The Research On Simulation And Optimization Of Injection And Mixing Of Diesel Engine Compact Aftertreatment System

The pollutants in the exhaust of the diesel engine will cause damage to the environment and human body.With the increasingly strict emission standards for diesel engines,the performance requirements of the aftertreatment system are getting higher and higher.In this thesis,the working performance of a compact aftertreatment system integrating DOC+DPF+SCR is simulated based on computational fluid dynamics.The movement of the airflow,the atomization of the urea solution and the working process of the catalyst in the aftertreatment system are analyzed,and a three-dimensional numerical model is built with appropriate simulation models and boundary conditions.The model is verified by the experimental data,which can be used for subsequent simulation research.The changes of urea decomposition efficiency and NO_X conversion efficiency in SCR catalyst with the rise of diesel exhaust temperature are studied.Three important indicators for evaluating the performance of the aftertreatment system are proposed.The effects of the installation position of urea nozzle,the number of orifices,the injection direction and the orifice diameter on the atomization of urea injection are studied by simulation,including the distribution and thickness of the wall film,the uniformity of the NH₃ distribution at the inlet of the SCR catalyst,the urea decomposition efficiency and NO_Xconversion efficiency,and the optimization plans are proposed.5 injection positions in the direction of air flow and 2 injection positions in the direction of reverse air flow is selected to be the installation position of urea nozzle.The results show that increasing the distance between the urea nozzle and the SCR inlet or injecting in the direction of reverse air flow can effectively improve the NH₃ uniformity.Position c and f can be selected as the installation position of urea nozzle for their better performance.The spray atomization with the injection of single hole,four holes,six holes and eight holes is studied.The results show that injection with six holes works best at position c,and the NO_X conversion efficiency reaches 88.6%.Increasing the number of injection holes at position f is not benefit to the performance of the aftertreatment system.The spray atomization is studied when the angle between the urea injection direction and the air flow is 0°,15°,30°,and 45°.When the direction is 15°,the NH₃ uniformity is the best,reaching 0.973,but the comprehensive performance become best at 0°injection.The spray atomization is studied when the orifice diameter is 0.4mm,0.5mm,0.8mm and 1.0mm.The results show that the highest urea decomposition efficiency and NO_Xconversion efficiency are achieved at the diameter of 0.5mm.The six holes urea nozzle installed at position c with an aperture of 0.5 mm and a direction of 0°is the best injection strategy.Three types of mixers are designed,the mixing per4formance of the grid mixer is not good.However,the blade mixer and the baffle mixer can increase the vortex and swirl in the flow field,they have a small pressure drop,as well as a higher NO_X conversion efficiency,reaching 89.6%and 89.1%respectively.The structure and installation position of the blade mixer and the baffle mixer are further simulated.The results show that the 45°blade mixer installed in position 2 and the 30°blade mixer installed in position 3 can improve NH₃uniformity,which are more suitable when the normalized stoichiometric ratio of NH₃ to NO_Xis bigger than 1.The baffle mixer has the best performance when installed at the initial position with 30°baffles.