The authors investigate the far-field noise emissions of a
Researchers have studied the link between the aerodynamic features of the fan rotor and its acoustic emissions extensively. In particular, Wright [
Researchers have identified a variety of mechanisms as causing noise signatures. The dominant sources, they believe, are the rotor blades, which generate noise as a result of turbulent wake shedding from the interaction between the end-wall boundary layer and the rotor tip. In view of the aerodynamic effect that tip-leakage flow exerts on wake and secondary flows, the industry widely recognises this mechanism as one of the most significant sources of noise [
The advent of stringent environmental regulations with respect to noise production has stimulated academics and practitioners alike to pursue the development of concepts and technologies that are likely to reduce fan noise either by attenuating noise propagation or by controlling the noise at source. In this regard, researchers have not given the deserved attention to the control and reduction of noise that the rotor-tip flow field generates primarily because of a lack of understanding of the complex flow fields involved. Nonetheless, some studies have addressed this question.
Marcinowski [
During the past decade, several researchers have proposed passive noise-control concepts based on modifications to the blade tip by means of antivortex appendages. Quinlan and Bent [
Test fan rotor blades and tip end-plates (not to scale) [
The objective of the study is to compare the acoustic merits of the proposed tip configurations, which, according to Corsini et al.’s [
The paper organisation is as follows. First, it describes the methodology in detail, including a description of (i) the family of fans under investigation (ii) passive noise-control devices (iii) flow conditions (iv) experimental set-up (v) aerodynamic performance measurements, and (vi) far field acoustic measurements. The paper next presents the authors’ findings, including (i) main aerodynamic and acoustic performance analysis and (ii) rotor noise sources’ directivity analysis. The paper concludes with a discussion of the findings and a summary of the major conclusions.
The authors designed the study’s methodology to achieve the following specific objectives: (i) to characterise various fan-blade configurations’ azimuthal distribution of the radiated sound pressure (
The authors conducted the study on a family of cooling fans, coded AC90/6, featuring good acoustic performance with respect to the state of the art according to in-service experience. The investigated fans were equipped with a six-blade unswept rotor, with modified ARA-D-geometry-type blade profiles originally designed for propeller applications. Table
Specifications of
Blade geometry | Hub | Tip |
l/ | 1.32 | 0.31 |
Pitch angle (deg) | 36 | 28 |
Camber angle (deg) | 46 | 41 |
solidity | 1.24 | 0.3 |
Fan rotor | ||
Blade number | 6 | |
Blade tip pitch angle (deg) | 16/28 | |
Blade tip stagger angle (deg) | 74/62 | |
Hub-to-casing diameter ratio | 0.22 | |
Tip diameter (mm) | 900.0 | |
Rotor tip clearance | 1.0 | |
Rated rotational frequency (rpm) | 935/950 |
The authors studied a
The improved blade-tip configurations of the
With a view to correcting the
The authors conducted the experiments in the Fläkt Woods semianechoic chamber at Colchester (UK). The chamber is a state-of-the-art anechoic facility used to certify noise emissions from industrial fans according to the BS 848-2.6:2000 standard, which is equivalent to the ISO 10302:1996 standard. The chamber cut-off frequency is 25 Hz.
The authors balanced the rotor to ensure vibration no greater than 4 mm/s, which is the accepted maximum value for this family of fans. Clearance between the blade tips and the casing was constant at 1% of blade span. Motor drove the rotor at a constant speed of 940 rpm, and the blade tip speed was 44.34 m/s. Under these conditions, the blade passing frequencies (BPFs) for all the tested configurations were
The test-rig was a standardised airway of type-A testing, Figure
Test rig set-up in the semianechoic room.
The authors set the microphone 2 meters from the rotor trailing edge plane. During the measurements, the authors protected the microphone diaphragm with a nose-cone windshield, upon which they conducted a preliminary test to quantify its self-induced noise, which thus enabled them to include a correction factor in data-processing calculations. The authors recorded signals from the far field microphone on separate channels of a 01-dB Symphonie digital signal processor. In all cases, the authors measured signals for 30 seconds. They repeated each measurement three times with an error band ±1%.
The authors measured the far field noise two fan diameters from the outflow sections, as recommended by Leggat and Siddon [
The authors investigated the fan rotor blades with a large tip-pitch angle of 28 degrees (when measured in the peripheral direction). This constituted the customary setting for these fans. The study accounted for a full performance test, in the way prescribed by the British Standard 848, spanning the operating range of the fan unit, that is,
The authors then examined the influence of the different end-plates on the fans’ noise emission by examining the noise performance when fitted with each of the three blade-tip configurations (
The tests established the aerodynamic performance of the three rotors. Figure
Comparison of (a) total pressure rise and (b) total efficiency curves.
As a consequence of its higher-pressure-developing capability, the
To compare the acoustic performance in the far field, first, Figure
Comparison of the A-filtered one-third octave band sound-power-level auto-spectra.
Compared with the
The authors achieved the larger noise reduction produced by the
To provide additional insight into the map of acoustic performance when spanning the fan operating margin, Figure
Comparison of A-filtered overall sound power level (
Notably, the
Table
Comparison of acoustic performance.
Volume flow | Unweighted | A weighted | Unweighted | A weighted | Unweighted | A weighted |
m2/s | dB | dB(A) | dB | dB(A) | dB | dB(A) |
8 | 100,1 | 92,5 | 96,7 | 89,3 | 98,6 | 88,1 |
7 | 96,8 | 92,5 | 96,6 | 87,9 | 94,7 | 86,7 |
6 | 95,4 | 93,2 | 92,2 | 87,1 | 89,9 | 85,0 |
The authors derived the
Directivity of the integrated
When moving away from the fan axis, all the modified impellers’ directivity patterns differed from that of the
The analyses showed that the
In order to give hints on the directivity patterns of the far field noise, Figures
Directivity map of the
Directivity map of the
Directivity map of the
Figure
When comparing the
The
The paper reports the results of an experimental study which assesses the acoustic improvement that occurs with three different fan impeller geometries. The authors also based the investigation on a developed technique to characterise noise sources along impeller’s radius. The authors have realised the aim of the work, with the investigation characterising the noise sources spanwise along the tested fans’ blades. The authors then established and discussed general acoustic proprieties of this family of fans.
The authors established tonal reduction, due to the enhanced blade-tip end-plates. They clarified the relevance of the tip features to influencing the radial distribution of the noise sources using coherence analysis. The modified multiple-vortex breakdown end-plate design was effective in reducing the broadband noise, improving the turbulent noise frequency range. The data that the authors obtained from the experiments demonstrated that the tip aeroacoustic emissions were differently sensitive when working in the presence of a single breakdown event, or multiple breakdowns as produced by the
The comparison of different tip features indicated that influence of the end-plate shaping positively impacted the fan blades’ global noise emission to which they were applied.
blade passing frequency (Hz)
frequency (Hz)
specific noise level (dB),
blade chord
sound pressure level (dB)
sound power level (dB)
overall sound power level (dB)
total pressure (Pa)
blade pitch
volume flow rate (m3/s).
fan efficiency based on total pressure rise
rotor tip clearance, % of the span.
The authors conducted the present research in the context of contract FW-DMA09-11 between Fläkt Woods Ltd and the