Experimental study on compressible flow in microtubes G.P. Celata a, * , M. Cumo b , S.J. McPhail a , L. Tesfagabir a , G. Zummo a a ENEA, Institute of Thermal Fluid Dynamics, Via Anguillarese 301, 00060 S.M. Galeria, Rome, Italy b University of Rome La Sapienza, Corso Vittorio Emanuele II, 244 Rome, Italy Accepted 27 April 2006 Available online 9 August 2006 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, regard- ing the global as well as the local approximations, and a very close agreement between the incompressible and quasi-compressible cor- relations. 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 syn- thesized in a limiting value of the pressure ratio at a given experimental uncertainty. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Gas flow; Microtube; Experimental; Friction factor; Fused silica pipe; Poiseuille number 1. Introduction The miniaturization of many appliances in biomedic, chemical and computer technology has brought with it increased demands for space-efficient high-performance heat dissipation and catalytic devices. Though much research on microscale level has already been done in recent years in the relevant fields, especially as regards hydrodynamic and heat transfer characterization, there is still much diversion of results to be discerned in the various experimental and numerical reports. The main difficulty in this new dimension of applied physics is the reliability of measurements, as conventional measuring apparatus is simply too big or coarse to implement in the tested system. As progress is being made, however, the tendency is for published results to be more orderly, and to converge towards the classical theories more than was originally expected. Conditions are fairly extreme and difficult to control at microscale, but in the case of microchannel gas flow, there is a major advantage over liquid flow in the distinctly lighter system requirements. Pressures do not achieve extreme values and no pumping equipment is strictly neces- sary. On the other hand, compressibility of the fluid becomes an issue, as also molecular rarefaction. For flow in microtubes, there are several theoretical stud- ies dealing with the effect of the compressibility on gas flow. Van der Berg et al. (1993a,b) and Choquette et al. (1996) solved the isothermal, compressible Navier–Stokes equa- tions for laminar flow in a circular tube. For low Reynolds number and Mach number flows the former obtained a local ‘‘self-similar’’ velocity profile with the product fRe still equal to 64. Beskok et al. (1996) numerically modelled the 0142-727X/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2006.04.009 * Corresponding author. E-mail address: celata@casaccia.enea.it (G.P. Celata). www.elsevier.com/locate/ijhff International Journal of Heat and Fluid Flow 28 (2007) 28–36