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Research Papers: Gas Turbines: Turbomachinery

Aeroacoustic Analysis of Low-Speed Axial Fans With Different Rotational Speeds in the Design Point

[+] Author and Article Information
Patrick Buchwald

Institute of Thermal Turbomachinery and
Machinery Laboratory (ITSM),
University of Stuttgart,
Stuttgart 70569, Germany
e-mail: buchwald@itsm.uni-stuttgart.de

Damian M. Vogt

Institute of Thermal Turbomachinery and
Machinery Laboratory (ITSM),
University of Stuttgart,
Stuttgart 70569, Germany
e-mail: damian.vogt@itsm.uni-stuttgart.de

Julien Grilliat

ebm-papst St. Georgen GmbH & Co. KG,
St. Georgen 78112, Germany
e-mail: julien.grilliat@de.ebmpapst.com

Wolfgang Laufer

ebm-papst St. Georgen GmbH & Co. KG,
St. Georgen 78112, Germany
e-mail: wolfgang.laufer@de.ebmpapst.com

Michael B. Schmitz

ebm-papst St. Georgen GmbH & Co. KG,
St. Georgen 78112, Germany
e-mail: michael.b.schmitz@siemens.com

Andreas Lucius

ebm-papst Mulfingen GmbH & Co. KG,
Mulfingen 74673, Germany
e-mail: andreas.lucius@de.ebmpapst.com

Marc Schneider

ebm-papst Mulfingen GmbH & Co. KG,
Mulfingen 74673, Germany
e-mail: marc.schneider@de.ebmpapst.com

1Present address: Siemens Switzerland Ltd., Zug 6301, Switzerland.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 3, 2017; final manuscript received August 8, 2017; published online November 14, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(5), 052601 (Nov 14, 2017) (12 pages) Paper No: GTP-17-1262; doi: 10.1115/1.4038122 History: Received July 03, 2017; Revised August 08, 2017

One of the main design decisions in the development of low-speed axial fans is the right choice of the blade loading versus rotational speed, since a target pressure rise could either be achieved with a slow spinning fan and high blade loading or a fast spinning fan with less flow turning in the blade passages. Both the blade loading and the fan speed have an influence on the fan performance and the fan acoustics, and there is a need to find the optimum choice in order to maximize efficiency while minimizing noise emissions. This paper addresses this problem by investigating five different fans with the same pressure rise but different rotational speeds in the design point (DP). In the first part of the numerical study, the fan design is described and steady-state Reynolds-averaged Navier–Stokes (RANS) simulations are conducted in order to identify the performance of the fans in the DP and in off-design conditions. The investigations show the existence of an optimum in rotational speed regarding fan efficiency and identify a flow separation on the hub causing a deflection of the outflow in radial direction as the main loss source for slow spinning fans with high blade loadings. Subsequently, large eddy simulations (LES) along with the acoustic analogy of Ffowcs Williams and Hawkings (FW–H) are performed in the DP to identify the main noise sources and to determine the far-field acoustics. The identification of the noise sources within the fans in the near-field is performed with the help of the power spectral density (PSD) of the pressure. In the far-field, the sound power level (SWL) is computed using different parts of the fan surface as FW–H sources. Both methods show the same trends regarding noise emissions and allow for a localization of the noise sources. The flow separation on the hub is one of the main noise sources along with the tip vortex with an increase in its strength toward lower rotational speeds and higher loading. Furthermore, a horseshoe vortex detaching from the rotor leading edge and impinging on the pressure side as well as the turbulent boundary layer on the suction side represent significant noise sources. In the present investigation, the maximum in efficiency coincides with the minimum in noise emissions.

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Figures

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Fig. 1

Required relative flow angle at the rotor exit for different rotational speeds to reach a total pressure rise between 400 Pa and 800 Pa (V˙=800 m3 h−1, r = (Dshroud + Dhub)/4). Axial direction: β2 = 0 deg.

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Fig. 2

Rotor blade angle distribution at the trailing edge

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Fig. 3

Hub and shroud definition

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Fig. 4

Example of a CAD model (fan9000rpm)

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Fig. 5

Example of the CFD domain (fan9000rpm)

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Fig. 6

Mean total to static pressure rise and efficiency, computed between the inlet and outlet (steady-state RANS, DP)

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Fig. 7

Mach number distribution on a plane parallel to the rotational axis in the outlet region, only a section of the outlet region is shown (steady-state RANS, DP)

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Fig. 8

Mean pressure rise through the CFD domain (steady-state RANS, DP): (a) total to static pressure rise and (b) total pressure rise

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Fig. 9

Off-design fan performance (steady-state RANS): (a) total to static pressure rise; (b) total to static efficiency; (c) nondimensional characteristic

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Fig. 10

Mean total to static pressure rise and efficiency (steady-state RANS versus time-averaged LES, DP)

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Fig. 11

fan9000rpm: Suction side; left: power spectral density (PSD) (100–4000 Hz); right: iso-surface of Q-criterion (Q = 3 × 107 s−2) colored with relative Mach number (LES, DP)

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Fig. 12

fan9000rpm: Pressure side; left: PSD (100–4000 Hz); right: iso-surface of Q-criterion (Q = 3 × 107 s−2) colored with relative Mach number (LES, DP)

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Fig. 13

fan9000rpm: Hub; left: PSD (100–4000 Hz); right: iso-surface of Q-criterion (Q = 3 × 107 s−2) colored with relative Mach number, blades partly hided (LES, DP)

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Fig. 14

fan9000rpm: Shroud; top: PSD (100–4000 Hz); bottom: iso-surface of Q-criterion (Q = 3 × 107 s–2) colored with relative Mach number (LES, DP)

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Fig. 15

Definition of the FW–H surfaces

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Fig. 16

Sound power level (SWL) for all investigated fans (100–4000 Hz)

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Fig. 17

Suction side, PSD (100–4000 Hz): (a) fan9000rpm and (b) fan7000rpm

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Fig. 18

Pressure side, PSD (100–4000 Hz): (a) fan9000rpm and (b) fan7000rpm

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Fig. 19

Shroud, PSD (100–4000 Hz): (a) fan9000rpm and (b) fan7000rpm

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Fig. 20

Overall SWL spectra for all fans

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Fig. 21

Fan efficiency and overall SWL

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