Urea Deposit Predictions on a Practical Mid/Heavy Duty Vehicle After-Treatment System 2018-01-0960
Urea/SCR systems have been proven effective at reducing NOx over a wide range of operating conditions on mid/heavy duty diesel vehicles. However, design changes due to reduction in the size of modern compact Urea/SCR systems and lower exhaust temperature have increased the possibility of urea deposit formation. Urea deposits are formed when urea in films and droplets undergoes undesirable secondary reactions and generate by-products such as ammelide, biuret and cyanuric Acid (CYA). Ammelide and CYA are difficult to decompose which lead to the formation of solid deposits on the surface. This phenomenon degrades the performance of the after treatment system by decreasing overall mixing efficiency, lowering de-NOx efficiency and increasing pressure drop. Therefore, mitigating urea deposits is a primary design goal of modern diesel after-treatment systems. The purpose of current study is to introduce the Computational Fluid Dynamics (CFD) approach to predict urea deposit formation in the Isuzu exhaust system using detailed urea decomposition mechanism. Conjugate Heat Transfer (CHT) is used along with the advanced splashing and film evaporation models to correctly predict the film temperature. Detailed decomposition mechanism approach with a modified multi component evaporation model was used to capture the urea deposit production process. The results were compared against engine dyno test data for mass accumulation prediction and gas-chromatograph (GC) - QTOFMS results for urea deposit chemical component prediction.