From our previous studies of fan casing vibroacoustics, it was found that noise caused by casing vibration was fairly small compared to its aeroacoustic counterpart. In the present work, a numerical study on the aerodynamic tonal noise of a centrifugal fan casing was carried out. A 3-D numerical simulation of turbulent unsteady flow on the whole impeller-volute configuration was performed using computational fluid dynamics (CFD) technique in order to obtain the pressure fluctuations on the casing wall which serve as aeroacoustic dipole sources. Three different flow rates were simulated: the best efficiency point (BEP), 1.382×BEP and the maximal flow rate (2.104×BEP). Fast Fourier Transform (FFT) was applied to the time series of pressure fluctuations to extract the blade passing frequency (BPF) component constituting the source term of the wave equation. Boundary element method (BEM) was used to solve the inhomogeneous frequency-domain wave equation. The influence of the casing on the sound field was taken into account in simulating the noise radiation by taking it as a rigid body. Results showed that the presence of the casing could greatly affect sound propagation. With the increase of flow rate, the radiated sound power rose drastically. The tonal blade noise was also investigated using Lowson’s formulation of rotor noise model, and the results showed that it’s smaller than the tonal casing noise.

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