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Research Papers

Clocking in Low-Pressure Turbines

[+] Author and Article Information
Kathryn R. Evans

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK
e-mail: kathryn.evans@cantab.net

John P. Longley

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK
e-mail: jpl@eng.cam.ac.uk

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 1, 2016; final manuscript received March 9, 2017; published online May 9, 2017. Editor: Kenneth Hall.

J. Turbomach 139(10), 101003 (May 09, 2017) (15 pages) Paper No: TURBO-16-1220; doi: 10.1115/1.4036341 History: Received September 01, 2016; Revised March 09, 2017

The effect of stator clocking has been experimentally and computationally investigated using a low-speed, two-stage, low-pressure turbine (LPT) which was specifically designed to maximize the clocking potential by aligning the stator 1 wake segments with the stator 2 leading edge along the span. It was verified that the wake segments are aligned to within 10% of stator pitch across the span. The measured clocking effect on the work extraction is 0.12% and on efficiency is 0.08%. Although the effect of clocking is small, it is repeatable, periodic across four stator pitches and consistent between independent measurements. Furthermore, factors to consider for a reliable clocking investigation are discussed. The measurements revealed that the majority of the clocking effect on the work extraction occurs in stage 2 and it originates at stator 2 exit. This indicates that the flow is being processed differently within stator 2. There is also an effect on the stage 1 work. In each blade row, the measured clocking effect on the lost work is similar across the span. The computations with meshed cavities do not capture any clocking effects in stage 1. This indicates that an unsteady viscid phenomenon within rotor 1 is not captured by the fully turbulent calculation, e.g., unsteady transition. However, the computations do capture the measured clocking effect on the stage 2 work extraction. It is hypothesized that the clocking effect on stator 2 flow turning is dominated by a steady, inviscid process.

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References

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Figures

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Fig. 1

Diagrams of the experimental facility: (a) schematic overview, (b) the working section (meridional view), and (c) circumferential positions of instrumentation (downstream view)

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Fig. 2

Streamtube tracking using the axial velocity profiles at measurement planes 1–7

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Fig. 3

Indexing stator 1 for a fixed stator 2 position (S2 0.0, square symbol): (a) clocking matrix and (b) blade-to-blade illustration

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Fig. 4

Six fixed stator 2 position clocking experiments (different symbols): (a) clocking matrix and (b) the variation in the numerically averaged exit stagnation pressure

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Fig. 5

Indexing stator 1 and stator 2 together for all six cases gives CLP0.0 (blue color, different symbols): (a) clocking matrix and (b) blade-to-blade illustration

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Fig. 6

Five Dual Clocking experiments: (a) negative sense clocking matrix, (b) positive sense clocking matrix, and (c) the variation in the turbine work coefficient (unaccounted)

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Fig. 7

A polar diagram for the turbine work coefficient for five Dual Clocking experiments (unaccounted)

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Fig. 8

Consistency of the exit stagnation pressure measurements (Dual Clocking average, MP7, and MP7 subset): (a) spanwise profiles and (b) polar diagram

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Fig. 9

Polar diagrams comparing the four approaches to determining the clocking effect on the overall two-stage (a) turbine work coefficient and (b) efficiency

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Fig. 10

Alignment of stator 1 wake segment (estimated from the minimum stagnation pressure) with the stator 2 leading edge: (a) CFD and (b) Expt. For convenience, the stator 2 leading edge has been clocked.

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Fig. 11

The fractional variation for five Dual Clocking experiments at the design flow coefficient and Reynolds number (accounted); against clocking position and in polar diagrams

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Fig. 12

Polar diagrams for the mass-averaged rvθ at measurement planes either side of the rotors: (a) measurements and (b) computations

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Fig. 13

Measured and calculated (2D and 3D computations) time-average wake segments at stator 2 inlet

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Fig. 14

Contours of the nondimensional axial flux of rvθ at stator 2 exit (MP5) for the five clocking positions and the estimated background flow

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Fig. 15

Measured circumferential variation of axial flux of rvθ at stator 2 exit (MP5) at 50% mass flow radius

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Fig. 16

Difference contours of the nondimensional axial flux of rvθ at stator 2 exit (MP5) compared to the background flow field; (a) Expt. and (b) CFD

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Fig. 17

Measured circumferential variation at rotor 1 exit (MP4) of at 50% mass flow radius: (a) Stagnation pressure and (b) nondimensional axial flux of rvθ

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Fig. 18

Subdivision into five streamtubes of the time average of the calculated flow through stator 2. The time average wake is shown entering the midstreamtube.

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Fig. 19

Change in the mass-averaged tangential velocity for each of the five streamtubes plotted against the time-average stagnation pressure entering that streamtube for the five clocking positions (colors match Fig. 18)

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Fig. 20

Decrease in flow turning due to the presence of the stator 1 wake segments plotted against the nondimensional curvature for each of the five streamtubes

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Fig. 21

Polar diagrams for the lost work across each blade row: (a) measured and (b) calculated

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Fig. 22

Polar diagrams for the measured lost work across each blade row for the hub, profile, and casing streamtubes

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