Reduction of Centrifugal Blower Noise Using Active Noise Source Cancellation Method

为了给出一种有效的评价和降低广泛使用的离心式鼓风机空气动力噪声的方法，进行了三年的实验和分析。实验的重点是辐射噪声的叶片通过频率(BPF)的基频和高阶分量，因为这些分量在功率谱上有明显的峰值，从而控制了整个噪声水平。首先，利用涡旋面压力脉动数据与远场噪声数据的相关方法，通过实验确定了有效噪声源区域。基本BPF的有效噪声源区域通常位于截止点附近，而高阶BPF的有效噪声源区域存在于涡旋上相对下游的部分。利用这种噪声源的分布模式，利用压电致动器直接振动涡旋面来实现对BPF分量的有源抵消。然而，所提出的主动控制仅对高阶BPF分量非常有效，因为基本BPF噪声源的压力脉动级远大于次级源的压力脉动级。然后，另一种降噪方法，即通过改变鼓风机的几何参数进行被动降噪，才有完整的降噪方法。被动降噪方法与噪声传递通道的特点有关，噪声传递通道由若干个叶片间通道和进气管道组成。通过将叶片通过频率调整到传输通道频响函数的最大衰减频率，基本带通滤波器的噪声水平可以降低20分贝以上。提出的主动和被动混合噪声控制方法对任何类型的离心式鼓风机辐射噪声都是有效的，是一种全新的降噪技术。

Experimental and Analytical Approach has been conducted for these three years in order to present an effective evaluation and reduction methods for aerodynamic noise radiated from widely used centrifugal type of blower.Experiments are focused on basic and higher-order of blade passing frequency (BPF) components of the radiated noise since these components show distinct peaks in power spectra and thus control the whole noise level. First, effective noise source region is experimentally determined using correlation method between the data of pressure fluctuation on the scroll surface and that of far field noise. The effective noise source region of basic BPF exists in the vicinity of the cut off apex as usually expected, while that of the higher-order BPF exists relatively downstream part on the scroll. Using this pattern of noise source distribution, active cancellation of BPF components are conducted by using piezoelectric actuators to oscillate the scroll surface directly. However, the presented active control is found to be very effective only for the higher-order BPF components because the pressure fluctuation level of basic BPF noise source is much larger than that of the secondary source. Then, another type of reduction method i.e. passive reduction method by changing geometric parameters of the blower, is necessary to have the complete noise reduction method. The passive reduction method is related to the characteristics of the noise transmission passage which is composed of some numbers of blade-to-blade passages and inlet duct. By adjusting the blade passing frequency to the maximum attenuation frequency of the frequency response function of the transmission passage, noise level of basic BPF can be reduced more than 20 decibels. The presented hybrid method of active and passive noise control is found to be effective against any types of radiated noise from centrifugal blower, and is the quite new type of noise reduction technique.