The influence of aspect ratio on the flow behavior in metallic rectangle microchannels was experimentally investigated. The results of flow behavior in silicon microchannels were compared to investigate the influence of roughness. The width of metallic microchannels was 0.4 mm and the aspect ratios were 0.50

0.67

1.00

respectively; and the relative roughness of the metallic microchannels ranged from 0.1609% to 0.2145%. The widths and heights of the silicon microchannels were both 0.4mm

and the relative roughness was 0.00325%. All experiments were performed with air. The experiments were completed with Reynolds number within the range of 250-3000. Results of experiments showed that the onset of transition from laminar flow to turbulent flow occurred earlier in unsmooth microchannels. And the critical Reynolds number decreased with the increase of aspect ratio. In laminar region

the friction factors for all the rectangle microchannels decreased roughly with the increase of Reynolds numbers. In transition region

the friction factors increased then decreased. At very low roughness

the transition enhancement was not found.