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.

Fourier Analysis of Beamforming Data at the Tip of an Axial Fan Rotor

“…The appropriate narrowband maps were then summed to create third-octave band maps centred on 2000 Hz, 2500 Hz, 3150 Hz, 4000 Hz, 5000 Hz, and 6300 Hz. These bands were chosen because earlier research has shown that below the 2000 Hz band, no distinct sources can be identified due to the large wavelength, while above the 6300 Hz, the emitted noise of the fan is significantly reduced [12].…”

Establishment of a Beamforming Dataset on Basic Models of Low-Speed Axial Fan Blade Sections

“…The aeroacoustic investigation of highly loaded / high incidence blade sections is therefore a topic of great practical importance, with special regard to discovering the correlation between the streamwise evolution of the boundary layer thickness and the spatial distribution of the noise sources associated with the thickened / separated boundary layer. In [3][4][5], a correlation has been found between the loss generated in the suction side boundary layer -represented by the momentum thickness -and the radiated broadband noise. This implies that by the detailed investigation of the boundary layer, additional knowledge can be gained regarding the noise formation mechanisms, contributing to the continuation of the research documented in [3][4][5].…”

“…In [3][4][5], a correlation has been found between the loss generated in the suction side boundary layer -represented by the momentum thickness -and the radiated broadband noise. This implies that by the detailed investigation of the boundary layer, additional knowledge can be gained regarding the noise formation mechanisms, contributing to the continuation of the research documented in [3][4][5]. Beyond the practical relevance as outlined above, the investigation of blade boundary layers exposed to pronounced adverse streamwise gradients, potentially leading to boundary layer separation, pose a challenge from basic research point view, in terms of experimentation, Computational Fluid Dynamics (CFD) as well as Computational Aeroacoustics (CAA) simulation.…”

Efficiency gain of low-speed axial flow rotors due to forward sweep

“…Fig. 1 presents the CAD image of the industrial fan rotor examined by Benedek and Vad in paper [7]. As detailed in [6], (i) Both CVD and FSW influence the three-dimensional interblade flow; (ii) Contrarily, the effects of sweep are often presented in the literature as features being independent from the fact that the blading has been designed for spanwise increasing circulation; (iii) Only very few papers comment on the combined effects of CVD and FSW.…”