Because of high thermal efficiency and low CO2(Carbon dioxide) emission, diesel engines are being used widely in many industrial fields although it emits considerable PM(Particulate matter) and NOx(Nitrogen oxide) which give both human health and environment a negative effect. However, simultaneous reduction of these emissions is quite difficult due to their trade-off characteristics.
For this reason, engines are getting more advanced and superior with the use of clean diesel technologies such as CRDI(Common Rail Direct Injection), VGT(Variable Geometry Turbocharger), and EGR(Exhaust Gas Recirculation). NOx regulations for diesel engines, however, are being strengthened and it can not be satisfied with pre-treatment engine technology only. Therefore, the after-treatment systems reducing NOx such as SCR(Selective catalytic reduction), LNC(Lean NOx catalyst), and LNT(Lean NOx trap) are considered as good solutions to overcome the limit of pre-treatment technology. Among the NOx reduction devices, Urea-SCR system is known as the most stable and efficient method to resolve the problem of NOx emission. But this device has some issues associated with the ammonia slip phenomenon which is occurred by shortage of evaporation and thermolysis time, and that makes it difficult to achieve uniform distribution of the injected urea in front of monolith.
This study, therefore, has focused on the mixing enhancement of urea water solution and exhaust gases using AVL FireTM to enhance the efficiency of the SCR catalyst equipped in catalytic muffler by changing installed injector angle and application conditions of mixer such as installation angle, number of mixer, and aspect ratio. Before conducting numerical simulation, pressure drop on the shape of the exhaust pipe and Arrhenius parameters on the thermal decomposition of urea water solution are validated by comparing the solutions with available data in the literature.
The results show that the increase of the mixer length results in the decrease of NH3 concentration because of wall-wetting of urea water solution on the mixer wall and the increase of NH3 uniformity index. Also, in case of Mixer 40(AR=1.0), horizontal and vertical elements indicate NH3 concentration of 231ppm and 240ppm and NH3 uniformity index of 0.96 and 0.94, respectively. Since the Mixer 40(AR=1.0) has higher pressure drop and stronger turbulent characteristic than Mixer 60(AR=1.5) and Mixer 80(AR=2.0) considered in this work. Finally, it is noticed that this study can be used to suggest the useful guidelines to achieve the optimum design of a Urea-SCR injection system for improving De-NOx performance and reducing ammonia slip.