On the effect of microbubble injection on cavitation bubble dynamics in liquid mercury

The effect of microbubble injection has been studied numerically to clarify the role of injected bubbles in the experimentally observed suppression of cavitation in liquid mercury. Recently, we attempted to inject gas microbubbles into liquid mercury in order to mitigate cavitation damage on mercury vessels, a critical issue in spallation neutron sources. From an experimental study using an electromagnetically driven impact test machine and a bubble generator, we found that by injecting microbubbles, the magnitude of the negative pressure generated in liquid mercury is slightly decreased and cavitation damage is remarkably reduced. In this paper, we have performed a numerical study using a multibubble model and experimentally obtained pressure-time curves in order to thoroughly explain the experimental findings. We have found that the observed slight change in negative pressure has a strong impact on cavitation bubble dynamics and was caused by the positive pressure wave that the injected bubbles radiated. Also, we have examined whether the injected microbubbles can cause significant erosion, and found that their collapse intensity is much smaller than that of cavitation bubbles since their expansion ratio is relatively small. Additionally we have examined high-frequency pressure pulses observed experimentally only when microbubbles were injected, and clarified that they are due to the free oscillation of injected bubbles.