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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

SPECTRAL ANALYSIS OF UNSTEADY TURBULENT FLOW AND THERMAL MIXINGS IN T-JUNCTIONS IN THE COOLANT LOOP OF PRESSURIZED WATER REACTORS

Kaushik Das
Thermal Modeling and Analysis Systems Engineer, The Giant Magellan Telescope Organization

Mohammed Hasan
UTC Aerospace Systems

Debashis Basu
Fire Technology Department, Chemistry and Chemical Engineering Division, Southwest Research Institute

Abstract

The phenomenon of turbulence-induced flow and thermal mixing occurs in a T-junctions by mixing of hot and cold water streams. Flow mixing in a T-junction is dominated by coherent structures, characterized by different spatial and temporal scales, that are the principal factors impacting mixing. The T-junction configuration and the associated thermal mixing problem often is encountered in commercial pressurized water reactor (PWR) plants, where thermal mixing occurs because of valve leakage. Thermal-mixing-induced fluctuating turbulent flow fields encountered in a T-junction pipe were thoroughly analyzed using high fidelity computational fluid dynamics (CFD) tools, as reported in a companion paper. The analysis presented in the companion paper used large eddy simulation (LES) for simulating the turbulence. The simulated 3D flow field using LES contains an instantaneous spatially filtered velocity field as well as a scalar concentration field describing the coherent flow structures. It is essential to identify and analyze these coherent flow structures, as the dynamics of the coherent structures will significantly affect the fatigue load experienced by T-junctions. In this study, two well-established statistical techniques, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), are used to analyze the simulated flow variables. The objectives are to (i) obtain a lower-dimensional representation of the flow field and (ii) analyze the dynamic contents involved in the turbulent mixing process. The POD analysis indicated that the change in the spatial distribution of the large coherent structures is due to the redistribution of the energy between the different modes, which, in turn, leads to energy redistribution between different scales. In other words, the large coherent structures and vortices break down in the downstream direction, forming smaller structures with lower energy. The DMD method successfully predicts the dynamic modes corresponding to the coherent structures. Results from the DMD analyses indicate that for both flow and thermal fields, the mean or "steady state" component dominates the overall field behavior. On the one hand, the DMD analyses of the flow field indicate the presence of some unsteadiness with relatively high modal energy content. The modal energy content for unsteady DMD modes in the temperature field is significantly lower than the velocity field.

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