# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 3 Results
### Sheet Breakup Dynamics  
#### 3.2.1 Breakup Morphology  
Different breakup patterns were observed for various flow rates such that, at low flow rates, the Rayleigh bubble regime (Figure 7a) was found with a certain bubble formation frequency at a given airflow rate with a slight increase in airflow rates.  
As observed in [5], the bubble breakup regime was visualized, forming ligaments and large globular droplets downstream. With the further increase in air mass flow rates or ALR, the aerodynamic interaction increases, leading to corrugated/wavy sheet contraction, forming a neck region where the bursting phenomenon was identified. This bursting occurs near the nozzle exit region, forming the annular sheet disintegration regime (Figure 7b). With the further increase in ALR, the ligaments/filaments shed directly from the near-nozzle region because of the very high-speed interaction that led to a ligament-type breakup regime (Figure 7c). As we increase the liquid flow rates at a low ALR, the jet formation (Figure 7d) occurs with some waviness. For a 350 kg/h liquid flow rate, the wavy sheet was formed near the nozzle with a further airflow rate, which is contracted to form a wavy sheet disintegration (Figure 7e), leading to ligaments interconnected in a three-dimensional fashion and satellite drops downstream. At high flow rates (ALR), ligaments shed from all azimuthal angles of the annular sheet neck region, forming a christmas-tree regime. Finally, at very high air-liquid flow rates (high ALR), a pure-pulsating regime (Figure 7f) is observed, almost like the '_Clristmas tree breakup_' observed in [9], in which ligament-like structures pulsate alternatively on the left and right side of the spray centerline.  
The breakup modes or regime diagram for converging and converging-diverging (CD) atomizers are similar, with a slight variation in the ALR range for different regimes. The regime diagram (see Figure 8) in a 3D manner with each axis corresponding to non-dimensional numbers is shown below.  
Figure 7: For the converging nozzle with 280 μm sheet thickness at a 100 kg/h water flow rate (**Top row**) and at a 350 kg/h water flow rate (**Bottom row**), respectively, with airflow rates of (**a,d**) \(1\) kg/h, (**b,e**) 10 kg/h, and (**c,f**) 35 Kg/h, respectively.  
The converging nozzle atomizer case for 280 \(\upmu\)m sheet thickness is plotted based on the air-to-liquid ratio (ALR) defined in Equation (2), along with the non-dimensional numbers such as Reynolds number (\(\text{Re}_{\text{g}}\)) based on gas flow in Equation (3), and Weber number (\(\text{We}_{\text{l}}\)) based on the liquid sheet in Equation (4), respectively, assuming the dynamic viscosity of air to be a relatively constant value of 18 \(\upmu\)Pa-s at 15 \({}^{\circ}\)C.  
\[ALR=m_{air}/m_{liquid} \tag{2}\]  
\[\text{Re}_{\text{g}}=\rho_{\text{g}}\cdot\text{u}_{\text{g}}\cdot\text{d}/ \text{u}_{\text{g}} \tag{3}\]  
\[\text{We}_{\text{L}}=\rho_{\text{L}}\cdot\text{u}_{\text{L}}{}^{2}\cdot\text{t} /\sigma_{\text{L}} \tag{4}\]  
where L = liquid, g = air, d = orifice (throat) diameter, and t = sheet thickness.