Thermal mixing characteristics of flows in horizontal T-junctions

Abstract

Thermal striping induced fatigue cracking incidents in the vicinity of a T-junction - where coolant streams at different temperatures mix together intensively - has been reported in several Nuclear Power Plants (NPPs) and is considered a challenge to the safe operation of NPPs. The complex underlying turbulent flow behavior following the T-junction makes it difficult to monitor the extent of fatigue damage employing the surface thermocouple instrumentation. While there are isolated guidelines issued by regulatory bodies on how to approach the issue, no general consensus exists internationally regarding the fatigue assessment approach induced by such incidents. The existing literature predominantly contains T-junction mixing experiments where the temperature difference (∆T) between the fluids is lower in relation to the ∆Ts experienced in NPPs. Experiments carried out at realistic ∆T between the fluids, on the other hand, lack detailed numerical studies analyzing the different aspects of the flow mixing behavior. This work deals with the coupled experimental and numerical studies of flow mixing occurring in a horizontal T-junction piping from a thermal-hydraulic standpoint. The chosen range of temperature difference (∆T) between the mixing fluids lie between 60 °C and 240 °C which is highly representative of operating temperatures encountered in an NPP. Experiments have been conducted at the horizontally aligned Fluid-Structure Interaction (FSI) test loop at the University of Stuttgart using deionized water as the working fluid. Numerical studies were performed using the large-eddy simulation (LES) turbulence model to capture the T-junction mixing flow behavior in greater detail using the ANSYS CFX solver. The observed mixing behavior could be summarized as follows: Thermally stratified flow behavior is observed in all the investigated cases with (i) an oscillating flow at lower ∆T (< 140 °C) between the fluids and (ii) a stably stratified flow at higher ∆T (> 140 °C) where buoyant forces significantly come into play. Hot flow penetration into the cold branch and vice versa occurs at higher ∆T (> 140 °C) resulting in the partial mixing of fluids even before they converge at the T-junction. Results from the study reveal that significant thermal gradients exist near the stratification layer, a potential region of high amplitude thermal fluctuations (a factor in thermal fatigue analysis). Frequency analyses of thermal fluctuations using the power spectral density (PSD) method highlight the absence of any specific dominant frequency (spectral peak) in the thermal fatigue relevant frequency range of 0.1 - 10 Hz. Comparison of measurement data and LES predictions exhibits very good agreement with one another highlighting the utility of LES as a useful tool in nuclear safety based research. In addition, LES calculations to analyze the flow mixing scenario at inflow conditions that could not be presently realized at the FSI test facility (e.g. higher branch velocity) were also performed in the present study. With rise in branch velocities, the flow nature changes from an unstably stratified flow to a completely mixed flow at lower ∆T (< 140 °C) between the fluids. At higher ∆T (> 140 °C) between the fluids, a transition from a stably stratified flow to an unstably stratified flow is observed.