Reviews of the experimental results related to the laminar-to-turbulent transition in microchannels have appeared in the last years in the literature. These studies indicate that the transition from the laminar to the turbulent flow in micro-scale passages has been experienced at critical Reynolds numbers ranging from 70 up to 10000. In this work the characteristics of the laminar-to-turbulent transition for gases flowing through commercial Peek microtubes used for gas-chromatography columns are experimentally investigated. The microtubes tested are characterized by an internal diameter ranging between 300 μm and 100 μm with a L/Dh ratio in the range between 167 to 4000. Since the absolute roughness of the microtubes tested was very low (less than 0.05 μm) it is possible to uncouple the effects of the roughness on the laminar-to-turbulent transition. In the paper the effects of the gas compressibility and of the L/Dh ratio on the critical Reynolds number for which the laminar-to-turbulent transition takes place will be analyzed and discussed by comparing the experimental results with the other data published in the literature.

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