Stratification breakup by a diffuse buoyant jet: The MISTRA HM1-1 and 1-1bis experiments and their CFD analysis

Density stratification and its breakup are important phenomena to consider in the analysis of the hydrogen distribution during a severe accident. Many research projects to understand the stratification behavior have been performed with large and small scale test facilities. Many previous experimental studies, using helium as mimic gas of hydrogen, focused on the stratification breakup by a vertical or horizontal jet. However, in a real containment vessel, the upward flow pattern can be considered “diffuse” and buoyant neither pure jet nor pure plume. HM1-1 and HM1-1bis tests in the MISTRA facility were performed to investigate such erosive flow pattern created from a horizontal hot air jet impinging on a vertical cylinder. The experimental results indicated that the jet flow was quickly mixed with the surrounding gas in the lower region of the initial stratification, and deaccelerated by buoyancy force therein. Consequently, the erosive process became slower at the upper region of the initial stratification. Those observed behavior was analyzed using the computational fluid dynamics (CFD) techniques focusing on models for turbulent Schmidt Sct and Prndtl Prt numbers. Some previous studies mentioned that these numbers significantly change in the stratified flow. The changes of Sct and Prt are very important factor to predict the stratification erosion process. The results have indicated that the simulation can be much improved by using appropriate dynamic models for those numbers. This research is a collaboration activity between CEA and JAEA.