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SAN ANDRES (2013)ENGINEERING ANALYSES FOR POCKET DAMPER SEALS (YEAR II) 1

ContinuationProposal to the TAMU Turbomachinery Research Consortium

ENGINEERING ANALYSES FOR POCKET DAMPER SEALS AND COMBINEDLABYRINTH-BRUSH SEALS

Dr. Luis San Andrs, Mast-Childs Tribology ProfessorWeilian Shan, Research Assistant

May 2013 (YEARII)

SIGNIFICANCE AND JUSTIFICATIONSeals leakage in centrifugal compressors and turbines represents a substantial loss in efficiency and

power delivery with an increase in specific fuel consumption. Labyrinth seals (LS) are the most commonand inexpensive means of reducing secondary leakage, albeit wearing out with operation and therebypenalizing performance and even affecting rotordynamic stability. Improperly designed and operatedlabyrinth seals can be the source of rotordynamic instabilities.

Pocket damper seals (PDS), adding baffles in (alternating)circumferential cavities of a LS and engineering the inlet andexit tip clearances, have demonstrated enormous benefits insystem stability by providing physically large dampingcoefficients [1]. These seals, when properly designed, can act asdamper bearings by offering also large stiffness coefficients.Bulk-flow analyses for pocket damper seals (PDS) are available[2] with predictions showing moderately good correlation withtest data. A recent seal development, the fully-partitioned PDS[3] lacks a proper physical modeling. This seal type, FPDS,has axial baffles covering the whole seal extent and the pocketsare separated by thick small clearance regions (not sharp teeth)which can amount to 20% of the whole sealing area. In

comparisons [4] with experimental results for a LS and a honeycomb seal, the FPDS showed the largestdestabilizing cross-coupled stiffnesses, strong functions of inlet preswirl and rotor speed. As a result, itappears a LS with a swirl brake could still be a favorable choice.

Brush seals (BS), although costlier, are common inspecialized applications (aircraft engines). BS mayincrease plant efficiency by up to 1/6 of a point andwith as little as 10% of the leakage in a similar sizelabyrinth seal. In addition, some steam turbinespresently incorporate a hybrid seal composed of a BSinstalled mid-way of a multiple-teeth labyrinth seal.The hybrid BS-LS [5] affords significant sealingimprovements over conventional labyrinth seals.Additionally, when retrofit into labyrinth seals with

radial movement, the individual labyrinth seal segments are free to move radially independently of oneanother during transients. [5] Retractable gland packing and inter-stage packing seals benefit most from

the change in seal configuration. Predictive tools for evaluating BS leakage are based on empiricalmodels, while their force coefficients are largely ignored. A computational tool modeling the combinedLS-BS would be a welcome addition into the engineering design process of steam and gas turbines.

STATUS OF WORK 2012-2013In 2012, the TRC funded a two-year project to develop a new computational tool for accurate

prediction of the force coefficients and leakage of pocket damper seals. Note that commercial FPDSs [4]have thick walls rather than blades with sharp edges as in conventional LSs and early PDSs [2].

Pocket damper seal and variation [3]

Stator (segmented)

flow

Stator (segmented)

flow

Labyrinth seal Combined brush seal -

labyrinth seal

Labyrinth seal and combination with a brush seal [4]Labyrinth seal and combination with a brush seal [5]

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SAN ANDRES (2013)ENGINEERING ANALYSES FOR POCKET DAMPER SEALS (YEAR II) 2

Weilian Shan, M.S. graduate student, studied an existing code, PDSEAL, developed by Li et al.[2]for prediction of leakage and force coefficients in labyrinth seals and pocket damper seals with sharpblades, and constructed an EXCEL graphic user interface (GUI) for ready interface with the originalFortran code. XLPDS, the GUI for pocket damper seals, is simple to use and obtains predictions formultiples cases varying the gas supply and exit pressures, rotor speed, and whirl excitation frequency. Thetool will be soon available in the XLTRC2software suite.

