Laminar convective heat transfer of supercritical CO2 in horizontal miniature circular and triangular tubes

Laminar mixed convective heat transfer of supercritical CO2 in horizontal miniature tubes with circular or triangular cross section was investigated numerically. The distributions of the velocity, temperature and wall shear stress inside the circular or equilateral triangular tubes with the hydraulic diameters less than 1.0 mm under cooling condition, with and without gravity, were obtained. The effects of the sharply varied physical properties of supercritical CO2 and the geometrical characteristics of tubes on the fluid flow and heat transfer were examined. It is found that the buoyancy is significant and enhances the heat transfer, especially near the pseudocritical point, unless the fluid and the tube wall are under the thermal equilibrium condition. More interesting transport phenomena and the detailed explanations to the physics were given. The results of this work benefit in the design and optimization of the high efficiency compact supercritical CO2 heat exchangers.