Mitigation of whistling in vertical corrugated pipes by liquid addition

Abstract

(FIVs) can occur, caused by an interaction between an acoustic pipe resonance and the unsteady shear layers spanning the corrugations. Under certain conditions, these FIVs result in the production of high-amplitude tonal noise (also known as whistling). This is not only inconvenient, but can lead to damage of equipment, failure of piping systems, and hazardous situations. This study focuses on whistling attenuation by liquid addition to vertical corrugated pipe flow, and the identification of the mechanisms behind this attenuation. For this purpose, a new approach is developed to identify the liquid accumulation within the cavities based on planar laser-induced-fluorescence (PLIF) measurements. These measurements are combined with acoustic measurements to identify the sound production from the corrugated pipes. Burstyn (1922) and Cermak (1922) were the first of many to study the sound production from single-phase corrugated pipe flow. Since then, many studies have been devoted to the phenomenon behind the whistling behavior, which are summarized in a review paper on corrugated pipe flow by Rajavel and Prasad (2013).Whistling in corrugated pipes originates from a fluidacoustic feedback. The free shear layers spanning the cavities in this kind of flows are intrinsically unstable and can act as a source of sound. Under certain conditions, discrete vortices can be shed in the cavity mouth. Vortex shedding in the shear layers exerts an unsteady force on the walls, causing a reaction force, which is associated with the sound generation (Curle 1955). This sound source is of a dipole nature due to the vortex-wall interactions (Howe 2003), and feeds an axial acoustic mode in the pipe when the shedding frequency is below the cut-off frequency of the pipe. The acoustic perturbation caused by this acoustic resonance is a source of instability in the shear layers over the corrugations, triggering vortex shedding and closing the feedback loop.Abstract When a corrugated pipe is subject to a dry gas flow, high amplitude sound can be produced (so-called 'whistling'). It was shown previously that liquid addition to corrugated pipe flow has the ability to reduce sound production. Small amounts of liquid are sufficient to mitigate whistling entirely. One of the mitigation mechanisms, cavity filling, is studied experimentally. Acoustic measurements are combined with a planar laser-induced fluorescence technique to measure the liquid accumulation in the cavities of a corrugated pipe. Using this technique, it is shown that the amount of filling of the cavities with liquid increases with increasing liquid injection rate and with reducing gas flow rate. The reduction in whistling amplitude caused by the liquid injection is closely related to the cavity filling. This indicates that the geometric alteration of the pipe wall, caused by the accumulation of liquid inside the cavities, is an important factor in the reduction in whistling amplitude.

Annular two-phase flow in vertical smooth and corrugated pipes

“…It is anticipated that the direction of gravity will have a strong effect on the obtained results. In a previous study, van Eckeveld et al (2017) found that the flow direction with respect to the gravity has significant consequences for the liquid cavity filling behavior. The liquid filling ratio is considerably higher for downward directed flow.…”

“…Ansari and Arzandi (2012) for horizontal channels with ribs. Recently, new attention was given to two-phase flows in corrugated pipes in the framework of the mitigation of flowinduced vibrations in these pipes ( Belfroid et al, 2013;van Eckeveld et al, 2017 ). The presence of liquid was found to reduce, and eventually mitigate, flow-induced noise.…”