Study On Spray Characteristics Of UWS And Influence Factors Of NO_x Conversion Efficiency In SCR System

With the rapid growth of car ownership and the increasingly stringent emission regulations,how to effectively control the NOx and PM emissions of diesel engines has become a key technical problem to be solved urgently.The Selected Catalytic Reduction?SCR?technology is the mainstream aftertreatment technology to effectively reduce the NO_X emission,but the atomization,evaporation,thermolysis and the effective mixing of the urea water solution?UWS?in the SCR system,as well as the effective improvement of the NOx conversion efficiency of the SCR system,especially the conversion efficiency of the low temperature operating conditions are still to be further studied.In this paper,based on the various characterization tests,the main components of urea crystallization,the thermolysis process of UWS evaporation crystallization and urea crystallization and the volatile components in urea crystallization thermolysis process were analyzed.Then,the urea spray atomization model was established.Based on the feasibility of the model,the effects of the nozzle injection pressure,nozzle injection angle?,spray cone angle?,nozzle offset angle?and diameter-to-length ratio of exhaust pipe on the spray characteristics of UWS were studied.Finally,the three-dimensional model of Urea-SCR system was established,and the influences of different system parameters on the UWS decomposition and NO_X conveision efficiency were analyzed,which provided the theoretical guidance for Urea-SCR system parameters.The main research contents were as follows:?1?Based on Fourier transform infrared spectroscopy?FTIR??X-ray diffraction?XRD?and Scanning Electron Microscope?SEM?and Thermogravimetric Analysis?TGA?,the main components of urea crystallization,the thermolysis process of UWS evaporation crystallization and urea crystallization and the volatile components in urea crystallization thermolysis process were analyzed.The characterization results showed that the decomposition process of UWS evaporation crystallization and urea crystallization was related to the exhaust temperature,but the maximum temperature stage of mass loss was different;The main components of urea crystallization are urea and cyanuric acid;During the process of urea crystallization heating,with the increase of temperature,ammonia,Gantrez AN-8194 and Mephenytoin gradually escaped.?2?Based on the urea water solution?UWS?spray atomization model that was established for SCR aftertreatment system in diesel engine,a series of physical processes of UWS droplets,such as turbulence disturbance,distortion,breakup,collision-merging and wall-impingement bounce in exhaust pipe were considered comprehensively.The effects of the nozzle injection pressure,nozzle injection angle?,spray cone angle?,nozzle offset angle?and diameter-to-length ratio of exhaust pipe on the decomposition of UWS droplets were analyzed.The results showed that when the injection pressure was low,the UWS spray tended to be a column of liquid or large droplets,the ability to spray atomization of UWS was poor,and the NH₃ average volume fraction increased linearly as the injection pressure increased;?had a significant effect on UWS decomposition,and the NH₃ average volume fraction increased as?increased;?had a significant effect on the NH₃ uniformity coefficient,and the distribution of NH₃ was equally distributed as?increased;The average volume fraction of NH₃ increased as?increased,but its uniformity coefficient decreased sharply;The average volume fraction of NH₃decreased as the exhaust pipe diameter increased,and then gradually became gentle.?3?Based on the bench test of SCR system,a Urea-SCR system model was established,and the influence of NH₃/NO,temperature,exhaust flow and location of mixer on the UWS decomposition and NOx conversion efficiency were analyzed.The results showed that the increase of NH₃/NO was beneficial to improve the NO conversion efficiency,wall film mass and NH₃ volume fraction of outlet increased as the increase of NH₃/NO;With the increase of temperature,urea crystallization was fully evaporated and decomposed by heat and NO conversion efficiency increased first and then decreased.Besides,wall film mass decreased continuously;With the increased of exhaust flow,the time that NH₃ mixed with exhaust gas was reduced,the NO conversion efficiency was linearly decreased,wall film mass and NH₃ volume fraction of outlet were increased;With the increase of the distance between the mixer and the nozzle,the NO conversion efficiency and NH₃ volume fraction of outlet first increased and then decreased,reaching the maximum when the distance was 238 mm.Besides,wall film mass decreased sharply.