Proceedings of ASME Turbo Expo 2011 GT2011 June 6-10, 2011, Vancouver, Canada GT2011-45004 SOUND DEADENING ON FANS Sascha Karstadt Chair of Fluid Systems Technology Technische Universit¨ at Darmstadt Darmstadt, Germany Email: Karstadt@fst.tu-darmstadt.de Berthold Matyschok Chair of Fluid Systems Technology Technische Universit¨ at Darmstadt Darmstadt, Germany Email: Matyschok@fst.tu-darmstadt.de Peter F. Pelz ∗ Chair of Fluid Systems Technology Technische Universit¨ at Darmstadt Darmstadt, Germany Email: Pelz@fst.tu-darmstadt.de ABSTRACT In addition to the objective of increasing the efﬁciency of fans, acoustic efﬁciency gains more and more importance in or- der 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 compara- tively 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 experimen- tal research investigates the acoustic and aerodynamic charac- teristics of an axial turbomachine and possibilities of reducing the emitted sound. Therefore a sound absorber ring is built di- rectly 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 determin- ing 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 dead- ening the sound of the fan. In order to measure the inﬂuence of the construction which contains the materials, the perforated ∗ Address all correspondence to this author. casing ring with a sound-reﬂecting termination and a plane ring with the same tip clearance are measured additionally as a ref- erence. The noise exposure is analyzed over the complete fre- quency spectrum in order to determine the absorbing frequencies of the materials. INTRODUCTION Fans have a very large range of application, from small fans for cooling CPUs to big fans for example in wind tunnels. Nature and size vary signiﬁcantly depending on their application. With increasing disturbances of daily life by different noise sources, especially in the vicinity of people, the interest in reducing noise emission grows. Possibilities are, on the one hand, a direct inﬂu- ence on sound development, e.g. by constructive changes of the rotor geometry or by reducing the rotational speed of the fan. On the other hand, passive or active measures of secondary sound reduction can be applied. In this experimental study, the effectiveness of a sound absorber with porous materials installed in the casing region around the rotor of a fan is analyzed. Seven different conﬁgurations con- sisting of ﬁve sound absorber materials installed in a perforated casing ring are tested. To determine the inﬂuence of the per- forated casing ring, two reference measurements are performed. Studies in an impedance tube show the absorbing coefﬁcients of the used materials and their frequency dependence. The fan and the sound absorber conﬁguration are installed in a test rig accord- ing to ISO 5136 [1], which deﬁnes a standard for determining of sound power radiated into a duct by fans. It has been shown, inter alia in [2] that a signiﬁcant part of the 1 Copyright c  2011 by ASME