# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 4 Discussion  
In this study, the breakup dynamics of the annular sheet in a twin-fluid atomizer was investigated to probe the primary atomization behaviour of sonic/transonic atomizers. Two distinct atomizer configurations (converging and converging-diverging (CD)) were tested for the range of operating conditions in terms of air and liquid flow rates. The gas flow study showed the wave patterns for different airflow rates or total pressure (P\({}_{\text{t}}\)). The irregular pressure distribution pattern along the visualized shock cells can lead to sudden acceleration or deceleration of the surrounding liquid sheet/ligament, which provides the sheet bursting effect. In the twin-fluid breakup study, the pressure distribution effect on the contracted sheet (due to the surface tension effect) dictates the breakup mechanics in the purview of the spray angle, breakup length, and so on. The bursting phenomenon (at the neck region) effect was observed in both kinds of atomizers, which can lead to the radial dispersion of the ligaments/globules, contributing substantially to the spray characteristics such as spray angle and so on. The breakup morphology was categorized into various observed breakup regimes: _annular sheet disintegration, ligament type breakup, wavy sheet breakup_, and _pure-pulsating breakup_. The air-to-liquid ratio (ALR) along with the liquid sheet Weber number (Weg) and Reynolds number (Rei) based on airflow are major parameters employed in the construction of the 3D breakup regime chart. There is no clear distinction of the regimes according to the ALR values. There is a certain overlap of breakup modes for the range of fluid flow rates such that certain breakup regimes gradually transit into another breakup regime. The breakup dynamics was studied with spray characteristics such as breakup length and spray angle. For low liquid flow rates (100 kg/h), the spray angle values showed a similar range for both the 70 \(\upmu\)m sheet and 280 \(\upmu\)m sheet. For higher flow rates (350 kg/h), the spray angle was narrower for the 70 \(\upmu\)m sheet than for the 280 \(\upmu\)m sheet because of the larger liquid axial momentum; spray diverges more for the 280 \(\upmu\)m sheet because of the relatively lower velocity of the sheet, resulting in a more intense aerodynamic interaction of the high-speed air jet with the liquid sheet. In general, the spray  
Figure 15: Breakup length measurement variation with ALR for converging and CD atomizer cases with 70 and 280 \(\upmu\)m sheet thickness.  
angle dictates the aerodynamic interaction of the liquid droplets with the surrounding air, which affects the droplet size distribution downstream. The breakup length is affected by the contact strength of the high-speed air jet, which drags the wavy (subjected to Kelvin-Helmholtz instability) liquid sheet leading to the perforations or holes, further leading to the detachment of the globules/ligaments. For 70 \(\upmu\)m, owing to the higher sheet velocity (momentum) for the given flow rate, the breakup length is relatively larger than for the 280 \(\upmu\)m sheet. The breakup length follows an inverse relationship with the ALR values for all the cases employed. The whole study has some limitations owing to the difficulty in the visualization of high-speed fluid flow rates. The in-depth visualization of the wave pattern (formed in the air jet only) interaction with the contracted wavy liquid sheet would give more insights into the instability and hydrodynamics of the sheet breakup. Further research is needed in other directions to quantitatively study the effect of converging and converging-diverging (CD) atomizer usage in terms of the droplet size pattern and drop size distribution (DSD) for various operating conditions employed in this spray.