Performance Study And Improvement Of Reactive Mufflers Based On The Acoustic Mode

Both the acoustic modes and inlet/outlet locations have great effects on the acoustic attenuation performance of muffler. But so far, studies about the effects of inlet/outlet locations on the pressure loss of muffler are few. In addition, previous studies are mostly based on the assumption that the medium inside the muffler is static ideal gas, the effects of airflow velocity and flow-induced noise on the acoustic attenuation performance of muffler are not considered. In order to explore the questions above, taken expansion chamber muffler and reversing chamber muffler two kinds of typical mufflers as illustrations, firstly, the effects of inlet/outlet locations on the pressure loss were discussed based on finite element method; then, the effects of airflow velocity on the acoustic attenuation performance were explored as well. On this basis, further study about the effects of inlet/outlet locations, airflow velocities and acoustic source types on the flow-induced noise was made based on boundary element method.First of all, the validity of finite element model and boundary conditions was verified, on this basis, the effects of inlet/outlet locations on the pressure loss of muffler were discussed, and the layout principles of inlet/outlet pipes were proposed. Results showed that the acoustic attenuation performance of reactive muffler can be improved significantly in a medium-high frequency region when the inlet/outlet pipes are located at the nodal point regions of higher order modes, but since the inlet/outlet pipes are not coaxial, the pressure loss increased obviously. By adding the transition arc at the entrance of outlet pipe, the pressure loss can be reduced effectively and the acoustic attenuation performance is not affected; moreover, this conclusion is a universal law and will not be changed with the change of type and size of the muffler. Secondly, the differences between transfer matrix method and three point method were compared based on the results of finite element method, for the no-flow, uniform flow and non-uniform flow three different cases. On this basis, comparative study about the effects of uniform flow and non-uniform flow on the acoustic attenuation performance of muffler was made. The results showed that both the transfer matrix method and three point method can accurately calculate the transmission loss of muffler under no-flow and uniform flow conditions. But, for the non-uniform flow case, both the results calculated by the two methods above have deviations on account of the complexity of flow field and the limitations of calculation methods. The difference of the effects of uniform flow and non-uniform flow on the acoustic attenuation performances is little when the flow-induced noise is ignored. Furthermore, both the acoustic attenuation performances of simple reactive muffler and complex car muffler are affected slightly by the airflow in the case of M<0.3. Therefore, the effect of airflow on the acoustic attenuation performance can be ignored during the initial phase of muffler design. Finally, a further study about the effects of inlet/outlet locations, airflow velocities and acoustic source types on the flow-induced noise of reactive muffler was made. Results showed that the flow-induced noise significantly increased in the whole analysis frequency range for the mufflers with offset outlet. But, once the offset distance reaches up to the pipe diameter of inlet and outlet pipes, the flow-induced noise would not be affected by the increase of offset distance. For the reactive muffler, quadrupole source caused by the turbulent velocity pulsation was the main acoustic source of flow-induced noise, and its pressure contribution is very big in the low frequency region.