Computational Studies of Urea-Derived Deposits in a Close-Coupled SCRF System

The close-coupled selective catalytic reduction (SCR) system integrated with particulate filters (SCRF) has been widely adopted for light/heavy-duty vehicles due to its compactness and faster warm-up performance. However, the performances of the SCRF system are negatively affected by the unwanted growth of deposits formed from the injected urea water solution (UWS), especially under low load engine conditions. This paper has developed a full 3D Computational Fluid Dynamics (CFD) model for a commercial close-coupled SCRF system based on the fluid-solid coupling method. The CFD model was successfully validated by comparing it with experimental results conducted under three diesel engine operating conditions. Our simulation results show that the area and thickness of wall film accumulated in the system are higher under a low exhaust mass flow rate. Under high wall temperature, the deposition rate of UWS is lower than the case under lower wall temperature. Nevertheless, the temperature of wall film is increasingly higher under high wall temperature which leads to the formation of more thermally stable compounds such as CYA and ammelide. Under low wall temperature, urea solid is found to be the main composition generated from the wall film comparing with other chemical compounds. The simulation results could be useful in providing some guidance for the design of SCRF system in terms of deposit reduction.