CFD validation of buoyancy driven jet spreading, mixing and wall interaction

Abstract

There are a number of scenarios that can occur in a PWR plant that require the injection of cold fluid into a plenum of warmer fluid. In systems code models of these scenarios, the level of mixing of the falling plume of cold fluid and how it interacts with structures in the plenum must be assumed. Higher fidelity CFD approaches have the potential to resolve the behaviour of the negatively buoyant plume within the plenum and could be used to reduce pessimisms associated with systems code models. This is, however, reliant on having sufficient validation evidence to support the findings of the CFD model. This paper describes validation of the capability of CFD to accurately capture the key phenomena involved, using data from the experiments reported by Kaye and Hunt (2007). Two experiments were modelled with the CFD code ANSYS CFX, using a transient RANS turbulence modelling approach: 1) Overturning of a buoyant plume that encounters a vertical wall; 2) Unconfined buoyant outflow of a plume impinging on a horizontal surface.It is shown that the key aspects of plume mixing and dynamics can be accurately captured by CFD simulations. Buoyant plume theory parameters are presented to enable scaling analysis and comparisons between CFD predictions for reactor cases, where buoyancy forces arise due to temperature differences, and the experiment, where buoyancy forces arise due to differences in salinity. The results are expected to be applicable to a range of practical situations where the interaction of buoyant plumes with solid boundaries occurs.