Validation of Bubble Distribution Measurements of the ABS Acoustic Bubble Spectrometer with High Speed Video Photography

Measurement of the bubble size distribution in a liquid is very important for cavitation inception studies. In this paper we describe an acoustics based device, the ABS Acoustic Bubble Spectrometer that measures bubble size distributions and void fractions in liquids based on the measurement of sound propagation through the tested liquid. Short monochromatic bursts of sound at different frequencies are generated by a transmitting hydrophone and received by a second hydrophone after passage through the liquid. These signals are processed and analyzed to obtain the frequency dependent attenuation and phase velocities of the acoustic waves. From these, the bubble size distribution (number of bubbles versus size) is obtained following solution of an inverse problem. In order to validate a new implementation of the instrument software, a fundamental experiment is conducted. Bubbles are generated in a controlled fashion, and then carefully mixed into a uniform distribution in a flowing system. A high-speed micro-video system is used to take videos of the bubbles at the same time and within the test volume interrogated by the ABS system. Both the acoustic data and the video frames are then analyzed using many datasets under the same conditions, and the results are compared. The two methods are seen to provide very close results within their limits of resolution and within the bubble distribution variations in the liquid. The ABS provides results very close to the time-consuming micro video photography in near real-time in a much more cost-effective fashion.