Acoustic and Aerodynamic Properties of the Petal-shaped Absorption Silencers

Abstract

Presented the mathematical modeling results of acoustic and aerodynamic properties of petal-shaped silencer. Petal-shaped silencers are used for noise reduction in cylindrical channels. Such channels are often formed behind axial fans, which provide high flow consumption. For example, axial fans of dry cooling towers or roof fans can be noise sources. A swirling stream moves in the channel behind the fan. The advantage of using petal-shaped silencers is that they have the least aerodynamic drag. A special twist of the petal-shaped plates of the silencer allows to achieve channel overlap. An extra effect of noise reduction is achieved by eliminating the sound energy transit through the channel. Two mechanisms determine the effectiveness of the petal-shaped silencer. Noise reduction occurs due to the absorption of sound energy by sound-absorbing material. But also, part of the sound energy is reflected from the surface of the plates back into the channel. In order to determine the acoustic efficiency of petal-shaped silencer, mathematical modeling was carried out in the Ansys. When modeling, the propagation of airborne noise along a cylindrical channel at the place where silencers are installed is considered. The acoustic efficiency of petal-shaped silencers with a twist angle of 50° and 75° is compared with cylindrical and radial ones. Authors determined that at the octave band frequency of 2000 Hz, the decrease in sound pressure level (SPL) in the channels of petal silencers is 15.4 dB. It is 4–11 dB higher than the acoustic efficiency of cylindrical and radial ones. The use of petal-shaped plates allows to increase the acoustic efficiency of the silencer. For different twist angles of the plates, dependences of the decrease in SPL in the channels of petal-shaped silencers for the octave bands are presented. The aerodynamic drag of the silencer as important as its acoustic efficiency. It is possible to increase the acoustic efficiency of a dissipative silencer due to a denser placement of the material in the channel. But, in this case, the aerodynamic drag of the silencer also increases. The cases when a flow with a uniform velocity profile and a swirling flow are considered. The aerodynamic drag calculated when using: a fairing in the central part at the inlet and outlet of the silencer, the installation of special blades on the output edges of the silencer, the use of spacers between the plates. Measures have been identified to achieve the lowest aerodynamic drag. Recommendations for the use of petal-shaped silencers in cylindrical channels are given. Such silencers allow you to reduce the aerodynamic drag by almost 4 times.