Numerical Investigations of Liquid Film Offtake by Transverse Gas Flow in a Downcomer Annulus Geometry

Abstract

Advances in computational power have enabled the application of computational fluid dynamics (CFD) to nuclear reactor safety analyses, which require accurate predictability for two-phase flow in complex geometries. This study is aimed at validating CFD simulation of liquid film off-take in a 1/10-scale downcomer of a reactor pressure vessel under emergency core coolant (ECC) bypass conditions. Even though different flow regimes can exist simultaneously in real flow, the traditional two-phase flow models used in CFD have a disadvantage with respect to regime dependency. In this study, VOF–slip, which is a hybrid model that combines volume-of-fluid (VOF) and mixture models and is offered in STAR-CCM+ 15.04, was used to simulate the film off-take phenomenon. The key parameters in the simulation were found to be the droplet diameter and interface turbulence damping coefficient. A parametric study was performed to determine the value of the parameters that yield a reasonable liquid film thickness and ECC bypass fraction which were measured at the Seoul National University (SNU) experiment facility. For the conditions of the SNU experiment, the droplet diameter was found to be 150 μm, and the interface turbulence damping coefficient was found to be in the range of 0–30. The validation results confirmed that the VOF–slip model can describe the behavior of the liquid film and the unresolved droplet appropriately. This resulted in an improvement in predicting the ECC bypass fraction in comparison to the results using the conventional VOF model.