# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 3 Results
### Sheet Breakup Dynamics
#### 3.2.3 Breakup Length  
The distance up to which the sheet is roughly intact is considered as the breakup length. The breakup length is measured using _Imagej software_[24], such that intensity values (grayscale values: 255 = white, 0 = black) suddenly jump, as shown in Figure 12. The pixel location was traced for a stack of 25 images (averaged) where there is discontinuity, then this location where breakage occurs is converted into a distance value by calibrating the pixel in terms of distance (mm). In this case, the 66-pixel distance is the actual breakup length, converted into the mm scale based on the nozzle diameter (mm) in pixels. This method is adopted for low flow rates where raw images after being contrast-enhanced and brightened are stacked into full intensity-based or standard deviation images (see Figure 13) using _Z-projection_. This method provides a greater depth of field image than source images, highlighting our breakup location. For higher flow rates, the breakup location is taken as where the neck-formation region starts to disrupt. It is also measured for a stack of 25 images, which gives us a breakup length with an uncertainty of 1-2% due to observation or measurement error.  
Figure 11: Spray angle measurement variation with ALR for the converging and converging–diverging (CD) atomizer cases with 70 and 280 μm sheet thickness.  
Figure 12: Breakup length measurement: (**a**) contrast-enhanced raw image, (**b**) maximum intensity (averaged) image, and (**c**) profile of grayscale values along the centerline (yellow).  
The breakup length has an inverse relation with the airflow rate, as shown in Figure 14; the breakup length has a more significant value for the higher liquid flow rate (350 kg/h), which converges to the same value for both types of atomizers with the increment in airflow rate for 70 \(\upmu\)m sheet thickness (Figure 14a). The CD atomizer shows reduced breakup length, especially at higher airflow rates (35 kg/h). The breakup length follows a similar pattern for the 280 \(\upmu\)m sheet thickness (Figure 14b). The breakup length reduces quite rapidly for higher water flow rates (350 kg/h) with an increase in airflow rates. The CD atomizer shows a greater decline in breakup length for higher liquid flow rates at a significant airflow rate. In contrast, the breakup length values converge for both types of atomizers for a lower liquid flow rate (100 kg/h) with increased airflow rates.  
The breakup length was plotted (Figure 15) against airflow rates for both 70 \(\upmu\)m sheet thickness and 280 \(\upmu\)m sheet thickness for the converging and CD atomizer, respectively. The breakup length follows an inverse relationship with the air-to-liquid ratio (ALR). For the thin sheet (70 \(\upmu\)m), the breakup length for the converging-diverging (CD) atomizer is lower than for the converging atomizer, which might be because of the sheet dragging effect due to the higher contact strength of air-jet ejecting from the former case, especially at a higher air-to-liquid ratio (ALR), whereas at lower ALR values, the breakup length for the converging-diverging (CD) atomizer is higher than for the converging atomizer, which is the condition before the flow is choked at the atomizer throat (atomizer exit in the case of the converging atomizer). Meanwhile, in the case of 280 \(\upmu\)m sheet thickness, the  
Figure 14: Breakup length measurement for both the converging and CD atomizer for two cases: (**a**) 70 \(\upmu\)m sheet thickness and (**b**) 280 \(\upmu\)m sheet thickness.  
Figure 13: Breakup length measurement with the 280 \(\upmu\)m sheet with a water and airflow rate of 100 kg/h and 20 kg/h, respectively, (**a,c**) for the converging atomizer and (**b,d**) for the converging–diverging (CD) atomizer.  
breakup length for the converging-diverging (CD) atomizer is slightly lower than for the converging atomizer, which can be attributed to the sheet contraction (owing to surface tension) occurring with both atomizers for all ranges of fluid flow rates.