Characterization and modeling of ultrasound propagation and cavitation in confined flow geometries

by Dwayne Savio Stephens
Doctoral thesis
Date of Examination:2023-10-06
Date of issue:2024-07-08
Advisor:Dr. Robert Mettin
Referee:Prof. Dr. Jörg Enderlein
Referee:Prof. Dr. Ulrich Parlitz
Persistent Address: http://dx.doi.org/10.53846/goediss-10589

 

 

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Abstract

English

Acoustic cavitation is a process by which small bubbles are formed in a liquid that is exposed to sound waves such as ultrasound. These bubbles are peculiar because they expand and collapse violently on themselves resulting in high temperatures (>5000C) and pressures (>100 atm) inside them. This process happens repeatedly and at the frequency of the ultrasound. Furthermore, these bubbles occur in large ensembles of thousands often forming larger structures far greater than the bubbles themselves. Acoustic cavitation has many industrial applications from cleaning of delicate electronic components to sonochemistry (the use of ultrasound to enhance chemical reactions in solution). Sonochemistry has gained attention because of its high energy efficiency, low waste production and synergistic capabilities with other green technologies like microwaves. Therefore, precise understanding of cavitation inside sonoreactors (chemical reactors with the capability to apply ultrasound to the reactants) is important to up-scale this technology. Despite its benefits, upscaling sonoreactors has been a major industrial challenge due to technical challenges as well as difficulties in predicting their behaviour due to the extremely complex nature of cavitation. Numerical methods can be used to predict the behaviour of cavitation, but it has been a numerical challenge for decades. These bubbles interact with each other by coalescing and breaking up and they effect the applied ultrasound as well by scattering on coming waves. The problem is highly non-linear. Early simulations focused on single bubble dynamics but over decades of computational advancements larger ensembles of bubbles could be simulated. However, dimensions of sonoreactors can range between a few millimeters to meters - which is much larger than the bubbles or bubble structures. Furthermore, the time scales on which bubble oscillation, translation, structure formation, and chemical reactions occur are spread between microseconds to seconds. Thus, modelling of the full n-body problem of cavitation and it’s non-linear coupling with the soundfield is nearly impossible. However averaging methods exist to simplify the problem and can be used in the design and development of sonoreactors. The work here tries to use existing models as well as develop them to find a middle ground in the complexity of modeling the dynamics inside sonoreactors. To this extent, experimental measurements of the soundfield of sonoreactors were compared to numerical simulations to verify and optimize the model.
Keywords: Acoustic cavitation; Sonication; Bubbles; Ultrasound; Cavitation
 
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