Improved urea-water solution spray model for simulations of selective catalytic reduction systems

Successive diesel engine emissions’ regulations impose increasingly tighter limits on the automotive industry. A well-established method to meet nitrogen oxides emissions’ requirements is selective catalytic reduction with a urea-water solution injection. State-of-the-art designs are based on close-coupled exhaust architecture which suffers from a lack of space for a proper decomposition of a urea-water solution. The optimization of such systems is very challenging and is always intensively supported by numerical simulations. Within this work a two-zone spray representation for a urea-water solution injection is proposed. The two-zone approach captures a non-uniform droplet distribution inside the spray cone while maintaining a reasonable calculation time, since it is still based on a Lagrangian method. The proposed model was applied into exhaust system simulations and tested against a standard spray representation. The results revealed that uniform spray representation led to a smaller wall film formation and different mass balance. Ammonia distribution at the catalyst inlet was not directly affected significantly; however, in real systems one could expect catalyst clogging due to wall film formation, followed by a decrease of ammonia distribution uniformity. The results clearly show that uniform spray representation may lead to misestimation of the selective catalytic reduction system’s performance and the novel two-zone approach should be used instead.