Development and Validation of a Simulation Model for Urea-Water-Solution Decomposition for Automotive SCR Systems

Stringent diesel emission regulations have been forcing constant reduction in the discharge of particulate matter and nitrogen oxide (NOx). Current state-of-the-art in-cylinder solutions are falling short of achieving these limits. For this reason engine manufacturers are looking at different ways to meet the emission regulations. Selective catalytic reduction (SCR) of oxides of nitrogen with ammonia gas is emerging as preferred technology for meeting stringent NOx emission standards across the world. SCR system designers face several technical challenges, such as avoiding ammonia slip, urea crystallization, low temperature deposits and other potential pitfalls. Simulation can help to develop a deep understanding of these technical challenges and issues, identify root causes of problems and help develop better designs.

This paper describes the modeling approach for Urea Water Solution (UWS) spray and its interaction with canister walls and exhaust gases. Custom subroutines are developed to capture the critical physical phenomena of spray-wall interaction and of heat and mass exchange between multi-component droplet and surrounding gases. These subroutines are validated by comparing simulation results with available experimental data. The proposed methodology uses the Lagrangian solver framework available within CFD code ANSYS Fluent. Owing to the ease of use and robustness of the models this approach can be extended to solve complex industrial problems related with SCR systems, to predict conversion efficiency and local reducing agent distribution.