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Abstract: Micro- and nanoscale gas flows are analyzed theoretically and numerically. The analyses of gas flow similarity show that the gas flows at different scales can be similar only when the gas is treated as a prefect gas. If the gas density is so high that the density effect cannot be ignored, the three dimensionless parameters, Re, Ma, and Kn, which characterize the micro gas flow, are independent of each other and cannot be equal for flows at different scales, so the similarity breaks down. The critical density for the similarity failure can be analytically determined for each kind of gas. The analytical results were validated by numerical simulations. High density, high Knudsen number gas flows were modeled using a generalized Monte Carlo method based on the Enskog theory which considers both the density effect on the collision rate and the molecular repulsive and attractive interactions for a Lennard–Jones gas. The predicted transport coefficients agree better with experimental data than previous predictions. The simulation results show that when the gas density is higher than the critical density, the denseness effect alters the flow velocity and temperature fields from the direct simulation Monte Carlo results. Higher densities lead to greater deviation. [Copyright &y& Elsevier]

Abstract: Gas flows in the continuum-transition regime often occur in micro-electro-mechanical systems. The relaxation time Monte Carlo (RTMC) method was modified by using an ellipsoid statistical model and a multiple translational temperature model in the BGK model equation to simulate continuum-transition gas flows. The modified RTMC method uses a simplified form of the generalized relaxation time, which is related to the macro velocity and the local Knudsen number. The results for Couette flow and Poiseuille flow in microchannels predicted using the modified RTMC and the DSMC are in good agreement with the modified RTMC being much faster than the DSMC for continuum-transition gas flow simulations. [Copyright &y& Elsevier]