Abstract This study evaluates the two-group interfacial area transport equation (2G IATE) coupled with the S-Gamma ( S γ ) population balance equation (PBE) model for beyond bubbly flow regimes in a vertical large-diameter pipe. The 2G IATE framework incorporates intergroup mass transfer mechanisms and is implemented within the S γ model, which assumes a log-normal bubble size distribution. The numerical approach is validated against experimental data from Schlegel et al. (2012), with a focus on the void fraction and interfacial area concentration (IAC) distributions. The results show that 2G IATE improves the predictions of the void fraction and IAC, although its accuracy varies with flow conditions. Dominant transport mechanisms, such as bubble interaction (IM), IAC from mass transfer between group-1 and group-2 bubbles (MT), and volume expansion (VE), are analyzed, revealing that the IM is the primary contributor to IAC variations, whereas MT effects become more significant at higher gas velocities. These findings contribute to the advancement of multiphase flow modeling, with potential applications in nuclear reactor safety, chemical processing, and CFD-based two-phase flow simulations.
Hong et al. (2026) studied this question.