Tonal noise control from centrifugal fans using flow control 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 tonal noise control method previously developed for axial fans to centrifugal fans using obstructions in the flow to destructively interfere with the primary non-uniform flow arising from stator/rotor interaction. 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. 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.