Ultrasonic Manipulation of Direct Contact Vapor Bubble Condensation

The growth, advection dynamics, condensation rate, and collapse of vapor bubbles injected into a subcooled liquid bath are manipulated using pulsed ultrasound (MHz) actuation. The vapor is injected vertically through a nozzle, forming bubbles with a characteristic diameter larger than that of the acoustic beam. Actuation exploits the mismatch of the acoustic impedance at the liquid-vapor interface, strongly deforming it to form a “spear” of liquid that penetrates the vapor volume, ultimately forming a torus; the acoustic beam focuses within the spear, ejecting atomized droplets that increase the condensation rate. The rapid deformation induces a vortex ring which is advected along the axis of the acoustic beam (absent actuation, a vortex ring forms during bubble collapse and instead advects upwards). High-speed Schlieren imaging and PIV are used to investigate the liquid flow fields around collapsing vapor bubbles in the absence and presence and ultrasonic actuation. It is shown that the impulse of the induced vortex rings increases with increased temperature difference between the vapor and the bulk liquid. The vapor collapse is compared with the acoustically-manipulated collapse of cavitation bubbles having a characteristic diameter much smaller than the acoustic beam.