Confidence in Modeling SCR Aftertreatment Systems

From 2012 Automotive Simulation World Congress: Complete SCR system calculations require modeling of liquid spray atomization and decomposition, spray/wall interaction, and catalytic reactions inside the monolithic SCR catalyst. Detailed validation studies of urea/water spray decomposition and wall film formation are performed in ANSYS Fluent to establish the suitability of alternative modeling approaches. The SCR porous structure and detailed chemistry are computed in an aftertreatment modeling software, axisuite®, and coupled to ANSYS Fluent through user-defined functions. In contrast to CFD efforts that calculate inlet flow and NH3 uniformity, this approach evaluates complete system performance in terms of NOx emissions. The results demonstrate that NOx conversion efficiency in SCR catalyst channels is a complex function of local temperature, flow rate and species availability. With advanced UWS modeling and surface reaction UDFs, the use of CFD can be extended from traditional uniformity index calculations to emissions-oriented SCR system design optimization.

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