Acoustic Detection of Gas Bubbles in a Pipe

This report describes an acoustic system designed to detect gas bubbles in a pipe. The system makes simultaneous use of multiple techniques for detection and sizing; these include six resonance-based indicators, as well as off-resonance scattering. The abilities of these acoustic indicators to detect, size and count bubbles are compared. It was envisaged that using several simultaneous bubble detection techniques would compensate for restrictions inherent in the isolated use of individual techniques. For example, whilst geometric scattering tends to be more successful at locating bubbles than sizing them, the converse tends to hold for techniques which exploit the bubble resonance. In the latter case, the greater the tolerance allowed in the estimation of the bubble size from the measured resonance frequency, the more approximate may be the conversion algorithm. Single bubbles tethered to a wire in the pipe, and a freely rising stream of similar size bubbles, are examined. Results measured in these two cases are presented to indicate the potential of the location and sizing techniques. The effect on bubble resonance frequencies of confining the bubble in a pipe is considered in detail. Standard interpretations of bubble resonances in terms of bubble size assume free-field conditions, and require modification for the pipe environment. The necessary corrections are presented for the frequency range below the first transverse acoustic resonance of the pipe.