Characterizing fluid structure interactions of a helical coil in cross flow

Mechanical vibrations compromise the integrity of key components of thermal power plants. Without careful design, strong resonances during steady state operation can wear these components to the point of failure, leading to an unsafe situation that may force a plant to shut down. The purpose of this research is to further the understanding of the vibrations induced in a helical coil subject to steady fluid flow. A helical coil steam generator, such as that found in most integral pressurized water reactors, appears to eliminate many flow-induced vibration concerns when compared to traditional steam generators; however this has yet to be clearly demonstrated experimentally. The objective of this study is to detail and demonstrate a new method to quantify the motion of a helical coil in an annulus subject to external axial flow of water and further characterizing the influence of pitch-to-diameter ratio on the fluidelastic instability of a helical coil. This is accomplished by observing the motion of a helical coil mounted to an inner opaque cylinder through an outer glass tube using a high speed video camera. A mirrored image-pair is used to observe this structure from two perspectives simultaneously, allowing for three-dimensional characterization of the coil motion. The experimental facility is described in detail. The method developed herein for identifying specific points on the coil from images and mapping them to the coil location using the law of refraction is described. An uncertainty analysis of the coil position measurement is conducted based on geometry and refractive index which can be readily applied to measurements obtained using this method. The outcome of empirical observations shows these helical coils to hold a slightly higher resonance frequency than that of cylinders in cross flow, their mechanical stiffness approximated through analytical means shows to produce relatively accurate natural frequencies when compared to the empirical data – 14.1 Hz and 12.5 Hz, respectively for first mode vibration. This study”s contributions present a new method for metering fluid structure interactions with high-fidelity, provide new empirical data which has not previously been produced, and make observations to the response of a helical coil which are new to the field of fluid-structure interactions.