Pressure Drop in Transitional and Turbulent Regime for Isothermal Gas Flows through Microtubes

This paper reports the main results of an experimental campaign aimed at studying the frictional behaviour of microtubes for gas flows in the transient and fully turbulent regimes. The specimens tested are eleven stainless steel pipes of nominal diameter between 127 m and 1016 m, and the Reynolds numbers investigated range from 500 to over 10,000. The results are discussed in the light of the classical theory for friction coefficients in turbulent flows, considering both qualitative behaviour and the Colebrook-White correlation for the turbulent region. It is found that two distinct kinds of frictional behaviour are present in microtubes: for smaller diameter there is an anticipated onset of turbulence (never below Re=1800) and friction factors in the turbulent regime are higher, with no meaningful dependence on the Reynolds number. For larger tubes, transition can also be delayed to Reynolds numbers around 4300, and the friction factors are smaller, sometimes falling below the Blasius correlation but exhibiting a slope which is slightly smaller, and seem to collapse in a single curve independent of the roughness for sufficiently high Reynolds numbers. The findings are commented in the light of the classical theory and analogies and discrepancies are highlighted.