Experimental and numerical investigation of forced convection of subsonic gas flows in microtubes

The aim of this paper is to investigate the impact of gas compressibility on forced convection through commercial stainless steel microtubes with an inner diameter of 750 μm, 510 μm and 170 μm by combining experimental data with numerical simulations. The analysis covers both transitional and turbulent flow regimes (3000 < Re < 12,000). The results have evidenced that compressibility effects can significantly enhance convective heat transfer when the gas flow is heated by the walls (H boundary condition). This enhancement turns out to be more remarkable for microtubes with smaller inner diameter (lower than 200 μm). In order to explore in-depth the heat transfer mechanism along the microtube in presence of non negligible compressibility effects, the experimental data have been integrated with the numerical results obtained by modeling the fluid flow through the microtube with the adoption of the Arbitrary–Lagrangian–Eulerian (ALE) method and the Lam–Bremhorst Low–Reynolds number turbulence model in order to evaluate eddy viscosity coefficient and turbulence energy within the gas flow. The results presented in this work put in evidence that the integration of the experimental data with the numerical results is strongly beneficial in order to obtain a deep investigation of the physics of micro convection for compressible flows. The experimental values of the Nusselt numbers obtained for three different microtubes have been compared with both classical correlations validated for conventional pipes and specific correlations proposed for microtubes. This comparison highlights that the conventional correlations still holds for gas flow convection through microtubes when the compressibility effect is not significant. On the contrary, when compressibility is no longer negligible, the conventional correlations tend to underestimate the value of the Nusselt number. It is also demonstrated that the specific correlations proposed for the prediction of the Nusselt number in microtubes fail in presence of strong compressibility effects.