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Research Papers

A Numerical Investigation of Rotating Instability in Steam Turbine Last Stage

[+] Author and Article Information
L. Y. Zhang

e-mail: luying.zhang@eng.ox.ac.uk

L. He

e-mail: li.he@eng.ox.ac.uk
Department of Engineering Science,
University of Oxford,
Oxford, OX1 2JD, UK

H. Stüer

Fossil Power Generation,
Steam, Energy Sector,
Turbine Technology,
Siemens AD,
45478 Mülheim an der Ruhr, Germany

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 24, 2011; final manuscript received August 16, 2011; published online October 22, 2012. Editor: David Wisler.

J. Turbomach 135(1), 011009 (Oct 22, 2012) (9 pages) Paper No: TURBO-11-1092; doi: 10.1115/1.4006330 History: Received June 24, 2011; Revised August 16, 2011

The unsteady flow phenomenon (identified as rotating instability) in the last stage of a low-pressure model steam turbine operated at very low mass flow conditions is numerically studied. This kind of instability has been observed previously in compressors and can be linked to the high structural stress levels associated with flow-induced blade vibrations. The overall objective of the study is to enhance the understanding of the rotating instability in steam turbines at off design conditions. A numerical analysis using a validated unsteady nonlinear time-domain CFD solver is performed. The 3D solution captures the massively separated flow structure in the rotor-exhaust region and the pressure ratio characteristics around the rotor tip of the test model turbine stage in good comparison with the experiment. A computational study with a multi-passage whole annulus domain on two different 2D blade sections is subsequently carried out. The computational results clearly show that a rotating instability in a turbine blading configuration can be captured by the 2D model. The frequency and spatial modal characteristics are analyzed. The simulations seem to be able to predict a rotating fluid dynamic instability with the similar characteristic features to those of the experiment. In contrast to many previous observations, the results for the present configurations suggest that the onset and development of rotating instabilities can occur without 3D and tip-leakage flows, although a quantitative comparison with the experimental data can only be expected to be possible with fully 3D unsteady solutions.

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References

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Figures

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Fig. 1

Frequency spectra of total pressure in axial gap of the last stage of a model turbine [9]

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Fig. 2

Steady pressures compared with experiment [19] (inlet flow angle 20 deg, Reynolds number 220,000)

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Fig. 3

Contours of steady total pressures

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Fig. 4

Computational domain of the model turbine stage (2 stator passages, 3 rotor passages)

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Fig. 5

Calculated streamlines in the model turbine stage

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Fig. 6

Experimental flow patterns. (ϕr=0.22) [20].

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Fig. 7

Calculated rotor tip pressure ratio compared with the measurement [20]. (P31 and P32 are taken from the locations shown in Fig. 5(b).)

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Fig. 8

2D multipassage configurations

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Fig. 9

Static pressure contour on 90% span section (nominal flow condition, 100%)

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Fig. 10

Static pressure contours on 90% span section (relative flow rate: 71.2%)

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Fig. 11

Frequency spectra (90% span section) of unsteady pressures (relative mass flow rate: 74.8%)

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Fig. 12

Frequency spectra (90% span section) of unsteady pressures (relative mass flow rate: 72.7%)

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Fig. 13

Frequency spectra (90% span section) of unsteady pressures (relative mass flow rate: 71.2%)

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Fig. 14

Time traces of static pressures of 90% span section (relative mass flow rate: 65.6%)

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Fig. 15

Frequency spectra from stator frame of reference (90% span, relative mass flow rate: 70.4%)

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Fig. 16

Frequency spectra from rotor frame of reference (90% span, relative mass flow rate: 70.4%)

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Fig. 17

Computational meshes (90% span) (left: original; right: refined)

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Fig. 18

Frequency spectra (90% span) of unsteady pressures from two meshes (relative mass flow rate: 70.4%)

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Fig. 19

Time traces of static pressures of 10% span section (relative mass flow rate: 14.7%)

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Fig. 20

Frequency spectra (10% span section) of unsteady pressures (relative mass flow rate: 14.7%)

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Fig. 21

Pressure contours on 10% span section (relative flow rate: 14.7%)

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Fig. 22

Relative velocity vectors on 10% span section (relative flow rate: 14.7%)

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Fig. 23

Rotor pressure ratios with mass flow rates (Pexit and Pinlet are taken from the locations shown in Fig. 8(a))

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