Microscopic particle image velocimetry (microPIV) was used to measure velocities in rectangular microchannels with aspect ratios ranging from 0.97 to 5.69 for 200<Re<3267. Mean velocity profiles, velocity fluctuations, and Reynolds stresses were determined from the microPIV data. Transition to turbulence was observed at Re=1765-2315 for the five aspect ratios studied, agreeing very well with both recent microscale experiments and macroscale duct flow and indicating no evidence of early transition for any of the aspect ratios studied. The onset of fully turbulent flow was observed at Re=2600-3200. For the fully turbulent flow, the uumax and vumax fluctuations at the channel centerline were 6% and 3%–3.5% and generally agreed well with macroscale results. As aspect ratio increased, the uumax and uumax profiles became flatter, with nearly uniform values extending for some distance from the centerline of the channel. This region of uniform uumax and uumax became larger with increasing aspect ratio. The Reynolds shear stress for fully turbulent flow also displayed a strong dependence on aspect ratio. For the WH=0.97 microchannel, uvumax2 steadily increased in value moving from the centerline to the wall, but for the higher aspect ratio microchannels, uvumax2 remained close to zero in the center region of the microchannel before increasing in value at locations close to the wall, and this region of near zero uvumax2 became larger with increasing aspect ratio. This behavior in the Reynolds shear stress is due to the region of uniform velocity and, hence, small mean shear, near the channel centerline of the high aspect ratio microchannels.

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