CFD Modeling of Urea Spray and Deposits for SCR Systems 2016-01-8077
Selective Catalytic Reduction (SCR) has become a mainstream approach to reduce diesel engine NOx emissions. Urea Water Solution (UWS) injection and interactions with mixers and exhaust gases affect the homogeneity of ammonia distribution at catalyst inlet and solid deposits formation on walls / mixer surfaces, therefore influencing SCR performance and durability.
Computational Fluid Dynamics (CFD) is used to simulate an EU V compliant SCR system with a dual baffle mixer for heavy duty diesel engines. The modeling procedure is carried out by a multi-dimensional CFD code CONVERGE that includes transient urea transport processes in an exhaust flow configuration, detailed spray break-up, evaporation, wall-film, turbulence, and Conjugate Heat Transfer (CHT) models as well as an automated mesh generation approach. Locations of urea deposits and system pressure drop are predicted and validated against measurements, providing uniformity index (UI) predictions at the catalyst inlet. A parameter called Deposit Formation Possibility (DFP) is proposed to quantify the risk of deposit formation.
More accurate description of the spray and urea behavior requires comprehensive capturing of spray transport in space, liquid film formation and breakup, liquid film boiling, and gravitation-induced liquid film movement under steady or transient state. Next step of work is planned to be published towards liquid film transport, boiling phenomena and associated deposit risk predictions.