Numerical investigation of the fluidelastic instability of two-phase flow in a parallel triangular tube array

Abstract

Steam generators are always susceptible to vibrations induced by the flow in either the shell or tube sides. The fluidelastic instability phenomenon (FEI) is considered one of the most devastating flow excitations since it may cause excessive wear and structural failure to the tubes in a short time span, yet the phenomenon is not well understood. The U-bend region of the steam generator is very prone to the FEI and the flow in this region is characterized by a two-phase nature. Most studies of this phenomenon have been carried out experimentally on specific tube arrays at certain conditions, and design guidelines were developed based on them. Thus, a need emerges to provide a model to predict the onset of FEI at any flow condition or geometry. Firstly, this research focuses on developing and validating a model to predict the onset of FEI in two-phase flows. Secondly, the work attempts to address the problem of varying the flow’s angle of attack inside the U-bend, known as flow’s approach angle, and how it influences the onset of the instability. Finally, due to the curvature of the tubes inside the U-bend region, they are not tuned to a single natural frequency, a case known as frequency detuning. The presented work inspects the effect of frequency detuning and the key parameters controlling its influence. In this study, a model based on Computational Fluid Dynamics was proposed to simulate the onset of FEI. The model was validated and tested for a two-phase air-water flow in parallel triangular array against FEI in transverse and streamwise directions. Predictions obtained were in good agreement with experiments in the literature. Furthermore, the influence of flow approach angle was relatively understood and an efficient approximate semi-analytical model was successfully developed to predict the FEI dynamic forces at any flow angle. Finally, isolation of FEI mechanisms was carried out. Generally, frequency detuning was found to stabilize the tube bundle and its effect is sensitive to the mass-damping parameter. This work is a step forward towards a better understanding and an accurate prediction of the onset of FEI in the U-bend region.