Selective Catalytic Reduction (SCR) based on urea water solution (UWS) has become a promising technology to reduce Nitrogen Oxides (NOx) emissions for mobile applications. However, urea may undergo incomplete evaporations, resulting in formation of solid deposits on the inner surfaces including walls and mixers, limiting the transformation of urea to ammonia and chemical reaction between NOx and ammonia.
Numerous design parameters of SCR system affect the formation of urea deposits [1] ; they are: exhaust condition, injector type, injector mounting angle, geometrical configurations of mixer, injection rate and etc. Research has been available in urea deposits, mixers, urea injection rates and others [2,4,5,6].
In this paper, focus is placed on improving mixing structure design from baseline design of EU IV to EU V. On-road tests indicate that deposits are highly likely to occur near locations where spray and exhaust gas interact most. Analysis of test data shows that mixing structure influences the UWS distribution uniformity and the residence time, thus often becomes the main factor in causing deposit formation on the wall surface.
To minimize urea deposit risks and improve the NH3 distribution uniformity, efforts have been taken to optimize the inlet and mixing configurations, the upstream baffle between the mixing chamber and catalyst chamber, and the injector seat of SCR system. Both Computational Fluid Dynamics (CFD) and tests are employed to identify areas of concern and to validate the improvement ideas. Distributions of Urea Water Solution (UWS), urea decomposition and droplet impingement on pipe wall are investigated. After new designs are proposed, engine bench emission tests, urea deposit tests, and on-road tests are used to validate the modified systems. It is found that the optimized design is able to improve ammonia distribution uniformity, eliminate urea deposits, improve NOx conversion efficiency, and satisfy the requirements of EU V emission regulations.