The Institute of Safety Research at the Forschungszentrum Rossendorf (FZR) focuses on safety assessments in nuclear plant and the chemical process industry. In this field, most of the passive safety components rely on natural convection to remove heat, and often involve large heat fluxes and boiling flows. We chose CFX-4 to help us in studying these flows because of its open interface that allows us easily to add our own models when required.

Before deciding on CFX-4, we undertook a number of validation projects. An example was the simulation of a scaled emergency condenser (NOKO) constructed in the laboratory of Forschungszentrum Jülich. We modelled the heat transfer from heated tubes in the primary circuit to the pool in the secondary circuit, accounting for the transient and laminar flow in the pool. CFX-4's calculation of the temperature stratification and the development of the plume were in good agreement with the experimental results.

The CFD results provided a detail understanding of the flow phenomena, showing that during the single-phase convection period, stable temperature stratification is established. Initiation of boiling leads to an enhancement of the velocity and mixing in the upper region where the temperature is close to saturation. The boundary between the upper well-mixed region and the lower region, which still shows strong temperature stratification, moves gradually downwards.

The model has allowed us to investigate countermeasures aimed at preventing the strong temperature stratification that leads to undesired boiling in an early stage of the heat-up process. We investigated the influence of attaching two horizontal baffle plates on the heated wall towards the top of the vessel. This proved to have a positive effect on the stratification, which postponed the gas release.

We plan to continue the experiments with improved measuring techniques and different geometries in order to investigate the modelling of high Rayleigh-number flows. These will account for turbulence and heat and mass transfer by boiling and condensation, which are particular points of interest.

Schematic view of the NOKO test facility.

Temperature distribution in the modelling plane after 2000 seconds heating.

Temperature and velocity distribution in a tank with side wall heating.

Investigation of constructive measures to reduce the temperature stratification.