DSMC Solution of Supersonic Scale to Choked Subsonic Flow in Micro to Nano Channels

In this study, the supersonic and choked subsonic flows through micro/nano channels are investigated using direct simulation Monte Carlo (DSMC) method. The supersonic case is simulated at different Knudsen numbers covering slip to transition flow regimes, while the effects of inlet Mach and back pressure are studied in details. The inlet/outlet pressure boundary conditions are suitably implemented benefiting from the basics of characteristics theory. A behavior similar to the one predicted by the Fanno theory is observed here; i.e., the supersonic flow velocity decelerates up to a choking condition where any further increase in Knudsen number is impossible unless strong normal/oblique shocks appear at the inlet and the inlet conditions change to the subsonic ones. However, a subsonic flow appears near the outlet section if one imposes a back pressure lower than the ordinary exit pressure at the outlet. Our investigation showed that applying the back pressure boundary condition right at the real channel exit would overwhelm the solution. A more realistic behavior can be achieved by inserting suitable buffer zone beyond the real channel exit, where the back pressure is applied there. This strategy results in capturing a more realistic physics of flow at the channel outlet and enforces choking condition at the outlet.