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Research Papers

Comparison of Steady and Unsteady Flows in a Transonic Radial Vaned Diffuser

[+] Author and Article Information
E. Benichou

Laboratoire de Mécanique des Fluides et
d'Acoustique,
UMR CNRS 5509,
Ecole Centrale de Lyon,
UCB Lyon I, INSA,
36 Avenue Guy de Collongue,
Ecully Cedex 69134, France
e-mail: emmanuel.benichou@ec-lyon.fr

I. Trébinjac

Professor
Laboratoire de Mécanique des
Fluides et d'Acoustique,
UMR CNRS 5509,
Ecole Centrale de Lyon,
UCB Lyon I, INSA,
36 Avenue Guy de Collongue,
Ecully Cedex 69134, France
e-mail: isabelle.trebinjac@ec-lyon.fr

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 3, 2015; final manuscript received April 14, 2016; published online June 1, 2016. Assoc. Editor: Ricardo F. Martinez-Botas.

J. Turbomach 138(12), 121002 (Jun 01, 2016) (10 pages) Paper No: TURBO-15-1197; doi: 10.1115/1.4033481 History: Received September 03, 2015; Revised April 14, 2016

Boundary layer suction can be effective in delaying compressor surge, if the surge is triggered by flow separation on the shroud- or hub-casing. This work aims at positioning a suction slot in a radial vaned diffuser, which is thought to be the limiting component in a centrifugal compressor, such as the one considered here. The location of the slot is determined based on the results of both steady and unsteady flow simulations of a transonic centrifugal compressor of a turboshaft. Although the overall performance of the compressor is well-described by steady RANS, large discrepancies are observed between the steady and unsteady simulations of the diffuser flow, discrepancies imply different flow-separation scenarios. Steady results show more low-momentum fluid near the hub, whereas it is concentrated near the shroud in the unsteady simulations, hence no valid physical conclusions can be expected from the steady simulations. Analysis of the instantaneous skin-friction distribution from the unsteady simulations reveals that the separation is fixed and leads to a slot location on the shroud casing, near the diffuser main-vane suction side, so that it covers the range of separation saddle positions as the operating point is changed.

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References

Figures

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Fig. 1

Meridional sketch of the compressor

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Fig. 3

Static pressure recovery coefficient of the diffuser

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Fig. 2

(a) Total-to-static pressure ratio and (b) isentropic efficiency of the stage

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Fig. 4

Static pressure recovery coefficient of the diffuser subcomponents: (a) vaneless space, (b) semivaneless space, and (c) vaned diffuser

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Fig. 13

Sketch of supersonic pockets attached to the LE of the diffuser: (a) instantaneous state from URANS and (b) steady RANS

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Fig. 14

RANS and time-averaged URANS fields at midspan, NS point: (a) static pressure, (b) absolute Mach number, and (c) stagnation pressure

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Fig. 6

Absolute Mach number at the hub at three operating points: (a) SB, (b) PE, and (c) NS

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Fig. 5

Evolution of the blockage coefficient along the diffuser: (a) at the LE, (b) at the diffuser throat, and (c) at the TE

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Fig. 10

Friction lines at the hub: (a) steady RANS and (b) time-averaged URANS

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Fig. 11

Friction lines at the shroud: (a) steady RANS and (b) time-averaged URANS

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Fig. 12

Friction lines on the suction side of the diffuser main vane: (a) steady RANS and (b) time-averaged URANS

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Fig. 7

Absolute Mach number at midspan at three operating points: (a) SB, (b) PE, and (c) NS

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Fig. 8

Absolute Mach number at the shroud at three operating points: (a) SB, (b) PE, and (c) NS

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Fig. 9

Streamlines near the splitter vane LE from steady RANS, NS point

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Fig. 16

Instantaneous friction lines on the suction side during half a diffuser period

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Fig. 17

Mean streamlines (smooth curves) and trajectories (irregular curves) in the corner separation

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Fig. 18

Ideal suction slot, based on URANS results (Mach number at the shroud, NS point)

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Fig. 15

Steady and time-averaged unsteady spanwise profiles at the diffuser LE: (a) flow incidence, (b) radial velocity, and (c) absolute Mach number

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