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Unshrouded Rotor Tip Clearance Effects in Expander Cycle Turbines

[+] Author Affiliations
Lennard Helmers

Volvo Aero Corporation, Trollhättan, Sweden

Jens Klingmann

Lund Institute of Technology, Lund, Sweden

Paper No. GT2002-30338, pp. 127-134; 8 pages
doi:10.1115/GT2002-30338
From:
  • ASME Turbo Expo 2002: Power for Land, Sea, and Air
  • Volume 5: Turbo Expo 2002, Parts A and B
  • Amsterdam, The Netherlands, June 3–6, 2002
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 0-7918-3610-X | eISBN: 0-7918-3601-0
  • Copyright © 2002 by ASME

abstract

Steady flow in axial one-stage turbines is assessed numerically and experimentally. The simulations are performed on coarse meshes using a standard numerical approach (3D, steady state, kε-turbulence model, wall function at solid boundaries). In order to allow for conclusions drawn from these rapid numerical studies, the approach was compared with an explicit LDA (Laser Doppler anemometry) mapping of the velocity field downstream the rotor on a representative turbine stage. A two-component LDA system allowed for measurements of axial and tangential velocity components at varying depth (radius) in the flow channel, Measurements thus correspond to a full plane at constant axial position in the rotating frame of reference of the rotor. Comparison between LDA velocity mapping and CFD results shows good agreement. A series of subsequent simulations is thus used to judge the impact of varied blade/stage design parameters. Two turbine layouts are defined for identical operating conditions and shaft power. The flow in the unshrouded rotor blade row is analyzed for the influence of varying tip clearance size and the dependency on stage velocity triangles. – Known correlations for tip clearance losses (typically used in mean line predictions) are used, though the blade row geometry considered is beyond the limits the correlations are intended for. The absolute loss level found in CFD simulations differs significantly from what is expected when using loss correlations. Still the variation with tip gap size is predicted well by some of the investigated models. As dependency of tip clearance losses on stage velocity triangles is considered, none of the tested correlations gives results consistent with the numerical simulations. The use of standard correlations ‘beyond the limits’ is thus considered to introduce high uncertainty. Due to the good consistency between LDA and numerical results, the conclusions are considered to be valid for stage designs similar to the ones analyzed.

Copyright © 2002 by ASME

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