Experimental Analysis of Gas Flow Forced Convection in Microtubes

This paper deals with the experimental analysis of forced micro-convection features of heated gas flows through commercial stainless steel microtubes having an inner diameter of 172 μm and 500 μm. The experimental results, in terms of Nusselt numbers, are compared to the classical correlations validated for conventional pipes and to the correlations proposed for gas flows through microtubes under laminar and transitional conditions (Re ∈ [400–3500]). The cross sections of the tested microtubes enabled the analysis of the effects of wall axial heat conduction on the Nusselt number which determines a dependence of the convective heat transfer coefficients on the Reynolds number even in the laminar regime, especially for low inner diameters. It is highlighted in the paper that the effects due to overall heat losses, to viscous dissipation and the problems in the right determination of the axial gas bulk temperature distribution cannot be ignored in the thermal analysis of gas flows through microtubes.