Turbulence Modelling for CFD-Methods for Containment Flows

Abstract

During a severe accident of a light-water reactor, hydrogen can be produced by a chemical reaction between the Zircaloy cladding and water and escape into the containment through a leak in the primary circuit. The prediction of the mass transport of hydrogen is vital for an optimised positioning of countermeasures like recombiners. It is possible that a stable stratification of hydrogen and air occurs, due to the different densities of those fluids. This stratification can be mixed with a free jet. This mixing is characterised by the time dependency of the flow, sharp velocity and density gradients as well as the non-isotropy of Reynolds stresses and turbulent mass fluxes.

With the use of a Reynolds stress turbulence model, the non-isotropic Reynolds stresses can be simulated. A similar approach is theoretically possible for the turbulent mass fluxes, but only the isotropic eddy diffusivity model is currently available in state-of-the-art cfd-software.

The shortcomings of the eddy diffusivity model to simulate the turbulent mass flux are investigated, as well as improvements with the use of a non-isotropic model. Because of the difficulties to get experimental data of flows in real containments, the THAI experimental facility was created to get experimental data for flows in large buildings. The experiments are performed by Becker Technologies. The analysis is using the experimental data of the THAI experiments TH-18, TH-20, TH-21 and TH-22 as the reference case. For safety reasons the used light gas for the TH-20 experiment is helium instead of hydrogen. Due to the rotational symmetry of the geometry as well as the boundary conditions, two-dimensional simulations are performed where applicable. The grids have been built following the best practice guidelines to ensure sufficient grid quality. Several simulations were carried out to investigate the numerical error caused by spatial and time discretisation.

During this reports time frame, simulations of the TH-20 and TH-22 experiments have been performed.