On the role of low frequency modes in the thermal and momentum mixing in parallel plane jets

Parallel plane jet mixing has been the subject of extensive study and its physics are characterized well in literature. However, the physics of parallel plane jet mixing in relatively small confined regions is not well known. In this study we investigate the effects of re-circulatory flow on thermal and momentum mixing in confined parallel plane jets. Studying confined jet interaction is important for a variety of engineering applications. One of these applications is sodium cooled fast reactors (SFR). It is common in SFRs to have coolant stream mixing in a confined domain, often the upper plenum. The Reactor Cavity Cooling System (RCCS) facility at the University of Michigan models this problem in a simplified enclosure with isothermal parallel plane jets as the incoming coolant streams. A Large Eddy Simulation (LES) of this simplified plenum was developed using optimal boundary conditions, proven mesh convergence, and consistent resolution. This model was then validated against experimental results from the RCCS facility. After model validation, flow structures in the jet interaction were discerned by means of Proper Orthogonal Decomposition (POD). Comparison of 3D POD to a clipped 2D POD region reveals that re-circulatory flow can lead to dominant oscillations within the jet mixing region. Subsequently, Power Spectrum Density (PSD) and Continuous Wavelet Transforms (CWT) were used to ascertain frequency-based phenomena that might occur in this mixing region. Frequency analysis uncovered a dominant low-frequency oscillation in both velocity and temperature PSDs. These frequencies fall within the known ranges of thermal striping in sodium. The data generated in this study can be used for the benchmarking of less computationally intensive approaches. This data will be made available upon request to ebm5351@psu.edu.