Design of a high intensity ultrasound dispersion cell

Master Thesis


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University of Cape Town

The aim of this project was to provide detailed research on the factors causing mechanical damage in a high power ultrasound environment, and to give recommendations for the production of an ultrasonic dispersion cell with a removable treatment vessel. The primary mechanism for causing this dispersion was cavitation: a void of air or vapour in a liquid medium that grows and collapses in an intense ultrasonic sound field. The secondary mechanism was a phenomenon called acoustic streaming which provides a macro mixing effect, also caused by intense ultrasound. Streaming and, even more so, cavitation were difficult to measure and for this reason a refinement of a method to map cavitation fields with aluminium foil was developed. This involved using digital image processing to extract quantitative information from damaged foil samples. A large portion of the project focused on the overcoming of absorption and subsequent rapid attenuation of sound between the transducer (ultrasonic source) and the treatment vessel. This absorption was due to a number of interrelated factors: reflection of sound at material boundaries; cavitation clouds causing sound scattering; energy absorption; and conventional absorption in liquids due to viscous damping. A number of strategies were employed to overcome this absorption problem: the use of increased static pressure to suppress cavitation in certain areas; the use of multiple transducers; and, as a result, multiple paths for the sound to enter the vessel. A combination of static pressure and multiple transducers were also tested. A number of different media were tested for their ability to transmit sound and an optimum solution was recommended. Streaming and the physical constraints affecting streaming in the treatment vessel were tested to give a practical guide to the factors producing streaming. Then, as the temperature of the liquid affects absorption, cavitation threshold, and the ability of a solvent to dissolve, a look at the thermal aspects of the system was discussed.

Includes bibliographical references.