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Research Papers

Quantifying Loss Mechanisms in Turbine Tip Shroud Cavity Flows

[+] Author and Article Information
Timothy R. Palmer

Massachusetts Institute of Technology,
77 Massachusetts Avenue, Building 31-267,
Cambridge, MA 02139

Choon S. Tan

Massachusetts Institute of Technology,
77 Massachusetts Avenue, Building 31-267,
Cambridge, MA 02139
e-mail: choon@mit.edu

Humberto Zuniga

Siemens Energy, Inc.,
4400 Alafaya Trail,
Orlando, FL 32826
e-mail: humberto.zuniga@siemens.com

David Little

Siemens Energy, Inc.,
4400 Alafaya Trail,
Orlando, FL 32826

Matthew Montgomery

Siemens Energy, Inc.,
1680 South Central Boulevard, Suite 103,
Jupiter, FL 33458-7395

Anthony Malandra

Siemens Energy, Inc.,
4400 Alafaya Trail,
Orlando, FL 32826
e-mail: anthony.malandra@siemens.com

1Corresponding author. Currently at ATA Engineering, Inc., 13290 Evening Creek Drive South, Suite 250, San Diego, CA 92128. E-mail: tpalmer@ata-e.com

2Currently at ATA America, 11360 N. Jog Road, Suite 200, Palm Beach Gardens, FL 33418. E-mail: matthew.montogery@doosan.com

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 4, 2015; final manuscript received December 7, 2015; published online April 12, 2016. Editor: Kenneth C. Hall.

J. Turbomach 138(9), 091006 (Apr 12, 2016) (10 pages) Paper No: TURBO-15-1246; doi: 10.1115/1.4032922 History: Received November 04, 2015; Revised December 07, 2015

Numerical calculations, steady as well as unsteady, of flow in a turbine stage with a tip shroud cavity elucidate that the loss-generating flow features consist of tip seal leakage jet, the interaction of cavity exit flow with main flow, the partially recirculating cavity inlet flow interaction with vane wakes, and injection of leakage flow into the shroud cavity. The first two flow features, namely, the tip seal leakage flow and mixing of cavity exit flow with main flow, dominate while the injection of leakage flow plays an indirect role in affecting the loss generation associated with cavity exit flow. The tip shroud cavity flow essentially consists of a system of toroidal vortices, the configuration of which is set by the cavity geometry and changes when subject to unsteady vane–rotor interaction. The role which the toroidal vortices play in setting the cavity inlet recirculating flow pattern and loss generation is delineated. It is suggested that there exists a link between the inlet cavity sizing and the toroidal vortical structure. The computed results appear to indicate that the main flow path approximately perceives the presence of the tip shroud cavity as a sink–source pair; as such a flow model based on this approximation is formulated. Loss variations with tip gap height and leakage flow injection are assessed. Results show that the expected loss due to mixing has a functional dependence on the square of the difference in their velocity magnitude and swirl. The tip seal leakage jet loss scales approximately linearly with the corrected mass flow rate per unit area over the range of tip gaps investigated.

Copyright © 2016 by ASME
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References

Figures

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Fig. 1

Computational domain and nomenclature for cavity domain regions and features. Stage features upstream stator, rotor with tip shroud and cavity, and downstream diffuser section.

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Fig. 2

Axial variation in loss from reference steady baseline value for three basic situations. The profile with circular markers is the unsteady baseline with no cavity. The profiles with square markers represent the case with a cavity but no injected sealing flow.

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Fig. 3

Regions of high loss production identified with the volumetric entropy generation rate

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Fig. 4

Schematic of division of subdomains to localize change in loss between major cases. The stator domain includes the interface between stator and rotor (i.e., it contains the mixing plane).

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Fig. 5

Differences in efficiency debit from the baseline (no cavity) case for different injected leakage mass flow rates. Difference in efficiency debit is normalized to the total difference in loss of the no leakage case (0.4%).

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Fig. 6

Variation of mixing loss between injected sealing flow and cavity flow as injecting sealing mass flow rate is increased

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Fig. 7

Change in relative flow angle between injected leakage flow and the cavity flow prior to mixing in the cavity exit region

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Fig. 8

Variation of the relative angle between the gas exiting the cavity and the main flow

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Fig. 9

Variation of mixing loss between the cavity exit and main flows as a function of the difference in swirl. Fit is a quadratic polynomial.

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Fig. 10

Variation of loss in the tip seal leakage jet due to rapid expansion and mixing as a function of the corrected mass flow rate per unit area

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Fig. 11

Schematic of the sink–source model for the generic cavity with single radial sealing fin. The black circle represents a sink; the white, a source.

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Fig. 12

(a) Change in mass flux for the nominal tip gap with respect to the baseline case, mixing plane approximation, (b) change in mass flux for the 1.8× tip gap with respect to the baseline case, mixing plane approximation, and (c) change in mass flux for nominal tip gap with respect to the baseline case, unsteady

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Fig. 13

Contours of circumferential vorticity for the unsteady no leakage, nominal tip gap case. Vorticity is normalized to blade angular velocity. Positive vorticity is directed into the page.

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Fig. 14

Contour of difference in circumferential vorticity between the 1.8× tip gap and nominal tip gap cases (1.8× minus nominal)

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Fig. 15

Contours of differences in volumetric entropy generation rate, comparing the unsteady, time-averaged 1.8× tip gap with the nominal tip gap. The values of the contours represent the 1.8× values minus the nominal values.

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Fig. 16

Percentage of mass flow which enters the cavity, circulates around the inlet toroidal vortex, and re-enters the main gas path as a function of tip gap

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Fig. 17

Variation of recirculating mass fraction in the cavity inlet as a function of the average vorticity of the inlet toroidal vortex

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