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Research Papers

Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow

[+] Author and Article Information
Benjamin Megerle

Alstom Power,
Baden 5401, Switzerland
e-mail: benjamin.megerle@power.alstom.com

Ivan McBean

Alstom Power,
Baden 5401, Switzerland

Timothy Stephen Rice

Alstom Power,
Rugby CV21 2NH, UK

Peter Ott

Group of Thermal Turbomachinery, EPFL,
Lausanne 1015, Switzerland

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 5, 2013; final manuscript received December 20, 2013; published online May 2, 2014. Editor: Ronald Bunker.

J. Turbomach 136(9), 091008 (May 02, 2014) (8 pages) Paper No: TURBO-13-1252; doi: 10.1115/1.4027373 History: Received November 05, 2013; Revised December 20, 2013

Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D computational fluid dynamics (CFD) has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multistage environment. The numerical results have been validated with measurement data from a multistage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.

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References

Megerle, B., Rice, T. S., McBean, I., and Ott, P., 2013, “Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow,” ASME J. Eng. Gas Turbines Power, 135(1), p. 012602. [CrossRef]
Lagun, V., Simoyu, Z., Frumin, Y., Povolotskii, L., and Sukharev, F., 1971, “Features of Operation of a Turbine Stage With Low DML Ratio Under Conditions of Low Loads,” Teploenergetika, 18, pp. 21–24.
Gerschütz, W., 2006, Experimentelle Untersuchung von rotierenden Strömungsinstabilitäten im Betriebsbereich der Ventiation einer Niederdruck-Dampfturbine, VDI Verlag, Düsseldorf.
Schmidt, D., and Riess, W., 1999, “Steady and Unsteady Flow Measurements in the Last Stages of LP Steam Turbines,” 3rd European Turbomachinery Conference, London, March 2–5, pp. 723–734.
Truckenmüller, F., 2002, “Untersuchung zur aerodynamisch induzierten Schwingungsanregung von Niederdruck-Laufschaufeln bei extremer Teillast, “ITSM, Universität Stuttgart, Stuttgart, Germany.
Day, I. J., and Cumpsty, N. A., 1978, “The Measurement and Interpretation of Flow Within Rotating Stall Cells in Axial Compressors,” J. Mech. Eng. Sci., 20(2), pp. 101–114. [CrossRef]
Usachev, I., Efimenko, E., Il’inikh, V., Kolyasnikov, V., and Neuimim, V., 1981, “Excitation of Axial Oscillations of Steam Turbine Rotors Under Operating Conditions,” Energomashinostroenie, 3, pp. 5–9.
McBean, I., Havakechian, S., and Masserey, P., 2010, “The Development of Long Last Stage Steam Turbine Blades,” ASME Paper No. GT2010-22747. [CrossRef]

Figures

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Fig. 1

LSMB tip pressure ratio as a function of volume flow

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Fig. 2

Meridional flow field

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Fig. 3

Model steam turbine at low volume flow LSMB inlet flow angles with 0 deg referring to axial flow

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Fig. 4

Rotating stall mechanism in axial compressors

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Fig. 5

(a) Cross-sectional view of the model steam turbine rear stage, (b) upstream view of plane 62, probe traverse locations S05, S06 and S07

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Fig. 6

Expansion lines for different volume flows, four stage CFD result—steam model turbine

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Fig. 7

FFT analysis of 10 rotor revolutions, static pressure in plane 61 at 95% span height

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Fig. 8

CFD model with realistic exhaust configuration

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Fig. 9

Time averaged flow field downstream of the model steam turbine LSMB in plane 62, see Fig. 5

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Fig. 10

Circumferentially averaged meridional stream lines from CFD at vx/uhub = 0.06

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Fig. 11

Normalized pressure amplitude (RMS) of the dominate cell patterns over span downstream of the LSMB air model

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Fig. 12

Unsteady pressure traverses for CFD and measurement, pressure amplitude of dominant cell pattern at vx/uhub = 0.06

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Fig. 13

Blade to blade view at 70% span with radial velocity contour normalized with uhub and projected flow vectors at vx/uhub = 0.06

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Fig. 14

Axial wall shear on LSMB blade surface looking down stream, regions with negative wall shear indicate separated flow (steam model) at vx/uhub = 0.06

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