Two-group interfacial drag force model for downward dispersed two-phase flows

The two-fluid model (TFM) is widely utilized in the analysis of many engineering systems, including nuclear power plants, chemical process equipment, and petroleum pipelines. Accurate modeling of interfacial momentum transfer is essential to ensure its reliability. The interfacial drag force (IDF) plays a dominant role in interfacial momentum transfer for dispersed two-phase flow. Bubbles exhibit diverse morphologies, ranging from spherical to distorted, cap-shaped, slug, and churn-turbulent forms, that strongly influence drag characteristics. Conventional one-group (1G) IDF models, implemented in codes such as TRACE and RELAP5, oversimplify bubble dynamics by treating all bubbles as a single category, thereby compromising physical fidelity. To enhance physical fidelity, this study proposed two two-group (2G) IDF models for downward two-phase flow based on theoretical analysis and compiled experimental data. First, the prediction performance of existing 1G IDF models adopted in TRACE and RELAP5 was assessed against downward flow data. Then, two 2G IDF models were formulated, explicitly distinguishing between spherical/distorted bubbles (G1) and cap/slug/churn-turbulent bubbles (G2). The models were established using two independent approaches: the extended Andersen-Chu approach and the drag coefficient-based approach. New constitutive correlations were established for group-wise relative velocity, drag coefficient, and bubble shape factor to achieve closure. The mean absolute relative errors for G1 and G2 IDFs are 13.4% and 8.83% for the extended Andersen-Chu approach, and 11.5% and 31.6% for the drag coefficient-based approach. These results confirm that the developed framework provides a robust and physically consistent basis for simulating IDF for downward two-phase flows. © 2026 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.