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

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/m(2) s and heat flux of 30-170 kW/m(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(4) to 3.8 x 10(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 10(-6) < Gr(b)/Re-b(2.7) < 2 x 10(-5) before entering a serious heat transfer deterioration regime. The heat transfer deteriorated when the value of the buoyancy parameter <(Gr(b))over bar>/Re-b(2.7) exceeded 2.0 x 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)) over bar /Re-b(2.7) reduced below the deterioration criterion 2.0 x 10(-5). It may justify the requirement of developing separate correlations for the normal and deterioration regimes, as proposed in this paper. (C) 2010 Elsevier Inc. All rights reserved.