The current research uses an unstructured direct simulation Monte Carlo (DSMC) solver to numerically investigate supersonic and subsonic flow behavior in micro convergent– divergent nozzles over a wide range of rarefaction regime. The unstructured DSMC solver has been optimized via using uniform distribution of particles, suitable subcell geometry, and advanced molecular tracking algorithm. The effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number, on the micronozzle flow field were studied. High viscous force manifesting in boundary layers prevents supersonic flow formation at the divergent section of nozzles as soon as the Knudsen number increases above a moderate level. In order to accurately simulate subsonic flow at the nozzle outlet, it is necessary to add a buffer zone to the end of nozzle. If we apply the back pressure at the outlet, boundary layer separation is observed and a region of backward flow appears inside the boundary layer while the core region of inviscid flow experiences multiple shock-expansion waves. We observed that the wall boundary layer prevents formation of normal shocks. Instead, Mach cores appear at the nozzle center followed by bow shocks and expansion region.

Keywords