Theoretical Analysis And Experimental Study Of The Split-Stream-Rushing Noise Reduction Structure

Because of its better power and economy,diesel engine has become the main power device for transportation,construction machinery,agricultural machinery and other equipments.However,the disadvantage of diesel engine is that the noise is large,and the main noise source is the exhaust noise.The most direct and effective way in reducing the diesel exhaust noise is to install an exhaust muffler.Because the exhaust gas flow in diesel engine is with high temperature and strong corrosion,the reactive muffler is normally used as the exhaust muffler.One of the most critical factors affecting the performance of muffler is airflow velocity in the muffler.If the exhaust gas velocity in the diesel engine is high,the regenerative noise caused by the high-speed airflow in the muffler will be large,which results the noise reduction performance worse and the back pressure will be increased.Based on this,our research team has proposed a new principle of split-stream-rushing from the perspective of reducing the airflow velocity internal the muffler.In this dissertation,the methods of both theoretical analysis and numerical simulations were used to study the acoustic performance and aerodynamic performance of the split-stream-rushing noise reduction structure.(1)The classical method of plane wave theory — transfer matrix method was used to deduce the integral transfer matrix of the split-stream-rushing noise reduction structure,revealed its acoustic performance and provided a theoretical basis for subsequent research.At the same time,the Matlab software was used to calculate the transmission loss of the given structure,and it was concluded that the split-stream-rushing noise reduction structure has a good noise reduction effect in the medium and high frequency ranges.(2)The acoustic properties of the basic acoustic units in the structure,the split unit as well as the rushing unit,were analyzed in detail.In particular,the ransfer matrix for the conical-ring split unit,as a new type of acoustics sub-structure,has been derived and the acoustic impedance was calculated in this dissertation,and thus its basic acoustic properties has been obtained.In addition,the transfer matrix of the rushing structure was derived by using the transfer matrix of the basic acoustic unit.Based on the transfer matrix of these basic acoustic units,the influence of structural parameters of muffler unit on transmission loss was analyzed.(3)In fluid dynamics software Fluent,the internal flow field of the split-stream-rushing structure and the velocity change of the airflow after rushing were analyzed.The simulation results show that the airflow velocity in the region of the rushing center is about 40% of that in the rushing hole exit,and the vortex intensity and vortex area were less than the traditional perforation expansion type muffler,proved that the split-stream-rushing mechanism can effectively reduce the exhaust airflow velocity in muffler.(4)Pressure loss of the split-stream-rushing muffler was studied by the way of the simulation and test,and the comparative analysis was performed with a traditional muffler with the same noise reduction volume.In the case of 6 different inlet airflow velocities,the pressure loss of the split-stream-rushing muffler is all about 30% of the traditional muffler.It can be seen that the split-stream-rushing structure can reduce the exhaust backpressure of the engine effectively while reducing the exhaust noise.(5)The insertion loss for both the split-stream-rushing muffler sample and the traditional muffler were measured in a diesel engine,the test results show that the acoustic performance of the two mufflers is similar in the low frequency range,but in the medium and high frequency ranges the insertion loss of the split-stream-rushing muffler is better than the traditional muffler,proved that the split-stream-rushing mechanism can effectively improve the comprehensive performance of the muffler in the case of the same muffle volume.The research conclusions achieved in this dissertation has laid a theoretical foundation for studying the design theory and design method of this new type of muffler.