Depth effect on the inertial collapse of cavitation bubble under ultrasound: Special emphasis on the role of the wave attenuation.

Acoustic cavitation concentrates and releases a very large amount of energy in localized areas, which can be used for many physical and chemical processes. Even though acoustic cavitation has been studied widely for decades in lab-scale sonoreactors, only few studies have been devoted to characterize this event in big-scale sonoreactors, where the liquid depth may have a critical influence on the bubble collapse. The present computational study furnished numerical data about the effect of depth (z = 0-10 m) on acoustic cavitation with special focus on the role of attenuation of the ultrasound wave on the dramatic conditions developed within bubbles at collapse. The used mathematical model takes into account the liquid compressibility, surface tension and viscosity, depth as well as the attenuation of the ultrasound wave with depth. It was found that the maximum bubble temperature (T) and pressure (p) at the collapse diminished considerably with deepening into water up to 10 m with a considerable contribution of the ultrasound wave attenuation in the overall reduction event. The reduction in T and p with depth was more pronounced at higher frequency (1000 kHz) and lower temperature (10 °C) in which losses of about up to 72% in T and till 94% in p (as compared with values at z = 0) were obtained at z = 10 m. Depending on operating conditions, i.e. frequency, acoustic intensity or liquid temperature, the ultrasound wave attenuation may contribute with up to 47% and 79% in the overall reductive effect of depth toward T and p, respectively. These results were discussed, interpreted and used to support some available experimental observations. Finally, the results of the present study may help in designing large-scale sonoreactors through providing data about the effect of one of the missing links between lab-scale sonoreactors and industrial large-scale sonoreactors.