Numerical analysis of rarefied gas flow dominated by wall reflection in a straight channel

When rarefied gas flows through a channel exposed to vacuum, wall reflections and inlet velocity differences strongly affect the outlet velocity. This study uses the Direct Simulation Monte Carlo method, incorporating the Cercignani–Lampis–Lord wall reflection model, to analyze the injection method (parallel and perpendicular) and channel aspect ratio (AR = length of the walls/width between the walls) effects on the outlet density and velocity. The results demonstrate that AR determines the dominant physical mechanism governing the flow. In low-AR channels (AR < 4), the injection conditions are strongly influential; at AR = 1, perpendicular injection increases the outlet density by 44.68 % compared to parallel injection. Conversely, for high-AR channels (AR > 4), the flow thermalizes through particle–wall interactions, and the outlet velocity converges to the root-mean-square thermal velocity irrespective of the injection method. These findings establish a quantitative relation between channel AR and the effectiveness of injection-based flow control in rarefied environments.