Cavitation bubble dynamics and sonochemiluminescence activity inside sonicated submerged flow tubes

TL;DR

This study investigates cavitation bubble behavior and luminol-based chemiluminescence in small flow tubes under ultrasonic waves, revealing bubble interactions, collapse dynamics, and high-temperature effects leading to water vapor dissociation.

Contribution

It provides new insights into cavitation bubble structures, interactions, and chemiluminescence activity inside submerged flow tubes at 27 kHz ultrasonic frequency.

Findings

Bubble clusters form via interface pinch-off.

Bubbles undergo merging and splitting events.

Peak temperatures reach around 3000 K, enabling water vapor dissociation.

Abstract

Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized PFA flow tubes, submerged in an ultrasonic bath reactor, are studied at 27 kHz driving frequency. Nucleation of cavitation inside the tubes only takes place via a free interface, realized here in form of an alternating water-air slug flow. High-speed recordings show that cavitation bubbles in the water slugs often develop localized structures in form of clusters or bubble plugs, and that such structures can be seeded via a single pinch-off from the free interface. Within the structures, bubbles strongly interact and frequently undergo merging or splitting events. Due to the mutual interaction and resulting motion, bubbles often collapse with a fast displacement, suggesting jetting dynamics. Bubble compression ratios are estimated on basis of observed individual bubble dynamics and numerical fitting by a single bubble model. The resulting peak temperatures around 3000 K allow for dissociation of water vapor. This is in accordance with observed sonochemiluminescence from luminol, originating from active bubble zones in the tubes.