Revealing the interplay between flow dynamics and cavitation activity in a flow-through sonoreactor

Acoustic cavitation is a well-known approach to intensify chemical and physical processes. Despite extensive studies in the understanding of batch ultrasonic reactors, flow-through sonoreactors, which are more relevant to continuous and large-scale operation, have received limited attention. Hence, in this study, the role of fluid flow rate and the relative direction between bulk flow and ultrasound propagation (co-current and counter-current) on sonochemical activity and cavitation cloud characteristics in a tubular flow-through sonoreactor (low frequency = 35 kHz) is systematically investigated under different ultrasonic amplitudes. Characterization techniques were employed, including calorimetry to quantify dissipated acoustic power and ultrasonic efficiency, shadowgraphy imaging to characterize the size of the cavitation cloud, sonochemiluminescence (SCL) to map chemically active cavitation zones, and KI dosimetry to quantify radicals. The results show that increasing flow rate slightly enhances calorimetric power, while ultrasonic efficiency remains largely unchanged. Shadowgraphy reveals that cavitation clouds elongate with increasing flow rate up to an intermediate value, beyond which the extent of the cavitation cloud decreases. In contrast, SCL mapping demonstrates that stagnant conditions and low flow rates are more favourable to induce larger and more intense chemically active cavitation zones, indicating that not all cavitation bubbles contribute equally to radical formation. The higher radical production at higher amplitudes and lower flow rates is shown through KI dosimetry. Moreover, counter-current operation enhances the distribution of chemically active cavitation and radical generation, as well as sonochemical efficiency when compared with co-current flow. This is attributed to modified hydrodynamics and increased residence time within the sonication zone.