Discrete-Frequency Radiated Noise and Unsteady Rotor Force from a Subsonic Axial Flow Fan

摘要

Noise radiated by a subsonic, axial-flow fan at its rotational frequency and harmonics is related to the non-steady force field created at the rotor blade/fluid interface. This unsteady field is highly dependent on the time-invariant flow distortions that enter the fan. In this study, a typical cooling fan used in the electronics industry was instrumented with a shaft unsteady axial force sensor. Its output is proportional to the total unsteady axial force created by the rotor. On-axis sound pressure levels were measured and compared to coherent output power spectra involving the unsteady force sensor and the microphone. Very good coherence at the discrete tones is observed. The fan's inflow field was systematically distorted by placing a small cylinder at various positions in the inlet plane. The non-uniform, three dimensional flow field entering the rotor was measured by traversing a set of miniature five-hole pressure probes. The total pressure outputs from this probe can be related to the axial, tangential, and radial velocity vectors. Fourier decomposition of the inflow velocity data is coupled with analysis to give information on the unsteady rotor force harmonic content. A simplified Curle's equation was then used to compute the discrete-frequency radiated noise at the Blade Passage Frequency(BPF) and its harmonics. The predicted and measured noise levels are in close agreement at the BPF and the first harmonic when the fan is a compact source. The fan inlet and fingerguard of the test fan were modified and evaluated acoustically. Evaluation shows that addition of a baffle does not reduce the fan noise. Keywords: Acoustic field; Acoustic variables measurement; Acoustic waves; Two phase fluid flow. (edc)