Abstract
Selective catalytic reduction (SCR), based on the injection of urea-water-solution (UWS), is one of the prevailing and more effective approaches to reduce NOx emissions in diesel engines. To improve the performance and durability of the system, it is crucial to develop reliable simulation tools able to correctly describe not only the urea conversion into ammonia and the mixing with exhaust gases but also the possible formation of solid deposits along with the engine’s exhaust line.In the present paper, two different exhaust systems for off-road applications are analyzed, both of them consisting of a diesel oxidation catalyst (DOC) followed by a diesel particulate filter (DPF), a UWS injection and a mixing device, and an SCR catalyst. Two alternative UWS mixing subsystems are evaluated, including a newly developed design. A 3D-CFD numerical analysis is carried out to assess the performance of both systems in terms of pressure drop, UWS spray, and liquid film development, in addition to flow velocities and species concentration uniformities at SCR catalyst inlet. A detailed analysis of droplet impingement on walls and liquid film development is enabled, thanks to a conjugate heat transfer (CHT) approach. Moreover, a deposit risk index is used to identify the areas of the systems where urea deposit formation is expected.Eventually, numerical results are compared with experiments on one operating condition chosen as the most challenging in terms of exhaust temperature and flow rate, both in terms of systems NOx conversion efficiency and deposit formation, showing a satisfactory agreement, thus paving the way to use the proposed synergetic numerical and experimental approach to further optimize the design and the system’s performance.
Highlights
European standards for off-road diesel engines have become gradually more stringent in terms of both particulate matter (PM) and NOx emissions
Images acquired within the system at the end of experimental tests are compared to simulation results representing the risk of deposit formation, to investigate the predictivity and the limitations of the numerical methodology
As far as deposits are concerned, 3D-Computational fluid dynamics (CFD) simulations performed in this study provide information concerning the risk of deposit formation rather than the exact location of deposit nuclei and their growth rate or disappearance over time
Summary
European standards for off-road diesel engines have become gradually more stringent in terms of both particulate matter (PM) and NOx emissions. Widespread adoption of NOx after-treatment systems (ATS) has been triggered, and urea-selective catalytic reduction (SCR) has become the mainstream approach for N Ox mitigation [1]. In this type of system, urea-water-solution (UWS) is injected into the exhaust gases, undergoes the atomization and evaporation processes, and decomposes to a mixture of ammonia, iso-cyanic acid, and other species. The mixture reacts with the exhaust flow in the SCR catalyst to abate NOx emissions. The presence of deposits results in an inefficient use of urea, with a consequent low catalyst conversion efficiency and corrosion of stainless
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