Abstract
As is generally known, discrete-frequency noises are radiated from fans due to rotor-stator interaction. Their fundamental frequency is the blade-passage frequency, which is determined by the number of rotor blades and their rotating speeds. To reduce such noises, several types of silencers have been designed. Among them, the authors noted a slitlike expansion chamber (hereafter referred to asslit, for simplicity) and have studied its performance. A slit is a simple expansion chamber with a very short axial length that is placed in a duct. A slit with a circular cross-section that is concentric with a circular duct may be studied using the same interpretation as is used for a side-branch resonator muffler (closed-end tube connected to a duct); that is, the resonant frequency of a slit depends on its depth (with an open-end correction). It is expected, hence, that a slit might be applicable as a simple and axially compact silencer that is effective on discrete-frequency noises. In this article, the properties of a slit are introduced, and the applicability of a slit to actual rotating machinery is described using experimental data.
Highlights
As is generally known, discrete-frequency noises are radiated from fans due to rotor-stator interaction
A slit with a circular cross-section that is concentric with a circular duct may be studied using the same interpretation as is used for a side-branch resonator muffler; that is, the resonant frequency of a slit depends on its depth
In order to indicate the effect of the slit, some examples of the measured spectrums are shown in Figure 13; the rotor speed was fixed at 925 rpm and the corresponding second peak appeared at 370 Hz
Summary
In order to indicate the slit property briefly, some examples of measured transmission coefficients are shown in Figure 3; they were obtained using the apparatus, which contained 25◦C air In this apparatus, a plane wave with an arbitrary monotonic frequency f was generated in a 41.3-mm-diameter pipe, using the speaker. Considering each calculated wave’s axial propagation (or attenuation, for nonpropagating modes) in an expansion, calculations of reflection and transmission are repeated many times at both ends of the expansion; this procedure is finished when summations of them converge Because this method does not take the viscous damping into account, each curve reaches zero at each resonant frequency, even when the slit length is extremely short.
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