Application of microperforated elements in axial fan noise control and silencer design
The microperforated panel (MPP) is considered to be an alternative sound absorbing material, which could replace the traditional glass fibers and other porous materials because of its sound absorbing characteristics in a wide frequency range. The MPP is also very useful in various human involved environments because it is less harmful to human respiratory system than traditional sound absorbing materials, meaning more hygienic. Therefore in this study, microperforated panels were used in fan noise control and in acoustic silencers, where both of the devices are easily accessible by humans. Axial fans are widely used to cool electronic devices, and the fans typically generate noticable amounts of noise during their operation. Among the various sources responsible for fan noise, tip clearance noise is one of the more critical components. It has been demonstrated, in gas turbine applications, that tip clearance noise can be reduced by installing a finite flow resistance, circumferential strip in the housing of the fan immediately adjacent to the turbine blade tips. It is possible, for example, that the finite level of flow resistance created by the slightly permeable housing may reduce turbulence levels in the tip region, thus decreasing the noise generation. In the present work, a similar approach was taken to the control of noise generated by a 120 mm axial fan. In this case, a microperforated film material was used as the finite flow resistance strip built into the scroll housing of the fan, spanning the axial region through which the blade tips sweep. Measurements of both sound radiation and of flow performance of a number of prototype fans having microperforated strips of varying flow resistances were conducted using an ISO plenum. A hemispherical array of ten microphones was used to measure the sound power of the fan as a function of fan operating point. The fan noise was quantified primarily on the basis of the blade passage sound power level. It was found that there were areas in the fan performance map within which tonal and/or overall noise levels could be consistently reduced by the use of the microperforated housing element. It was also found that the flow resistance needed to obtain an optimal noise reduction was a function of the fan operating point. Further, it was found that the inclusion of the microperforated strip in the fan housing had a negligible impact on the fan performance: that is, there was no performance penalty associated with the fan noise reduction. The MPP treatment in the housing area of the fan was extended to the upstream and downstream sides of the fan so that the housing could make itself as the duct. Therefore the MPP treatment effects in the upstream and the downstream side of the fan were also considered. In addition to the sound power measurement, the differences in the sound field around the fan due to different housing treatments were visualized and investigated by adopting Nearfield Acoustic Holography (NAH). The objective of the study on the acoustic muffler was to develop a compact, multi-chamber silencer incorporating dissipative microperforated elements that could be used to reduce transmitted noise in a flow system. Two expansion mufflers in series were used to create a relatively compact system that attenuated sound effectively over the speech interference range. The microperforated elements were used both to increase the acoustic performance of the silencer and to reduce the system pressure drop with respect to a muffler without a microperforated lining. Both Finite Element Modeling (FEM) simulation and experimental methods were employed in the detailed design of the multi-chamber silencer. In the FEM simulations, the microperforated lining was modeled as a fluid layer having complex properties, and the model was used, for example, to identify the optimal flow resistance of the microperforated lining. The predicted results were successfully compared with full-scale experimental results that were obtained by using a four-microphone standing wave tube. Additionally, mean flow effect inside the silencer and different structural internal designs of silencers such as inlet and outlet extensions, were considered.
Bolton, Purdue University.
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