# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 3 Results
### Flow Dynamics
#### 3.1.2 Twin-Fluid Study  
The sheet disintegration process is examined for a converging atomizer case with two sheet thicknesses (70 and 280 \(\upmu\)m) for a given flow rate (100 kg/h), as shown in Figure 5. The air-liquid interaction gives rise to sheet instability in the form of waviness, which leads to ligaments formation (Figure 5a). The high-speed interaction between the liquid and airflow leads to the high shear layer effects, resulting in the Kelvin-Helmholtz instability waves or Rayleigh-Taylor instability [19] depending upon the side (windward or leeward, respectively) of the ligaments resulting in the very fine mist formation, as seen in Figure 5c,d. Wave growth and ligament formation depend mainly on the surface tension force and aerodynamic forces, which define the droplet size formation further downstream. The sprays exhibit a radial periodic liquid elements' ejection, attributed to the high-speed core air-jet. The bursting phenomenon (Figure 5) in the converging atomizer is due to airflow behavior. In general, for both above cases, the frequency of the bursting phenomenon at the neck region (which was visually analyzed in this study) may vary depending on the aerodynamic interaction effects, the natural pulsating frequency of the liquid sheet, and pulsations caused by the slight variation in airflow rates (<2%). For the 70 \(\upmu\)m sheet, owing to higher axial momentum, the breakup length (which is considered as an indicator of the stability of the liquid sheet) (Figure 5a) is longer than the 280 \(\upmu\)m sheet (Figure 5e). With increasing ALR, owing to the high-speed aerodynamic interaction, the mist-like droplets (tiny) formed downstream axially in the 70 \(\upmu\)m case (Figure 5c,d), whereas some thread-like droplets (bigger) ejecting laterally out of the sheet are visible with 280 \(\upmu\)m sheet thickness (Figure 5g,h). The sheet formed was corrugated/wavy in both of the above cases, forming a cellular pattern (not visible here, whose cell size may depend upon the air jet velocity), as well as stretched-sheet/ligament structure in both the spanwise and streamwise direction (also observed in planar sheet configuration [20,21]), which is attributed to the three-dimensional (3D) nature of the annular sheet. The pressure difference, the surface tension effect, and the aerodynamic forces dictate the liquid sheet breakup characteristics, such as breakup length, spray angle, and so on. For the converging-diverging (CD) atomizer, the sheet breaks up into larger angles for lower fluid  
Figure 4: Wave pattern observed in shadowgraph imaging at 20,000 frames per second for the (**a**) converging nozzle and (**b**) CD nozzle at 35 kg/h airflow rate.  
flow rates (Figure 6a); with an increase in ALR values, the ligaments/droplets emanate sideways owing to the air jet pattern (Figure 6b). At a higher air-to-liquid ratio (ALR), the coherent pattern of airflow and mist is observed in the near spray centerline, leading to smaller droplet clustering/segregation (Figure 6,d). The neck formation region is more visible at higher fluid flow rates, where then liquid fragments eject radially, forming a larger spray angle (Figure 6f). The boundary layer stripping (Figure 6g,h) is observed at a higher air-to-liquid ratio (ALR) owing to the intense aerodynamic interaction between the fractured/perforated sheet and high-speed airflow, which was also observed in [22].  
Figure 5: For the converging atomizer with 70 μm (**top row**) and 280 μm (**bottom row**) sheet thickness at a 100 kg/h water flow rate with airflow rates of (**a,e**) 5 kg/h, (**b,f**) 15 kg/h, (**c,g**) 25 kg/h, and (**d,h**) 35 Kg/h, respectively.  
Figure 6: For the converging–diverging (CD) atomizer at 100 kg/h (**top row**) and 350 kg/h water flow rates for 280 μm sheet thickness with airflow rates of (**a,e**) 5 kg/h, (**b,f**) 15 kg/h, (**c,g**) 25 kg/h, and (**d,h**) 35 Kg/h, respectively.