Centrifugal fans are widely used due to their ability to achieve relatively high-pressure ratios in a short axial distance compared with axial fans. At the same time, the noise generated by these machines causes one of the most serious problems. In general, the centrifugal fan noise is often dominated by tones at BPF(blade passage frequency) and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cutoff in the casing. To reduce these tones, many experiments have been performed. On the other hand, there is little research to predict the noise numerically because of the difficulty in obtaining detailed information about the flow field around the impeller and the casing. The objective of this study is to understand the generation mechanism of fan noise through flow field calculation and to develop a prediction method for the acoustic pressure field of the centrifugal fan.
A discrete vortex method (DVM) is used to model the centrifugal fan and to calculate the flow field. Lowson's method is used to predict the acoustic pressure in a free field. In order to compare the experimental data, numerical calculation about centrifugal impeller and wedge, introduced by Weidemann, is carried out and the results are compared with the experimental data..
In order to analysis the scattering effect from the wedge, a concept of combined Kirchhoff - BEM (Boundary Element Method) is introduced in this paper. A Kirchhoff surface around the impeller is constructed to calculate a fictitious impeller noise source on the surface. The results of the Kirchhoff - BEM shows good agreement within 2~3dB with measured data.
Thin body Kirchhoff - BEM is also developed to calculate the acoustic field of a centrifugal fan with a casing including a finite duct. The hypersingular integral is regularized by using the Maue's less singular normal derivative integral equation. A standard Gaussian quadrature is used for the Maue's integral equation. The developed method is validated by comparison with analytic and measured results for acoustic scattering and radiation.
Neise's centrifugal fan with casing and duct is introduced to validate the prediction method. The results of the thin body Kirchhoff - BEM also shows good agreement with the measured acoustic pressure.
