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TECHNICAL PAPERS

Aeroelastic Stability of Welded-in-Pair Low Pressure Turbine Rotor Blades: A Comparative Study Using Linear Methods

[+] Author and Article Information
Roque Corral1

 Technology and Methods Department, Industria de Turbopropulsores S.A., 28830 Madrid, Spainroque.corral@itp.es

Juan Manuel Gallardo, Carlos Vasco

 Technology and Methods Department, Industria de Turbopropulsores S.A., 28830 Madrid, Spain

1

Also Associate Professor at the Department of Engine Propulsion and Fluid Dynamics of the School of Aeronautics, UPM.

J. Turbomach 129(1), 72-83 (Mar 01, 2004) (12 pages) doi:10.1115/1.2366512 History: Received October 01, 2003; Revised March 01, 2004

The aerodynamic damping of a modern low pressure turbine bladed-disk with interlock rotor blades is compared for the first time to that obtained when the rotor blades are welded in pairs through the lateral face of the shroud. The damping is computed solving the linearized Reynolds averaged Navier-Stokes equations on a moving grid. First the basics of the stabilizing mechanism of welding the rotor blades in pairs is investigated using two-dimensional analyses and the Panovsky and Kielb method. It is concluded that the stabilizing effect is due to the suppression of unsteady perturbations in one out of the two passages providing for the first time a physical explanation to engine data. Three-dimensional effects are then studied using the actual mode shapes of two bladed disks differing solely in the shroud boundary conditions. It is concluded that the increase in the aerodynamic damping, due to the modification of the mode shapes caused by welding the rotor blades in pairs, is smaller than that due to the overall raise of the reduced frequencies of a bladed disk with an interlock design. The modification of the flutter boundaries due to mistuning effects is assessed using the reduced order model known as the Fundamental Mistuning Model. A novel extension of the critical reduced frequency stability maps accounting for mistuning effects is derived and applied for both, the freestanding and welded-in-pair airfoils. The stabilizing effect of mistuning is clearly seen in these maps. Finally, the effect of mistuning on low-pressure-turbine bladed disks is studied. It is shown that the modification on the stability limit of the interlock bladed disk is negligible, while for the welded-in-pair configuration a 0.15% increase of the damping relative to the critical damping is found. This qualitative difference between both configurations had not been reported before.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 3

Typical hybrid-cell grid and associated dual mesh

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Figure 4

Definition of fundamental mode shapes

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Figure 5

Mode shapes of a welded-pair configuration. Left: flap mode. Right: 1st torsion mode.

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Figure 6

Phase of the flap (left) and 1st torsion (right) mode-shapes at the mid-section

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Figure 7

Damping as a function of IBPA for the three fundamental modes. Top: freestanding airfoil. Bottom: welded-in-pair configuration.

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Figure 8

Critical reduced frequency maps for the freestanding (left) and welded-in-pair (right) configurations

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Figure 1

Evolution of LPT mid-section airfoils during the last two decades

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Figure 2

Sketch of cantilever, welded-in-pair and interlock shrouds

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Figure 9

Modulus of the unsteady pressure for a single airfoil vibrating in torsion about the leading edge for different IBPA

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Figure 10

Unsteady pressure phase along the blade surface. Filled symbols: suction side. Open symbols: pressure side.

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Figure 11

Modulus of the unsteady pressure for a welded pair of airfoils vibrating in torsion about the leading edge for different IBPA

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Figure 12

Modal characteristics of the bladed-disk assembly. Left: cantilever. Middle: welded pair. Right: Interlock.

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Figure 13

Damping as a function of the inter-blade phase angle for an interlock (left) and its equivalent welded-in-pair (solid lines) and freestanding configuration (right)

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Figure 14

Aerodynamic damping as a function of the IBPA for the WP1 (solid line) and WP2 (dashed line)

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Figure 15

Critical reduced frequency maps for mistuned freestanding (left) and welded-in-pair (right) airfoils

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Figure 16

Minimum damping as a function of the standard deviation of the rotor blade airfoil frequency for an interlock (solid symbols) and a welded-pair (open symbols) blade disk

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Figure 17

Damping relative to the critical damping as a function of the rotor blade frequency standard deviation

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Figure 18

Minimum damping as a function of the standard deviation of the rotor blade frequency for the WP1 (solid symbols) and the WP2 (open symbols) blade disks

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