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Ultrasonic atomization formation and influencing factors

2020-09-24 13:30:55

 

Liquid atomization is a process in which a thin liquid film that is sufficiently disturbed by the surface in the normal direction is separated from the surface and split into small water droplets, such as mist in the gas phase. Liquid atomization plays an important role in industrial processes such as spray drying, coating, spray cooling, liquid fuel and waste incineration and combustion, fine powder preparation, and emulsion preparation. In these applications, most of the droplets are required to have the required size distribution.

 

Classification of atomization

Different types of atomization processes are used, and the effects of atomization on the surface of the liquid film can be classified according to the energy transfer method. Mechanical or traditional atomization processes, such as two-fluid atomization, pressure atomization and rotary disk atomization, use mechanical energy to pressurize the liquid or increase its kinetic energy so that it can be broken down in the form of droplets. These processes require more energy and cannot control the final size and ejection speed of the droplets.

 

Unlike traditional atomization, ultrasonic atomization can be more efficient, and only requires electric energy to be transmitted to the piezoelectric transducer to drive the nozzle to resonate. The droplet has no moving parts, only the mechanical vibration generated by the supplied electrical energy is used to generate the droplet. Since no additional energy is required, ultrasonic atomization can better control the size distribution of the droplets.

 

Different working fluids (including water, oil and molten wax) have the average diameter of the droplets generated by the capillary peak at the forced vibration frequency of 10~800 kHz, and establish the relationship between the average diameter of the ejected droplets. dp = 0.34*8π / ρf2

 

Crude oil and cavitation effects:

The generation of ultrasonic atomization is based on the capillary wave effect and the cavitation effect. When a 20KHz ultrasonic atomization head is applied with a lower power, a grid-like regular structure is observed on the surface of the atomization head, with the same number of peaks and valleys per unit area, called capillary waves. This low power input creates surface interference without actual droplet ejection.

 

Cavitation is a microscopic phenomenon and cannot be directly observed on the surface of the atomizer with the naked eye. Through camera time-lapse shooting, it is found that there are two different types of droplets, namely, nearly spherical droplets and stripes. The stripes have a higher speed and the nearly spherical droplets have less speed, which can confirm the existence of cavitation.

 

The formation of cavities near the surface of the atomizer and in the liquid film and the subsequent collapse of these cavities result in the local release of large amounts of energy; therefore, compared with the low ejection velocity observed in the case of droplet ejection caused 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, so that it is difficult to capture the capillary waves on the surface.

 

The formation of cavities near the surface of the atomizer and in the liquid film and the subsequent collapse of these cavities result in the local release of large amounts of energy; therefore, compared with the low ejection velocity observed in the case of droplet ejection caused 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, so that it is difficult to capture the capillary waves on the surface.



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