Modeling free surface flows relevant to a PTS scenario: Comparison between experimental data and three RANS based CFD-codes. Comments on the CFD-experiment integration and best practice guideline

This paper presents some results concerning a benchmark for stratified two-phase flows conducted in the frame of the European Platform for NUclear REactor SIMulations (NURESIM). This benchmark relies on the FZD slug flow experiment performed in the horizontal air/water channel (HAWAC). For this benchmark special experimental arrangements have been taken in order to be able to properly model the boundary and initial conditions with CFD. A picture sequence recorded with a high-speed camera was used as reference for comparison with the simulations. For this benchmark, three different codes have been tried out. CFX was used with a turbulent two-fluid model in which a special turbulence damping function was implemented in the specific dissipation rate of the turbulent kinetic energy. This allowed a good qualitative representation of the slug dynamics, even though quantitative comparison were less successful because of difficulties in modeling the inlet instabilities. The VOF approach in its laminar and turbulent form was also tried out through the FLUENT code and was found to be inappropriate for those conditions due to the high velocity slip between phases. Moreover, NEPTUNECFD was tested with a newly implemented model allowing free surface location and the computation of momentum transfer across this interface. This Large Interface Model (LIM) is able to detect 'stratified cells' from the other and so that appropriate closure law can be applied. With this model, the results agreed well with experimental data qualitatively and quantitatively. This benchmark experience also allowed us to draw basis recommendations for a best practice guideline in numerical simulation related to free surface flows in nuclear thermal hydraulics.