STUDY ON THE EFFECT OF INLET GEOMETRY ON THE NOISE OF AN AXIAL FAN, WITH INVOLVEMENT OF THE PHASED ARRAY MICROPHONE TECHNIQUE

摘要

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 The original short-tapered entry has been replaced by a standardized bellmouth entry, resulting in remarkable noise reduction.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 on-site 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.