In this work, numerical simulation was carried out to study the injection and mixing of ammonia solution spray in the exhaust pipe of diesel engines. The ammonia solution was injected into the hot gas of 623 K. The spray was vaporized into gas and mixed with the hot exhaust gas and together the mixture was transported in the exhaust gas pipe downstream where a selective catalytic reduction (SCR) system was implemented to convert NOx to H2O and N2. The effects of exhaust pipe geometry and gravity on the droplet evolution and vapor mass fraction were investigated. The results show that the influence of gravity is negligible, while the exhaust pipe geometry has a great impact on the vapor distribution. In straight pipe case the NOx reduction is only achievable in a small area of the catalyzer, which is in good agreement with the experiments. Analysis of the velocity field and streamlines shows that the ammonia vapor is blocked by a recirculation zone; as a result, the vapor mass fraction will be centralized into a small zone because radial component of vapor velocity is too slow to penetrate into the center of main flow. In addition, the spray droplet size distribution indicates that a certain amount of ammonia enters into catalyzer as liquid phase with medium diameter around 19 and 17 m in straight and bending pipe, which further deteriorates the catalytic efficiency. The results provide a guidance for the design of ammonia injector and the exhaust pipe geometry.
Keywords exhaust gas after treatment, selective catalytic reduction, ammonia