| With the development of agricultural mechanization,the noise pollution caused by the exhaust of tractor diesel engine is increasingly prominent.The exhaust noise of diesel engine is too high,which caused serious impact to the surrounding environment and the physiological health of operators.How to effectively reduce the exhaust noise of tractor diesel engine has been paid more and more attention.Therefore,the research of tractor diesel engine noise reduction is of great significance to the development of human society.The LMS Virtual.Lab acoustic simulation software and Computational Fluid Dynamics simulation software ANSYS Fluent were used in this thesis.The internal flow field and sound field of tractor exhaust pipe and muffler were studied in detail,the aerodynamic performance and noise reduction characteristics of muffler were analyzed,and the vibration characteristics of muffler shell were studied.The corrugated perforated pipe,corrugated lining structure and bionic exhaust tailpipe were designed to reduce noise of exhaust muffler.The noise reduction effect of three kinds of muffler structure units was simulated analyzed.The main contents are as follows:(1)Based on the improved design of the straight-through perforated pipe resistance muffler,a corrugated perforated pipe resistance muffler was proposed.The acoustic attenuation performance of the corrugated perforated pipe muffler under the action of nonuniform flow and temperature gradient was analyzed by using the one-way flowacoustic coupling method,which combined computational fluid dynamics and the acoustic finite element method.The influence of the structural parameters of the corrugated perforated pipe mufflers on the transmission loss was analyzed.The significance level of the perforation diameter,peak height,distance between adjacent peaks and peak width on the transmission loss of the corrugated perforated pipe muffler was studied by multiple linear regression analysis.In the calculated frequency range,compared with the straight perforated pipe resistant muffler the transmission loss of the corrugated perforated pipe resistant muffler was improved.The maximum increase was33.02 d B,the minimum increase was 0.53 d B,and the average increase was 8.50 d B.As the temperature increased,the transmission loss curve shifted to a higher frequency and the transmission loss decreased.With the increased of airflow Mach number,the transmission loss curve shifted slightly to the low frequency direction,and the transmission loss increased at most frequencies.The peak height and distance between adjacent peaks were the main factors affecting acoustic attenuation performance.(2)The noise characteristics of the straight-through perforated pipe resistance muffler with perforated corrugated lining was analyzed considering the airflow velocity and temperature inside the muffler.The influence of temperature and velocity on the noise reduction performance of muffler was studied by using numerical simulation method,which took the flow field solution results inside muffler as boundary conditions of sound field analysis.The influence of changing the internal structure of muffler on the aerodynamic performance of muffler was discussed and the pressure loss was analyzed.The research showed that adding perforated corrugated lining inside muffler could effectively improve the transmission loss of muffler and the noise reduction performance of prototype straight-through perforated pipe resistant muffler.(3)The straight-through perforated pipe resistant muffler was improved to designe a straight-through perforated pipe resistant muffler with a corrugated lining.Structural modal analysis,acoustic modal analysis and transmission loss analysis of the two mufflers were carried out.The coupling of the acoustic cavity and structure was considered,and the radiation noise of two kinds of resistant mufflers was comparatively analyzed by the direct-boundary-element method.The acoustic radiation of two kinds of resistant mufflers was studied.The pressure loss of the two kinds of mufflers was studied by aerodynamic analysis.From the sound pressure level amplitude radiation noise cloud diagrams and the sound power level curves,it can be seen that the corrugated lining can effectively reduce the sound pressure amplitude and the muffler radiation noise.The total sound power level was reduced by 8.23 d B in the range of 850 Hz-1850 Hz,and 12.83 d B in the range of1870 Hz-3570 Hz.By CFD analysis,the pressure loss value of the straight perforated pipe resistance muffler with corrugated lining was slightly increased compared with that of the straight-through perforated pipe resistance muffle.(4)The exhaust tailpipe of the Dongfanghong wheeled tractor was improved,and the bionic triangular convex texture was added to the inner wall of the exhaust tailpipe.The simulation softwares were used to analyse the aerodynamic noise and transmission loss of the exhaust tailpipe.The exhaust tailpipes with bionic triangular convex textures of different numbers of circumferential columns,texture height and top angles were analyzed to study the noise reduction performance.In order to obtain the maximum transmission loss,a genetic algorithms(GA)optimized back-propagation neural network(BP)was used to optimize the bionic triangular convex texture parameters.The noise reduction mechanism of the bionic exhaust tailpipe was analyzed and verified by test.The results showed that when the texture top angle is 61° and the texture height is0.95 mm,the corresponding transmission loss reached the maximum.In the calculate frequency range,the transmission loss of the optimized bionic exhaust tailpipe was increased by 9.62 d B at the maximum,1.01 d B at the minimum,and 2.99 d B on average compared with the prototype exhaust tailpipe.By comparing the simulation results of the bionic exhaust tailpipe with the optimized structural parameters and the test results,the results are basically consistent.(5)Taking the Dongfanghong wheeled tractor muffler exhaust pipe as the research object,the bionic triangular convex texture was added to the inner wall of exhaust tailpipe,corrugated perforated pipe structure was used and the perforated corrugated lining was added in the expansion cavity.The noise reduction effect of the three parts of the structure were studied.The results showed that the modified tractor muffler exhaust pipe could increase the muffler transmission loss under the boundary conditions of mean flow,nonmean flow and temperature.Considering both temperature and velocity,compared with the prototype tractor muffler exhaust pipe,the transmission loss of the modified tractor muffler exhaust pipe was improved in the calculated frequency range,with the maximum increased of 69.20 d B,the minimum increased of 0.19 d B,and the average increased of5.33 d B.The displacement amplitude of the tractor muffler exhaust pipe shell vibration caused by sound waves and the sound radiation of the tractor muffler exhaust pipe shell can be reduced by adding bionic triangular convex texture,corrugated perforated pipe and perforated corrugated lining inside the tractor muffler exhaust pipe.The total sound power level was reduced by 5.36 d B in the range of 790 Hz-1510 Hz and reduced by15.41 d B in the range of 1310 Hz-1510 Hz.The modified tractor muffler exhaust pipe can effectively reduce the radiated noise of the muffler shell. |
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