Modeling axial void fraction profile of upward subcooled boiling flows in vertical round tubes

Precise prediction of the void fraction is essential for analyzing the heat transfer in boiling flows. Existing methods typically use correlations for the onset of significant void (OSV) point as the starting point for predicting the axial void fraction, which often introduces substantial prediction errors in the subcooled boiling region, especially in the pre-OSV region. To address this issue, a new model was proposed here to predict the axial void fraction profile in upward boiling flows in vertical round tubes. First, a mathematical model for flow quality was formulated for the subcooled boiling region. Second, this model was integrated with the flow quality model for the saturated boiling region to establish a comprehensive flow quality model covering the entire boiling process. Finally, a new void fraction model was developed for the entire boiling region by coupling with the full-range drift-flux model. To assess the performance of this final model, an extensive experimental database consisting of 788 data points from seven sources was utilized. In a comparison between prediction results of the developed void fraction model against those of the existing model, the developed model not only accurately predicted the OSV points for most operating conditions but also provided precise axial void fraction profiles across the entire boiling region, especially in the subcooled boiling region near the OSV point. Furthermore, the developed model effectively captured the influences of key parameters on the axial void fraction profile, confirming its robust applicability. © 2025 Elsevier Ltd