Mixed convective heat transfer of CO2 at supercritical pressures flowing upward through a vertical helically coiled tube

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The mixed convective heat transfer of CO2 at supercritical pressures inside a vertical helically coiled tube was experimentally investigated under constant heat flux conditions. Experiments were conducted at three supercritical pressures for various heat and mass fluxes. The buoyancy force was found to have two opposing effects on the heat transfer. When Bo∗≤4×10−8, the forced convection is dominant, and the density is only reduced at the near-wall region, which leads to flow acceleration, relaminarization of the turbulence, and deterioration of the heat transfer. When 4×10−8<Bo∗≤8×10−7, as the buoyancy number increases, natural convection starts to have a positive effect on the heat transfer, and part of the heat transfer ability is recovered. When Bo∗>8×10−7, the natural convection is fully developed, and the relaminarization of the turbulence at the near-wall region is suppressed, which enhances the heat transfer. The coupling effects of the buoyancy force, centrifugal force, and variations in the physical properties were found to determine the temperature and heat transfer coefficient distributions along the circumference edges. Based on the current experimental data, two correlations of the Nusselt number were presented.