Verification and Supplemental Information for the Influence of Tube Frequency Differences on Fluid-Elastic Instability of Tube Arrays In Crossflow

Abstract This paper studies the effect of tube natural frequency differences on fluid-elastic instability (FEI) in tube arrays in fluid crossflow. FEI in heat exchanger tubes, particularly in steam generators, is a significant concern due to its potential to cause tube damage. Design guidelines are essential to mitigate the risk of excessive tube vibration, especially in U-bend steam generator tubes. In the U-bend region, tubes experience high-flow, two-phase transverse conditions, leading to vibrations and accelerated wear, which can result in operational failures. Although extensive research has been conducted on FEI, most of the data documented in the ASME Boiler and Pressure Vessel Code Section III Appendix N pertains to straight-tube arrays in heat exchangers. Experimental data on the onset of fluid elastic instability are 1) surveyed for U-bend and straight tube arrays with asymmetric tube supports that contribute to frequency detuning and 2) compared with the onset of FEI in symmetrically supported straight tubes without frequency detuning. The results show FEI exists in U-bend and straight tube arrays with natural frequency differences but the velocity for the onset of their instability is higher than for comparable straight tube bundles where all tubes have the same natural frequencies. This paper provides supplemental literature data and basis that is relevant to U-bend tube arrays for Appendix N considerations with FEI. This paper discusses the discrepancies observed in U-bend steam generator measurements, focusing on their effects on the FEI constant (K), mass damping, pitch-to-diameter ratio, and critical velocities. We provide a digital dataset to support follow-on experimental or modeling and simulation activities. Based on these additional data points regarding the onset of instability in tube bundles of various array geometries, the damping ratios and the existing mean and lower bound fits will be verified and updated as needed.