# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 3 Results
### Sheet Breakup Dynamics
#### 3.2.2 Spray Angle  
The spray angle variation was observed for a change in fluid flow rates. Spray angle measurement is based on the tangent lines fitted at the spray edges at some downstream location, but the spray periphery is curved owing to air-interaction effects. In our case, spray angle was measured such that it covers the majority (approximately 99%) of the droplet mass of the whole spray, as is depicted (see Figure 9a,b). The set of 25 images (frames) were pre-processed (sharpness and contrast enhancement) in the ImageJ software for each data set before the angle measurements were taken. The angles were obtained by taking the mean (average) value of these 25 images, with an uncertainty of 2-3% due to observation error as the spray boundary line is vague. The spray boundary is more unclear in the case of sprays at higher ALR values, which can be attributed to the high density of the spray, which makes it more difficult to detect the spray edge [23]. The intensity-averaged images (Figure 9c,d) were not taken for measurement as they underpredict the spray angle for most cases.  
The spray angle was plotted (Figure 10) against airflow rates for both 70 \(\upmu\)m sheet thickness and 280 \(\upmu\)m sheet thickness for the converging and CD atomizer, respectively. For the thin sheet (70 \(\upmu\)m), as shown in Figure 10a, the spray angle rapidly increases with an increase in airflow, especially for low flow rates (say, 100 kg/h); the spray angle is relatively smaller for higher flow rates (say, 350 kg/h) owing to the higher axial momentum. The  
Figure 8: Regime chart for the converging atomizer for various flow rates for 280 \(\upmu\)m sheet thickness.  
spray angle increased with an increase in the air flow rate; the increment is more continuous with a thicker sheet (280 \(\upmu\)m) than with the thin sheet (70 \(\upmu\)m) because of the lower axial momentum in the former sheet case. The increment is slightly more in the CD atomizer case than the converging atomizer (Figure 10b) for the higher liquid flow rate owing to the pulsating airflow, resulting in the more pronounced bursting phenomenon.  
The spray angle against the air-to-liquid ratio was plotted for both converging and CD atomizers, for both sheet thicknesses (70 \(\upmu\)m and 280 \(\upmu\)m), in Figure 11. The spray angle increases with an increase in ALR values for all cases. For the 70 \(\upmu\)m sheet, the spray angle is slightly higher for the converging atomizer than for the CD atomizer case, though the difference is apparent at relatively higher air-to-liquid ratios. In contrast, the spray angle is higher for the converging-diverging (CD) atomizer than the converging atomizer for the 280 \(\upmu\)m sheet owing to the high contact strength, ejecting the sheet ligaments/parcels in a more pronounced fashion, resulting in the spreading of the spray periphery. The spray angle increment is more evident for higher airflow rates (high ALR).  
Figure 10: Spray angle measurement for both the converging and converging–diverging (CD) atomizer case with 70 and 280 \(\upmu\)m sheet thickness.  
Figure 9: Spray angle measurement for the CD nozzle atomizer for two cases: (**a,c**) water and an airflow rate of 100 kg/h and 5 kg/h, respectively; (**b,d**) water and an airflow rate of 350 kg/h and 35 kg/h, respectively.