Assessment of State-of-the-art multiphase CFD modeling for subcooled flow boiling in reactor applications

Abstract

Multiphase Computational Fluid Dynamics (M-CFD) has the potential to provide high fidelity simulation of complex boiling phenomena in Light Water Reactors (LWRs), thereby accelerating the development cycle and reducing the need for expensive large-scale experiments. M-CFD relies on two-phase closure models to consistently represent the relevant physical phenomena in flow boiling. However, the still incomplete understanding of the ability of these closures to accurately capture the underlying physics, limits the adoption of M-CFD in reactor development and design optimization. Due to the interaction of complex physical phenomena present in subcooled flow boiling, local measurements are necessary to assess the performance of existing closures. Furthermore, because previous validation was performed mostly on low pressure data, measurements at high pressure are needed to understand the performance of multiphase closures at PWR conditions. In this work, benchmarking was conducted using measured radial profiles of void fraction from subcooled flow boiling experiments of R12 refrigerant that reproduced density ratios and scaled flow conditions corresponding to PWR operation, called the DEBORA experiments. This work leverages recent advancements in momentum closures to perform a systematic assessment of the wall boiling representation by evaluating heat flux partitioning formulations and the associated closure relations. The influence and sensitivity of bulk boiling and condensation models were also evaluated. Using separate effect assessments and recent advancements in experimental understanding, this work presents a closure representation that demonstrates consistent predictions and is applicable to prototypical PWR conditions, while identifying areas for future improvement.