Generation mechanisms of low-frequency centrifugal fan noise

Earlier investigations by Stahl & Argiiello (1986), who studied the sound field in a circular duct generated by a jet flow, indicated that flow separation occurring in a diffuser attached to the jet nozzle leads to increased sound levels in the low frequency regime. This observation formed the basis for the present study. Experimental investigations were carried out with five centrifugal fan impellers of different geometries. Cross correlation studies involving the unsteady wall pressures on various parts of the impeller and the fan casing lead to the following insights: The low frequency noise of centrifugal fans is generated by classical flow separation regions located on the shroud and the blade suction sides of the impeller. The design of the impeller shroud is of particular importance: A large radius of curvature in connection with a smaller impeller exit width improves the flow along the shroud. As a result, flow separation is reduced, the flow leaving the impeller is more uniform and less turbulent, and the low frequency noise is reduced. The flow in the fan casing also contributes to the low frequency noise: Here the flow along those parts of the volute is important which is close to the impeller periphery, specifically the cut-off area.