Improving DSMC Inlet/Outlet Flow Boundary Conditions for Gas Mixtures Using the Chapman-Enskog Distribution Available to Purchase

Prescribed pressure is the most common flow boundary condition used in microchannel flow simulations. In the Direct Simulation Monte Carlo (DSMC) method, boundary pressure is controlled by the number flux of the simulating molecules which enter the domain through the boundary. This number flux, in the conventional DSMC algorithm, is calculated iteratively using sampled values of velocity and number density and an expression derived from the Maxwell distribution function. This procedure does not work well for low speed flows where the role of the molecules entering from the flow boundaries becomes important. The statistical scatter of the DSMC results is generally known to be the main reason; however, the Maxwell distribution used in the pressure boundary treatment is valid just for equilibrium conditions. Accordingly, current implementations of the DSMC pressure boundary treatment are limited to boundaries with sufficiently small variations of flow variables. This is not, however, the case for many practical cases in which high gradients of the flow variables close to the boundaries lead to considerable non-equilibrium effects. In this study, therefore, an expression for the inward number flux of species is derived using the Chapman-Enskog velocity distribution to improve the pressure boundary condition in dealing with gradients of the flow properties close to the boundary. The resulting algorithm is then used for modeling a micro-channel binary gas mixture flow with prescribed pressure boundary conditions. The results are compared to those obtained from the conventional DSMC simulations using the Maxwell distribution.