Compressibility effects of gas flows in microtubes

The characteristics of gaseous flow in commercial stainless steel microtubes for gas-chromatography having an inner diameter of 0.751, 0.508 and 0.254 mm are experimentally investigated. The friction factor is calculated as a function of the Reynolds number and plotted in a Moody chart. A comparison among three different methods to calculate the friction factor is conducted in order to evidence limits and advantages of each method. It has been observed that in laminar regime the Poiseuille law can be used to predict the value of the pressure drop. The experimental data shown in this work highlight that the compressibility effects are experienced only for high Reynolds numbers (Re>2000-3000). Compressibility seems to influence the laminar-to-turbulent transition in microtubes. For the shorter microtubes tested, the transition to turbulence seems to be influenced by the L/D (length/diameter) ratio of the tube. The shorter microtubes having a large value of the L/D ratio exhibit a drastic and well localized rise of the friction factor in the transitional region. On the contrary, the microtubes having a low values of the L/D ratio demonstrate a gradual transition to turbulence. However, no early transition to turbulence has been evidenced in the tests but the transition takes place at Reynolds numbers ranging between 1800 and 2900.