ACOUSTIC DETECTION OF GAS-BUBBLES IN A PIPE

This report describes an acoustic system designed to detect gas bubble s 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 env isaged that using several simultaneous bubble detection techniques wou ld compensate for restrictions inherent in the isolated use of individ ual techniques. For example, whilst geometric scattering tends to be m ore successful at locating bubbles than sizing them the converse tends to hold for techniques which exploit the bubble resonance. In the lat ter case, the greater the tolerance allowed in the estimation of the b ubble 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 exa mined. Results measured in these two cases are presented to indicate t he potential of the location and sizing techniques. The effect on bubb le resonance frequencies of confining the bubble in a pipe is consider ed in detail. Standard interpretations of bubble resonances in terms o f bubble size assume free-field conditions, and require modification f or the pipe environment. The necessary corrections are presented for t he frequency range below the first transverse acoustic resonance of th e pipe.