Experimental study of the collapse strength of a bubble merged from a spark-induced bubble and a pre-existing air bubble

This research investigates the variation of the collapse strength of a merged bubble when a spark-induced bubble merges with a pre-existing air bubble. The dynamic evolution process of the merged bubble is recorded by a high-speed camera, and a hydrophone monitoring system provides the collapse sound pressure of merged bubble. Results show that relative distance and relative size of the air bubble and the cavitation bubble have significant effects on the collapse strength of the merged bubble. When the maximum radius of cavitation bubble and the radius of air bubble are fixed, with the increase in the relative distance between the cavitation bubble and the air bubble, the merged bubble gradually changes from an approximate spherical shrinkage shape to a non-spherical shrinkage shape, and the collapse shock wave stratification phenomenon becomes more evident. When the maximum radius of cavitation bubble and the relative distance between the cavitation bubble and the air bubble are fixed, with the increase of the air bubble radius, the minimum shrinkage radius of the merged bubble gradually increases, the maximum shrinkage speed and the collapse sound pressure of the merged bubble are gradually attenuated, and the contrast of the collapse shock wave image is significantly weakened. •Studying cavitation bubble dynamics when air bubble merges with cavitation bubble.•The size and distance of air bubble influence the cavitation bubble dynamics.•Shock wave of merged bubble is observed for the first time.•Proposing the key value that has a significant effect on the cavitation bubble.