3D-Semi 1D Coupling for a Complete Simulation of an SCR System

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Abstract
The presented work describes how numerical modeling techniques were extended to simulate a full Selective Catalytic Reduction (SCR) NOx aftertreatement system. Besides predicting ammonia-to-NOX ratio (ANR) and uniformity index (UI) at the SCR inlet, the developed numerical model was able to predict NOx reduction and ammonia slip. To reduce the calculation time due to the complexity of the chemical process and flow field within the SCR, a semi-1D approach was developed and applied to model the SCR catalyst, which was subsequently coupled with a 3D model of the rest of the exhaust system. Droplet depletion of urea water solution (UWS) was modeled by vaporization and thermolysis techniques while ammonia generation was modeled by the thermolysis and hydrolysis method. Test data of two different SCR systems were used to calibrate the simulation results. Results obtained using the thermolysis method showed better agreement with test data compared to the vaporization method. Decreasing the distance between the injector and SCR inlet decreased ANR and UI, while NOx concentration at the SCR exit increased but ammonia slip decreased. For a constant distance between the urea injector and the SCR inlet, putting a mixer closer to the injector changes ANR slightly while UI remains nearly the same.

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