Study of subsonic–supersonic gas flow through micro/nanoscale nozzles using unstructured DSMC solver

• Published in: Microfluidics and nanofluidics Vol. 10; no. 2; pp. 321 - 335
• Main Authors: Darbandi, Masoud, Roohi, Ehsan
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.02.2011; Springer; Springer Nature B.V
• Subjects: Analytical Chemistry; Applied sciences; Biomedical Engineering and Bioengineering; Boundary conditions; Boundary layers; Electronics; Engineering; Engineering Fluid Dynamics; Exact sciences and technology; Fluid dynamics; Fundamental areas of phenomenology (including applications); Micro- and nanoelectromechanical devices (mems/nems); Monte Carlo simulation; Nanotechnology and Microengineering; Nozzles; Physics; Rarefied gas dynamics; Research Paper; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Slip flows; OpenFOAM; Unstructured mesh; Subsonic regime; DSMC; Navier–Stokes; Supersonic regime; Micro/nanoscale nozzles; Slip boundary condition; Rarefied flow; Slip flow; Supersonic flow; Monte Carlo method; Rarefied gas flow; Navier―Stokes; Convergent divergent nozzle; Nanoelectromechanical device; Nanostructure; Boundary condition; Modeling; Navier Stokes equation; Subsonic flow; Numerical simulation; Microstructure; Microelectromechanical device; Mesh generation
• ISSN: 1613-4982; 1613-4990
• DOI: 10.1007/s10404-010-0671-7

We use an extended direct simulation Monte Carlo (DSMC) method, applicable to unstructured meshes, to numerically simulate a wide range of rarefaction regimes from subsonic to supersonic flows through micro/nanoscale converging–diverging nozzles. Our unstructured DSMC method considers a uniform distribution of particles, employs proper subcell geometry, and follows an appropriate particle tracking algorithm. Using the unstructured DSMC, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number on the flow field in micro/nanoscale nozzles. If we apply the back pressure at the nozzle outlet, a boundary layer separation occurs before the outlet and a region with reverse flow appears inside the boundary layer. Meanwhile, the core region of inviscid flow experiences multiple shock-expansion waves. In order to accurately simulate the outflow, we extend a buffer zone at the nozzle outlet. We show that a high viscous force creation in the wall boundary layer prevents any supersonic flow formation in the divergent part of the nozzle if the Knudsen number exceeds a moderate magnitude. We also show that the wall boundary layer prevents forming any normal shock in the divergent part. In reality, Mach cores would appear at the nozzle center followed by bow shocks and expansion region. We compare the current DSMC results with the solution of the Navier–Stokes equations subject to the velocity slip and temperature jump boundary conditions. We use OpenFOAM as a compressible flow solver to treat the Navier–Stokes equations.