Numerical analysis on the mixing and urea crystallization characteristics in the SCR system

Abstract To meet the demand of the stricter emission limitation of NOx emissions in the new emission standards, selective catalytic reduction system has become an essential part in diesel engines. Currently, mitigation of urea crystallization and improvement of the mixing quality of ammonia are two crucial issues in diesel engine equipped with SCR system. In this paper, the flow dynamics, heat transfer, spray atomization and evaporation as well as the formation of urea deposits were predicted by a validated CFD model using fluid-solid coupling method. On the basis of this model, different mixer designs and spray cone angles were assessed. According to the results, the change of film mass is consistent with that of solid urea. In this case, solid urea could be used as an indicator for urea crystallization prediction. In view of different mixer designs, mixer with a cylindrical region in the center and two curved baffles around the cylindrical region could mitigate the formation of urea crystallization and achieve high mixing quality. When this kind of mixer is adopted, using smaller spray cone angle could further reduce the tendency of urea crystallization, raise the evaporation rate and improve the mixing uniformity.