Experimental study on compressible flow in microtubes

Abstract

Gas flow in microchannels has practical advantages compared to the use of liquids, in that system requirements are generally of a lighter and less fragile nature. However, the aspect of compressibility becomes an issue to take into account. Due to the high frictional head loss in microscopic channels, gas density can vary up to an order of magnitude between inlet and outlet, and correspondingly alter the shape and magnitude of the velocity profile. In this article the possible effects of this microscale phenomenon are investigated. To this effect, an experimental procedure for determining a local friction factor is implemented, and compared with a linearized global friction factor. Furthermore, a quantitative comparison between the linearized (incompressible) approximation and two “quasi-compressible” correlations for the friction factor is included. The results show a remarkably accurate prediction of the friction factor by the reference curve of Hagen-Poiseuille, f = 64/ Re, regarding the global as well as the local approximations, and a very close agreement between the incompressible and quasi-compressible correlations. This goes to indicate that the afore-mentioned effects of density-change-induced acceleration are extremely limited on a practical scale, and that an incompressible characterization is valid within the studied conditions of flow. The range of validity is synthesized in a limiting value of the pressure ratio at a given experimental uncertainty.