Assessment of a URANS CFD model for gravity driven flows: A comparison with OECD/PKL2 ROCOM experiments

Abstract

The present investigation makes use of unsteady Reynolds Average Navier-Stokes equations written for multi-component incompressible liquid to describe gravity driven flows and the mixing of an active scalar in a four-loop KONVOI German type Reactor Pressure Vessel. The types of asymmetric flow conditions considered here are characteristic of main steam line break with loss of off-site power accident scenarios. Numerical results are compared with three selected tests performed in the ROCOM experimental facility and released as part of the OECD-NEA PKL2 Project. The test matrix spans quasi-steady state and transient flow conditions, low and high flow rates as well as low and high density ratios of the injected coolant.A critical screening of the experimental data using fundamental principles has suggested that for certain tests a correction of the experimental data is required in order to take into account non-linearities induced by the chemical properties of the employed mixture. Furthermore, the influence of the turbulent Schmidt number on the overall mixing is investigated. A reduction of the turbulent Schmidt number generally shows a better agreement with the spatial averaged measurements for all the chosen tests but at the same time it decreases the performance of the model in predicting the evolution of the local maximum.The study shows that URANS simulations are rather successful in describing quantitatively the mixing phenomenon induced by gravity driven turbulent flows, in a global sense, inside complex geometries like reactor pressure vessels.