Numerical Modeling of Liquid Film Boiling, Urea Deposition and Solidification in SCR Applications

The proposed Euro 7 regulation aims to substantially reduce the NO_x emissions to 0.03 g/km, a trend also seen in upcoming China 6b and US EPA regulations. Meeting these stringent requirements necessitates advancements in Urea/Selective Catalytic Reduction (SCR) aftertreatment systems, with the urea deposit formation being a key challenge to its design. It’s proven that Computational Fluid Dynamics (CFD) can be an effective tool to predict Urea deposits. Transient wall temperature prediction is crucial in Urea deposit modeling. Additionally, fully understanding the kinetics of urea decomposition and by-products solidification are also critical in predicting the deposit amount and its location. In this study, we introduce (i) a novel film boiling model (IFPEN-BRT model) and (ii) a new urea by-product solidification model in the CONVERGE CFD commercial solver, and validate the results against the recent experiments. The IFPEN-BRT model handles the spray-wall heat transfer in various boiling regimes, and the urea by-product solidification model separates solid deposits from liquid film parcels and renders them inert on the walls. We couple the by-product solidification model with the detailed decomposition model for urea. We use surface morphing feature developed in CONVERGE to enable realistic representation of solid surface topology once the solid deposits are formed on the testing plate. Multiple acceleration schemes have been employed to achieve a faster turnaround time while maintaining high fidelity. Additionally, the fixed flow approach has been used to accelerate the simulation and reach the time scale required for appreciable deposit formation. The simulations, incorporating both the IFPEN-BRT model and the urea by-product solidification model, matches the experiments very well on several fronts: the wall temperature contours, the temporal evolution of wall temperature profiles, and film/deposit patterns. The simulations also correctly predict cyanuric acid (CYA) as the primary solid deposit, aligning with experimental findings after 20 minutes real time simulation.