separation. The goal of these simulations is to identify the parameters providing optimal separation performance.