Concerted Aerodynamic and Acoustic Diagnostics of an Axial Flow Industrial Fan, Involving the Phased Array Microphone Technique

Abstract

The paper presents a methodology for on-site investigation of short-ducted industrial axial flow fans, as an easily realizable and effective means for concerted diagnostics on fan aerodynamics and acoustics along the rotor radius. The methodology relies on the accessibility of the fan from the upstream direction only. It involves experiments such as i) measurement of inlet axial velocity profile along the radius, combined with ii) beamforming studies using the Phased Array Microphone technique. The application of the methodology has been demonstrated in a case study of a ventilating fan. The semi-empirical data processing demonstrated significant changes of aerodynamic properties along the blade span. The acoustic studies regarded a frequency range being significant from the viewpoint of human audition. The phased array data have been processed and evaluated on the basis of two in-house developed beamforming codes, based on the Delay and Sum as well as the Rotating Source Identifier (ROSI) methods. The measurements revealed that the detected noise is dominated by rotating sources of broadband noise. By means of the ROSI code, pitchwise-resolved information has been obtained on the rotor noise. By such means, noise sources such as locally thickened suction side blade boundary layers and tip leakage flow have been identified. The spanwise variation of sound pressure has been compared with cascade loss indicators used in fan analysis and preliminary design, such as the total pressure loss coefficient and the Lieblein diffusion factor. The sound pressure has been found to increase locally with the total pressure loss and diffusion along the dominant portion of blade span, in the frequency bands being the most significant from the viewpoint of audibility of the noise generated by the fan.