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Research Papers

Turbine Hub and Shroud Sealing Flow Loss Mechanisms

[+] Author and Article Information
Metodi Blagoev Zlatinov1

Gas Turbine Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139zlatinov@alum.mit.edu

Choon Sooi Tan

Gas Turbine Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

Matthew Montgomery, Tito Islam, Melissa Harris

Gas Turbine Engineering, Siemens Energy Inc., Orlando, FL 32826

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Personal correspondence with David Little, Siemens Energy Inc.

1

Corresponding author. Currently employed at Altran Solutions, Boston.

J. Turbomach 134(6), 061027 (Sep 04, 2012) (12 pages) doi:10.1115/1.4006294 History: Received July 17, 2011; Revised July 22, 2011; Published September 04, 2012; Online September 04, 2012

Purge air is injected through seals in the hub and shroud of axial turbines in order to prevent hot gas ingestion into the inter-stage gaps. An investigation into the losses involved with the injection of purge air has been undertaken, with the objectives of answering where the losses are generated, how they are generated, and what are the most effective ways for reducing them. In order to address these questions, a consistent framework for interpreting entropy generation as a measure of loss is developed for turbomachinery applications with secondary air streams. A procedure for factoring out distinct effects is also presented. These tools, applied to steady computations, elucidate four mechanisms by which change in loss generation is brought about due to injection of purge air: a shear layer between purge and main streams, interaction with the passage vortex system that generates radial velocity gradients, changes in wetted loss and tip clearance flow due to an increased degree of reaction, and the potential for reducing tip clearance flow for the case of purge flow injected from the shroud. An emphasis is placed on tracing these effects to specific purge flow characteristics that drive them. The understanding gained provides a rationale for the observed sensitivity of purge flow losses to the design parameters purge air mass fraction and swirl, compared to purge slot axial inclination and gap width. Preswirling of purge flow is less effective in mitigating losses in the case of shroud-injection, since there is a tradeoff with the tip clearance flow suppression effect.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Axisymmetric and three-dimensional models

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Figure 2

T-s diagram of multistream expansion

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Figure 3

Accumulated viscous loss for baseline case with no purge flow

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Figure 4

Entropy generation rate per unit volume at axial planes through rotor

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Figure 5

Volumetric entropy generation rate due to viscous effects in shear layer. (a) Axisymmetric model and (b) three-dimensional rotor passage.

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Figure 6

Parametric study of shear layer loss using analytical mixed out analysis. (a) Effect of mf and sf and (b) Effect of gf and φ.

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Figure 7

Accumulated loss. (a) Three-dimensional stage, with and without purge flow. (b) Net loss due to purge flow injection for axisymmetric and 3D stage. (c) Net loss due to purge flow injection 3D stage with axisymmetric shear layer factored out.

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Figure 8

Change in entropy generation rate per unit volume due to injection of 1.5% purge flow at the hub, upstream of the rotor

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Figure 9

Circumferentially averaged cross-flow velocity (a) at downstream edge of purge slot and (b)-(d) at 0, 20, 100% axial chord

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Figure 10

Entropy generation rate per unit volume in region (A) of Fig. 4 in the plane X/Cx  = 0.8

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Figure 11

Decomposition of entropy generation rate per unit volume in terms of streamwise terms and secondary flow terms. Location and scale same as Fig. 1.

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Figure 12

Change in operating point for NGV and rotor, mf = 1.5% sf = 0%

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Figure 13

Entropy generation rate per unit volume in regions (B) and (C) in Fig. 4 for the baseline case (Bi,Ci) and a case with 1.5% purge flow injected at the shroud (Bii, Cii)

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Figure 14

Velocity field in rotor frame of reference half way between rotor tip and shroud, and blade loading at 95, 96, 97, 98.5 and 50% span. (a) No purge flow and (b) 1.5% purge flow at shroud

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Figure 15

Procedure for isolating and comparing purge flow loss mechanisms. Losses normalized by baseline stage loss.

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Figure 16

Comparison of present results to experimental data published by Reid [3]

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Figure 17

Lost work in turbine expansion

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