# Study on Spray Characteristics and Breakup Mechanism of an SCR Injector
## 4 Conclusions  
In this study, a NPFPS numerical model is proposed to simulate the spray process of a commercial non-air-assisted pressure-driven UWS injector. The coupled simulation includes the primary breakup process of the liquid jet by the VOF approach and the subsequent droplet development process by the DPM approach. The primary breakup simulation can capture relevant details of liquid jet propagation and disintegration, which reveals the primary breakup mechanism. The primary breakup simulation obtains a reasonable prediction on the parameters such as jet inclination angle, location of primary droplet formation, primary droplet size and primary droplet velocity. Furthermore, the influence of injection pressure on the parameters including jet breakup length and jet inclination angle are also discussed. The results obtained by the primary breakup simulation are as follows. Figure 12: Droplet radial velocity distribution at 30 mm from the nozzle exits. Figure 13: Droplet axial velocity distribution at 30 mm from the nozzle exits. The primary breakup of the injector spray has the relevant characteristics of the first wind-induced breakup regime according to the liquid phase volume distribution, _We_ number and _Oh_ number. It was also found from the primary breakup simulation that the characteristic diameter of the liquid jet is related to the connected domain between the pressure chamber and the nozzle, which can be used for optimal design of the injector structure to improve the droplet size distribution. The results show that the jet breakup length increases with the increase in the injection pressure, and the jet inclination angle of the liquid is insensitive to the injection pressure. Furthermore, a droplet development simulation based on the DPM approach is performed and the primary breakup parameters such as jet inclination angle, location of primary droplet formation, primary droplet size and primary droplet velocity are taken as the initial injection conditions. A TAB model is coupled with a droplet development simulation to determine whether a secondary breakup occurs. The main results of the droplet development simulation are:  
It is proven that the droplet development model can well characterize the far-field spray by comparing the droplet size distribution and droplet velocity distribution obtained by the simulation and experiment separately. It can be concluded that the relative velocity between the spray and still gas is insufficient to induce a substantial secondary breakup according to the aerodynamic Weber number and the particle size distribution data of different cross-sections. In other words, for the low-pressure non-air-assisted injector, the primary breakup characteristics determine the distribution characteristics of the spray. We also found that small droplets are more susceptible to aerodynamic forces to change their state of motion. Overall, the NPFPS numerical model presented in this study is a valuable alternative to study low pressure non-air-assisted injector sprays. The primary breakup model can be used to study the breakup mechanism and to optimize the injector structure. The droplet development model is conducive to improve the prediction accuracy of the numerical studies related to the SCR system. Conceptualization, C.B. and K.L. ; methodology, C.B. and T.Z. ; formal analysis, C.B. and J.L. ; validation, C.B. and J.L. ; writing--original draft preparation, C.B. ; writing--review and editing, C.B. and K.L. ; visualization, C.B. ; supervision, K.L. ; funding acquisition, K.L. and T.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 51876175. Not applicable. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: