## 5 Conclusions  
Aiming at urea crystallization in the SCR system of diesel engines, the ammonia supply process of the SCR system is modeled in detail in this article, and a prediction model that can accurately assess the risk of urea crystallization is established by considering the processes of gas flow, heat transfer, spray atomization, component evaporation, spray wall interaction, urea film formation, and urea thermal decomposition reaction. A urea crystallization risk assessment method based on urea film thickness and urea film mass accumulation was proposed, and the feasibility of the model and assessment method was verified through bench experiments, solving the key problem of crystallization risk prediction. The main contents and conclusions of the article are as follows:  
1. To ensure the efficiency of nitrogen oxide catalytic conversion under low exhaust temperature conditions and timely ammonia storage, the maximum urea film thickness method and urea film mass accumulation method are used to suppress the risk of urea crystallization. 2. Based on the established crystallization risk prediction model, the influence of exhaust temperature, exhaust flow rate, and urea spray rate on the crystallization risk of urea was deeply explored. During the process of increasing the exhaust temperature from 190 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, the maximum urea film thickness decreased by 142.55 \(\upmu\)m, and the urea film accumulation decreased by about 5%. During the process of increasing the exhaust flow rate from 60 kg/h to 300 kg/h, the maximum urea film thickness decreased to 352.77 \(\upmu\)m but had a small impact on the urea film accumulation; after increasing the urea spray rate from a low level of 0.16 g/s to 0.32 g/s, the maximum urea film thickness increased by 428.51 \(\upmu\)m, and when increasing from 0.24 g/s to 0.08 g/s, the urea film accumulation increased by 27%. At the same time, the urea film retention ratio was affected by exhaust temperature, exhaust flow rate, and time, and there was no positive correlation between the urea film retention ratio and urea film accumulation. Figure 28: Changes in mass accumulation of urea and UWS film with a single spray beam cone angle. 3. Considering the effects of the structure of the integrated mixer and the spray parameters on the inhibition of urea crystallization risk, the optimized scheme of using a 40\({}^{\circ}\) rotating-stream tray angle, a 6-hole injector, a 25\({}^{\circ}\) spray angle, and a 19\({}^{\circ}\) spray cone angle can reduce the maximum urea film thickness to 34.4% of the pre-optimization value, while slightly reducing the urea film mass accumulation, providing support for inhibiting urea crystallization risk. Conceptualization, K.S. and S.B. ; methodology, K.S. ; software, K.Z. and W.S. ; validation, K.Z. and W.S. ; formal analysis, K.Z. and G.Z. ; investigation, K.S., K.Z. and G.Z. ; resources, S.B. and C.L. ; data curation, K.S. and G.Z. ; writing--original draft preparation, K.S., K.Z., and W.S. ; writing--review and editing, K.Z., G.Z., W.S., and H.C.; visualization, G.Z. and K.Z. ; supervision, G.L., S.B., and C.L. ; project administration, K.S. ; funding acquisition, G.L. and S.B. All authors have read and agreed to the published version of the manuscript. This work was funded by the Construction Machinery Intelligent Equipment Innovation and Entrepreneurship Community of Shandong, China (grant number GTT2021105), Department of Science & Technology of Shandong Province, China (grant number 2021 TSGC1334), Asset & Laboratory Management Department of Shandong University, China (grant number sy20232305), and the Undergraduate School of Shandong University, China (grant number 2022Y155). Institutional Review Board Statement: Institutional review is not applicable, as the paper did not involve humans or animals. Informed Consent Statement: Informed consent is not applicable, as the paper did not involve humans. Thedata presented in this study are available in the article.