Research Papers: Gas Turbines: Turbomachinery

Three-Dimensional Blade-Stacking Strategies and Understanding of Flow Physics in Low-Pressure Steam Turbines—Part I: Three-Dimensional Stacking Mechanisms

[+] Author and Article Information
Said Havakechian

Baden 5401, Switzerland
e-mail: said.havakechian@power.alstom.com

John Denton

Emeritus Professor
Whittle Laboratory,
Department of Engineering,
University of Cambridge,
North Yorkshire DL84LG, UK
e-mail: jdd1@cam.ac.uk

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 20, 2015; final manuscript received September 10, 2015; published online November 3, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(5), 052603 (Nov 03, 2015) (10 pages) Paper No: GTP-15-1353; doi: 10.1115/1.4031597 History: Received July 20, 2015; Revised September 10, 2015

Optimization of blade stacking in the last stage of low-pressure (LP) steam turbines constitutes one of the most delicate and time-consuming parts of the design process. This is the first of two papers focusing on the stacking strategies applied to the last stage guide vane (G0). Following a comprehensive review of the main features that characterize the LP last stage aerodynamics, the three-dimensional (3D) computational fluid dynamics (CFD) code used for the investigation and options related to the modeling of wet steam are described. Aerodynamic problems related to the LP last stage and the principles of 3D stacking are reviewed in detail. In this first paper, the results of a systematic study on an isolated LP stator row are used to elucidate the effects of stacking schemes, such as lean, twist, sweep, and hub profiling. These results show that stator twist not only has the most powerful influence on the reaction variation but it also produces undesirable spanwise variations in angular momentum at stator exit. These may be compensated by introducing a positive stagnation pressure gradient at entry to the last stage.

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Fig. 1

(a) Meridional sections of typical double flow LP cylinder and (b) magnified view of LP last stage

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Fig. 2

Mach number variation after the last stator

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Fig. 3

Absolute Mach numbers at stator exit for differing midspan Mach numbers. The dashed lines correspond to DR = 0.5.

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Fig. 4

Relative Mach number at rotor inlet for different midspan stator exit Mach numbers. The red lines correspond to DR = 0.5.

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Fig. 5

Effect of streamline curvature on the Mach number variation at the stator exit. The dashed lines correspond to DR = 0.5.

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Fig. 6

Specific mass flux, ρVx, kg/m2 s, through a choked cascade of stator blades. The inset shows the streamlines.

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Fig. 13

Stator with radial stacking. Left—streamlines and right—pitchwise average Mach numbers.

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Fig. 12

Surface pressure distribution on the hub, midspan, and tip of a flared cascade

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Fig. 11

Effects of sweep on a cascade

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Fig. 8

Effect of blade twist on the streamline curvature

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Fig. 7

Illustration of induced streamline curvature generated by hub profiling

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Fig. 10

Effects of stator lean on an LP stage

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Fig. 14

Streamlines for the forward curved stator and the stator with hub curvature

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Fig. 15

Pitchwise-averaged Mach numbers behind the isolated stator

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Fig. 16

Surface pressure distribution at hub, midspan, and tip. Top—radially stacked stator. Bottom—forward curved stator.

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Fig. 17

Angular momentum variation after the stator



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