In this article, we perform a series of simulations to analyze the gaseous flow in two-dimensional (2D) and three-dimensional (3D) microchannels. The geometry effects of entrance and exit, applied pressure ratios and rarefaction effects on the flow characteristics are thoroughly investigated. In addition, a modified Poiseuille number correlation for air flows is obtained. This calculation solves the compressible Navier-Stokes and energy equations under velocity slip and temperature jump conditions with varying inlet to outlet pressure ratios (from 1.76 to 20), the outlet Knudsen numbers (from 0 to 0.22) and the aspect ratios (from 0 to 0.47). The calculated mass flow rate, pressure distribution and friction factor are compared with analytic solutions and experimental data in both the slip flow and earlier transition flow regimes. In the case of higher applied pressure ratio, both experiments and numerical modeling show pressure drop at upstream and downstream. Finally, we discuss the adequacy of the friction factor correlation for the 2D flow and the 3D flow in microchannels with both inlet and outlet chambers.

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