The investigation of laminar mixed convection with heat and mass transfer of supercritical carbon dioxide in horizontal miniature tubes was performed numerically. The flow patterns, isotherms, distributions of wall shear stresses and Nusselt numbers in typical cross sections with tube diameters less than 1.0 mm under both cooling and heating conditions with and/or without gravity were obtained. The variations of skin-friction coefficients, heat transfer coefficients and Nusselt numbers with bulk temperature were also provided. The results presented in this paper reveal that the buoyancy effects on supercritical carbon dioxide laminar flow and heat transfer in horizontal miniature tubes are significant, where the secondary motions in cross sections are very severe due to the sharply varied physical properties of supercritical fluid. The results of this work are of significance for the design of high efficiency compact heat exchangers of supercritical carbon dioxide.
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