Abstract
This study simulates the subsonic gas flows in two-dimensional microchannels using the Direct Simulation Monte Carlo (DSMC) method. Because the subsonic inflow/outflow (I/O) boundary conditions are so different from those in conventional DSMC applications, new numerical schemes applying the particle flux conservation at the I/O boundaries are developed. The two types of I/O boundary conditions considered in this study are (1) those with specified inlet and exit pressures, and (2) with a specified mass flow rate and a given exit pressure. Among the developed numerical schemes, Procedure A is established for boundary conditions of type 1, while Procedure C is for those of type 2. These two DSMC procedures have been verified to be successful in simulating the gas flow in microchannels. Especially, they converge to the steady state rather rapidly. Also, a theoretical model based on the Navier-Stokes equations, applied with a VHS (Variable Hard Sphere) slip velocity, is developed for isothermal gas flows with boundary conditions of type 1. This model has been shown to be adequate for weakly compressible flows in the slip flow regime. By comparing with the available experimental data and theoretical results, the DSMC method is found reliable for a wide range of compressibility and rarefaction, the two key but competing effects in microchannel gas flows. The DSMC Procedures A and C are further applied to the simulation of micro-step gas flows with success. The backward-facing and forward-facing micro-step flows under variant rarefaction are investigated. A vortex behind the step is predicted by both procedures at lowly rarefaction.