Efficient CFD modelling of bulk condensation, fog transport and re-evaporation for application to containment scale

Abstract

Formation of fog due to bulk condensation during a Beyond Design Basis Accident (BDBA) in a water-cooled nuclear reactor is of high safety relevance with regard to the flammability of the containment atmosphere and its interaction with several other safety relevant phenomena such as aerosol transport. Bulk condensation of steam in the presence of non-condensable gases is modelled using ‘return to saturation in constant timescale’ method in single-phase system as mass sink. The fog droplets formed as a result of condensation is transported as passive scalar in an Eulerian frame by considering the effects of convection by the gas mixture, turbulent and Brownian diffusion and droplet drift due to gravitation, drag and inertial forces. The model formulation is based on the assumption that fog volume fraction is fairly low (<1%) so that droplet–droplet interactions and the effect of droplets on the gas flow are negligible. The present work considers only spherical fog droplets of fixed diameter and evolution of droplet by coalescence and breakup is neglected. The effect of different droplet sizes is investigated in the validation cases to understand its consequence on the results. It is also assumed that the fog droplets are always in thermal equilibrium with the surrounding gas mixture. The re-evaporation of the transported fog droplets in regions where gas temperature is higher than saturation temperature is also incorporated into the solver. The model was numerically implemented to the containmentFOAM CFD package based on OpenFOAM. The verification of the phase change(condensation & evaporation) model by simulating the mixing of a cold and hot air–steam–fog mixture gave good agreement with Mollier diagram theory in terms of both final mixture temperature and quantity of steam and fog. The droplets drift transport model was validated by aerosol flow through a bent pipe investigating the effect of droplet Stokes number on deposition efficiency. The performance of the bulk condensation model was further validated and assessed by including the interaction with other relevant containment phenomena (e.g., wall condensation, multispecies transport) on SETCOM experimental facility (Forschungszentrum Jülich, Germany) and a technical scale THAI experiment (Becker Technologies, Germany). The results indicated that the inclusion of bulk condensation model improved the predictive capabilities of containmentFOAM, i.e., the consistency of simulation and measurements for these experiments.