MicroPIV measurements of turbulent flow in square microchannels with hydraulic diameters from 200 μm to 640 μm

Microscopic particle image velocimetry (microPIV) experiments were performed on square polydimethylsiloxane (PDMS) microchannels with hydraulic diameters ranging from 200 μm to 640 μm and for Reynolds numbers ranging from 200 through 3971. Deionized water mixed with fluorescent seed particles was used as the working fluid. There was no evidence of early transition to turbulence, with transition observed at Reynolds numbers between 1718 and 1885 for the different sized microchannels. The 200 μm microchannel structurally failed before fully turbulent flow was reached, but for the other microchannel geometries studied, the flow was found to become fully turbulent at Reynolds numbers ranging from 2600 to 2900. The measured fully turbulent 〈u′〉/umax velocity fluctuations agreed well with results for turbulent duct flow for the 320 μm, 480 μm and 640 μm microchannels. The measured fully turbulent 〈v′〉/umax fluctuations agreed well with turbulent duct flow results for the 480 μm and 640 μm microchannels, but were 20–40% lower than turbulent duct flow results for the 320 μm microchannel. Similar results were observed in the measured Reynolds shear stresses. Spatial correlations of velocity fluctuations were also measured in the transverse direction, and in the Ru′u′ spatial correlation results, the 480 μm and 640 μm microchannel results agreed very well with turbulent pipe flow results, but the 320 μm results showed significant differences. The Rv′v′ correlations were very similar for all three microchannels, with the Rv′v′ correlations decaying more slowly than the Ru′u′ correlations.