Long-term reliability is one of the major requirements for the operation of automotive exhaust aftertreatment systems based on selective catalytic reduction (SCR). For an efficient reduction of nitrogen oxides in the SCR catalyst it is desirable that the thermolysis of the injected urea water solution (UWS) is completed within the mixing section of the exhaust system. Urea might undergo a number of secondary reactions leading to the formation of solid deposits on system walls. A deeper understanding of the mechanisms and influence factors is a basic requirement to prevent and predict undesired decomposition products.
This paper outlines the mechanisms of UWS transport and deposition on a typical mixing element geometry. The conditions leading to deposit formation were investigated based on optical and temperature measurements in a box with optical access. A good correlation with the deposit location observed at the close-to-series exhaust system was found. A chemical analysis complemented the investigations. Wall film formation and propagation within the SCR system were simulated with a validated CFD model. Numerous influence parameters were studied and integrated into a predictive post-processing routine.
The observations show that deposits can be formed up to Leidenfrost temperature. The liquid film pathways did not change significantly depending on the operating conditions. An increase of the UWS injection rate solely shifted the deposit location.
Further work is necessary to enhance and verify the modelling approach with respect to wall film and deposit chemistry. It will also focus on decreasing simulation wall time.