Nikuradse’s  work on friction factors focused on the turbulent flow regime in addition to being performed in large diameter pipes. Laminar data was collected by Nikuradse, however only low relative roughness values were examined. A recent review by Kandlikar  showed that the uncertainties in the laminar region of Nikuradse’s experiments were very high, and his conclusion regarding no roughness effects in the laminar region is open to question. In order to conclusively resolve this discrepancy, we have experimentally determined the effects of relative roughness ranging from 0–5.18% in micro and minichannels on friction factor and critical Reynolds numbers. Reynolds numbers were varied from 30 to 7000 and hydraulic diameters ranged from 198μm to 1084μm. There is indeed a roughness effect seen in the laminar region, contrary to what is reported by Nikuradse. The resulting friction factors are well predicted using a set of constricted flow parameters. In addition to higher friction factors, transition to turbulence was observed at decreasing Reynolds numbers as relative roughness increased.
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Effects of Low Uniform Relative Roughness on Single-Phase Friction Factors in Microchannels and Minichannels
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Brackbill, TP, & Kandlikar, SG. "Effects of Low Uniform Relative Roughness on Single-Phase Friction Factors in Microchannels and Minichannels." Proceedings of the ASME 2007 5th International Conference on Nanochannels, Microchannels, and Minichannels. ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels. Puebla, Mexico. June 18–20, 2007. pp. 509-518. ASME. https://doi.org/10.1115/ICNMM2007-30031
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