## 4 Results
### Influence of Urea Crystallization Risk under Structural Factors
#### 4.2.3 Influence of the Spray Angle on Crystallization Risk  
The variation of maximum urea film thickness with time under different spray angles is shown in Figure 25. The maximum urea film thickness under different spray angles gradually increases with the extension of spray time at different growth rates. At 200 ms ASOI, the maximum urea film thickness corresponding to different spray angles is between  
Figure 23: Variations of maximum urea film thickness with time under different nozzle numbers.  
Figure 24: Accumulation of urea and UWS film under different nozzle numbers.  
100 and 1000 \(\upmu\)m. This indicates that changing the spray angle will have a profound impact on the maximum urea film thickness. If the spray angle is larger, such as 32.5\({}^{\circ}\) and 35.0\({}^{\circ}\), the \"two-stage\" growth trend of the maximum urea film thickness is more obvious. Before the 100 ms ASOI moment, the difference of the maximum urea film thickness at each spray angle is inapparent. After the 100 ms ASOI moment, there began to be a significant difference in the maximum urea film thickness at each spray angle. This indicates that the impact of spray angle on urea film formation is mainly reflected in the stable development and accumulation stage of the urea film.  
Extract urea and UWS film mass accumulation data under different spray angles at 200 ms ASOI time, and obtain the variation of urea and UWS film mass accumulation with the spray angle, as shown in Figure 26. The accumulation of urea film mass increased at first and then decreased with the increase of spray angle. In the range of 25.0\({}^{\circ}\)\(\sim\)30.0\({}^{\circ}\), increasing the spray angle will increase the amount of UWS hitting the wall, promote the conversion of UWS droplets into urea films, and accelerate the accumulation of urea films. Within the range of 30.0\({}^{\circ}\)\(\sim\)35.0\({}^{\circ}\), the cumulative mass of urea film decreases with the increase of spray angle and then changes slightly. The reason is that when the spray angle increases from 32.5\({}^{\circ}\) to 35.0\({}^{\circ}\), a portion of UWS droplet particles do not interact with the wall surface of the porous tube, but pass through the holes of the porous tube and enter the space between the swirling plate mixing tube and the porous tube, resulting in a decrease in the collision area and a decrease in the overall accumulation of urea film mass.  
Figure 26: Changes in mass accumulation of urea and UWS film with spray angles.  
The trend of the accumulation of urea film mass is different from that of the maximum urea film thickness because some droplet particles do not undergo wall collision behavior, which only affects the accumulation of urea film and does not affect the process of other droplets colliding with the wall and forming urea film accumulation at the wall collision point. Based on the variation pattern of the maximum urea film thickness and the accumulation of urea film mass, increasing the spray angle will have a significant impact on the crystallization risk, and excessive spray angles should be avoided. In addition, in terms of improving the uniformity of the reducing agent in the system, increasing the spray angle is beneficial for the diffusion distribution of UWS droplets in the film field, thereby promoting the uniform distribution of the reducing agent. Therefore, when determining the spray angle parameters, consideration should be given to the increase in crystallization risk caused by a large spray angle and the decrease in reducing agent uniformity caused by a small spray angle.