Interfacial area concentration for dispersed boiling flows in internally heated annuli

Boiling flows in annulus channels are commonly encountered in thermal-hydraulic systems. To advance complicated boiling flow analyses, a two-group (2 G) approach treating bubbles in two groups has been considered. In the 2 G approach, bubbles are classified into group-one (G1) bubbles, which correspond to distorted spherical bubbles, and group-two (G2) bubbles, which encompass churn-turbulent, slug, and cap bubbles. The 2 G drift-flux models accurately predicted void fractions (VFs) for each group. They acted as the foundation for determining 2 G interfacial area concentrations (IACs) and Sauter mean diameters (SMDs). The current study developed and validated novel 2 G correlations for predicting 2 G SMDs and IACs of boiling flows in upward, internally heated annulus channels. The developed 2 G SMD correlation achieved the mean average percentage error ( M A P E ) of 10.6 % for G1 SMD and 27.0 % for G2 SMD. The 2 G IAC correlation achieved an M A P E of 28.4 %, 37.4 %, and 21.5 % for G1, G2, and total (or 1 G) IACs, respectively. Moreover, the 2 G correlation demonstrated reliable performance in predicting 1 G SMD and IAC for 1 G datasets, with M A P E as low as 36.3 % and 28.3 % (when VF > 0.03). The 2 G correlation was validated over wide boiling conditions: steam-water and R-134a as the working fluids, hydraulic diameters from 17.1 to 25.4 mm, pressures from 0.10 to 2.7 MPa, mass fluxes between 140 and 2100 kg/m²·s, superficial velocities ranging from 0.0 to 2.0 m/s (for gas) and 0.1 to 2.0 m/s (for liquid), VFs from 0.0 to 0.6, and density ratios of 0.0006 to 0.17. © 2025 Elsevier Ltd.