Optimization Design And Research Of Diesel Engine Exhaust Mixing Chamber

Diesel engines are widely used in transportation,agricultural,engineering and other fields.In order to control the emission pollution of diesel engines,the relevant laws are increasingly strict.There are some problems in the mixer of diesel exhaust gas treatment system,such as incomplete decomposition of urea,uneven distribution of ammonia and crystallization.These factors will lead to poor purification effect of pollutants.In this paper,the exhaust gas treatment system of 28 R diesel engine is the research object.The performance of tail gas emission was improved by optimizing the mixing structure of catalytic converter and analyzing the characteristics of flow field.The developed hybrid structure enables the exhaust emission of the diesel engine to meet the existing regulations.Specific research contents are as follows:The short-pitch screw mixer is designed to reduce heat transfer losses and promote gas mixing.The design of conical plate mixer is used to improve the uniformity of gas distribution.Two mixers are combined to form a mixing chamber.The flow field characteristics of traditional structures,new structures and combined structures are studied based on CFD.The rationality of the new structure design is verified through the analysis of airflow direction,velocity distribution uniformity,heat transfer loss and back pressure under different working conditions,which lays a foundation for the follow-up research.The decomposition capacity of urea in the tail gas treatment system was evaluated by the distribution of urea droplets and the average concentration of ammonia gas in the front end of the carrier.Furthermore,the distribution uniformity and deviation rate of ammonia gas distribution were studied to analyze the distribution of ammonia gas in tail gas treatment system.Combined with the decomposition capacity of urea and the distribution of ammonia gas,the optimal design scheme of the mixed structure was selected.The optimal scheme was evaluated by the whole working condition NOx purification test and three cycle tests.The crystallization risk of different structures was predicted by the thickness and distribution of liquid film in the simulation results.Based on the results of low temperature crystallization bench test,the correlation between liquid film and crystallization risk was established.In order to make the tail gas treatment system not only provide enough urea to remove NOx but also effectively avoid the formation of crystallization,the critical urea injection rate without crystallization risk was studied.The maximum injection rate of urea without crystallization risk was discussed according to different conditions.In this paper,the flow field characteristics,urea decomposition,ammonia distribution,crystallization risk and other problems of the mixing structure were studied by simulation analysis with experimental verification.The structure of the new mixing chamber has excellent ability of urea decomposition,uniform distribution of ammonia and anti-crystallization at low temperature.The structure can improve the nitrogen oxide purification capacity of the engine and make it meet the existing emission regulations.The research can provide some guidance for the mixing structure design of exhaust treatment system of various diesel engines.