The objective of the project is to redesign a reactive-dissipative muffler for engines to optimize its overall attenuation in a frequency range of 16 to 2000 Hz. Since silencers usually present a maximum attenuation at high frequencies imposed by acoustic considerations, and this maximum can be reached if the disposition and quantity of absorbent material is adequate, the studies carried out in this project will prioritize the improvement of attenuation at low frequencies.
The equipment to be optimized is a reactive-dissipative type silencer, used to attenuate the noise generated by the gases of a combustion engine. It is therefore a silencer designed to work at high temperatures (200-500 ºC) and moderate gas speeds (15-40 m / s). The muffler geometry is cylindrical, which tends to work quite well for these types of applications.
This silencer consists of a reactive part (where geometric discontinuities produce interference of some waves with others) and a dissipative part (in which absorbent material is used to convert acoustic energy into heat energy). Reactive chambers are effective at attenuating low-frequency noise, while dissipative chambers work best at eliminating high-frequency noise.
Specifically, the silencer to be improved has five chambers, of which four are reactive and one dissipative. It should be noted that the entire silencer is made of steel, except for the absorbent material, which is rock wool. The model is parameterized according to its main dimensions.
• Inlet / outlet diameter
• Core diameter
• Baffles internal / external diameter
• Absorbent internal / external diameter
• Inlet / outlet length
• Chamber 1 length
• Chamber 2 length
• Intermediate chamber length
• Chamber 3 length
• Chamber length 4
The optimal modification of all those carried out in the expansion chamber consists in extending the inlet pipe into the chamber itself. This introduces a new shock surface for the sound waves within this chamber. After analysing the results of this modification, it was found that it offered higher attenuation values. Specifically, with the optimal extension length, the low frequency attenuation was improved, increasing the maximum attenuation peak and shifting it slightly towards a lower frequency (from the 1000 to 500 Hz band). In all the octave bands analysed, a similar or better behaviour of the modified model was observed, except for the 1000 Hz band, motivated by the displacement of the maximum attenuation peak. However, this decrease is clearly offset by the corresponding increase at 500 Hz.
The reactive chamber was also modified, so that the perforated crowns were replaced by five long, perforated and smooth canals. Comparing the acoustic behaviour of these modifications with the original model, it was observed that the use of smooth tubes presented a great improvement in low frequency attenuation, but significantly impaired high frequency attenuation. The use of perforated tubes made it possible to achieve much greater or similar attenuations at high frequencies, however, it reduced the attenuation at the lower frequency values.
After simulating the overall model of the silencer, it was found that the use of perforated tubes was much more beneficial than the use of smooth tubes, since the attenuation drops caused by the perforations were clearly compensated for by the rest of the silencer.
Finally, it is possible to design a silencer that improves the attenuation in the entire low frequency range, maintaining the maximum values of noise reduction at high frequency. Furthermore, as the air passage area through the internal ducts of the equipment has been maintained, the pressure drop is hardly affected, presenting a value very similar to that of the original model.