Effect of rod bundle casing geometry on drift-flux correlation for boiling two-phase flows

Rod and tube bundles (hereafter referred to collectively as rod bundles) are utilized in various components of power plants, including heat exchangers, nuclear reactor cores, and steam generators. In rod bundles, the accurate prediction of void fractions is critical for understanding flow dynamics and heat transfer characteristics. The drift-flux model (DFM) is extensively employed for predicting area-averaged void fractions in engineering calculations, formulating mass, momentum, and energy conservation equations of a two-phase mixture, and modeling interfacial drag forces in two-fluid model-based codes. Accurate constitutive equations for the distribution parameter and drift velocity are essential for implementing the DFM. These constitutive equations are referred to as drift-flux correlation (DFC), which is significantly influenced by flow channel geometries. However, there is no systematic research on the effect of rod bundle casing geometry on DFC. This study considers three rod bundle casing geometries: square, hexagonal, and circular casings, and addresses the impact of rod bundle casing geometry on DFC. An asymptotic distribution parameter of 1.10 is indicated by the existing DFC for square casing rod bundles. However, the newly developed DFCs for hexagonal and circular casing rod bundles indicate an asymptotic distribution parameter of 1.40 and 1.02, respectively. The DFCs for square, hexagonal, and circular casing rod bundles have the void fraction prediction performance of 2.18%, −1.78%, and − 0.84%, respectively. This study demonstrates that the rod bundle casing geometry significantly affects the void fraction prediction. © 2026 Elsevier Ltd.