Numerical Simulation of the Discrete Noise and Noise Redution of Centrifugal Fan
|Keywords||centrifugal fan noise prediction boundary element method noisereduction|
With the higher demand to the environmental noise, it makes people attach more attention to the aerodynamic noise of centrifugal fan. At present, the study on reducing noise of the centrifugal fan is focus on the experiment. While the high cost and long period are adverse to the development of fan products. From this point, this paper aims at studying a method of numerical research on predicting and reducing noise of the centrifugal fan.The aeroacoustic source of centrifugal fans can be divided into three types, monopole source, dipole source and quadruple source, while dipole source is main. During the aeroacoustic noise produced from dipole source radiating outwardly, volute have an obvious effect on scatterer and reflection of aeroacoustic noise, whereas the recent calculation of aeroacoustic noise usually takes inadequate account of the this effect, resulting in the numerical prediction with large error.In this paper the noise source is simplified, namely ignore the influence of monopole source and quadruple source, and the dipole is the main source and study the discrete noise producing from volute and impeller of centrifugal fan. The study method is based on the theory of Lighthill’s acoustic analogy, the source and radiation are calculated respectively. Firstly, the unsteady internal flow of fan is simulated to obtain the static pressure fluctuation on the surface of volute and impeller. Secondly, using FW-H equation to transform the static pressure fluctuation of volute surface into the dipole source, and using Lowson equation, to transform the static pressure fluctuation of rotator outlut into the rotating dipole source. In order to considerate the effect of scatter and reflection of volute, the boundary element method is used to discretize the Helmholtz equation under the acoustic boundary condition of the volute dipole source, rotating dipole source and volute surface, solve these discrete equations to get the acoustic field produced by the volute dipole source and the rotating dipole source. Boundary element method is divided into direct boundary element and indirect boundary element method, due to the huge computation of direct boundary element when calculating the unclosed structure, the indirect boundary element method is used to calculate the acoustic radiation.In order to verify the prediction model of centrifugal fan noise9-19No.4A centrifugal fan is selected, the simulation result is coincident with the experiment, it shows the prediction model is reliable. After further analysis, it indicates that the discrete noise produced from volute is obviously higher than the rotator. Furthermore, since the effect of scatter and reflection from volute already has been considered, the accuracy of predicting noise has been raised obviously.The numerical calculation method is applied to the study of reducing noise in multi-blade centrifugal fan. The result shows that the impeller is the main source of dipole noise. A further research, influence of tongue shape and tongue location on discrete noise, is done, the results show that the level of discrete noise is decreased with the increase of gap between tongue and rotor, radius of tongue and declining gradient of tongue, however, the aerodynamic performance of multi-blade centrifugal fan is worse. The achievements in the paper can be used to guide the design of reducing noise in multi-blade centrifugal fan.