The liquid vortex flow field plays a crucial role not only in the transfer of matter and heat but also in significantly affecting the distribution of sound fields, which in turn influences the behavior of bubbles in the flow. This ultimately impacts the phenomenon of acoustic cavitation. Based on the combination of the theory of bubble fragmentation and the theory of funnel-shaped vortex, in a three-dimensional vortex field. the effect of the vortex flow field (flow field generated by stirring) on the bubble breakup probability, as well as its modulation of acoustic cavitation, is investigated in this paper. In addition, the paper provides explanations for the phenomena observed in experiments: when the stirring speed reaches 1000 rpm, the degradation effect no longer shows a monotonous increase, but instead begins to decline.
The study demonstrates that with the increase in stirring speed, the probability of bubble breakup increases significantly. For instance, when the stirring speed is 1000 rpm, the probability of bubble breakup is approximately 0.17%. At a stirring speed of 1500 rpm, the breakup probability rises to 23%, and at 2000 rpm, it reaches 44%. Moreover, the critical radius for bubble breakup also decreases. The critical radius, as defined in this study, refers to the bubble radius at which the probability of breakup becomes nonzero. Experimental data show that at 600 rpm, the critical radius for bubble breakup is about 200 μm, while at 2000 rpm, it shrinks to 55.5 μm. This indicates that in the high-speed rotating vortex field, bubbles may rupture before reaching their maximum cavitation radius, thus losing their effective cavitation effect.
Further analysis shows that in the vortex flow field, for bubbles with an initial radius smaller than 22.5 μm, the temperature inside the bubble upon collapse can reach as high as 2217.3 K (corresponding to an initial radius of 22.5 μm). For bubbles with an initial radius of 20 μm, the collapse temperature can even reach 2264.3 K. For bubbles with an initial radius of 40 μm, when the stirring speed does not exceed 1500 rpm, the bubbles can still collapse under the influence of the sound field, and the temperature inside the bubble upon collapse can reach 1659.6 K, which is sufficient to trigger the cavitation effect. However, when the stirring speed exceeds 1500 rpm, bubbles may break up too quickly and lose their cavitation capacity, thus failing to produce the expected cavitation effect.
Experimental results further verify that at moderate stirring speeds (600-1000 rpm), the acoustic cavitation effect is most pronounced, while excessively high stirring speeds suppress the enhancement of the degradation effect. This phenomenon suggests that the introduction of the vortex flow field makes the factors affecting acoustic cavitation more complex. The optimization of the acoustic cavitation effect requires not only consideration of the sound field distribution and mass transfer but also the comprehensive factors such as gas entrainment, bubble aggregation, and breakup. Therefore, a thorough analysis and regulation of these factors is essential for the wide application of acoustic cavitation technology in engineering, providing important theoretical value and practical significance, and offering scientific basis and directions for further optimization of the acoustic cavitation process.