A numerical investigation of laminar convection ofsupercritical carbon dioxide in vertical mini/micro tubes

Laminar convective heat transfer of supercritical carbon dioxide flowing in miniature vertical tubes was investigated numerically. Typical velocity profiles, temperature profiles, Nusselt numbers, and skin-friction coefficients for circular tubes having diameters of 0.5, 0.7, 1.4 and 2.16 mm, under both cooling and heating conditions, with and without gravity, were obtained. It is shown that for the supercritical carbon dioxide tube flow, hydrodynamically and thermally fully developed flow regions can be reached only when the fluid and the pipe wall are under the thermal equilibrium condition, which usually occurs at a location a long way downstream from the tube inlet (x/d > 1000). In addition, it has been revealed that buoyancy effects are significant for all cases considered, even for small tubes and at high Reynolds numbers. The results of this paper are of significance for the design of high efficiency compact supercritical carbon dioxide heat exchangers.