## 3 Experimental Results and Analysis  
The influence of related factors on the atomization performance of the nozzle was experimentally studied according to the orthogonal design scheme in Table 1, and the results are shown in Table 2. Figure 3 shows the distribution of droplet size under 25 operating conditions, in which the red curve represents the cumulative percentage of the droplet size, and the blue column represents the volumetric frequency of the droplet size.  
The orthogonal experimental results in Table 2 and Figure 3 show the wide distribution of the nozzle's atomization characteristic parameters, which can basically meet the requirements of investigation and analysis. The wide distribution of the experimental results indicated that the level of factors was not limited to local areas but can accurately reflect the overall situation of the factors, suggesting that the orthogonal experiment design scheme was reasonable and effective [47; 48]. At industrial production sites, according to the requirements on the nozzle atomization parameters, a parameter combination that approximated the requirements of the conditions can be selected from the orthogonal experimental results in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & \multicolumn{4}{c}{**Influence Factor**} & \multicolumn{4}{c}{**Experiment Result**} \\ \cline{2-7}  & \(p_{air}\)/MPa & \(p_{l}\)/MPa & \(d\)/mm & \(Q_{L}\)/(L-min\({}^{-1}\)) & \(Q_{air}\)/(L-min\({}^{-1}\)) & **SMD/\(\mu\)m** \\ \hline  
\hline \hline \end{tabular}
\end{table}
Table 2: Orthogonal experimental results.  
By comprehensively averaging the SMD data in Table 2, the influencing significance of various factors on the SMD can be obtained through range analysis. Figure 4 shows the average value and range of SMD. From Figure 4, among the three influencing factors for the droplet size, the range of air pressure is the largest. The influencing significance of various factors can be ranked as \(R_{air}>R_{\text{L}}>R_{d}\). From the overall trend of SMD, the SMD was negatively correlated with air pressure, positively correlated with water pressure and outlet diameter. The compressed air and low-speed water flow collided in the mixing chamber to achieve the primary atomization (initial atomization). Then, the gas-liquid two-phase flow was sprayed from the nozzle outlet at a high speed. The impulse of the fluid excited the ultrasonic generator to generate ultrasonic waves, which led to the secondary atomization of water.  
The experimental results on the flow rate showed that at a higher air pressure, the water flow rate of the nozzle was lower and the air flow rate was greater. As the air flow rate increased, the velocity of the gas-liquid two-phase flow at the outlet increased, the intensity of the ultrasonic waves increased, and the generated effect on the surface of the droplets became more significant. As a result, the oscillation frequency of the bubbles was larger, and the particle size of droplets decreased. The increase of the outlet diameter led to a continuous increase in water flow rate. In contrast, at a relatively stable air flow rate, the atomization energy was reduced for a unit mass of water, which affected the effect of the primary and secondary atomization of the liquid. Consequently, the droplet got larger and larger. The increase in water pressure led to a greater water flow rate and a lower air flow rate. As a result, the air-liquid flow ratio continued to decrease, as shown in Figure 5. At a  
Figure 3: The droplet size distribution under 25 operating conditions.  
lower air-liquid flow ratio of nozzle, the required atomization energy for a unit mass of droplets was decreased, thus both the primary atomization and the secondary atomization effects became weaker, and the droplet size increased.