Flow field of rarefied gases in suddenly expanded / contracted nano-channels using Direct Simulation Monte Carlo method

Micro electro mechanical systems(MEMS) are devices with characteristic length between 1 mm and 1 μm. [1]. Further miniaturization of MEMS device leads to nano electro mechanical systems (NEMS). An emerging application of nano devices is nano-porous membranes used to interface with micro fluidic channels [2]. Based on the fact that experimental study of these systems are expensive, and time intensive, implementation of simulation techniques to study behavior of micro devices is highly valuable. MEMS and NEMS devices mostly contain gaseous environments at standard condition where the molecular mean free path (λ) is less than 70 nm. Thus for micro and sub micron devices, continuum approach breaks down and the flow should be described considering a molecular point of view [3]. As a result, particle based methods such as Direct Simulation Monte Carlo method (DSMC) are attractive tools for studying rarefied gas behavior in MEMS and NEMS. Fluid flows through complicated, micro scale, geometries have already been reported. Rathakrishnan and Sreeknath (1994) [4] studied rarefied gas flow through circular tube with sudden enlargement. Their experimental results show that flow and geometrical parameters affect, very strongly, on discharge in transition regime. Lee and Zohar [5] performed experiments in two channels connected in series with angles of 5°, 15°, 90°, 180°, using integrated pressure sensors. Flow field in their system shows Hele-Shaw flow without separation. Lie et.al. (2000) [6] compared flows through an orifice and a venture meter at small Re .According to their results vortices appear at the corners. Alexeenko et al. [7] (2005) applied Arcilik model [3] and DSMC method to simulate long micro channels with constriction. They observed a good agreement between results. Agrawal et al. (2005) [8] reported results from Lattice Boltzmann Method(LBM) simulations in two dimensional sudden expanded / contracted micro channels in a slip flow regime. They have shown that the complex channels could be modeled with knowledge of their primary units, i.e. straight channels. So far, all studies were for low-pressure ratio through channels and long micro channels. However, fluid flow through nano-channels has not yet been reported. This research represents numerical result of rarefied gas flow through sudden expanded / contracted short nano-channels using DSMC method. Computations are performed for axially symmetric configurations.