Acoustic Performance Research Of Expansion Chamber Muffler Based On Pipeline Acoustic Mode

The high-order acoustic modes have great influence on the low-frequency silencing of large-section mufflers.Although the relevant researchers have done a series of in-depth studies on the acoustic modes,few researches have been done on the low-frequency silencing based on the pipeline acoustic mode theory.In view of the fact that low frequency noise is the peak noise of coal mine wind shaft noise,which is the most serious and difficult to eliminate,a large section resistance muffler composed of several silencing units is designed in this paper.And take one of the silencing units as the research object.Based on the theory of pipe acoustic mode,the adverse effect of high-order acoustic mode on the lowfrequency silencing of expansion chamber muffler is studied.Firstly,according to the muffler model,the conditions for higher-order modal waves to be excited are derived based on the acoustic mode theory of pipes.By using this condition formula,the excitation frequencies of the acoustic modes in the inlet and outlet pipes of the muffler and the expansion chamber are calculated,and the position of the nodel lines corresponding to the sound pressure of the various modes are calculated.It is found that the high-order acoustic mode has a serious effect on the low-frequency muffler expansion chamber.After acoustic simulation of the muffler and the control model,the results are in agreement with the previous theoretical results,so the correctness of the acoustic mode theory of the pipe is proved.Then,using the cut-off frequency formula of plane wave in rectangular pipe,it is found that cutting and expanding chamber body can effectively improve the excitation frequency of first-order modal wave.It is also proved by acoustic simulation that the first-order acoustic mode excitation frequency of muffler is increased from 578 Hz to 756 Hz by parallel structure.Because 756 Hz is still in low frequency band,the structure of muffler is improved again by using the pitch line position of modal sound pressure calculated earlier.The improved muffler deflected the outlet tube to the nodel line position of(0,2)order acoustic mode,so that the first order acoustic mode excitation frequency of the muffler was raised from 756 Hz to 1165 Hz,and the low frequency silencing is completely removed from the adverse effects of high-order acoustic mode.After the acoustic simulation of the muffler,it is found that the noise reduction near 750 Hz frenquency of the single stage expansion champer muffler is about 30 dB,and the noise reduction near 1000 Hz frenquency can also keep about 25 dB,which greatly improves the acoustic performance of the expansion champer muffler.Finally,the aerodynamic analysis of the bias muffler is carried out.It is found that due to the different axes of the inlet and outlet tube,the pressure loss of the muffler may be greater.The flow field simulation of the muffler shows that the pressure loss is almost doubled.Therefore,the structure of the muffler is optimized again,and the perforated plate is used instead of the solid plate to split the expansion chamber,which increases the air flow efficiency.The simulation results show that the pressure loss of the muffler is reduced by about half after the optimization of the structure,so the optimization of the structure is successful.The acoustic transmission loss of the final structure of the muffler is checked by means of the transmission admittance attribute.It is found that the acoustic transmission loss is basically consistent with that the structure before the optimization.Through the verification of the acoustic mode theory of muffler and a series of structural improvements,the expansion champer muffler has achieved good acoustics performance and good aerodynamic performance.The research on acoustic performance and acoustic mode of expansion champer muffler has some guidance and reference significance to the engineering noise reduction in the future.