Rarefied gas flow in micro/nano electro mechanical systems (MEMS/NEMS) does not perform exactly as that in macro-scale devices. The main goal in this study is to investigate mixed subsonic-supersonic flows in micro/nano channels and nozzles and to provide physical descriptions on their behaviors. We use DSMC method as a reliable numerical tool to extend our simulation. It is because the DSMC provides accurate solution for the Boltzmann equations over the entire range of rarefied flow regime or Knudsen numbers. As is known, the appearance of oblique/normal shocks at the inlet of a channel or a nozzle adds to the complexity of internal flow field analyses. We found some very unique physical aspects of micro/nano flows including mixed supersonic-subsonic flow regimes in constant area ducts and the attenuation of emitted shocks, which are attributed to the strong viscous forces and dominant rarefaction effects in micro/nano scales. We simulated nozzle flow under different flow conditions including different Knudsen and Reynolds numbers and inlet-outlet pressure ratios. It was observed that as the Knudsen number increases, the viscous dissipation forces increase and the flow in nozzle would not be choked at its throat and no supersonic flow is observed in the divergent part. Contrary to the classical gas dynamics, no shock stands in the divergent part despite specifying a back pressure at the outlet. Alternatively, we observed that multiple expansion-compression waves would be generated and amplified as the back pressure was decreased.

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