Micro-electro-mechanical systems (MEMS) are the integration of mechanical elements and electronics on a common silicon substrate through micro-fabrication technology. The most popular application of these devices, such as, micro ducts, micro pumps, micro turbines and micro valves, involves gaseous flows. Further miniaturization of MEMS device causes nano electro mechanical systems (NEMS). An emerging application of nano devices is nanoporous membranes which use to interface micro fluidic channels. Although micro fabrication grows very fast, there is a minimum level of understanding of the fluid dynamics and heat transfer processes in these devices. Therefore, simulation of behavior of micro devices becomes important. In this research the rarefied gas behavior have been considered through suddenly expanded/contracted micro/nano channels under slip, transitional, and free molecular regimes. Flow of argon due to pressure gradient through micro/nano channels with sudden expansion and contraction is simulated using Direct Simulation Monte Carlo method. Velocity, pressure and temperature fields, velocity vector plot and velocity contours for different Knudsen numbers are presented. Results reveal that: 1) Pressure distribution is nonlinear because of compressibility and rarefaction effects. The discontinuity in pressure distribution is due to both density and cross section variations. Large pressure loss exists in first half of the expanded channel and second half of the contracted channel. 2) Velocity profile have a negative or positive jump regularly in diverging and converging channels according to continuity equation (?uA=cte). 3) The temperature drops drastically in 0.8 x/L 1 for both geometries. Also for diverging channel there is a minimum temperature at x/L=0.5. Temperature drops can be attributed to the conversion of thermal energy in to kinetic energy. 4) Temperature, pressure and velocity distributions are similar for nano and micro scales. 5) Separation does not happen in contracted geometry for all flow regimes. 6) In microchannels with sudden expansion stable vortices form at Kn=0.058 (Knudsen number). 7) In nanochannels with sudden expansion stable vortices form at Kn=0.043 (Knudsen number). 8) Results obtained for microchannel can not be used to determine the critical Knudsen number for nano channels.
