In this research, an improved gas-surface molecule collision rule is
proposed for adiabatic wall boundary conditions in DSMC (Direct
Simulation Monte Carlo) calculations of rarefied gas natural convection in a micro enclosure. This gas-surface interaction model keeps the particle velocity magnitude invariant and the velocity direction of the reflected particle is set based on isotropic scattering distribution in the half space.This boundary treatment is physically more reasonable for the description of reflected molecules on an adiabatic solid surface. To examine the performance of this rule, the natural convection of rarefied gas in a micro scale rectangular enclosure with length-to-width aspect ratios of AR = 2.016 and 4, heated from below, is employed as the test model. The Modified No Time Counter (MNTC) method is used for the selection of representative collision pairs. The parameters of Knudsen numbers Kn = 0.01, 0.016, 0.02 and Rayleigh numbers Ra ranging from 126 to 4671 are considered. The present results demonstrate that the improved collision rule generates satisfactory predictions of thermal-fluid behaviors both in qualitative and quantitative aspects at micro scales.