A new visual experimental study is performed on flow field for laminar to turbulent transition in three different roughness microchannels with almost the same aspect ratio. The red trace is a thin straight line in microchannels at the low Reynolds number. The exit, middle and entrance region become diffuse and transitional flow occurs successively with increasing Reynolds number. The transition Reynolds number is about 1700 for relative roughness smaller than 3%, but the transition Reynolds number is about 1500 with relative roughness is 3.15%, which show that transition occurs earlier than the traditional values. The friction factor and Poiseuille number were also measured in microchannels. It is found that the friction factor and Hagen-Poiseuille number increased with surface roughness, and were significantly greater than the classical values.

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