Flow characteristics of nitrogen flows in the three different microchannels (hydraulic diameter Dh ranging 30–3000 μm) have been investigated numerically considering of the effect of compressibility and viscosity heating. It indicated that pressure drop at inlet and outlet is nonlinear with Reynolds number. Transition Re is as low as about 1200 for microchannels with Dh = 300 μm. To incompressible flow, Dh, has no effect on friction factor f. However, L/Dh has great effect on f, and f increases with the decrease of L/Dh. It was found from numerical results that fRe can be expressed as a function of Mach number. By compared with the experimental results, the function has been proved right. When pressure drop at inlet and outlet is over 10kPa, the effect of compressibility could not be neglected. This implies that the effect of compressibility in microchannels can be described better by pressure drop at inlet and outlet than Ma, which is in contrast to the practice in the conventional scale. At last, the viscosity heating effect was simply analyzed. It was found that the effect of viscosity heating increases with the increase of Re and decrease of size.

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