| dc.description.abstracteng | In the framework of intensification of chemical processes, miniature continuous flow reactors have come into focus in recent time. As a specific way of “non-classical” energy supply to the liquid in such a flow reactor, an intense ultrasonic field can be employed to enable sonochemistry. This type of chemistry is typically mediated by cavitation bubbles which emerge in the strong sound field, and which can trigger chemical reactions during their collapse. The present work investigates ultrasonic cavitation conditions in small flow channels to gain a better understanding of the physical background of sonicated flow reactors. In prototypical experimental setups with sub-millimeter diameter flow tubes submerged in an ultrasonic bath, cavitation inception, cavitation bubble structures, and bubble dynamics are studied. The methods comprise high-speed imaging, acoustic pressure mappings, and sono-chemiluminescence measurements. Additionally, sound fields and bubble oscillations are partly simulated numerically. The main results show that a sonicated small diameter flow tube differs in several aspects from a larger volume of liquid that is exposed to ultrasound. This concerns bubble nucleation, bubble dynamics, and bubble activity. For nucleation to take place at all, usually gas slugs have to be introduced to form a Taylor flow, which leads to bubble nuclei being entrained into the liquid via the acoustically agitated interface between gas and liquid slugs. Cavitation bubbles in the tube appear in several typical structures, like streamers, clusters, and “plugs” that develop a peculiar sharp boundary of regions populated with cavitation bubbles. The bubbles in the channel are observed to show a modified oscillation behavior as compared to the unbound liquid. In particular, a certain phase shift appears, i.e.,
bubble expansion starts earlier, and maximum expansion can be restricted. Chemical activity of bubbles in the tubes can emerge in zones of high acoustic pressure, while in batch reactors, high pressure zones might be void of active bubbles which are driven away by acoustic forces. Taking into account these findings, sonicated submerged channels can be employed successfully for intensified sonochemical processing in miniature flow reactors. | de |