Cavitation bubble dynamics and sonochemiluminescence activity inside sonicated submerged flow tubes

Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized perfluoroalkoxy alkane (PFA) flow tubes, submerged in an ultrasonic bath reactor, are studied at 27.2 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 that takes the tube wall into account. The resulting peak temperatures around 3500 K allow for dissociation of water vapor. This is in accordance with observed sonochemiluminescence from luminol, originating from active bubble zones in the tubes.