# Study on Spray Characteristics and Breakup Mechanism of an SCR Injector
## 1 Introduction  
Vehicle exhaust is one of the main culprits of air contamination, which mainly includes carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (PM). Diesel vehicles have been identified as the main contributors to NOx [1]. Selective catalytic reduction (SCR) has been proved by industrial practice to be one of the most effective technologies to eliminate NOx [2]. Ammonia (NH\({}_{3}\)) is generally adopted as a reducing agent in the SCR system and is stored in the form of a urea water solution (UWS). UWS is sprayed into the SCR system through an injector, and NH\({}_{3}\) is finally generated through evaporation, pyrolysis, and hydrolysis [3, 4]. The spray characteristics of the injector have a significant impact on the decomposition of urea and the formation of a deposit, which eventually affect the denitration efficiency of the SCR system [5, 6]. A UWS injector can be divided into two categories: air-assisted injector and non-air-assisted injector [7, 8]. The former one is less used in diesel vehicles due to its complex structure, including the air circuit and metering device, whereas the latter one has an extensive application as a result of its simple structure and easy control. Therefore, many studies related to UWS sprays are based on non-air-assisted injectors. [PERSON] and [PERSON] [9] provided an indication of the predominant spray distribution properties in exhaust configurations, and the results indicate that the fluid mechanical differencesbetween the spray and gas flow are insufficient to induce substantial secondary breakup. [PERSON] et al. [10] adopted Mie scattering and Phase Doppler Anemometry (PDA) to assess the effects of cross-flow velocity and temperature on the spray structure, droplet size and velocity distributions, which realized a comprehensive characterization of the spray under different exhaust conditions. [PERSON] et al. [11] reported the heat transfer characteristics of the spray/wall interaction in typical diesel exhaust flow conditions and deduced the influence of the gas flow conditions on the heat transfer characteristics. [PERSON] et al. [12] carried out a series of investigations including UWS spray, droplet breakup, urea decomposition and deposit formation in a combined experimental and numerical approach. The investigation revealed that the injection pressure influences deposit formation and droplet size; furthermore, the cooling effects of the spray injection prevent urea decomposition. The studies mentioned above focus on the influence of the exhaust on spray distribution, urea decomposition and deposit formation, etc., rather than on the spray characteristics of the injector. However, accurate characterization of the spray is of great importance for the quantitative study of numerical simulations for the SCR system. Therefore, some studies have characterized the spray characteristics by experimental methods such as Mie, PDA, particle image velocimetry (PIV), high resolution laser backlight imaging (HRLBI) and high-speed microscopic imaging (HSMI). The droplet size distribution along the spray axis was measured through PDA, and the spray developing process was investigated through a high-speed photography method in reference [13]. The results indicate that the droplet size distribution of a non-air-assisted injector is wide. [PERSON] and [PERSON] [14] used laser diagnostics and a high-speed camera to analyze the spray characteristics, such as spray angle, injection quantity and Sauter mean diameter (SMD) to clarify the distribution of the sprayed droplets. [PERSON] et al. [15] compared the global and local spray parameters of UWS and pure water under the same conditions. The experimental studies have shown differences in almost all considered spray parameters. [PERSON] et al. [16] compared the reliability of PDA, HRLBI and HSMI for the characterization of spray properties.