Two-group interfacial drag force for dispersed gas-liquid flows in medium-to-large pipes

Accurate prediction of thermal-hydraulic parameters is essential to the safety and efficiency of nuclear and chemical infrastructure. One-dimensional system analysis codes extensively utilize the two-fluid model to simulate dynamic gas-liquid flow behaviors. Within this framework, the interfacial drag force (IDF) constitutes the dominant component of the interfacial momentum transfer. Currently, system codes employ one-group (1 G) models using two approaches: the Andersen and Chu’s approach, which is based on a local force balance under steady-state and fully developed conditions, and the drag coefficient-based approach, which expresses the IDF as the product of the interfacial area concentration (IAC), relative velocity, drag coefficient, and shape factor. In the 1 G framework, bubbles are treated as a single group, regardless of differences in bubble size and drag characteristics. However, bubbles of different sizes and shapes exhibit distinct interfacial transfer behaviors; in particular, between group-one (G1) bubbles, including spherical and distorted bubbles and group-two (G2) bubbles, including cap, slug, and churn-turbulent bubbles. Therefore, a two-group (2 G) approach is more effective in regimes where G1 and G2 bubbles coexist. The current study developed 2 G IDF models for dispersed gas-liquid flows in medium- and large-sized pipes using two approaches. The first approach extended Andersen and Chu’s approach by integrating a 2 G drift-flux correlation. The second approach developed a 2 G IDF model based on the drag coefficient, incorporating rigorous correlations for 2 G IAC, relative velocity, and shape factor. Specifically, the G1 shape factor was mechanistically modeled to account for hindering effects in bubbly flow and wake-accelerating effects in beyond-bubbly flows, while the G2 shape factor incorporated the influence of hydraulic diameter for medium-sized pipes. Results demonstrated that the proposed models effectively capture the distinct interfacial dynamics across different pipe sizes. © 2026 Elsevier Ltd.