A three-dimensional incompressible model of microchannel is proposed. Flow characteristics of nitrogen flow in different microchannels (hydraulic diameter ranging 100–500μm, the ratio of length-to-diameter ranging 60–150, the ratio of height-to-width ranging 0.2–1) have been investigated numerically. It is found that the velocity distribution in microchannels is obviously different from that in conventional channels, and the maximum velocity occurs not in the channel core as conventional theory expected but near the walls due to the surface effect. These phenomena result in the reduction of the thickness of hydrodynamic boundary layer. So the hydrodynamic entry length in microchannels is much larger than that in conventional channels. Theoretical analysis was given to explain these phenomena. The effects of Reynolds number, hydrodynamic diameter, length-to-diameter ratio and height-to-width ratio on hydrodynamic entry length were analyzed. The correlation between L/D and Re and height-to-width ratio, which is useful for designing and optimizing the microchannel heat sinks and other microfluidic devices, was suggested.

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