Classification of regimes determining ultrasonic cavitation erosion in aqueous solutions containing dissolved air

Abstract

Ultrasonic cavitation is crucial for wastewater treatment, as it enhances the oxidation and degradation of contaminants. However, cavitation erosion of acoustic horn tips poses a significant challenge due to reduced treatment efficiency and increased operational costs. Since the air dissolved in liquids not only affects sonochemical yields but also complicates ultrasonic cavitation erosion, the quantitative analysis of dissolved air affecting cavitation erosion is required. This study experimentally investigated the effect of dissolved air on erosion by increasing the dissolved oxygen content from 1.18 to 18.30 mg·L^–1 via pre-supersaturation and degasification methods. Employing ultrasonic vibrations, material removal results indicated that erosion was initially aggravated and then alleviated, with an increase in dissolved air shifting the state of the solution from an undersaturated to a supersaturated state. The most severe erosion was observed when the dissolved oxygen content reached 4 mg·L^–1, which corresponded to a half-saturated state. Theoretical examinations of heterogeneous nucleation rates and energy dissipation following asymmetrical bubble collapse revealed four regimes: homogeneous nucleation, vaporous and gaseous cavitation bubble nucleation, and microbubble formation. With increasing dissolved air, accelerated vaporous cavitation aggravates erosion, while gaseous cavitation and microbubble formation alleviate erosion, which provides a classification of regimes determining cavitation erosion affected by dissolved air. These findings highlight the significant effect of dissolved air on ultrasonic cavitation erosion and, with a better understanding of these regimes, can aid in optimizing the design and operation of sonoreactors used for wastewater treatment.