The destruction of per- and polyfluoroalkyl substances (PFAS) remains a significant environmental challenge. Acoustic cavitation has shown promising potential for PFAS degradation; however, its broader application is limited by the low treatment capacity and insufficient energy efficiency of conventional sonication systems. This study investigates the transition from batch to flow-through multi-frequency sonication. A high-frequency unit was numerically designed and acoustically optimized to enhance acoustic pressure focusing and cavitation intensity. The optimized unit was implemented in a single-frequency flow-through configuration (135 kHz, 0.2 kWh/L), achieving 66% PFOS and 55% PFOA removal. In addition, conventional batch and hybrid batch/flow-through configurations were evaluated as intermediate steps toward the development of a flow-through sonication system. The dual frequency batch system (21 and 31 kHz, 0.8 kWh/L) achieved 40% PFOS and 45% PFOA degradation, whereas the triple-frequency hybrid sonicator (21, 31, and 135 kHz, 0.8 kWh/L) achieved up to 77% PFOS and 81% PFOA removal. The progressive formation of short-chain PFAS indicated sustained chain scission during sonication. To further investigate treatment capacity and energy efficiency under larger-volume operation, a dual-frequency flowthrough reactor (21 and 38 kHz) was proposed and evaluated, achieving 74% PFOS and 36% PFOA degradation at an energy density of 0.10 kWh/L. These findings demonstrate the influence of frequency and reactor configuration on PFAS degradation mechanism and highlight the potential of multifrequency flow-through sonication to improve the energy efficiency of PFAS degradation.

Thermal pasteurization is widely applied to ensure the microbial safety of fruit juices; however, it can degrade heat-sensitive nutritional and sensory attributes. In this study, a reduced-temperature processing approach based on combined acoustic and hydrodynamic cavitation was investigated using a flow-through sonication system. The sonicator design was validated through both simulation and experimental results. It enables an efficient combination of acoustic and hydrodynamic cavitation, improving energy transfer, pressure localization, generation of transient cavitation, and increased field complexity. The effects of excitation mode (single and dual-frequency), temperature (30–55 °C), and treatment time were evaluated in terms of microbial inactivation, microstructural modification, and physical stability. Dual-frequency excitation at 50–55 °C (DF-4) showed the highest performance, achieving a log reduction of 3.6 for yeast, 2.7 for mold, and 2.8 for aerobic microorganisms as a hygiene indicator after 450  s efficient time. SEM observations confirmed progressive cellular disruption, while sedimentation tracking indicated improved stability due to reduced particle size and improved dispersion. The proposed approach improves microbial inactivation and modifies the microstructure and stability of the juice, highlighting its potential for processing at reduced-temperatures of apple juice.

Railway curve squeal is a significant source of environmental noise, arising from friction-induced instabilities in the wheel–rail contact. These instabilities are influenced by prevailing friction conditions which are affected by environmental factors such as humidity and temperature. This study presents a statistical analysis of long-term curve squeal measurements from a curve operated by commuter trains in Sweden. The analysis focuses on the relationship between environmental variables and squeal occurrence, distinguishing between squeal generated on the low and high rail. The results reveal distinct differences in squeal tendencies. Low rail squeal is most likely during relative rail humidity 55%–75%, absolute humidity 7–9 g/m3, and temperatures 7–15 °C, with peak occurrence in the morning hours and during the autumn season. The probability decreases notably outside these ranges. A specific range of estimated friction coefficients is also associated with low rail squeal. Conversely, high rail squeal exhibits increased probability during dry conditions and elevated temperatures. Both low rail and high rail squeal probabilities are reduced during low temperatures, rail temperatures close to the dew point, high relative humidity, and during the winter season. The observed differences suggest that separate mechanisms may be responsible for squeal on the low and high rail, involving wheel tread contact on the low rail, and wheel flange or two-point contact on the high rail. The results provide new insights into the environmental dependencies of low rail and high rail generated curve squeal, which can support the development of targeted squeal noise mitigation strategies. 

The cellulose pulp refining process is crucial for achieving high-quality paper characteristics. This research aims to enhance energy efficiency while maintaining good fiber quality using hydrodynamic and acoustic cavitation (HAC). Experiments were conducted with an in-house developed flow-through sonicator combined with a novel Venturi nozzle for hydrodynamic cavitation. The Venturi design was determined by analytical modeling and verified by CFD simulation with multi-phase turbulence models to balance cavitation intensity and turbulence against the acoustic cavitation effect. Experimental evaluation of two batches of CTMP fibers, pre-processed in different ways, showed significant improvements in paper strength and fiber properties. The best results for Batch 1 (HC and LC) were obtained with 386 kWh/bdt for AC and 350 kWh/bdt for HC (60 °C, 2 % concentration). The tensile strength index increased by 26 %, and the TEA-index, related to freeness, increased by 55 %. HAC treatment (750 kWh/bdt, 70 °C, 1.5 % concentration) of the less refined Batch2 (HC) yielded results better than the Batch 1 reference. These findings confirm the energy-efficient potential of the sonicator concept compared to traditional industrial processes. The conclusion is that HAC-refining of softwood pulp requires a proper balance between hydrodynamic and acoustic cavitation intensities. Both fiber concentration by weight and temperature are critical for an energy-efficient process.

The study focuses on developing a comprehensive design approach for a flow-through ultrasonic reactor (sonicator) to tackle challenges like low energy transfer efficiency and unstable system performance. The simulation accounts for structural vibrations, structural-fluid interactions, and pressure distributions within the cavitation zone under single-frequency excitation. Different geometrical designs of cylindrical sonicators are analyzed, with input parameters tailored to acquire higher acoustic cavitation intensity. The findings reveal a novel hexagonal ring-shaped excitation structure that reduces coupling losses, ensures uniform acoustic pressure distribution, and generates symmetric vibration mode shapes. The study emphasizes the separation of parasitic modes from the desired resonance frequency response and simulates the influence of bubbly liquid properties through complex wave numbers and harmonic responses. Experimental validation on a manufactured prototype, including mechanical and electrical impedance, sound pressure spectrum, and cavitation intensity, supports the simulated results. Ultimately, the sonicator exhibits three feasible resonance frequencies to be used pairwise at the certain temperature and input power interval for different applications.

Fibrillation is identified as the most energy intensive process step in pulp and paper manufacturing and improved energy efficiency is the motivation for development of alternative technologies. The aim of this study is to explore the potential of a new refining concept based on cavitation, focusing on the optimization of acoustic cavitation efficiency of the proposed flow-through sonicator concept. The simulations utilize the linearized wave equation in the frequency domain with an addition of nonlinear attenuation introduced by cavitation bubbles. Verification is made by pressure measurements, calorimetry, and foil tests. The fibrillation capability was validated on chemi-thermo mechanical pulp fibers at low consistencies. Fiber properties was characterized by ultrasonic spectroscopy, fiber analysis and SEM. The objective is to optimize the energy transfer efficiency from electrical input power to acoustic cavitation intensity for efficient fibrillation of cellulose fibers. Results showed changes in fiber dimensions and fiber morphology, however, improvements in tensile strength index, measured and predicted by ultrasonic spectroscopy, was limited to 20 % at an energy level of 804 kWh/bdt. To enhance energy efficiency and paper strength properties, it is suggested to add a hydrodynamic cavitation device prior to the sonicator to initiate cavitation bubbles and to increase turbulence intensity.