A novel ultrasonic field-focusing nozzle for enhanced sonochemical reactor performance

Ultrasonic reactors operating at megahertz frequencies typically exhibit low cavitation intensity and sonochemical yields. This study introduces an innovative ultrasonic field-focusing nozzle that improves the performance of a 1.7 MHz sonoreactor by concentrating the acoustic field geometrically into a defined focal zone. The sonochemical performance of the reactor with and without the focusing nozzle was compared across solution volumes ranging from 50 to 200 mL. Comprehensive characterization techniques were employed, including calorimetry, KI and Fricke dosimetries, hydrogen peroxide quantification, 4-nitrophenol oxidation, Sunset Yellow FCF (SSY) degradation kinetics, and sonochemiluminescence (SCL) imaging. The focused configuration outperformed the unfocused system across all metrics. Calorimetric power increased by up to 15 %, and radical production rates, as measured by triiodide, Fe³⁺, and 4-nitrocatechol yields, rose by over 50 % under optimal conditions. SSY degradation rates improved by as much as 78.5 %, and SCL imaging revealed a bright, narrowly confined cavitation zone, which is indicative of elevated local pressure amplitudes. The most significant enhancements occurred at a liquid volume of 80 mL (liquid height of 11.3 cm). This corresponded to the upper boundary of the transducer's Fresnel (near-field) zone. In this zone, standing wave coherence and constructive interference are maximized. Even at larger volumes (up to 200 mL), the focusing nozzle sustained significant improvements in cavitational activity and sonochemical yield. These results highlight the importance of geometric focusing in overcoming the limitations of high-frequency ultrasonication, especially when acoustic energy would otherwise diminish.