## 3 Results and discussion
### Wind Tunnel Test
#### 3.1.2 Variation of Droplet under Different Airflow Velocities  
Figure 6 shows the variation of \(D_{V0.1}\), \(D_{V0.5}\), and \(D_{V0.9}\) for the three types nozzles at different airflow velocities. The droplets sizes of the CP02, CP03, and CP04 nozzles showed a significant step-like difference. Taking \(D_{V0.9}\) as an example, the \(D_{V0.9}\) average values of the three types of nozzles were 366.6 \(\upmu\)m (CP02), 524.7 \(\upmu\)m (CP03), and 648.6 \(\upmu\)m (CP04), respectively, due to the different orifice sizes. It was also found that the \(D_{V0.1}\) and \(D_{V0.5}\) values of the corresponding three types nozzles showed a steady increase trend with the airflow velocity increases from 0 m/s to 27.8 m/s. However, the value of \(D_{V0.9}\) was of great volatility, even the same type of nozzles showed obvious differences. According to the statistical analysis, the maximum difference of \(D_{V0.9}\) was between CP04-1 and CP04-2 when the airflow velocity was 22.2 m/s (Fig. 6c), up to 259.4 \(\upmu\)m, but it also showed a trend of increasing with the airflow velocity. The reason why the droplets size variation fluctuated greatly was mainly due to the performance problem with the tested nozzles. The tested nozzles had been used for a long time, and there was some wear, which may lead to a large deviation of the spray in the repeated test. It was inevitable for older non-reference nozzles. Secondly, studies had shown that laser measurement using LDD had sampling bias and certain limitations under different airflow conditions. In the absence or low airflow velocity, small droplets will be measured multiple times, resulting in a small measurement of the droplet size. Therefore, it appeared that the measured droplet size increased with the airflow velocity increased, which was similar to the results in Fritz's Fritz (2017). In addition, it was also related to the principle of liquid atomization of the pressure nozzle. The formation of droplets in the pressure nozzle depended on the fracture of the flat liquid film. In essence, the generation of fine droplets was caused by the aerodynamic fragmentation of the relative movement between the two phases (liquid phase and surrounding gas medium). The most important parameter affecting the droplet stability was the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number of Nozzles** & **Pressure (kPa)** & \multicolumn{2}{c}{**Flow Rate (L/min)**} \\  & & **Measured** & **Reference [a]** \\ \hline CP02-1 & & 1.02 & \\ CP02-2 & 345 & 0.96 & 1.00 \\ CP02-3 & & 1.08 & \\ \hline CP03-1 & & 1.65 & \\ CP03-2 & 330 & 1.44 & 1.51 \\ CP03-3 & & 1.50 & \\ \hline CP04-1 & & 2.40 & \\ CP04-2 & 310 & 1.95 & 1.93 \\ CP04-3 & & 2.25 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: CP nozzle flow rates of the three selected nozzles used in the droplet size test. nondimensional Weber number. As the airflow velocity increased, the flat liquid film was gradually increased by the shearing force of the air, the droplets deformation rate accelerated, and the Weber number also increased. When a certain critical state was reached, the surface tension was not enough to resist the liquid flow, the droplets were continuously stretched, so that the droplets were more likely to be broken and secondary broken, and the droplet size changed. Some studies also showed that for different types of nozzles, when the Weber number was the same, the larger the droplet size, the lower the relative gas/liquid velocity. As a result, the deformation rate of the larger size droplets was slightly lower than that of the smaller size droplets. Moreover, in the above process, there were accompanied by the effect of droplet dispersion and droplet aggregation. The interaction of the two effects on the droplets also caused a series of uncontrollable changes in droplets size [28; 29]. Except for the droplets size, other spray parameters of each nozzle were also affected by the change of airflow velocity.