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Research Papers

Effect of Blade Skew Strategies on the Operating Range and Aeroacoustic Performance of the Wells Turbine

[+] Author and Article Information
R. Starzmann

e-mail: ralf.starzmann@uni-siegen.de

Th. Carolus

e-mail: thomas.carolus@uni-siegen.de
Institut für Fluid und Thermodynamik,
University of Siegen,
Siegen 57068, Germany

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 15, 2012; final manuscript received July 9, 2013; published online September 20, 2013. Assoc. Editor: Zoltan Spakovszky.

J. Turbomach 136(1), 011003 (Sep 20, 2013) (11 pages) Paper No: TURBO-12-1068; doi: 10.1115/1.4025156 History: Received June 15, 2012; Revised July 09, 2013

One of the most intensively studied principles of harnessing the energy from ocean waves is the oscillating water column (OWC) device. The OWC converts the motion of the water waves into a bidirectional air flow, which in turn drives an air turbine. The bidirectional axial Wells turbine as a candidate for OWC power takeoff systems was the object of considerable research conducted in the last decades. The vast majority of the investigations focused on the aerodynamic performance. However, aiming at minimizing the overall environmental impact of this technology requires a new effort to reduce the aeroacoustic noise associated with a Wells turbine's operation. As for other turbomachinery, rotor blade skew is hypothesized to affect aeroacoustic noise sources favorably. Because of the unique symmetry of the blade shape of any Wells turbine, skew here means an inclination of the stagger line exclusively in circumferential direction and hence incorporates a combination of blade sweep and dihedral. Based on a blade element momentum theory, a new blade design methodology for a Wells turbine with skewed blades is established. Then, the effect of blade skew is assessed systematically by numerical simulations and experiments. As compared to a state-of-the-art rotor with straight blades, optimal backward/forward blade skew from hub to tip delays the onset of stall and increases the range of unstalled operation by approximately 5% in terms of static pressure head. As a Wells turbine in an OWC power plant operates cyclically along its characteristic, any extension of stall-free operating range has the potential of improving the energy yield. The flow-generated sound in unstalled operation was decreased up to 3 dB by optimal backward/forward blade skew. However, the predominate noise benefit in terms of equivalent sound power along complete operating cycles is due to the extended operating range without excessive sound due to stall.

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Figures

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Fig. 1

Schematic coaxial cascade section; left: turbine blade element (BE) with control volume (CV), velocity triangles, and induced angular force δFu; right: vector mean relative flow velocity and flow angle

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Fig. 2

Skewed Wells turbine rotors; left: backward skewed blade (δ < 0 deg); middle: straight blade (δ = 0 deg); right: forward skewed blade (δ > 0 deg); all forms can be combined in one blade

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Fig. 3

Wells turbine rotor; left: loss analysis; right: blade element (BE)

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Fig. 4

Local lift correction μloss as a function of the distance from a wall at y/L = 0 for various constant skew angles δ

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Fig. 6

Test facility: (a) housing, (b) splitter attenuator, (c) centrifugal fan, (d) splitter attenuator, (e) plenum, (f) honeycombs and turbulence control screens, (g) nozzle, (h) static pressure measurement, (i) static pressure measurement, (j) honeycombs, (k) turbine section; dimensions in mm

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Fig. 7

Acoustic measurement setup (j) honeycomb screen, (l) nose cone, (m) rotor with torque flange, (n) flow traverse position, (o) generator, (p) generator struts, (q) reference sound source (RSS), (r) microphone; dimensions in mm

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Fig. 8

Comparison of turbine (Lp) and background sound pressure (Lp,bgn) at microphone position according to Fig. 6; turbine is operated at its design point; frequency resolution: Δf = 8 Hz

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Fig. 9

Complete set of aeroacoustic steady-state characteristics; (a) flow coefficient, (b) power coefficient, (c) total-static efficiency, (d) specific sound power level

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Fig. 10

Measured and predicted flow field data: radial distributions of circumferentially averaged rotor exit flow quantities (traverse position (n), Fig. 7) at ψts = 0.38; (a) axial velocity, (b) circumferential velocity, (c) blade work

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Fig. 11

Blade loading at the hub from RANS (ψts = 0.38, r/rtip = 0.51)

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Fig. 12

Blade area with separated flow versus turbine pressure head (from RANS)

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Fig. 13

Skin friction coefficient and “surface” stream lines at suction surface; ψts = 0.38 (from RANS)

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Fig. 14

Measured narrow band sound power spectra; (a) ψts = 0.38, (b) ψts = 0.60; frequency resolution of Δf = 8 Hz

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