| In 2023,the Limits and measurement methods for emissions?CHINA?b?will be fully implemented.Requirements of NOx emissions in the regulations are very strict.Currently,SCR?selective catalytic reduction?technology with urea as a reducing agent is mainstream solution to meet NOx emission requirements.It is an urgent problem to design excellent SCR system to meet emission requirements for all kinds of diesel engines.In this paper,the key issues studied about SCR system for diesel engine include spray evaporation and decomposition,mixing of NH3 and exhaust gas,pipeline crystallization and control.Through the analysis of urea transamination and the mechanism of wall film crystallization,the design process of SCR system is established following optimization of injection parameters,mixer design,anti-crystallization structure design,and optimization the structural parameters.The main content and research results are as follows:?1?Analyze the atomization mechanism of urea injection.According to the droplet energy and quality control equation,the accuracy of the model is improved by 12.7%by introducing temperature-based optimization parameters.The initial simulation characteristics of the flow field are obtained by using CVM method to establish the 3D model.The flow field characteristics of the system under two working conditions are compared to guide the design of the injection parameters.The influencing factors of the spray atomization distribution are studied.The premise parameters such as the injection position,the injection mode,the orifice distribution and the angle of the spray are determined before 12 groups of different spray directions and orifices are compared with the distribution and uniformity of NH3.The result indicates that the jetting with 6-hole and direction of 15°shows optimal effect.?2?Analyze the differential equation of urea decomposition mass,optimize the activation energy and kinetic parameters of the model according with the relationship between temperature and reaction rate.Summarize the purpose and principle of mixer design,and propose the uniformity evaluation index based on area and variance.Summarize the design feasibility standard of mixer.10 common mixer forms are generalized with advantages and disadvantages.Three kinds of adjustable parameter mixer structures are designed.The simulation results and causes of the three sets of mixers under optimized injection parameters are compared.The selection and application of the mixer are analyzed.?3?Analyze the wall collision-control model and the liquid film formation mechanism.The combination of Kuhnke and Birkhold models is used to refine the simulation process.The wall-breaking results are divided into six forms using the dimensionless parametersT'and K.It is concluded that the crystallization can be effectively prevented by controlling the hitting type of wall and droplet.Analyze the crystallization process of the wall film and the composition of sediment.The chemical reactions of the main components under various temperature conditions are expounded,and finding that the temperature is the key effect of the conversion between the reactants.The sedimentation has a critical point and the critical section of urea is used to guide the design of the mixer.Four kinds of adjustable parameters of the anti-crystallization structure are designed,and the flow field characteristics under each scheme are compared.?4?Summarize the design process of SCR system and the general methods of designing three modules.For the long tube of this paper,select the injection parameters,mixer and embedded pipe respectively,and determine the solution of net mixer combined with cylinder pipe.Three sets of control parameters were designed according to the selection structure.The pressure loss,NH3 distribution and NO conversion rate of the three parameters are compared and analyzed.The influence of various parameters on the flow field characteristics of the pipeline was obtained. |
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