The numerical and experimental works dealing with the behaviour of gas flow through microchannels have shown results which are by no means univocal, sometimes agreeing with the classical correlations, and other times contradicting them. It is now agreed upon that the effects due to both rarefaction and compressibility must be accounted for. In addition, the experimental works have demonstrated that sometimes compressibility and rarefaction effects can be coupled in microchannels: since these two actions contrast each other, the scatter of the friction factor data for gaseous flows is remarkably large. This paper is aimed at determining the friction factor for commercial short and long Peek microtubes with nominal internal diameters between 300 and 100 μm and different values of the length-to-diameter ratio L/D, ranging between 167 and 5000. Nitrogen flows inside the microtubes, with a maximum value of the supply pressure equal to 10 bar. Very low Knudsen numbers (Kn<0.001) are considered in order to uncouple the rarefaction effects from the compressibility effects. The role of compressibility on the friction factor is analyzed and discussed in order to draw its limit of significance.

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