Size effect on compressible flow and heat transfer in microtube with rarefaction and viscous dissipation

In a microtube, the flow and thermal fields have very different features to the conventional tube flows. Fluid properties such as compressibility, viscous dissipation, and rarefaction are important for the flow and heat transfer changes. However, with a decrease in the tube diameter, the relative effect of each of these properties on flow development is unclear. In this study, various compressible microtube flows were numerically investigated for different tube diameters of 1 ∼ 100 μm to examine the size effect on the flow and heat transfer. The slip-flow regime 0.001 < Kn < 0.1, where the continuum-based equations are valid under velocity slip and temperature jump conditions, was selected. The composite effects of compressibility, viscous dissipation, and rarefaction are examined for four cases based on Br and Kn. On the basis of the results, compressible flows were analyzed by varying Kn and the relation Kn ≈ Ma/Re was identified. Furthermore, flow fields related to the Knudsen paradox were examined. The size effect and the effects of viscous dissipation and compressibility on the microtube heat transfer are discussed.