# Modeling the Formation of Urea-Water Sprays from an Air-Assisted Nozzle
## 1 Introduction  
Emission legislation is nowadays a crucial factor for the development of internal combustion engines in a wide range of applications. An adherence to the latest thresholds for pollutant emissions requires the use of aftertreatment systems with high conversion rates. Thereby, selective catalytic reduction (SCR) has proven to provide an effective method for cleaning the exhaust gas of diesel engines from nitrogen oxides, with efficiencies higher than 90% [1]. Thus, SCR systems are already state-of-the-art, from small engines for passenger cars up to large engines for the maritime transport [2, 3]. The working principle of an SCR system is based on the injection of an aqueous urea solution into the hot exhaust gas. Following the evaporation of the water content, urea undergoes thermo- and hydrolysis reactions and forms the reducing agent ammonia, which reacts with nitrogen oxides to pure nitrogen and water at the catalyst [4]. The high reliability and efficiency of the system require an appropriate preparation of the urea-water spray to ensure fast evaporation and to avoid the formation of deposits from urea. Therefore, depending on the general mixing strategy of the system, an atomization process producing a fine spray with a small initial Sauter mean diameter (SMD) may be favorable. Pressure-driven urea dosing systems usually work with small injection pressures less than 10 bar, which is not sufficient to provide an appropriate atomization. These concepts rather rely on wall wetting and liquid film formation on the hot surfaces of mixing elements that promotes the fast preparation of the aqueous urea solution, as well as the homogeneous distribution of gaseous ammonia in the turbulent flow of the mixing pipe upstream the SCR catalyst [5]. However, increasing the injection pressure as shown by the authors in [6] promotes the atomization process. Air-assisted atomizers, on the other hand, constitute an interesting alternative to achieve a fine spray and high ammonia formation rates without wall interactions. This kind of nozzle is exclusively used in heavy-duty vehicles, as these are often already equipped with compressed air systems [7]. A comparison between pressure-driven and air-assisted systems was conducted by [PERSON] et al. [8] by means of optical experiments. They measured a decrease of the SMD from 70 \(\upmu\)m for a pressure-driven system to 15 \(\upmu\)m for an air-assisted nozzle, which leads to less wall wetting and an improvement of the ammonia distribution. Similar findings were made by [PERSON] et al. [9], who carried out optical investigations of different pressure-driven and air-assisted injection systems. The fine spray generated from the air-assisted nozzle follows the exhaust gas flow and is highly susceptible to its flow conditions, which enables an appropriate prevention of wall wetting and deposit formation. Numerical flow simulations have nowadays an important role in the development process of aftertreatment systems. In addition to the exhaust gas flow, spray preparation, wall wetting, deposit formation and ammonia distribution are part of numerical models. An overview of the modeling approaches used for SCR systems and the ongoing challenges is given by [PERSON] in [10]. Many investigations concerning spray preparation can be found in literature. Those studies are, for the most part, focused on the modeling of pressure-driven systems; for example, in [11; 12]. Thereby, even advanced methods like large eddy simulation (LES) and volume-of-fluid approaches, which require a high computational effort, are applied to study the breakup of the liquid jet and the formation of initial droplets [13; 14]. However, less is reported about the modeling of air-assisted systems in the context of SCR. [PERSON] et al. [15] modeled a twin-fluid atomizer for stationary SCR systems with an external mixing of urea and compressed air. The urea injection was depicted using a blob injection approach assuming an intact liquid core, and the compressed air was introduced by an annular mass flow boundary.