Research Of Microscale Gas Flow And Mixing In Direct Simulation Monte Carlo Method

With the extensive application of Micro-electromechanical System (MEMS) in the domain of BME, the study of gas flow and mixture in micro- and nano-channels has gradually become one research hotspot, which is the key for micromixer performance optimization and MEMS development.The Knudsen number becomes large enough due to the decreased characteristic length. The gas flow is described by the nonlinear Boltzmann equation in slip flow regime, transition regime and free molecule regime. The classical simulation methods based on continuum hypothesis are no longer applicable. Numerical methods are required based on the molecular kinetic theory and statistical law, of which the DSMC method is very successful for high-Knudsen-number flows. Instead of calculating the complex Boltzmann equation, the DSMC method simulates the physical process described by the Boltzmann equation. Calculating time is greatly reduced by decoupling the molecular movement and collision, which makes the DSMC method more effective than others.Firstly, the dissertation summarizes the present development of MEMS and the characteristics of microscale gas flow and mixing. From three aspects of theoretical, experimental and numerical simulation studies, it reviews the development of microscale gas flow both at home and abroad, and makes an introduction to the present DSMC simulation of microscale gas flow and mixing.Secondly, the principles of the Monte Carlo (MC) method and random sampling are introduced. The program flow and process of the DSMC method is analyzed in detail. With the analysis of binary molecular collision model and the nonlinear Boltzmann equation, the mechanism of microscale gas flow is studied and the agreement of the DSMC method with the Boltzmann equation is obtained, which provide bases for the application of the DSMC method in the microscale gas flow and mixture in theory.Thirdly, the DSMC program of the flow field simulation is designed, which improves Bird's standard program to be applicable for complex gas flows. It numerically simulates the microscale gas Couette flow using the DSMC program. The results show that the Knudsen number has an effect on the velocity, shearing stress and temperature. It validates the program to be accurate and the DSMC method effective by comparing the simulation results with the analytical solutions to the N-S equation in slip boundary conditions and the numerical solutions to the linearized Boltzmann equation. It is discussed that the effect on the simulation accuracy of the velocity and temperature in the Couette flow of the mesh size and the number of simulated molecules in each grid as the two main parameters.Finally, numerical simulation of gas mixing in a T-shape microchannel is implemented by the DSMC program as well as the CFD analysis software Fluent. By studying the effects on the mixing length and coefficient of gas flow velocities, inlet pressures, gas and wall temperatures, microchannel sizes and Knudsen numbers, it analyzes various factors affecting micro gas mixing, which offers the guidance for further optimization of the gas micro-mixer structure design.