Vibration Behavior of a Channel Subjected to an Internal Two-Phase Flow

Abstract

Vibrations of a channel wall subjected to an external two-phase flow have been widely investigated in the past. However, there is very little information on the effect of internal two-phase flows on the vibrations. To this end, this paper presents results of an experimental investigation of changes to wall vibrations in a vertical square channel, which are caused by introducing a uniform bubble cloud into the flow. The effects of void fraction, characteristic bubble size and flow velocity on wall vibrations are measured. Results show that wall vibrations are greatly enhanced, by up to 25 dB, compared with the same flow without bubbles. The enhancement and spectra of the vibration are mainly dependent on the bubble void fraction. To understand the physics behind the results, evolution of the spectra along the streamwise direction are examined. The primary mechanisms for increased vibrations are: the lateral fundamental acoustic mode excitation of a bubble cloud whose frequency decreases due to change in sonic speed and normal modes of the bubble cloud. The fundamental mode of a bubble cloud persists along the entire channel and its harmonics enhance the vibrations over a broad frequency range. The cloud normal modes are generated due to the process of bubble formation, and they decay with increasing distance from the source.