One-group and two-group drift-flux models for dispersed gas-liquid flows in subchannels of a vertical rod bundle

Rod bundles are essential structures for enhancing heat transfer efficiency in heat production processes accompanied by gas-liquid two-phase flows. Subchannel analysis divides the rod bundle flow area into subchannels enclosed by a rod lattice and casing box, and it performs two-phase flow thermal-hydraulic simulations at the subchannel scale. The drift-flux model (DFM) is critical for modeling the relative velocity and interfacial drag force in subchannel analysis codes, and the model requires reliable constitutive equations for the distribution parameter and drift velocity. Conventional two-phase flow modeling considers the gas phase in dispersed flows as a whole. Namely, it takes a one-group (1G) approach. A two-group (2G) bubble classification was proposed considering bubble shape and size characteristics in interfacial mass, momentum, and heat transfer modeling, where group one bubbles include spherical and distorted bubbles, and group two bubbles include cap, slug, and churn-turbulent bubbles. The present study develops 1G and 2G DFMs for dispersed gas-liquid flows in interior, edge, and corner subchannels in vertical rod bundles. The existing 1G subchannel distribution parameter correlations are examined and improved for rod bundles with different casing box designs. The 1G subchannel drift velocity is modeled and validated independently by experimental data. Then, the 2G distribution parameter and drift velocity are proposed and validated by experimental data. The developed 1G and 2G DFMs are rigorously consistent with each other. Their general prediction performances regarding gas velocity and void fraction are validated. The validation results show high accuracies for 1G and group-wise gas velocities and void fractions predicted using the DFMs. © 2025 Elsevier Ltd