Large eddy simulations of erosion of a stratified layer by a buoyant jet

Abstract

During a Loss of Coolant Accident (LOCA) accident, leaked hydrogen from the primary circuit can form a stable stratified layer at the top of the containment building. The formation and erosion of a stratified layer is a challenging numerical problem due to the interaction mechanism of the jet flow with the stratified layer. The OECD-NEA conducted an experiment to investigate the erosion of the stratified layer by a vertical air-helium jet from the bottom of the large vessel (height 8m., diameter 4m). During the experiment, CFD grade experimental data was generated that could be used for comparative studies.In this study, the LES (Large Eddy Simulation) numerical methodology is used to validate the PANDA experiment for the nuclear reactor containment safety analysis. Specific attention is given to the analysis of the interaction between a buoyant jet and stratified layer. An understanding of the interaction mechanisms will help to quantify turbulent mass transport of the gas components. Good agreement between the numerical results and the experimental data is observed for temporal velocity, gas concentration and temperature data. FFT (Fast Fourier Transform) analysis is applied to check the numerical grid resolution and POD (Proper Orthogonal Decomposition) is applied to investigate coherent flow structures and turbulent mass transport mechanisms. Horseshoe vortices, which has previously been observed experimentally and numerically for buoyant jets, are observed near the round jet.