Mixing and evaporation analysis of a high-pressure SCR system using a hybrid LES-RANS approach

A hybrid Large Eddy Simulation (LES) - Reynolds-Averaged Navier-Stokes (RANS) method (HLR) has been applied to simulate an engine related selective catalytic reduction (SCR) system. Typical SCR systems utilize low pressure urea injection together with a mixer for vapor field homogenization. Simultaneously, it is also desirable to reduce spray-wall interaction to avoid urea crystallization. The present study considers an SCR system where a high pressure (150 bar) urea spray is injected towards hot exhaust gases in exhaust pipe. The system has been shown to work well in a previous experimental study but detailed characterization of the system is missing. The novelty of the present study rises from: 1) the creation of phase diagrams with single droplet simulations that predict the optimum operation regions for the SCR system, 2) validation of the HLR method in a high Reynolds number (Re=49,900) compressible pipe flow, 3) the use of HLR simulations in an engine SCR system for the first time, and 4) the detailed characterization of the present SCR system. As a result of the study, new non-dimensional timescale ratios are proposed to link the droplet size and liquid injection velocity to the exhaust pipe dimensions in future SCR systems.