RANS and LES Analyses on a Density Stratified Layer Behavior of Multicomponent Gas by Buoyant Jet in a Small Vessel

Abstract

The analysis on a density stratified layer consisting of multiple gases in the reactor containment vessel is important for the safety assessment of sever accidents. Computational Fluid Dynamics (CFD) code has a potential to clarify detailed stratification phenomena in the containment vessel. In this paper, CFD analyses were carried out in order to investigate the erosion of the stratified layer by a vertical buoyant jet injected from the bottom of a small vessel. Although the Reynolds-Averaged Navier-Stokes (RANS) model is commonly used in industrial applications, it is known that the RANS analyses tend to overpredict effects of turbulent mixing and stratification erosion for these phenomena. This study carried out the RANS and Large-Eddy simulations (LES) in order to understand the detailed phenomena of the stratification erosion in a containment vessel, and clarify the problems of the RANS analysis from the comparison. As a result, although both the RANS and LES models calculated the erosion, the erosion rates calculated by the RANS models were faster than that by the LES model. The calculated erosion behavior was qualitatively different: the LES analyses showed the vertical helium turbulent transport was enhanced only in the radial region directly affected by the impinging jet, while the RANS analyses indicated the occurrences of such transportation at all the radial locations. Although more detailed validation is required using appropriate experimental data, this difference among the calculated cases suggests the importance of the improvement of the turbulence models in order to accurately predict turbulence damping in the stratification layer.