Advancing State-of-the-art Multiphase CFD Modeling for PWR 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. Additionally, because previous validation was performed using 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 local measurements from subcooled flow boiling experiments that reproduced density ratios and scaled flow conditions corresponding to PWR operating conditions. Measured radial profiles of void fraction and bubble diameters from the DEBORA experiments were used to assess the performance of two-phase closures. The DEBORA experiments consist of vertical flow boiling of R12 in a circular pipe, in which experimental conditions were scaled to replicate PWR conditions. Eleven test cases at various flow conditions, heat flux, inlet subcooling, and pressure have been used in this systematic validation. This work leverages advancements in momentum closures to perform a systematic assessment of wall boiling representation by evaluating heat flux partitioning formulations and the related 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 an optimal closure representation that demonstrates consistent predictions and is applicable to prototypical PWR conditions while identifying areas for future improvement.