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Reading Room :: Theses 1995
Thomas Francis Maher's dissertation
by
Thomas Francis Maher, Ph.D.
University of Texas at Austin, 1995
Supervisors: Steven Lee Morriss
Alfred Daniel Hill
Ultrasonic extinction measurements were carried out in a specially
constructed acrylic tube filled with bubbly air-water mixtures.
Several configurations were utilized to obtain bubble size
distributions that significantly differed from each other. The
systems were all oriented vertically, some with air bubbles rising
through stagnant columns of water and others in simultaneous
air-water flow. In all experiments a series of ultrasonic pulses was
transmitted through the mixture and the received signals digitally
acquired to computer disk for analysis. Ensemble averages of the
acquired pulses were calculated in order to find the ultrasonic
extinction caused by each bubble swarm. The extinction level of each
individual pulse was also used to generate a time series which was
referred to as the extinction signal in this report.
The extinction was observed to provide a sensitive indication of the
volume fraction of air (void fraction) even at low air
concentrations. The level of extinction was found to be a strong
function of the bubble size distribution but only an indirect
measurement of void fraction. However, the extinction was observed to
correlate well with void fraction for each configuration studied so
that relative changes in the void fraction for any one system could
be accurately determined from changes in the extinction. This result
leads to the potential for an ultrasonic tool to identify gas entry
points in a borehole with liquids present. The extinction signal was
presented as a possible means of determining the flow regime and as a
qualitative indication of the bubble size distribution pattern.
Experiments performed with swarms of bubbles whose average diameter
diminished over time indicate that the bubbles most influential on
the extinction measurement were quite small (on the order of a
wavelength) and not volumetrically important. Contrary to most
published correlations, there was no significant variation in the
observed sound speed as air was introduced into the system.
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