Characterisation of the acoustic cavitation cloud by two laser techniques

An experimental investigation of the size and volumetric concentration of a coustic cavitation bubbles is presented. The cavitation bubble cloud is gen erated at 20 kHz by an immersed horn in a rectangular glass vessel containi ng bi-distilled water. Two laser techniques, laser diffraction and phase Do ppler interferometry, are implemented and compared. These two techniques ar e based on different measuring principles. The laser diffraction technique analyses the light pattern scattered by the bubbles along a line-of-sight o f the experimental vessel (spatial average). The phase Doppler technique is based on the analysis of the light scattered from single bubbles passing t hrough a set of interference fringes formed by the intersection of two lase r beams: bubble size and velocity distributions are extracted from a great number of single-bubble events (local and temporal average) but only size d istributions are discussed here. Difficulties arising in the application of the laser diffraction technique are discussed: in particular, the fact tha t the acoustic wave disturbs the light scattering patterns even when there are no cavitation bubbles along the measurement volume. As a consequence, a procedure has been developed to correct the raw data in order to get a sig nificant bubble size distribution. After this data treatment has been appli ed the results from the two measurement techniques show good agreement. Und er the emitter surface, the Sauter mean diameter D(3, 2) is approximately 1 0 mu m by phase Doppler measurement and 7.5 mu m by laser diffraction measu rement at 179 W. Note that the mean measured diameter is much smaller than the resonance diameter predicted by the linear theory (about 280 mu m). The influence of the acoustic power is investigated Axial and radial profiles of mean bubble diameters and void fraction are also presented. (C) 1999 Els evier Science B.V. All rights reserved.