Relevant studies believe that ultrasonic atomization is the process of using ultrasonic energy to make liquid form fine droplets in the gas phase, that is, ultrasonic waves are generated on the surface of the vibrating liquid, and the vibration peak composed of amplitude separates and breaks the droplets from the surface. As the ultrasonic frequency increases, the atomized droplets become thinner and finer. Generally, under the action of the ultrasonic vibration frequency, fine droplets can be obtained. In addition, the ultrasonic frequency field can eliminate or thin the temperature boundary layer near the heat transfer surface, thereby promoting heat transfer.
Different types of atomization processes are used, which can be classified according to the effect of energy transfer on the atomization of the liquid film surface. Mechanical or traditional atomization processes, such as two-fluid atomization, pressure atomization and rotary disk atomization, use mechanical energy to pressurize or increase the kinetic energy of a liquid so that it can be broken down in the form of droplets. These processes require more energy and have no control over the final size and ejection velocity of the droplets.
Different from traditional atomization, it can be more efficient and only requires electric energy to be transmitted to the piezoelectric transducer to drive the nozzle to resonate. The droplets have no moving parts, only the mechanical vibrations generated by the supplied electrical energy are used to create the droplets. Since no additional energy is required, the droplet size distribution can be better controlled.
The average diameters of droplets generated by capillary peaks at forced vibration frequencies of 10–800 kHz for different working fluids (including water, oil, and molten wax), and the relationship between the average diameters of jetted droplets was established. dp = 0.34*8π / ρf2
Capillary waves and cavitation effects
The generation of ultrasonic atomization is based on capillary wave effect and cavitation effect. When acting on the 20KHz atomizing head with lower power, it is observed that there is a grid-like regular structure on the surface of the atomizing head, with the same number of peaks and troughs per unit area, called capillary waves. This low power input produces surface disturbance without actual droplet ejection.
Cavitation is a microscopic phenomenon that cannot be directly observed on the surface of the atomizing head with the naked eye. Two different types of droplets were found through camera time-lapse, namely near-spherical droplets and streaks, with streaks having higher velocities, and near-spherical droplets having less velocity, where the presence of cavitation can be identified.
The formation of cavities near the atomizer surface and in the liquid film and the subsequent collapse of these cavities results in the local release of large amounts of energy; thus, compared to the low ejection velocities observed in the case of droplet ejection induced by capillary wave propagation, The cavitation effect greatly increases the droplet ejection velocity. At the same time, the surface area occupied by the liquid on the tip of the atomizing head decreases as the frequency of the atomizer increases, making it difficult to capture capillary waves on the surface.