Forced and mixed convection heat transfer to supercritical CO sub(2) vertically flowing in a uniformly-heated circular tube

An experiment of heat transfer to CO sub(2), 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/m super(2) s and heat flux of 30-170 kW/m super(2) at pressures of 7.75 and 8.12 MPa, respectively. The corresponding Reynolds number at the tube test section inlet ranges from 1.8 x 10 super(4) to 3.8 x 10 super(5). 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 before entering a serious heat transfer deterioration regime. The heat transfer deteriorated when the value of the buoyancy parameter exceeded 2.0x10 super(-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 reduced below the deterioration criterion 2.0x10 super(-5). It may justify the requirement of developing separate correlations for the normal and deterioration regimes, as proposed in this paper.