Assessment of Multi-Phase-CFD Frameworks for High Void Fraction Flow in Large Diameter Systems

Abstract

Multi-phase CFD is widely applied to low void fraction bubbly flows in small geometry applications. At high void fraction flows, complex interactions occur that make their modeling challenging. Hybrid models have been proposed for application to such conditions as they resolve large structures using interface capturing methods while modeling dispersed structures with mixture approaches adding slip correlations or the Eulerian framework. Beyond simply bubbly flows, these hybrid formulations and multiphase frameworks remain widely untested, particularly for large nuclear reactor components. In this work, the Volume of Fluid framework (VOF), a hybrid Mixture-Multiphase with Large-Scale Interface capturing (MMP-LSI) framework, and the Eulerian-Eulerian framework are validated against experiments performed at the TOPFLOW and HUSTLE facilities. These facilities resemble the two-phase conditions of recent Small Modular light water reactor designs. This work’s objective is to assess the model performance consistently at different mesh resolutions and support future hybrid adoption. The results show that, on sufficiently resolved meshes, void fraction profiles are well predicted by the VOF method for the conditions of the TOPFLOW experiment, while also showing good resolution of the shape and distribution of the large gas structures. However, when applied to the mist flow conditions of the HUSTLE facility, the void fraction profiles deviate from the experimental results and do not show sufficient grid convergence, especially in the near wall regions. The MMP-LSI method is still challenged in these applications since its inability to resolve smaller structures leads to larger errors than the VOF method. The Eulerian-Eulerian framework, although powerful for such large-scale industrial applications, was found to be limited due to the lack of validated interfacial closure models for the flow conditions of interest. Further work would be required to advance the capabilities of the Eulerian framework for high void fraction flows. As a result, the VOF framework was found to be the most applicable for BWR simulations as it was able to model key characteristics of the flow that are relevant for validation and understanding of the flow conditions.