Effect Of Airflow On Noise-Reduction Performance And Structural Property Of The Split-Stream-Rushing Muffler

The pollution problem of diesel engine noise has always been a hot issue of concern.In the whole machine noise,the exhaust noise accounts for the largest proportion.Therefore,reducing the exhaust noise is of great significance in reducing the noise of the diesel engine.At present,the installation of exhaust muffler is the most direct and effective way to control the exhaust noise of internal combustion engines.In the past,the noise of muffler pipeline was mostly considered in the design of muffler,but the influence of wall vibration on the performance of muffler was not considered.Because the exhaust muffler is a thin-walled structure,when its inner chamber is stimulated by engine and airflow,it is easy to cause structural vibration and change its internal sound wave propagation law,which will affect the acoustic performance of the muffler.At the same time,the structure will also be affected by the sound field,which in turn will affect the vibration of the muffler housing structure as an acoustic load.In this thesis,split-stream-rushing exhaust muffler was studied.Firstly,the acoustic finite element module of the acoustic software Virtual.Lab was used to simulate the transmission loss of the exhaust muffler under different inlet velocity conditions.The internal sound pressure cloud diagram of the muffler,the sound pressure response curves at the inlet and outlet ends,as well as the transmission loss curve were analyzed.The results showed that with the inlet flow velocity increased,the transmission loss increased in the low frequency range of 20?420 Hz,and the transmission loss curve of the muffler moved toward the low frequency in the medium and high frequency band.Thus it can be seen that the high-speed airflow is an important reason of the change in the muffling performance of the exhaust muffler.Secondly,the insertion loss test of the muffler was carried out on a self-made muffler test bench.The results showed that in the low frequency range of 20?420 Hz,the insertion loss increased with the increase of the flow velocity.However,when the flow velocity increased to above 36 m/s,the average insertion loss of the split-stream-rushing muffler decreased sharply.The results were different from those obtained by numerical calculation,which was mainly due to that the regenerative noise generated by the internal airflow impact to structure components was not considered in simulation.Finally,the structural mode,acoustic cavity mode and coupled mode of the exhaust muffler were simulated,and the structural mode was tested and verified.Based on the structural mode,the acoustic cavity mode and the external excitation conditions,the modal superposition method was used to study the acoustic-vibration coupling characteristics.The results indicated that the first-order structural modal frequency(69.6 Hz)of the exhaust muffler was close to the excitation frequency of the engine(74 Hz),which was prone to the resonance.By adjusting the wall thickness of the muffler,it made the natural frequency avoid the excitation frequency of the engine,which could effectively reduce the radiated noise in the low frequency band near the structural mode and excitation frequency.