Nowadays, there has been great interest in micro- and nano-applications. Understanding the flow and heat transfer in microscale is useful. The primary goal of this investigation is to reveal the flow behavior in microchannels. The deionized water flow in a microchannel was experimentally investigated. The pressure drops were measured under the conditions of different Reynolds numbers. The friction factors were obtained based on the experimental measurements. The friction factors deviate significantly from the conventional theory. A series of numerical simulation was also carried out to explore the mechanism of the deviation in the present paper. The numerical simulation adopting the slip boundary condition shows the numerical results coincide with the experimental results very well. The velocity slip was obtained based on the comparison with the experimental results. By introducing the slip length Ls, the velocity slip of water flow in the microchannel can be characterized. The velocity slip increases with the increase of the inlet velocity and the slip length increases with the increase of the shear rate. The linear Navier boundary condition breaks down when the shear rate reaches a critical value. A correlation between the velocity slip and the inlet velocity based on our experimental measurements and numerical results was obtained.

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