Case study: Evaluation of discrete frequency noise generated from object in wake of centrifugal impeller: Case study on tonal noise of a centrifugal fan Centrifugal Fan

Abstract

In order to clarify the physical mechanism of unpleasant fan noise, the discrete frequency noise (DFN) generated from a centrifugal fan is evaluated quantitatively based on the wake characteristics. The volute tongue of the scroll casing is replaced by a cylinder as an object in order to simplify the mechanical model. Moreover, an analytical evaluation method for the DFN generated from the volute tongue of the scroll casing based on the wake characteristics of the centrifugal impeller is proposed. From the comparison on the prediction and measurement of the noise level, the influence of the clearance of the volute tongue on the DFN of a centrifugal fan was evaluated from the viewpoint of fluid dynamics. The aerodynamic noise source of the DFN was formed at the rear side of the impeller, which had large momentum in the wake. The momentum was rapidly lost by the diffusion in the near-wake field. Therefore, when the scroll casing of the fan is designed with suitable clearance for the volute tongue, the DFN will be decreased because of the diffusion of the wake. The evaluated noise indicated that the noise level of the DFN was lower in the far-wake field than the broadband noise level. These results imply that the sinusoidal pressure fluctuation caused by the impingement of the jetwake flow became weaker than the pressure fluctuation induced by the turbulence on the blade surface. © 2019 Institute of Noise Control Engineering.