Proof of existence of monopole-type sound generation mechanism in a centrifugal blower

Abstract

Previous studies have shown that sound radiation from a centrifugal blower is primarily of the dipole and quadrupole characteristics, which describe the effects of fluid-structure interaction and turbulence generated by rotating blades. However, in many engineering applications the sound powers from centrifugal blowers are found to be proportional to the speed to the power of 4–6, which implies the existence of a monopole-type sound according to the Lighthills acoustic analogy [Lighthill, Proc. R. Soc. London, Ser. A 222, 564–587 (1952)]. This paper demonstrates that such a monopole sound generation mechanism indeed exists in a centrifugal blower used in an HVAC unit of a passenger vehicle. LDV measurements show a velocity fluctuation at the trailing edge tangential to the blade surface of an impeller. Laser tachometer measurements further indicate a fluctuation in shaft speed for all models of blowers tested. The resulting sound powers from these blowers show a predominant dependence of the speed to the power of 4, indicating the characteristic of a monopole. When these blowers are installed on a nonfluctuating shaft, however, the flow fluctuations are significantly reduced and the resulting sound powers are found to be proportional to the speed to the power of 6, which implies that the monopole sound generation mechanism is effectively eliminated.