Measuring void fraction in bubbly air-water mixtures with ultrasonic extinctions

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, no significant variation in sound speed was observed as air was introduced into the system.