Study on the Effect of Inlet Geometry on the Noise of an Axial Fan, With Involvement of the Phased Array Microphone Technique

Abstract

The paper presents a comparative case study in which a free-inlet, free-outlet industrial ventilating fan has been equipped with various inlet geometries. The original short-tapered entry has been replaced by a standardized bellmouth entry, resulting in remarkable noise reduction. The experimentation presented herein is adaptable to industrial onsite diagnostics. The upstream-radiated broadband noise associated with rotating sources has been mapped in a spatially resolved manner, by means of a Phased Array Microphone system and a Rotating Source Identifier beamforming algorithm. The acoustic measurements have been supplemented with aerodynamic measurements on the inlet velocity profile, and with Computational Fluid Dynamics studies. The acoustic data have been processed for enabling their evaluation in association with the aerodynamic operation of the elemental rotor cascades in a two-dimensional approach, and also for their interpretation in relationship to three-dimensional flow phenomena such as tip leakage flow. For this purpose, the acoustic data have been presented in the form of circumferentially-averaged noise profiles along the blade span, as well as noise source imaging maps. The studies reveal the following acoustic benefits of reconfiguring the original short-tapered entry to the bellmouth entry. A peripheral separation zone is characteristic for the short-tapered entry, provoking double-leakage tip clearance flow, being the dominant source of noise at higher frequencies. Such a peripheral separation zone is suppressed by the bellmouth inlet, and thus, the double-leakage flow and the related noise is eliminated. Farther away from the tip, along the dominant portion of the span, the moderation of endwall blockage due to suppressing the peripheral separation zone has led to the reduction of the rotor inlet velocity, thus moderating the noise associated with the suction side boundary layer developing on the blades.

Algorithmic localisation of noise sources in the tip region of a low-speed axial flow fan

“…The method can be extended to localise sources in rotating systems [4,5,6]. In the present investigation, the Rotating Source Identifier (ROSI) [4] method is applied, providing beamform results characterising the locations and strengths of noise sources related to the rotating fan blades [7,8,9,10].…”

“…The issue of the last point, related to the spatial resolution of the technique, is often attempted to be tackled with use of optics-based resolution criteria [8,13,14,15]. These were historically established for human vision, and therefore, their application is dubious in acoustic beamforming.…”

“…This points to the fact that the Sparrow limit cannot be applied when the sources are to be distinguished along a line, such as the tip circumference in the present case study. In evaluation of beamform maps, involving subjectivity (visual inspection), the arbitrary use of the Rayleigh limit [8,13,14], as well as the Sparrow limit [13,15] is widespread. The application of the Rayleigh criterion and the Full Width at Half Maximum criterion is investigated in [18], where alternative limits are also introduced.…”

“…In fan noise investigations however, the distance between the fan and the array is often confined to the order of magnitude of the rotor tip diameter. In [8], serving as a basis for the present investigation, the array was installed approximately 2 d t tip diameters away from the fan inlet, while the size of the array is approximately 3 d t . With such a geometrical configuration, the validity of the plane wave approximation is doubtful, similarly to the case in [15].…”

Comparison of the Rotating Source Identifier and the Virtual Rotating Array Method

“…The applied weight vector was chosen to conform with previous investigations reported in literature [5,20,21]. One element of this vector is shown in Eq.…”