Tonal Noise Control from Centrifugal Fans Using Flow Control Obstructions

A1 - Tonal Noise Control with Flow Obstructions

Tonal noise originates from non-uniform flow that causes circumferentially varying blade forces and gives rise to a considerably larger radiated dipolar sound at the blade passage frequency (BPF) and its harmonics. The approach presented in this paper adapts a method previously developed for axial fans to control tonal noise using obstructions in the flow to destructively interfere with the primary non-uniform flow arising from stator/rotor interaction.

To target the circumferential modes of the unsteady forces to be controlled, the analytical model of tonal noise radiation from centrifugal fans in free field developed by Khelladi et al. is first analysed. Three major circumferential modes of the rotor unsteady forces radiate tonal noise at BPF: mainly the circumferential B-1 and B+1 orders for radial and tangential forces and the Bth order for the axial forces, where B is the number of blades. Controlling one of these modes can lead to significant BPF tone attenuation. The flow control obstruction is located such that the secondary radiated noise is of equal magnitude but opposite in phase compared to the primary noise. The magnitude of the secondary noise is controlled by the axial distance between the rotor and the obstruction or the size of the lobes of the obstruction. The phase of the secondary noise is controlled by the angular position of the control obstruction.

Experiments were carried out for a centrifugal fan test bench to validate the method for controlling BPF tonal noise by carefully positioning obstructions in a duct in the upstream flow.