A numerical investigation of turbulent convective heat transfer of supercritical CO_2 in vertical mini tubes

摘要

This article presents a numerical investigation on the turbulent convective heat transfer of supercritical CO_2 in vertical mini tubes. For buoyancy-aided cases, buoyancy-opposed cases and buoyancy-free cases, the profiles of velocity, temperature and turbulent kinetic energy, as well as heat transfer coefficients and Nusselt numbers inside the tubes with the diameters less than 2 mm, under heating and cooling conditions, are obtained. It is found that the buoyancy is significant near the pseudocritical point due to the sharply varied properties of supercritical CO_2, leading to the variations of the flow pattern, turbulent production and hence heat transfer coefficient in the tube. Detailed explanations to the physics of the complex but interesting transport phenomena are given. Furthermore, the values of the buoyancy parameter Gr_b/Re_b~(2.7) are calculated, and Jackson and Hall's criterion for assessing the buoyancy's influence is examined for cooling flows in mini tubes.