Among the NOx after-treatment devices, the urea-based Selective Catalytic Reduction (SCR) has become the new standard, due to its high conversion efficiency with respect to the other alternatives. However, the formation of solid deposits caused by urea and its by-products is an issue, since it can severely affect the functionality of the entire after-treatment system, due to increased back-pressure and reduced conversion efficiency. This Thesis activity is meant to explore the mechanisms behind this phenomenon aiming to develop a CFD methodology capable to assess the risk of deposit formation.