Vibration Excitation Forces Due to Two-Phase Flow in Piping Elements

Abstract

Severe vibrations were observed in a small piping system comprising two elbows and straight sections configured in the form of a U and subjected to air-water internal two-phase flow. An experimental study was undertaken to investigate the governing vibration excitation mechanism. Vibration response, excitation forces, and fluctuating properties of two-phase flow were measured over a wide range of flow conditions. The results show that the observed vibrations are due to a resonance phenomenon between periodic momentum flux fluctuations of two-phase flow and the first modes of the piping system. The excitation forces consist of a combination of narrow-band and periodic components, with a predominant frequency that increases proportionally to flow velocity. For a given void fraction, the force spectra for various flow velocities and elbow geometries show little scatter on a plot of a normalized power spectral density as a function of a dimensionless frequency. The predominant frequencies of excitation agree with recent results on the characteristics of periodic structures in two-phase flow.