Analysis of micro-scale gas flows with pressure boundaries using direct simulation Monte Carlo method

The development of a two-dimensional direct simulation Monte Carlo program for pressure boundaries using unstructured cells and its applications to ty pical micro-scale gas flows are described. For the molecular collision kine tics, variable hard sphere molecular model and no time counter collision sa mpling scheme are used, while the cell-by-cell particle tracing technique i s implemented for particle movement. The program has been verified by compa rison of simulated equilibrium collision frequency with theoretical value a nd by comparison of simulated non-equilibrium profiles of one-dimensional n ormal shock with previous reported work. Applications to micro-scale gas fl ows includes micro-manifold, micro-nozzle and slider air bearing. The aim i s to further test the treatment of pressure boundaries, developed previousl y by the first author, by particle flux conservation for gas flows involvin g many exits, complicated geometries and moving boundaries. For micro-manif old gas flows, excellent mass flow conservation between the inlet and two e xits is obtained at low subsonic flows. For micro-nozzle gas flows, with fi xed inlet pressure, the mass flow rate increases with decreasing pressure r atio (exit to inlet), but remains essentially the same at pressure ratios m uch lower than that obtained by continuum inviscid analysis. For higher spe cified pressure ratios, the locations of maximum Mach number moves further downstream as the pressure ratio decreases; while, for lower specified pres sure ratios, the Mach number increases all the way through the nozzle to th e exit. Eventually, supersonic speed is observed at the exit for pressure r atios equal to or less than 0.143. Finally, for slider air bearing gas flow s of the computer hard drive, the simulated gas pressures, at different rot ating speeds, agree very well with previous studies. However, there exists strong translational nonequilibrium in the gas flows at the high rotating s peeds. The applicability of the treatment of pressure boundaries using the equilibrium Maxwell-Boltzmann distribution function is discussed in terms o f the magnitude of the local Knudsen number at the pressure boundary for mi cro-nozzles and slider air bearing applications. (C) 2001 Elsevier Science Ltd. All rights reserved.