A combined acoustic-aerodynamic diagnostics methodology is presented herein, involving the phased array microphone technique and an in-house rotating source identifier processing algorithm. The methodology makes possible the industrial onsite investigation of unducted or short-ducted rotor-only axial fans. Its application is illustrated in a case study of a free-inlet, free-exhausting fan, surveyed from the upstream direction. A methodology is outlined for guaranteeing the appropriateness of averaging of the phased array microphone records. A technique is proposed for eliminating the ambiguity of the angular position of the rotor noise sources. A complete set of noise source maps of various frequencies have been evaluated. Noise sources, such as tip leakage flow and suction side blade boundary layer flow, have been identified. Empirical correlations have been pointed out between indicators of aerodynamic noise and aerodynamic loss along the blade span. Such correlations may contribute to redesign guidelines for simultaneous reduction of noise and improvement of efficiency at prescribed fan performance.

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.

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.

This paper summarizes a case study example on the methodology used for phased array microphone measurements in a realistic acoustic environment. A ducted fan rotor is measured in an environment, where structure born noise and aerodynamically generated noise cannot be separated using conventional single microphone acoustic analysis. The conditions of the measurement are similar to an industrial acoustic measurement scenario, where no acoustics treatment can be implemented for aiding in the source localization. The acoustics propagation paths are complex and combined with the high level of wind noise which the microphones exposed to, the interpretation of the results is difficult. The paper presents the processing and analyses of the many microphone worth of data of the phased array microphone system. The wind noise dominated cross spectral matrix is analyzed and source positions are obtained with beamforming procedures for both stationary and rotating sources.

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