## 4 Results
### Influence of Urea Crystallization Risk under Structural Factors
#### 4.2.1 Influence of the Rotating-Stream Tray Angle on Crystallization Risk  
The spray parameters have a direct correlation with the performance of atomization evaporation, which in turn affects the crystallization risk. The structural characteristics of the SCR mixer impact fluid dynamics, further affecting the spatial distribution and crystallization risk of UWS atomized droplets. In order to clarify the influence of the rotating-stream tray on the crystallization risk of the mixer, three different layouts of rotating-stream tray angles were selected in Figure 20.  
The variation of the maximum urea film thickness with time under different rotating-stream tray angles is displayed in Figure 21. The maximum urea film thickness under different rotating-stream tray angles gradually increases with the extension of spray time, and the difference in the maximum urea film thickness value is small. At the time of 200 ms ASOI, the maximum urea film thickness corresponding to each rotating-stream tray angle is only in the thickness range of 340\(\sim\)400 \(\upmu\)m, indicating that the rotating-stream tray angle has a certain impact on the maximum urea film thickness, but this effect is smaller than other factors. Before the ASOI time of 100 ms, the maximum urea film thickness under the three different rotating-stream tray angle layouts is basically the same. After the ASOI time exceeds 100 ms, the maximum urea film thickness corresponding to a rotating-stream tray angle of 20\({}^{\circ}\) is always slightly lower than the corresponding values of the other two angles. In the ASOI range of 100\(\sim\)160 ms, the maximum urea film thickness corresponding to a rotating-stream tray angle of 40\({}^{\circ}\) is slightly higher than the other two angles. Overall, the development process of the maximum urea film thickness is less impacted by the layout of different rotating-stream tray angles.  
Extract urea and UWS film mass accumulation data at different rotating-stream tray angles at 200 ms ASOI time, and compare the urea and UWS film mass accumulation at different rotating-stream tray angles, shown in Figure 22. When the angle of the rotating-stream tray increases from 20\({}^{\circ}\) to 30\({}^{\circ}\), the accumulated urea film mass increases by 3%. When the angle of the rotating-stream tray increases from 30\({}^{\circ}\) to 40\({}^{\circ}\), the accumulated urea film mass decreases by 5%. When the angle of the rotating-stream tray is selected as 20\({}^{\circ}\) or 40\({}^{\circ}\), the amount of UWS wall collision is reduced, which is beneficial for reducing the risk of crystallization. Based on the results of the maximum urea film thickness and the accumulation of urea film mass, it is reasonable to choose a rotating-stream tray angle of 20\({}^{\circ}\) or 40\({}^{\circ}\). In addition, the pressure loss and reducing agent uniformity index corresponding to the angle layout of three sets of swirl sheets were extracted. The pressure loss was 1525.9 Pa, 1305.6 Pa, 1239.8 Pa, and the reducing agent uniformity index was 95.4%, 96.3%, and 95.6%, respectively. Taking into account multiple indicators, it can be concluded that a layout with a rotating-stream tray angle of 40\({}^{\circ}\) is more reasonable, ensuring the minimum  
Figure 20: Three types of rotating-stream tray angles layout.  
risk of crystallization and pressure loss while meeting the requirements for reducing agent uniformity.