Forced and mixed convection heat transfer to supercritical CO2 vertically flowing in a uniformly-heated circular tube

Abstract An experiment of heat transfer to CO2, which flows upward and downward in a circular tube with an inner diameter of 6.32 mm, was carried out with mass flux of 285–1200 kg/m2 s and heat flux of 30–170 kW/m2 at pressures of 7.75 and 8.12 MPa, respectively. The corresponding Reynolds number at the tube test section inlet ranges from 1.8 × 104 to 3.8 × 105. The tube inner diameter corresponds to the equivalent hydraulic diameter of the fuel assembly sub-channel, which is being studied at KAERI. Among the tested correlations, the Bishop correlation predicted the experimental data most accurately, but only 66.9% of normal heat transfer data were predicted within ±30% error range. The Watts and Chou correlation, which is claimed to be valid for both the normal and deteriorated heat transfer regime, showed unsatisfactory performance. A significant decrease in Nusselt number was observed in the range of 10 - 6 Gr b / Re b 2.7 2 × 10 - 5 before entering a serious heat transfer deterioration regime. The heat transfer deteriorated when the value of the buoyancy parameter Gr ¯ b / Re b 2.7 exceeded 2.0 × 10 - 5 close to the Jackson and Hall’s criterion. As soon as the heat transfer deteriorated, it entered a new regime and did not return to a normal heat transfer regime, although the value of buoyancy parameter Gr ¯ b / Re b 2.7 reduced below the deterioration criterion 2.0 × 10 - 5 . It may justify the requirement of developing separate correlations for the normal and deterioration regimes, as proposed in this paper.