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Predicted Rotordynamic Behavior of a Labyrinth Seal as Rotor Surface Speed Approaches Mach 1

[+] Author Affiliations
Manish R. Thorat, Dara W. Childs

Texas A&M University, College Station, TX

Paper No. GT2009-59256, pp. 699-709; 11 pages
doi:10.1115/GT2009-59256
From:
  • ASME Turbo Expo 2009: Power for Land, Sea, and Air
  • Volume 6: Structures and Dynamics, Parts A and B
  • Orlando, Florida, USA, June 8–12, 2009
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-4887-6 | eISBN: 978-0-7918-3849-5
  • Copyright © 2009 by ASME

abstract

Prior one-control-volume (1CV) models for rotor-fluid interaction in labyrinth seals produce synchronously-reduced (at running-speed), frequency-independent stiffness and damping coefficients. The 1CV model, consisting of a leakage equation, a continuity equation, and a circumferential-momentum equation (for each cavity) was stated to be invalid for rotor surface speeds approaching the speed of sound. However, the present results show that, while the 1CV fluid-mechanic model continues to be valid, the calculated rotordynamic coefficients become strongly frequency dependent. A solution is developed for the reaction-force components for a range of precession frequencies, producing frequency-dependent stiffness and damping coefficients. They can be used to define a Laplace-domain transfer-function model for the reaction-force/rotor-motion components. Calculated rotordynamic results are presented for a simple Jeffcott rotor acted on by a labyrinth seal. The seal radius Rs and running speed ω cause the rotor surface velocity Rs ω to equal the speed of sound c0 at ω = 58 krpm. Calculated synchronous-response results due to imbalance coincide for the synchronously-reduced and the frequency-dependent models. For an inlet preswirl ratio of 0.5, both models predict the same log decs out to ω≈14.5 krpm. The synchronously-reduced model predicts an onset speed of instability (OSI) at 15 krpm, but a return to stability at 45 krpm, with subsequent increases in log dec out to 65 krpm. The frequency-dependent model predicts an OSI of 65 krpm. The frequency-dependent models predict small changes in the rotor’s damped natural frequencies. The synchronously-reduced model predicts large changes. The stability-analysis results show that a frequency-dependent labyrinth seal model should be used if the rotor surface speed approaches a significant fraction of the speed of sound. For the present example, observable discrepancies arose when Rs ω = 0.26 c0 .

Copyright © 2009 by ASME
Topics: Rotors

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