Aeroacoustics of Air Purifier Centrifugal Fan Design

The sound generation mechanism of an existing air purifier centrifugal fan is firstly investigated by three dimensional numerical flow calculations. The simulation is carried with the Large Eddies Simulation (LES) approach to account the turbulent effect of the flow. The sound pressure level of the fan at the observer location is predicted separately by; 1. the Ffowcs Williams and Hawkings (FW-H) formulation (solution obtained free space Green's function) alone and; 2. the formulation with inclusion of reflection and scattering effects of fan housing (regarded as complete way of noise estimation with correction of scattering and reflection by boundary element method). Both calculations acquire the unsteady aerodynamic forces acting onto the fluid by the rotating blades obtained from the flow calculations as the source term. In the first part, the unsteady noise source is identified. The flow interaction with the volute tongue is the dominant noise source as identified from the flow field results. In the aeroacoustic calculation, it is found that the noise level calculated by FW-H equation is over-predicted; the difference from the measured value at BPF is about 3 dB. There are also some deviations from the first harmonic of the BPF. On the contrary, when the effect of scattering and reflection are taken into account, the noise level is better predicted. The difference from measurement at BPF narrowed to about 1 dB and the SPL spectrum agrees well with measurement.