Thermo-fluid characteristics at the points of onset of nucleate boiling and onset of significant void in vertical upward boiling flows

The onset of nucleate boiling (ONB) and the onset of significant void (OSV) are critical transition points that demarcate heat transfer regimes and govern the development of two-phase flow in subcooled boiling flows. Accurate prediction of these two characteristic points is of great significance for the thermal-hydraulic design and safety analysis of industrial heat exchange equipment, such as nuclear reactors and steam generators. This study focused on the local characteristics at the ONB and OSV points in vertical upward flow channels. Firstly, a low-pressure ONB experimental database covering various channel geometries (including circular tubes, rectangular channels, and rod bundles) was compiled, and the existing classical ONB prediction models were systematically evaluated. The results indicated that the existing models exhibited considerable scatter and significant deviations. To address this issue, a new ONB correlation was developed based on a correlation analysis of the active nucleation site density with dimensionless parameters. The developed correlation reduced the mean absolute relative deviation for wall superheat prediction at ONB to 19.7%, outperforming existing models. Secondly, for the OSV point, a comprehensive drift-flux model was employed to analyze the cross-sectional void fraction and its axial development rate, revealing an exponential correlation between these parameters and the modified dimensionless Sauter mean diameter. Furthermore, by incorporating a bubble layer thickness model, the local parameters of the bubble layer (i.e., bubble layer thickness and void fraction) at the OSV point were quantitatively characterized for both externally heated channels and internally heated annular channels, and corresponding predictive correlations were established. The results demonstrated that the average void fraction within the bubble layer remains essentially constant (approximately 0.1) in externally heated channels, whereas it increased exponentially with the cross-sectional void fraction in internally heated annular channels. The quantitative relationships established in this study addressed the deficiency of existing bubble coalescence-based OSV models in terms of closure relations for local parameters, thereby providing crucial theoretical support for the accurate modeling and engineering applications of subcooled boiling two-phase flows.