Experimental investigations on the collapse of cavity cluster in high power ultrasound fields

Applications of high power ultrasound span many industrial sectors, from healthcare, through sonochemistry, to ultrasonic cleaning. These applications depend on the dynamics of cavity cluster. This study was conducted to investigate the collapse of cavity cluster in high power ultrasound fields. The cloud cavitation was produced by a 20 kHz ultrasonic horn used for sonochemistry. The dynamics of cavity cluster below the ultrasonic horn was recorded with high-speed photography at framing rate of 100,000 fps. PIV (particle image velocimetry) techniques were also used to get a velocity field of acoustic streaming. The motion of cavitation bubbles was strongly influenced by the acoustic streaming. They moved from the perisphere downward towards the axis and convergent at a point about 2.5 mm below the horn (where small bubbles incorporated into big bubbles) and then move downward along the axial direction. However, bubbles were found stray from the acoustic streaming upwards towards the center of the cavity cluster when the cavitation bubbles collapse. The growth and collapse of incipient cavitation bubbles in the cavity cluster in two acoustic cycles were recorded with high-speed photography and the variation of the diameter of the bubbles with time was investigated. The primary Bjerknes force and jets directed toward the center of the cloud produced during the bubbles collapse may be responsible for the motion of bubbles towards the center of cavity cluster. The first collapse of incipient bubbles is influenced by the maximum diameter of cavitation bubbles, while the subsequent rebound and collapse make the cluster very complicated. Asynchrony of the collapse of cavitation bubbles may be responsible for the propagation of shock wave in the cavity cluster.