Munjal, M. and Kumar, V., "Design and Analysis of a Plug-Muffler Modified for Low Back-Pressure and Improved Acoustic Performance," SAE Int. J. Veh. Dyn., Stab., and NVH 1(1):44-57, 2017, doi:10.4271/2017-26-0190.
High insertion loss is desirable and can be achieved by using plug-muffler elements which consist of two cross-flow perforated sections. However, the plug-mufflers have an inherent disadvantage of high back-pressure which may affect the engine performance adversely. In this paper, a novel structural modifications has been introduced to the plug-muffler to obtain better acoustic performance as well as low back-pressure. Three configurations have been analyzed here including the classical plug-muffler configuration. Back-pressure has been calculated using the lumped flow-resistance network theory for all three configurations and compared. To evaluate the transmission loss, the 1-D (plane wave) analysis has been carried out using the Integrated Transfer Matrix (ITM) method and the results so obtained are validated against 3-D FEM using a commercial software. For the 1-D ITM method as well as the 3-D FEM, an existing perforate impedance model has been used for evaluating and comparing the acoustic performance of the three configurations. Incidentally, however, a simplified expression for the acoustic impedance of the perforate has been developed, based primarily on the existing models for the acoustic perforate impedance. The proposed perforate impedance expression is shown to predict the acoustic performance with reasonable accuracy as compared to the existing complicated models. Thus it provides a simple expression for perforate impedance which includes the bias flow and grazing flow effects as well as the viscous dissipation and inertance effects. Finally, the modified plug-muffler configuration (configuration 3) proposed here is shown to have superior acoustic performance, particularly at low frequencies where it is crucial, and lower back-pressure when compared to the other two configurations studied here. Physical explanation of the improvement in acoustic performance has been provided in terms of phase cancellation that is typical of the multiply-connected elements.