The increasing demand for energy-efficient and sustainable process technologies has led to the exploration of alternative methods for process intensification in all industrial sectors. Among these, cavitation has emerged as a promising approach, with applications ranging from biological treatments such as fruit juice pasteurization to chemical processes, including the degradation of persistent contaminants. Although cavitation-based processes have demonstrated significant potential as green and energy-efficient technologies, their large-scale industrial implementation remains limited due to challenges associated with process scale-up, including process design complexity and energy losses within the system.
The underlying mechanisms of cavitation are governed by the formation, growth, and implosive collapse of microbubbles in a liquid subjected to alternating pressure cycles in acoustic cavitation, whereas in hydrodynamic cavitation similar effects arise from pressure drops in constricted flow regions. These processes generate localized extreme conditions characterized by high temperatures, pressures, and the formation of reactive radical species.
This thesis addresses these challenges through a systematic design approach for efficient generation and control of acoustic cavitation that integrates multiphysical simulation with experimental validation. The work investigates key parameters that influence cavitation performance, including geometrical design, coupled resonances, impedance matching, operating frequency, and excitation signal characteristics. A flow-through sonicator concept was developed, incorporating six to eighteen transducers arranged in hexagonal or triangular configurations for different frequencies, further intensified through the integration of hydrodynamic cavitation using Venturi-shaped flow constrictions. The sonicator performance was analyzed with particular emphasis on energy dissipation and hydrodynamic conditions that influence cavitation behavior, evaluated through calorimetric measurements and optimization of resonant acoustic and structural mode coupling supported by simulations.
The first part of the work focuses on structural acoustic design and excitation strategies. Optimized sonicator geometries and tailored excitation signals were shown to significantly enhance acoustic pressure localization and improve energy transfer within the system. The combined analysis of single- and multi-frequency excitation revealed the critical role of signal characteristics in controlling cavitation activity and improving overall performance.
In environmental applications, the transition from batch to flow-through multi-frequency sonication enabled improved degradation of PFAS. Optimized high-frequency integration improved acoustic pressure focusing, resulting in removal efficiencies of up to 77% for PFOS and 81% for PFOA under triple-frequency operation. At the same time, energy-conscious dual-frequency flow-through configurations achieved up to 73% PFOS degradation at substantially lower energy input. The formation of short-chain PFAS confirmed sustained chain scission during sonication.
In fruit processing, the developed flow-through sonicator enabled reduced-temperature pasteurization of apple juice. Dual-frequency excitation at 50-55 °C achieved microbial reductions of up to 3.6 log for yeast, 2.7 for mold and 2.8 for aerobic microorganisms within 450 s efficient time. These effects were supported by microstructural modifications, including cellular disruption and improved dispersion, as evidenced by SEM analysis, leading to enhanced physical stability confirmed by sedimentation measurements.
In hydrometallurgical applications, a recirculating sonication system was developed for thiosulfate leaching of gold. The process improved kinetics extensively and gave 40 % gold recovery in 4 h, compared to 34 % in the conventional method. The results indicate that, unlike degradation processes, precise control of cavitation intensity is more critical than maximizing cavitation strength, as temperature and reagent consumption govern process efficiency. This highlights the importance of application-specific cavitation control strategies.
By integrating sonicator design, excitation methods, and process requirements, the thesis shows how acoustic and hydrodynamic cavitation systems can be integrated and reduce energy losses and improve process intensification. The consistent improvements achieved in PFAS degradation, Apple juice pasteurization, and thiosulfate gold leaching support the transition of cavitation technologies from laboratory research to industrial-scale implementation.