Subsonic axial flow fan noise and unsteady rotor force

Abstract

The noise radiated by a subsonic, axial flow fan at its rotational frequency and harmonics is related to the unsteady blade force created by the rotor blade/fluid interaction. This force is highly dependent on the time-invariant flow distortions that enter the fan. The objectives of this study are to quantify the correlation between the inflow nonuniformity, unsteady blade loading, and the discrete-frequency radiated noise. The inflow field of a typical cooling fan used in the electronic and computer industry was distorted by placing a small cylinder in the inlet plane. The fan was instrumented with a sensor to measure the axial rotor force. The on-axis sound pressure spectra were compared to the coherent output power spectra conditioned on the unsteady rotor force and the radiated sound. Very good coherence at the discrete tones was observed. In the inlet plane, the time-invariant, spatially nonuniform inflow was measured using miniature, five-hole pressure probes. Fourier analysis of the inflow velocity data was used to compute the harmonic content of the unsteady rotor force. A simplified form of Curle’s equation was then used to compute the discrete-frequency radiated noise at the blade passage frequency (BPF) and its lower harmonics. The predicted and measured noise levels are in close agreement at the BPF and the first harmonic, for which the fan is a compact source.