Study On Acoustic Characteristics And Flow Characteristics Of SCR Catalytic Muffler

The motor vehicle emission standards of fifth phase were gradually implemented in China in 2016.The outside purification has become an integral part of the diesel engine.Selective Catalytic Reduction(SCR)technology is considered the most promising diesel engine post-processing technology.In order to achieve a dual effect of exhaust gas purification and exhaust noise reduction,SCR catalytic muffler combines the functions of SCR catalytic converter and muffler.The layout space is also saved.A SCR catalytic muffler is taken as research object in this paper.The effects of internal structure on acoustic and flow performance are studied.In order to meet the emission requirements of the fifth stage,the mixer which used for improving the mixing and evaporation performance is optimized.The paper's major tasks are as bellows:1.The acoustic performance of catalytic muffler is studied based on acoustic finite element method.The acoustic finite element model of catalytic muffler is created and the simulation is conducted with the acoustic module of Virtual.Lab.The influence of mixer and carrier on the transmission loss is studied to provide guidance for the acoustic design of catalytic muffler.2.Hybrid modeling of catalytic muffler based on experimental model.In order to improve the accuracy of flow field simulation,the mathematical model of nozzle and carrier are established in FLUENT with hybrid modeling method.The carrier is simplified as porous medium and the mathematical model of the carrier is created with the inertial resistance and viscous resistance coefficients from the cold flow back pressure test.The atomization characteristic of nozzle is obtained with laser particle size analyzer.The atomization model of nozzle is created according to the distribution index and the distribution diameter of Urea droplet.3.The flow field performance of catalytic muffler is studied based on the coupling calculation of continuous phase and discrete phase.The finite element model of catalytic muffler is created in ICEM and import the model into FLUENT.The influences of the mixer and carrier on pressure loss,velocity uniformity and ammonia vapor uniformity are analyzed through the single flow field calculation and the discrete phase coupling calculation.4.Mixer design for improving the evaporation performance of urea solution.The structure of the mixer affects the mixing,evaporation and crystallization of urea droplets,which is closely related to the purification performance of the catalytic muffler.In order to meets the emission requirements of the fifth phase,a new mixer structure which combined with hemispherical surface and shutter structure is proposed to improve the evaporation rate of Urea droplet.5.The acoustic and flow field performance of catalytic muffler are analyzed through simulation before and after improvement.The result shows that the pressure loss of improved catalytic muffler is slightly improved.The acoustic performance,velocity uniformity,ammonia vapor uniformity and urea droplet evaporation performance are significantly better than the original structure.6.Test verification.The noise test,emission test and crystallization test are conducted on the engine test bench.Results show that the insertion loss increased by 11.9%and the back pressure increased from 8kPa to 9.5kPa under the rated condition.In the ESC and ETC experiments,the NOx emission decreased by 20.6%and 16.6%than before and meets the emission requirements of the fifth phase.At the 20000 airspeed,the single point conversion rate of NOx is significantly improved.The average conversion of single point increased by 7.3%at low temperature condition below 265?The crystallization performance of the improved catalytic muffler meets the requirements.