Generation mechanisms of low-frequency centrifugal fan noise

Earlier investigations by Stahl and Arguello (Stahl, B,, and Arguello, G., "Schallerzeugung und Schalldampfung in einer Rohrstromung stromab einer unstetigen Querschnittserweiterung," Acustica, Vol. 65, No. 2, 1988, pp. 75-84), who studied the sound field in a circular duct generated by a jet how 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. Experiments were made with five centrifugal fan impellers. Cross-correlation studies involving the unsteady wall pressures in the impeller and the casing led 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 shroud is of particular importance: a large radius of curvature in combination with a smaller impeller exit width improves the flow along the shroud. As a result, how 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 how along those parts of the volute that are close to the impeller periphery, i.e., the region close to the cutoff is Important.