Research Papers: Gas Turbines: Turbomachinery

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

[+] Author and Article Information
Benjamin Megerle

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

Timothy Stephen Rice

Alstom Power,
CV21 2NH Rugby, United Kingdom

Ivan McBean

Alstom Power,
5401 Baden, Switzerland

Peter Ott

Group of Thermal Turbomachinery,
1015 Lausanne, Switzerland

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 21, 2012; final manuscript received July 16, 2012; published online November 26, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(1), 012602 (Nov 26, 2012) (7 pages) Paper No: GTP-12-1199; doi: 10.1115/1.4007334 History: Received June 21, 2012; Revised July 16, 2012

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

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Fig. 1

Characteristic meridional flow field under ventilation

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Fig. 2

Typical spectra of low volume flow events

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Fig. 3

Meridional view of the rig with traverses

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Fig. 4

Carpet intensity plot recorded by the strain gauge (normalized contour scale) and the dynamic pressure probe (contour is in (mbar) (RMS))

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Fig. 5

Meridional flow field (blue arrows: time average measurement; red arrows: steady state CFD)

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Fig. 6

Circumferential velocity in front and behind the rotor blade

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Fig. 7

Moving blade leading edge plane and axial velocity

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Fig. 8

One cell passing in the relative frame at 50% span with relative velocity vectors in the blade to blade plane and radial velocity as contour and indicative white arrows

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Fig. 9

FFT for one static pressure monitor point between the stator and rotor, 80% span, CFD, and relative frame of reference

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Fig. 10

Unsteady pressure amplitude for different cell counts



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