## 3 Experiments
### Simulation Results and Experimental Verification  
From an energy perspective, it can be considered that the risk of crystallization is the result of a game between the available heat in the exhaust and the heat required for the complete evaporation and decomposition of UWS. At a certain exhaust temperature, the size of the urea spray rate determines the crystallization situation. This section selected a set of typical operating conditions at low exhaust temperature and a set of rated operating conditions at high exhaust temperature to testify to the reliability of the model and characterize the SCR system's ability to resist crystallization risks under all operating conditions. The operating parameters are listed in Table 4. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Rated Condition** & **Typical Condition** \\ \hline Exhaust temperature/\({}^{\circ}\)C & 539 & 225 \\ Exhaust flow rate/kg/h & 1384.8 & 218.2 \\ Injection rate/mg/s & 1385.5 & 47.0 \\ Ambient temperature/\({}^{\circ}\)C & 25 & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Selected operating condition parameters. Figure 3: Mesh independence verification. Figure 2: Mesh model diagram. The thickness and distribution of the urea film 10 ms after spraying of the injector (ASOI) are shown in Figure 4. It can be seen that a urea film is formed on the inner wall of the porous tube and the pore wall of the porous tube, with an overall macroscopic morphology similar to a \"spiral band\", and no urea film is produced in other parts. The maximum local urea film thickness is \(6.92\times 10^{-6}\) m, appearing closer to the injector, with a common urea film thickness of about \(1\times 10^{-6}\) m in other areas. As the spray distance increases, the maximum urea film thickness gradually decreases. The droplets of urea aqueous solution collide with the wall and transform into a urea film; thus, the distribution of the urea film is related to the evolution process of the droplets. The distribution results of spray particles are displayed in Figure 5. The droplets did not form a uniform spatial distribution after leaving the injector, and the UWS droplets sprayed into the exhaust from each orifice do not penetrate forward along the axis of the single spray beam but have a significant deflection. Non-colliding droplets continue to move along the spiral trajectory; they continue to interact with the rear wall surface and form a urea film under the action of centrifugal force. Finally, a \"spiral ribbon\" macroscopic form of the urea film is formed. The flow state of exhaust gas can significantly affect the movement process of urea droplets. To clarify the influence of the exhaust flow field on droplet distribution, the distribution results of exhaust flow lines inside the mixing section of the SCR system are shown in Figure 6. The uniform airflow undergoes a significant change in its motion state after entering the SCR mixer. The left view shows the exhaust flow state inside the porous tube. It can be seen that there is a regular vortex distribution inside the porous tube, and the closer it is to the vortex center, the greater the airflow velocity, reaching a maximum of 127.4 m/s. Figure 4: Urea film thickness and distribution (t = 10 ms ASOI). Figure 5: Spatial distribution of droplets in UWS atomization. The mass accumulation of urea and urea film over time is shown in Figure 7. It can be seen that at 4 ms ASOI, a urea film begins to form, and the mass accumulation of the urea and urea film increases linearly with time. Due to the continuous spray of UWS into the exhaust pipe at a certain rate, the total mass of the urea film will continue to increase at a certain rate. When the evaporation rate of urea film components is much lower than the mass replenishment rate of the urea film, the total mass of the urea film shows an almost linear increase trend. From the variation curve of the urea loss rate in the figure, it can be seen that the SCR system has caused considerable urea loss due to the formation of a urea film.