Research On The Local-structure Effects On The Performance Of The Silencing Element

With the improvement of the living standard day after day, people have higher requirements on the living security and comfortability, coming with a requirement to a stricter control on nosie from vehicle, fans, engineering machinery, and et al. Muffler has been a important measure to control the noise pollution and to reduce the nosie radiation, which has been broadly used in many engineering projects. So it is significant to design the muffler with better performances for controlling nosie pollution. Complex mufflers are made of silencing elements which are organized by some effective principles. And the silencing elements thoroughly determines the noise absorption and insulation performance of the complx muffler. Therefore, the silencing elements are the key objects for research in this paper. The local-structure effects to the performance of the mufflers elements have been studied and some design principles for silencing elements have been put forward, which can be valuable for the design of noise elimination structure and nosie pollution controlling.Supported by the National Science Foundation project 51275275 and Shandong Young Scientist Research Award Fund BS2010ZZ006, The expansion chamber and micro perforated panel are the two key silencing elements set as the objects of study in this paper. And the local-structre effects on these two silencing elements have been studied, aiming to get some valuable conclusions and principles for the design of muffler. Several researches have been done as follow.First, the expansion chamber has been studied in depth. The characteristics of the three-dimensional acoustic modes and the inlet and outlet layout principle based on acoustic modes of cylindrical expansion chamber, which leads to a high pressure loss, were specifically introduced. The local pressure loss generation mechanism were studied in an expansion chamber whose model is from a literature. The analysis showed the local pressure loss was caused by the viscous friction of the shear layers, the fierce velocity fluctuation near compression vent, and forcely altered velocity of the inlet air flow. This mechanism can be a guide for pressure loss reduction and the local-structure design in expansion chamber.Second, based on the inlet and outlet layout principle and pressure loss mechanism, the local-structure effects on the pressure loss of expansion chamber was analyzed on tthe premise of promoting acoustic performance. The effects of the offset patterns and the oblique angle of inlet and oulet tubes to pressure loss of expansion chamber were studied and got the best oblique angle of inlet and outlet tubes. That best angle was the rotating angle when the axes of inlet and outlet tubes and the link line of the centers of the compression vent and the compression vent.Several simple expansion chamber mufflers with the same acoustic performances but different offset patterns and oblique patterns were modeled. And the effects of the expansion ratio, the length-diameter ratio, and the transition arc on pressure loss had been studied. And the results showed the best model and it is better not to offset outlet tube.Research had been done on the effect of the length of oblique insert tube to pressure loss and transmission loss. Results showed that oblique insert tube could further lower pressure loss. But the oblique insert tube can greatly change the excitation or suppression state of acoustic modes and acoustic performance. The essence of this phenomenon is that the spatial locations of expansion vent and compression vent are essencially the key to the excitation or suppression state of acoustic modes. Thus, the oblique insert tubes layout principle has been concluded. Locate the expansion vent and the compression vent on the intersectant area of nodal points of all acoustic modes, which can suppress acoustic modes as many as possible.Third, study had been carried on micro perforated panel. The effects of the depth, length, and punching rate of the micro perforated panel on the absorption characteristics has been analyzed in the simplified muffler. And analyzed the overlap absorption phenomenon between the two kinds of resonant peaks, one brought by back chamber acoustic modes and the other brought by micro holes. Several different length micro perforated panels were paralleled in the heating section of a high speed train ventilation tube to get different attenuation schemes to decrease noise radiation. Simulations and experiments had been done.Experiment showed that the scheme of inserting the micro perforated panel could effectively decrease the noise level in the heating section. And the noise level radiated from the end of the ventilation tube declined 2.8dB(A). The micro perforated panel scheme increased the pressure loss leading to a drop of velocity of flow in the ventilation tube. And the air flow clashing the side walls of the micro perforated panel caused a big turbulene noise, leading to an increasement of noise level of microphones installed in the heating section. Therefore, it’s better to make the micro perforated plate and the wall of the heating section as a whole to lower the turbulence noise.