Technical report TRC-SEAL-0213 [6] reports the progress and delivers predictions of rotordynamicforce coefficients for a test LS and a test FPDS in Ref. [4]. The predicted leakage and dynamic forcecoefficients correlate well with the test data for the labyrinth seal [4]. Predicted force coefficients for theFPDS are in gross error when compared to the experimental coefficients. Hence, the currentcomputational program shows severe limitations to predict the dynamic force performance of PDSs withthick walls. The physical model is a one-control volume, turbulent bulk flow model that includes theeffects of circumferential flow velocity within a seal pocket and uses Neumanns leakage equation acrossthe seal blades. The model ignores the flow resistance along the circumferential direction, badly neededfor PDS with blunt bladesof sizable axial thickness. Needless to state that, it is in this region the sealdevelops a cross-coupled stiffness as the gas is whirled because of shaft rotation.

PROPOSED WORK 2013-2014(YEAR II)To develop computational models for prediction of leakage, drag power loss and force coefficients of

fully partitioned PDSs and combined labyrinth-brush seals. During Year II, the specific tasks are: Update the physical model for PDS [2] by replacing the empirical leakage formulas with a bulk flow model that

incudes flow conservation and circumferential and axial momenta transport equations in the flow region under athick blade and the spinning rotor.

Perform further calibration of the physical model predictions against test leakage and force coefficient, those inRef. [4] for example.

Begin extensions of the bulk-flow model to include two-component mixtures (liquid in a gas) as in Ref. [7].

Search and evaluate leakage models for brush seals; empirical models are preferred, perhaps using equivalences toporous media flow models.

The models will integrate real gas properties, including steam and supercritical CO 2. The analysis will be limitedto a centered rotor thus giving force coefficients of the form KYY=KXXand KXY=-KYX

BUDGET FROM TRCFOR 2013-2014 YEAR ISupport for graduate student (20 h/week) x $ 1,950 x 12 months $ 23,400Fringe benefits (0.6%) and medical insurance ($185/month) $ 2,360Travel to (US) technical conference $ 1,200Tuition & fees three semesters ($362 credit hour x 24 ch/year) $ 8,686Other (Mathcad and portable data storage) $ 220

Total Cost: $ 35,866

REFERENCES[1] Childs, D. W., Vance, J. M., 1997, Annular Gas Seals and Rotordynamics of Compressors and Turbines, Proc. 26 th

Turbomachinery Symposium, September, Houston, TX, pp. 201-220.[2] Li, J., San Andrs, L., and J. Vance, 1999, "Bulk Flow Analysis of Multiple-Pocket Gas Damper Seals," ASME Journal of

Gas Turbines and Power, Vol. 121, 2, pp. 355-362.[3] Ertas, B., Vance, J., 2007, "Rotordynamic Force Coefficients for a New Damper Seal Design," ASME J. Tribol., 129, pp.

365-374.[4] Ertas, B.H., Delgado, A., and Vannini, G., 2011, Rotordynamic Force Coefficients for Three Types of Annular Gas Sealswith Inlet Preswirl and High Pressure Differential Ratio, ASME GT2011-45556.

[5] Skinner, D.R., et al., 2001, Combined Brush Seal and Labyrinth Seal Segment for Rotary Machines, US Patent 6,257,586B1 (see also http://www.turbocare.com/retractable_brush_seals.html .

[6] Shan, W., and San Andrs, L., 2013, Predictions vs. Test Results for Leakage and Rotordynamic Force Coefficients of aFully Partitioned Pocket Damper Seal and a Labyrinth Seal Limitations and the Current Computational model, ProgressReport to the Turbomachinery Research Consortium, TRC-SEAL-02-2013.

[7] San Andrs, L., 2012, Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals, ASME J. Eng. GasTurbines Power, 134, p. 022503.