In addition to the objective of increasing the efficiency of fans, acoustic efficiency gains more and more importance in order to reduce fan noise exposure. Whereas previously, research in this field was focused basically on blade design, nowadays the peripheral devices of fans are more often the object of study, since technical understanding of noise development is comparatively incomplete. Therefore, fundamental studies are essential to gain insight in the patterns of noise development and relatively easy opportunities to reduce noise level. In order to combine good aerodynamic properties with a silent fan, this experimental research investigates the acoustic and aerodynamic characteristics of an axial turbomachine and possibilities of reducing the emitted sound. Therefore a sound absorber ring is built directly around the rotor of a fan in order to absorb the sound very close to the origin. The fan assembly is installed in a test rig according to ISO 5136, which defines a standard for determining sound power radiated into a duct by fans. Acoustic signals are recorded with two microphones in the test duct, one on the pressure side, the second on the suction side, each close to ane-choic terminations at the ends. The aerodynamic characteristics are determined with a calibrated inlet nozzle and static pressure measurements over the fan stage. To confirm the expectation that a significant part of the emitted sound power is tip clearance noise, which changes with the operating point, the volume flow of the fan is varied over its entire operating range. In this study, five different porous materials are tested for their ability of deadening the sound of the fan. In order to measure the influence of the construction which contains the materials, the perforated casing ring with a sound-reflecting termination and a plane ring with the same tip clearance are measured additionally as a reference. The noise exposure is analyzed over the complete frequency spectrum in order to determine the absorbing frequencies of the materials.
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