Atomization of Liquids with Widely Differing Material Properties by Ultrasonic Standing-Waves

Ultrasonic standing wave fields in a gaseous atmosphere, generated by two opposed, identical transducers (20 kHz) are used as standing wave atomization (USW A) to produce very fine narrow sized spherical droplets. USW A may be operated under ambient gas pressure or under gas pressures of up to 2,0 MPa (and more) thus allowing disintegration of fluids with high viscosities (synthetic resins, glass melts, suspensions) and/or high surface tensions (metal melts). The feed is led from above into the central sound pressure node of the standing wave field. Here the cylindrical liquid jet is deformed into a flat circular disk parallel to the transducer surfaces. At the rim of this disk atomization into ultra-fine droplets takes place, the size distribution of which can be fitted by a log-normal distribution with (in the case of tin particles) a mean diameter of 10 μm and a geometrical standard deviation between 1.6 and 1.9. In order to gain quantitative insight into the dependence of the droplet size on the atomization conditions, a dimensional analysis was carried out, leading to two equations for the mean drop size of either metal melts or highly viscous liquids and showing significant influences of either the Weber-number or the Reynolds­number together with the ratio of the kinetic energy density of the gas flow (in the USWA field) and the feed. A description of the apparatus and of the process conditions gives an impression about the necessary hardware of this new technique which has its main claim in the disintegration of liquids with very high viscosities and/or high surface tensions and in the contamination free atomization into ultra-fine spherical particles.