Investigation of capillary wave, cavitation and droplet diameter distribution during ultrasonic atomization

Droplet size distribution during ultrasonic atomization was investigated by laser diffraction based on two phenomena—capillary waves and cavitation. The former was visually illustrated using the photographic method, and the latter was examined by the potassium iodide decomposition rate test and Particle Image Velocimetry technique. This study also analyzed the effects of operating ultrasonic parameters such as input power, the flow rate of liquid and gas, liquid temperature, distance from the vibrational tip to laser, and physicochemical properties of liquids (surface tension and viscosity). The relationship among capillary waves, cavitation, and the droplet size distribution was developed for the first time. Results suggest that uniform capillary waves and low cavitation intensity contribute to narrow droplet size distribution. Bimodal droplet size distribution was observed with an increasing liquid flow rate and an assisted gas flow rate. The droplet size did not keep increasing or decreasing with increasing input power and liquid viscosity. A new correlation based on all data using dimensionless numbers was then proposed to predict droplet sizes. This work presents an effective ultrasonic atomization approach to obtaining desirable droplet size distribution by choosing optimum operating parameters, and establishes the controlling mechanism for ultrasonic atomization.