Transitional and Turbulent Convective Heat Transfer of Compressible Gas Flows Through Microtubes

This paper presents the results of experimental and numerical investigation of forced convection of gas flows through stainless steel microtubes having inner diameters of 750 μm, 510 μm and 170 μm. The study covers both transitional and turbulent flow regimes (3000<Re<12000). In these regimes the flow is highly compressible, inducing conversion from thermal energy to kinetic energy inside microtubes. Moreover, reverse energy conversion takes place immediately after the fluid is vented to the outlet chamber where the measurement of fluid outlet temperature is performed. In this work the effects of fluid compressibility on the forced convection at microscale is quantitatively discussed by combining experimental data with numerical predictions. It is evidenced that compressibility effects can distinctively enhance convective heat transfer in terms of Nusselt number. This enhancement turns out to be more pronounced for microtubes with smaller inner diameter even at medium Reynolds numbers. In order to explore in-depth the heat transfer mechanism, the system is numerically simulated adopting the Arbitrary-Lagrangian-Eulerian (ALE) method and the Lam-Bremhorst Low-Reynolds number turbulence model to evaluate eddy viscosity coefficient and turbulence energy. The crossing of the numerical data, which provide the local value of pressure and temperature, with the experimental ones helps to explain the physical sense of the experimental results. In addition, the convective heat transfer coefficients obtained in the present work are compared with both classical correlations validated for conventional pipes and the correlations proposed for gas flows through microtubes.