# Study on Spray Characteristics and Breakup Mechanism of an SCR Injector
## 1 Introduction
The droplet size distribution and velocity distribution of the spray can be accurately obtained by these three approaches and applied to the numerical models to optimize exhaust geometries and mixers. The optical methods in the above studies can accurately characterize the macroscopic characteristics of the spray, which are not qualified to study the near-field breakup process because the nozzle size of the injector is small, and the primary breakup occurs in the near-field area of the nozzle exits. For this reason, it is necessary to use computational fluid dynamics (CFD) to study the spray process of a non-air-assisted pressure-driven injector. The numerical methods for spray modeling are mainly divided into volume of fluid (VOF) [17; 18; 19] based on the Eulerian-Eulerian method and discrete phase model (DPM) based on the Eulerian-Lagrangian method [20; 21] at present. The VOF approach can be employed to visualize the breakup process of the liquid jet and to obtain a clearer insight into the breakup mechanism [22]. On this basis, it can establish the relationship between the internal flow field of the injector and the external flow field, which is helpful for the optimal design of the injector. However, since the droplet size can be as small as a few microns, the mesh scale of the VOF model also needs to reach this level to capture clear phase interfaces. If the VOF model is adopted to simulate the full process of the spray, the count of computation will be expensive. Therefore, most studies employ the DPM model to characterize the far-field characteristics of the UWS injector. [PERSON] et al. [12] discussed the effect of injection pressure on the risk of wall impingement and wall wetting. [PERSON] et al. [15] studied the influence of different spray characteristics of water and UWS on wall film formation. [PERSON] and [PERSON] [23] developed a model used for spray simulation of UWS droplet evaporation to determine the urea-to-ammonia conversion efficiency. [PERSON] et al. [24] proposed a two-zone approach capturing a non-uniform droplet distribution inside the spray cone. The spray models used in these studies can accurately characterize the spray but cannot be used to study the mechanism of the primary breakup and to optimize the injector structure. Moreover, it is known from reference [9] that fluid mechanic differences between the spray and gas flow are insufficient to induce a substantial secondary breakup due to the low working pressure of the non-air-assisted UWS injector. Hence, the primary breakup characteristics of the non-air-assisted UWS injector play a dominant role in the characterization of the spray. To the best of our knowledge, the numerical studies on the primary breakup of a non-air-assisted UWS injector are relatively limited, and thus, this study is necessary. However, it does not mean that the far-field spray model is not important. Parameters such as droplet size distribution, velocity distribution, and inclination angle are of great significance for the quantitative numerical study of the urea-to-ammonia conversion efficiency, ammonia uniformity, and deposit formation, etc. For this reason, it is necessary to establish a numerical model to study the full spray process of the UWS injector. Some studies in other research fields [25; 26] have combined the Eulerian-Eulerian and Eulerian-Lagrangian methods to simulate the primary and secondary breakup processes, respectively, and realized the full process simulation of the spray. This method provides a new idea for the study of the spray characteristics of the UWS injector. In this work, a typical commercial non-air-assisted pressure-driven UWS injector is taken as the research object, and its spray process is modeled and simulated by FLUENT. The simulation based on the VOF approach starts from the pressure chamber of the injector and characterizes the propagation and primary breakup process of a liquid jet in a simplified computational domain.