High-Resolution Experiments and Computations on Mixing of Turbulent Buoyant Round Free Jets in Uniform and Stratified Environments

Abstract

Fluid jets interacting with stratified environments play an essential role in the safety of several nuclear reactor designs. In the containment of some power plants like a Light Water Reactor (LWR), fluid jets dominate the transport and mixing of gaseous species and consequent hydrogen distribution in case of a severe accident. The mixing phenomena in the containment are driven by buoyant high-momentum injections (jets) and low momentum injection plumes. Mixing near the postulated break is initially dominated by high flow velocities, while plumes with moderate flow velocities are instead relevant in the break compartment during the long-term pressurization phase. Phenomena of interest include free plumes (as produced by the efflux from the break compartment in a larger room), wall plumes (such those produced by low mass flows through inter-compartment apertures), and propagating stratification fronts in the ambient. These phenomena have been highly ranked for nuclear reactor safety analyses. For example, during a Pressurized Thermal Shock (PTS) scenario in a Pressurized Water Reactor (PWR), the interaction between the cold injection plume of the Emergency Core Cooling Systems (ECCS) and the stratified fluid present in the cold (or hot) leg is important to determine the time evolution of the temperature at the inlet of the reactor pressure vessel (RPV) and therefore the potential to cause a thermal shock on the RPV wall. All the above phenomena can be characterized by the interaction of buoyant jets with stratified environments. In stratified layers, baroclinic forces create a significant redistribution of turbulent kinetic energy and scales, which leads to anisotropy. This important physical phenomenon is highly three dimensional. Results reported in the literature have shown that the turbulence models currently implemented in Computational Fluid Dynamics (CFD) commercial software tend to overestimate thermal stratification and underestimate turbulent mixing when buoyancy effects become dominant for momentum effects. Because of the local isotropy assumptions used in most subgrid-scale models, even Large Eddy Simulations (LES) might fail to reproduce thermal stratification when buoyant jets interact with stratified environments. To gain insight into the relevant physical phenomena and improve the existing models for the buoyancy-driven flows, it is pivotal to establish a database of high-resolution experiments for turbulent buoyant jet flow fields in the presence of uniform and stratified environments. To shed new light on this crucial phenomenon, non-intrusive optical methods of flow visualization, like Particle Image Velocity (PIV) and Planar Laser-induced Fluorescence (PLIF), are applied to obtain highly-resolved velocity and concentration fields. However, high-resolution measurements of turbulent jet flows with density differences are extremely challenging because tiny changes in density correspond with changes in refractive indices, yielding blurred images. Refractive indices must precisely match (up to 0.0002) throughout the mixing process. The experiments in this dissertation make use of a novel Refractive Index Matching (RIM) methodology based on mixing behaviors of ternary-component systems that were developed in our lab. This technique has allowed performing experiments with density differences up to about 8.6% (3 times larger than previously published). Implementing a high-fidelity synchronized PIV/PLIF system combined with the new RIM technique, a novel high-resolution experimental database for the mixing of turbulent buoyant jets in both uniform and stratified environments has been built. The experimental data are analyzed to extract fundamental features of buoyant jets in stratified environments and to assess and improve the predictive capabilities of the current CFD models.