Characterisation of bubbles in liquids using acoustic techniques

Ramble, David Gary (1997) Characterisation of bubbles in liquids using acoustic techniques. University of Southampton, Doctoral Thesis.

Abstract

This thesis is concerned with the characterisation of air bubbles in a liquid through the use of a range of acoustic techniques, with the ultimate aim of minimising the ambiguity of the result and the complexity of the task.

A bubble is particularly amenable to detection by using acoustical methods because there usually exists a large acoustic impedance mismatch between the gas/vapour inside the bubble and that of the surrounding liquid. The bubble also behaves like a single degree-of-freedom oscillator when excited, and as such exhibits a well-defined resonance frequency which is related to its radius. Tough techniques which exploit the resonance property of the bubble are straightforward to apply, the results are prone to ambiguities as larger bubbles can geometrically scatter more sound than a smaller resonant bubble. However, these drawbacks can be overcome by using acoustical methods which make use of the nonlinear behaviour of bubbles. A particular nonlinear technique monitors the second harmonic emission of the bubble which is a global maximum at resonance. In addition, a two-frequency excitation technique is used which involves exciting the bubble with a fixed high frequency signal (the imaging signal, ω_i) of the order of megahertz, and a lower variable frequency (the pumping signal, ω_p) which is tuned to the bubble's resonance. The bubble couples these two sound fields together to produce sum-and-difference terms which peak at resonance. The two most promising combination frequency signals involve the coupling of the bubble's fundamental with the imaging frequency to give rise to a ω_i±ω_p signal, and the coupling of a subharmonic signal at half the resonance frequency of the bubble to give rise to a ω_i±ω_p/2 signal.

Initially, theory is studied which outlines the advantages and disadvantages of each of the acoustic techniques available. Experiments are then conducted in a large tank of water on simple bubble populations, ranging from stationary single and paired bubbles, to a single rising bubble stream. The techniques are first applied sequentially for calibration purposes and then a selection are applied simultaneously to enable a direct comparison of these methods. Following this, the techniques are applied to the more challenging and practical acoustic environment of a fluid-filled pipe, where the first experimental measurement in a pipe of the ω_i±ω_p/2 signal is obtained.

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