A proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) study on the effects of different momentum ratios and Reynolds number in a T-junction with an upstream elbow

Abstract

T-junctions are critical components within the primary circuit of pressurised water reactors (PWRs). They connect the pressuriser (PRZ), with the steam generator (SG), and the reactor pressure vessel (RPV). As such, it is crucial to have a mechanistic understanding of the turbulent fluid flow and thermal mixing within such T-junctions. Computational fluid dynamics (CFD) studies of flows within T-junctions usually involve understanding the effects of variations in momentum and Reynolds numbers on the turbulent flow structures and thermal mixing phenomena. In this paper, we utilise proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) methods to analyse the complex flow structures arising from various test cases for a PWR T-junction with an upstream elbow.

The first aim is to compare the flow structures resulting from different momentum ratios and/or Reynolds numbers of the inlet branch flow to identify the dominant factor. These parameters are adjusted by varying the branch pipe diameter and inlet branch velocity. The final aim is to compare flow structures obtained using POD and SPOD analysis. While both methods produce explicable patterns, they reveal vastly different structures that can be interpreted differently.

Momentum ratios have traditionally guided engineering design optimization. Our findings indicate that altering the Reynolds number of the inlet branch flow can help avoid turbulent flow structures that may compromise the structural integrity of nuclear components in NPPs. While POD is widely used for fluid flow analysis, SPOD offers a more detailed examination of turbulent fluid flow structures.