# Innovative Ultrasonic Spray Methods for Indoor Disinfection
## 2 Development of Ultrasonic and Combined Spraying Devices for Disinfectant Liquids
### Innovative Acoustic-Dynamic Methods and Devices for Spraying Disinfectant Liquids  
#### 2.3.1 Multi-Stage Spraying  
In ultrasonic liquid spraying, droplet formation occurs in a single stage, where droplets detach from the liquid film. In this case, the higher the ultrasound frequency, the higher the dispersion of aerosols, which is preferable in the problem under consideration. However, the generation of high-frequency oscillations (more than 80 kHz) with an amplitude sufficient for spraying limits the area of the radiating surface. These oscillations are dampened in the sprayed liquid and absorbed by the ultrasonic emitter material, reducing spraying efficiency. To overcome this limitation, a method of multi-stage ultrasonic spraying with the supply of acoustic energy to the liquid through gas is proposed [22]. The concept of implementing the proposed method of ultrasonic spraying is shown in Figure 7.  
Figure 6: Ultrasonic wearable aerosol sprayer with an average particle size of 65 microns and a capacity of no more than 60 mL per second: (**a**) three-dimensional model, (**b**) structural diagram. 1—piezoelectric transducer; 2—flexural–oscillating disk (spray surface); 3—sprayer body; 4—fan for cooling the piezoelectric transducer of the atomizer; 5—handle; 6—flow regulators; 7—holder of the disinfect liquid distributor; 8—tubes for supplying liquid; 9—distributor of the sprayed liquid; 10—nipple for connecting the liquid supply system. The initial aerosol is formed by any method of atomization with sufficient productivity. In the case of technologically simple methods (e.g., hydraulic, low-frequency ultrasound, etc. ), the initial aerosol will have low dispersion (droplet size of about 1-10 mm). Primary droplets, produced at high rates, move with a velocity under the influence of the ultrasonic field. Ultrasonic action causes successive droplet fragmentation each time they interact with a power antinode in the standing wave. The primary flow of large droplets moves either due to gravity or is carried away by the transport gas flow towards the next stages of atomization. At the next stages, the droplets enter the ultrasonic field (standing ultrasonic wave) in the gas. The droplets are broken down into smaller fragments at the antinodes of the standing wave due to deformation and a loss of stability (Kelvin-Helmholtz instability). Ultrasonic energy and air interaction atomize droplets, following mechanisms categorized by morphological characteristics [23; 24; 25]. As a result of such an interaction of the droplets with the antinodes of the ultrasonic field, a gradual destruction of the droplets occurs up to a certain limit (minimum size). The developed method of ultrasonic atomization is implemented by means of a bending-diametrically oscillating tubular ultrasonic emitter. It is a hollow cylinder with a stepwise changing external and constant internal diameter. The internal diameter of the tubular emitter is selected from the condition of ensuring the resonance of ultrasonic vibrations in the internal air cavity of the tubular emitter (the formation of a standing acoustic wave), at the operating frequency of the emitter. The shape of the external surface of the emitter is determined from the condition of ensuring a uniform distribution of the amplitude of ultrasonic vibrations along the length of the emitter. A drawing of the tubular emitter and the distribution of the oscillation amplitude are shown in Figure 8.  
Figure 7: Conceptual diagram of multi-stage ultrasonic atomization. The results of the calculations of the distribution of sound pressure created by the emitter, as well as a photograph of the manufactured emitter, are shown in Figure 9. The calculation of the distribution of the sound pressure level inside the tubular emitter was carried out in the ANSYS program. The Harmonic Acoustics harmonic acoustic analysis module was used. The volume of the calculation area was limited by the inner surface of the tubular radiator.