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<div class="section abstract"><div class="htmlview paragraph">The permanently tightening emission regulations for nitrogen oxides (NOx) pollutants force further development of mobile exhaust aftertreatment systems with selective catalytic reduction (SCR). Of particular interest is the long-term reliability of SCR-systems with regard to unfavorable operating conditions, such as high injection rates of urea water solution (UWS) or low exhaust gas temperatures. Both may lead to the formation of solid deposits which decrease system efficiency by increasing backpressure and impairing ammonia formation.</div><div class="htmlview paragraph">In order to study the most relevant processes of deposit formation, an optical box with heat resistant glass was designed. Three UWS injectors with different spray characteristics were used to study their influence on the deposit formation under a wide range of stationary and transient operating conditions. Infrared thermography was applied to observe spray-induced wall cooling, both below and above the Leidenfrost point. The formation of a liquid fluid film on a hot surface as well as deposit growth and decomposition were monitored by video recording. A chemical analysis of obtained solid deposits complemented the experimental investigations.</div><div class="htmlview paragraph">This paper describes influences on deposit formation and decomposition. A strong impact of spray properties, such as droplet Weber number and spray area load, on the critical wall temperature for film formation was found. Different types of liquid film propagation were observed for different surface temperatures. The impact of operating conditions, such as exhaust gas temperature and gas flow rate, on the amount and the chemical composition of the deposits was determined. The experimental observations revealed different impact factors on the persistence of solid deposits.</div><div class="htmlview paragraph">The obtained experimental results show the complexity of physical and chemical processes leading to formation of solid deposits in SCR systems. Based on that, the requirements of CFD models for modeling of deposit formation and decomposition were defined in terms of physical and chemical properties as well as the necessary simulated time scales.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The permanently tightening emission regulations for NOx pollutants force further development of automotive exhaust aftertreatment systems with selective catalytic reduction (SCR). Of particular interest is the long-term reliability of SCR systems with regard to unfavorable operating conditions, such as high injection rates of urea water solution (UWS) or a low exhaust gas temperature. Both of them may lead to formation of solid deposits which increase backpressure and impair ammonia uniformity.</div><div class="htmlview paragraph">A fast modeling approach for numerical prediction of deposit formation in urea SCR systems is desired for optimization of system design. This paper presents a modified methodology for the modeling of deposit formation risk. A new determination of the initial footprint of the spray, where the deposit formation is inhibited, is proposed. The threshold values for the evaluation of the film transport were validated based on experimental results. To achieve a more realistic simulation in terms of wall wetting and cooling, the impingement heat transfer as well as the impingement model were modified based on optical investigations.</div><div class="htmlview paragraph">In order to accomplish the modeling of deposit formation with typical time ranges of several minutes, a recently developed injection source approach was applied. The substitution of the Lagrange-particles with source terms of mass, momentum and energy allowed to reduce simulation time by a factor of 30. The presented modeling approach was validated against both, the experimental data from an optical box and an exhaust aftertreatment system. The comparison of measured and simulated results shows the capability of the presented modeling approach to predict the position and the severity of solid deposits.</div></div>

The understanding of impingement processes is crucial for optimizing automotive selective catalytic reduction (SCR) systems. An accurate description of this behavior helps design exhaust systems and increases the validity of modeling approaches. A component test bench was set up, featuring a droplet chain generator for producing droplet sizes typically found in the urea–water solution sprays of SCR systems. A heatable impingement plate with an interchangeable surface enabled investigation of the influence of surface roughness. Data were acquired using a high-speed camera and image postprocessing. The droplet–wall interaction could be described using different regimes. An approach to characterizing impingement behavior based on weighted-regime superposition enabled gradual transitions between regimes, instead of step-like changes. It was observed that the surface roughness increased the droplet–solid contact area and generated thermal-induced secondary droplets at lower temperatures. A region of enhanced mechanical disintegration of the droplet was found, caused by peaks of the surface shearing off parts of the droplet. The probability of a droplet rebounding from the wall was reduced on a rough surface, due to the interference of the surface spikes with the droplet’s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface’s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed.