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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.