Numerical Investigation of Motion Behavior Interference of Double Quasi-spherical Bubbles in Ultrasonic Standing Wave Fields

The intervention of an ultrasonic field can effectively control the kinematic behavior of bubbles, leading to an increase in the efficiency of mass and heat transfer between liquid and gas phases. Since bubbles rarely appear individually in liquid, the mechanism of multiple bubble motion affected by the ultrasonic standing wave must be thoroughly understood. The authors numerically investigated the motion process of centroid of double quasi-spherical bubbles in ultrasonic standing wave fields and the corresponding alteration of the velocity field. The effects of sound pressure amplitude, acoustic frequency and bubble radius on double quasi-spherical bubble motion were fully analyzed. It was found that the above three variables have an important effect on the double bubble motion and the surrounding flow field. When the secondary Bjerknes force is the attractive force between two bubbles, three types of motion pattern are recognized: two bubbles approaching towards each other and then coalescing into one bubble, two bubbles travelling along the same direction and then coalescing into one bubble, and two bubbles remaining in levitation respectively without coalescence. When two bubbles move together in ultrasonic standing wave fields, the appearance of the secondary Bjerknes force breaks the equilibrium relationship between the time-averaged primary Bjerknes force and buoyancy force acting on each bubble, the centroid motion of bubbles changes from levitation to rising or from rising to sinking. When two bubbles coalesce into a single bubble, its motion follows the motion law of a single bubble.