Development of a 3D Numerical Model for Predicting Spray, Urea Decomposition and Mixing in SCR Systems 2007-01-3985
In a typical automotive SCR system, the urea-water solution is injected into the exhaust gas stream and breaks up into small droplets through the atomization process due to the interaction with the exhaust gas flow. The water vapor is released by vaporization and leaves the urea component in the droplets. The urea component is converted to ammonia and iso-cyanic acid and other species through thermal decomposition and hydrolysis processes. A multi-dimensional CFD model is developed in the present study to simulate injection and mixing in the automotive SCR systems. The urea-water solution is modeled as a bi-component liquid mixture with evaporation laws for each component. A Lagrangian method with spray sub-models is used to track the liquid droplets and the atomization process. The thermal decomposition and hydrolysis processes of the urea and iso-cyanic acid are simulated by simplified two-step reaction mechanisms.
The spray and mixture qualities are investigated in the paper for an exhaust system. A uniformity index is developed to evaluate the mixture quality at the inlet of the SCR catalyst, which could also serve as an indirect indication of the de-NOx efficiency of the SCR system. The effects of mixers on the spray and mixing are studied in this paper.