Supercritical Water Heat-Transfer Correlation for Vertical Bare Tubes

This paper presents an analysis of heat transfer to supercritical water in bare vertical tubes. A large set of experimental data, obtained in Russia, was analyzed and an updated heat-transfer correlation for supercritical water was developed. This experimental dataset was obtained within conditions similar to those for proposed SuperCritical Water-cooled nuclear Reactor (SCWR) concepts. Thus, the new correlation presented in this paper can be used for preliminary heat-transfer calculations in SCWR fuel channels. The experimental dataset was obtained for supercritical water flowing upward in a 4-m-long vertical bare tube. The data was collected at pressures of about 24 MPa for several combinations of wall and bulk-fluid temperatures that were below, at, or above the pseudocritical temperature. The values for mass flux ranged from 200–1500 kg/m2s, for heat flux up to 1250 kW/m2 and inlet temperatures from 320 to 350°C. Previous study (Pioro et al., 2008) confirmed that there are three heat-transfer regimes for forced convective heat transfer to water flowing inside tubes at supercritical pressures: (1) Normal heat-transfer regime; (2) Deteriorated heat-transfer regime, characterized by lower than expected heat transfer coefficients (HTCs) (i.e., higher than expected wall temperatures) than in the normal heat-transfer regime; and (3) Improved heat-transfer regime with higher-than-expected HTC values, and thus lower values of wall temperature within some part of a test section compared to those of the normal heat-transfer regime. The HTC data were compared to those values calculated with the Dittus-Boelter and Bishop et al. correlations. The comparison showed that the Bishop et al. correlation represents more closely HTC profiles along the heated length of the tube than the Dittus-Boelter correlation. The latter correlation deviates significantly from experimental data within the pseudocritical range. However, outside the pseudocritical region, the Dittus-Boelter correlation can predict closely experimental HTCs. It should be noted that neither of these correlations can be used for prediction of HTCs within the deteriorated heat-transfer regime. An updated heat-transfer correlation is presented in this paper for forced convective heat transfer in the normal heat-transfer regime to supercritical water in a bare vertical tube. It has demonstrated a good fit (±25%) for the analyzed dataset. This correlation can be used for future comparisons with other independent datasets, with bundled data, for the verification of computer codes for SCWR core thermalhydraulics and for the verification of scaling parameters between water and modeling fluids.