Florian Zenger

Influence of blade skew on axial fan component noise

Microphone arrays can be used to detect sound sources on rotating machinery. For this study, experiments with three different axial fans, featuring backward-skewed, unskewed, and forward-skewed blades, were conducted in a standardized fan test chamber. The measured data are processed using the virtual rotating array method. Subsequent application of beamforming and deconvolution in the frequency domain allows the localization and quantification of separate sources, as appear at different regions on the blades. Evaluating broadband spectra of the leading and trailing edges of the blades, phenomena governing the acoustic characteristics of the fans at different operating points are identified. This enables a detailed discussion of the influence of the blade design on the radiated noise.

Efficient and Noise Reduced Design of Axial Fans Considering Psychoacoustic Evaluation Criteria

Axial fans are the largest group of blowers and thus have a significant impact on their environment: Their demand for electrical power and the generation of a significant noise emission, have a negative impact on the environment.In this work the improvement of efficiency and reduction of noise emission are subject of the investigations. Several types of low-pressure axial fans are subject of this investigation. Measurements of pressure rise, efficiency and acoustic behavior will be presented for different blade geometries. The modifications of forward- and backward-skewed blades are compared to a unskewed blade geometry. The forward-skewed configurations show an improvement on the hydraulic properties, the efficiency and reduce the noise emission whilst the backward-skewed configurations improve the acoustic behavior at low flow rates with flow detachment occurring. Besides an acoustic quantification of the noise emission using the sound power level, psychoacoustic methods like loudness and sharpness were used.

Cross Wind Influence on Noise Emission and Computed Vibrational Noise of an Axial Fan

The total noise emissions of two commercial axial fans were measured in a semi-anechoic fan test rig in comparison. The total sound pressure levels and the respective spectra were found to change with the fans’ operating points. Increasing fan flow rates lowered the total acoustic pressure, with a broadband shift towards higher frequencies, keeping perceived (A-weighted) sound pressure levels approximately constant over a wide range of operating points. In a second step, Laser Scanning Vibrometry measurements of the fan blades’ axial motion were conducted in comparison inside a wind tunnel fan test rig. Rotating blade surface vibration data was used as sole input to a Ffowcs Williams and Hawkings algorithm, to estimate noise emission from vibration. The computed noise from surface vibration was found to be hardly affected by the change of fan flow rate.In the application of an axial fan subject to natural wind or induced cross flow at its inlet, the flow field and possible noise emission of the fan changes. Microphone measurements of the cross flow influence inside a semi-anechoic wind tunnel revealed increasing broadband noise with ambient flow field velocity, and an amplification of the sound at the blade passing frequency harmonics. Similar excitations of the blade passing frequency harmonics under cross flow influence were also found in sound pressure spectra computations based on the Laser Scanning Vibrometry measurement data captured in the wind tunnel fan test rig. Blade vibration is considered to contribute to the low frequency tonal noise emission of axial fans operating under cross flow conditions.Copyright

Correlation of flow field and acoustic output from hearing aids under influence of wind

Wind noise in hearing aids occurs even at low wind speeds and is a confounding factor for hearing aid wearer, hence leading to a reduction of speech intelligibility. In this submission, a study on the correlation of the flow field around a hearing aid to its acoustic output is made. The BTE (behind the ear) hearing aid is mounted on an artificial head with three different ear geometries. The flow field is captured using a two component PIV (particle image velocimetry) system. For exposing critical flow phenomena, a POD (proper orthogonal decomposition) of the PIV measurement data is made. The hearing aid output is measured with a microphone inside the artificial head. On the one hand, wind noise in hearing aids is generated by the fluctuating velocity field of the boundary layer on the hearing aid. On the other hand, based on the PIV data and the POD results, flow patterns around the artificial head and the hearing aid are detected, which cause further noise, that is captured by the hearing aid microphone...

Investigation of the flow- and sound-field of a low-pressure axial fan benchmark case using experimental and numerical methods

An extension of a benchmark case for a low-pressure axial fan is presented. The generic fan is a typical fan to be used in commercial applications. The fan design procedure, as well as the experimental setups are described in detail. The numerical approach is based on a forward coupling between a flow simulation with ANSYS Fluent and an aeroacoustic source term and wave propagation computation with multiphysics research software CFS + +. Experimental and numerical data for aerodynamics and aeroacoustics are compared. This includes aerodynamic performance (volume flow rate, pressure rise and efficiency), fluid mechanical quantities on the fan suction and pressure side (velocity distribution and turbulent kinetic energy), wall pressure fluctuations in the gap region and acoustic spectra at various microphone positions. Finally, a comprehensive data base of an axial fan was generated. Flow field properties at the fan suction and pressure side from the CFD simulation are in good agreement and spectra from the...

Computational Aeroacoustics for Rotating Systems with Application to an Axial Fan

A hybrid aeroacoustic formulation, which is well suited for the computation of rotating systems, is presented. It is based on a decomposition of flow (incompressible part) and acoustic (compressibl...

Efficient and Noise Reduced Design of a Side Channel Blower Considering Psychoacoustic Evaluation Criteria

Commonly used turbo machines like axial fans or side channels blowers have a large impact on their environment: Large demand for electrical power and production of strong aeroacoustic sound sources, generating a significant noise emission, lead to a environmental footprint. The current work addresses both tasks: improvement of efficiency and reduction of noise emission. In this work a side channel blower is in the focus of the investigations. Since the noise emission of side channel blowers mainly occurs in the region of the stripper, pressure measurements on the blade surface are performed. The pressure trend during the blade passage along the stripper provides information on the exact location of and the contribution to the acoustic noise generation. Modifications of the stripper geometry with the objective of pressure drop linearization along the stripper are evaluated with respect to efficiency, pressure rise and noise reduction. All acoustic measurements are also evaluated with psychoacoustic quantities, like loudness and sharpness, to account for the human perception.