Assessment of interfacial area model for two-phase flows in rod bundles implemented in RELAP5 code

Gas-liquid two-phase flows are widely encountered in thermal systems with mass and energy transfers. The interfacial area concentration (IAC) determines the characteristics of interfacial mass, momentum, and energy transfers. Bubbles are classified into two groups due to different characteristics in interfacial drag force. Group one comprises spherical and distorted spherical bubbles. Group two comprises cap, slug, and churn-turbulent bubbles. The two-group IAC modeling approach is prevalent, and it has been successfully applied to dispersed flows by separately considering IAC features for two groups of bubbles. The present study aims to thoroughly evaluate and substantially improve the two-group IAC model implemented in the RELAP5 code (hereafter referred to as the RELAP5 two-group IAC model) for its application to rod bundles. Bubble coalescence caused by bubble-bubble interactions and bubble breakup caused by bubble-turbulent eddy interactions are two major factors governing group one bubble sizes. The Weber number defined based on the turbulent energy dissipation rate per unit mass is utilized to consider the balance between bubble coalescence and breakup rates in steady state and fully developed flows. The bubble Weber number is applied to group one Sauter mean diameter modeling in dispersed flows. The turbulence also influences bubble shapes and causes wavy interfaces on large group two bubbles, affecting group two bubble sizes. The present study models the group two Sauter mean diameter based on an interface roughness coefficient and rod bundle characteristic length. The developed two-group Sauter mean diameter model is integrated with the validated drift-flux-based two-group void fraction model to improve IAC predictions. The developed IAC model provides good IAC predictions in dispersed flows in rod bundles and superior IAC prediction performance compared to the IAC model implemented in the RELAP5 code. © 2025 Elsevier Ltd<br/>