The Application of Microperforated Material to Control Axial Fan Tip Clearance Noise

A3 - Tip Leakage Noise of Axial Fans

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. Therefore, optimized microperforated housing designs for certain operating conditions of axial fans will be suggested in this presentation.