In this paper, nitrogen gas flow in long microchannels with square cross section is simulated numerically with a three-dimensional continuum model. The governing equations of the model are solved by a control volume method. Numerical results are verified with analytical solutions for the range of hydraulic diameters investigated at constant L/Dh ratio of 1000 and pressure ratio of 1.0001. As the pressure ratio is small and therefore the compressibility effect is negligible in this study. A parametric study is conducted at the same L/Dh and pressure ratio. By reducing the hydraulic diameter down from 10000 μm to 0.01 μm, the gas flow inside the microchannels switches from continuum to free molecular flows. The no-slip flow simulation results provide a constant fRe = 54.8, which is very close to the analytical result of fRe = 57. Slip flow condition reduces the friction factor and starts to drop fRe at Dh = 60 μm in the continuum regime.

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