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Research Papers

Forced Response in Axial Turbines Under the Influence of Partial Admission

[+] Author and Article Information
Jens Fridh

Chair of Heat and Power Technology
e-mail: jens@energy.kth.se

Torsten Fransson

Royal Institute of Technology (KTH),
Stockholm, Sweden

Contributed by International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 11, 2012; final manuscript received August 14, 2012; published online June 5, 2013. Assoc. Editor: David Wisler.

J. Turbomach 135(4), 041014 (Jun 05, 2013) (9 pages) Paper No: TURBO-12-1138; doi: 10.1115/1.4007599 History: Received July 11, 2012; Revised August 14, 2012

High cycle fatigue (HCF) due to unforeseen excitation frequencies, underestimated force magnitudes, or a combination of both causes control-stage failures for steam turbine stakeholders. This paper provides an extended design criteria toolbox, as well as validation data, for control-stage design based on experimental data to reduce HCF incidents in partial-admission turbines. The upstream rotor in a two-stage air test turbine is instrumented with pressure transducers and strain gauges. Admission degrees extend from 28.6% to 100%, as one or two admission arcs are simulated by blocking segmental arcs immediately upstream of the first stator vanes with aerodynamically shaped filling blocks. Sweeps across a speed range of 50%–105% of design speed are performed at a constant turbine pressure ratio during simultaneous high-speed acquisition. A forced-response analysis is performed and results presented in Campbell diagrams. Partial admission creates a large number of low-engine-order forced responses because of the blockage, pumping, loading, and unloading processes. Combinations of the number of rotor blades and low-engine-order excitations are the principal sources of forced-response vibrations for the turbine studied here. Altering the stator and/or rotor pitches changes the excitation pattern. We observed that a relationship between the circumferential lengths of the admitted and nonadmitted arcs dictates the excitation forces and may serve as a design parameter.

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References

Figures

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Fig. 1

Cross section of the test object

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Fig. 2

The test turbine setup

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Fig. 3

Typical sensor locations: left—pressure sensor; right—strain gauges

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Fig. 4

Acceleration sensitivity of pressure transducer

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Fig. 5

Comparisons of sensor response at 4450 rpm, 100% speed (300 and 3 rotational cycles) at two admission degrees (full and partial)

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Fig. 6

Nodal circle (NC) and nodal diameter (ND) examples; upper image from Hållberg, lower image from Srinivasan

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Fig. 7

ZZENF diagram for the test turbine (natural frequencies courtesy of Siemens Industrial Turbomachinery AB); zig-zag lines here plotted at design speed (4450 rpm)

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Fig. 8

General characteristics of blade forces (adapted from Pigott [16])

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Fig. 9

Qualitative forcing function at ε = 0.762 (design speed 4450 rpm); QNUM adapted from Hushmandi [6]

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Fig. 10

Campbell diagrams at two admission degrees (full and partial)

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Fig. 11

Magnitude ratios (frequency domain)

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Fig. 12

Magnitude ratios (frequency domain)

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Fig. 13

ZZENF diagram plotted at 4309 rpm (97% speed) with resonance order 53 highlighted

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