Research Papers

Aeromechanical Optimization of a Winglet-Squealer Tip for an Axial Turbine

[+] Author and Article Information
Zbigniew Schabowski, Howard Hodson, Davide Giacche, Bronwyn Power

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK

Mark R. Stokes

Rolls-Royce Plc.,
P.O. Box 31,
Derby DE24 8BJ, UK

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received December 19, 2012; final manuscript received June 23, 2013; published online January 2, 2014. Editor: Ronald Bunker.

J. Turbomach 136(7), 071004 (Jan 02, 2014) (12 pages) Paper No: TURBO-12-1248; doi: 10.1115/1.4025687 History: Received December 19, 2012; Revised June 23, 2013

The possibility of reducing the over tip leakage loss of unshrouded axial turbine rotors has been investigated in an experiment using a linear cascade of turbine blades and by using CFD. A numerical optimization of a winglet-squealer geometry was performed. The optimization involved the structural analysis alongside the CFD. Significant effects of the tip design on the tip gap flow pattern, loss generation and mechanical deformation under centrifugal loads were found. The results of the optimization process were verified by low speed cascade testing. The measurements showed that the optimized winglet-squealer design had a lower loss than the flat tip at all of the tested tip gaps. At the same time, it offered a 37% reduction in the rate of change of the aerodynamic loss with the tip gap size. The optimized tip geometry was used to experimentally assess the effects of the opening of the tip cavity in the leading edge part of the blade and the inclination of the pressure side squealer from the radial direction. The opening of the cavity had a negligible effect on the aerodynamic performance of the cascade. The squealer lean resulted in a small reduction of the aerodynamic loss at all the tested tip gaps. It was shown that a careful consideration of the mechanical aspects of the winglet is required during the design process. Mechanically unconstrained designs could result in unacceptable deformation of the winglet due to centrifugal loads. An example winglet geometry is presented that produced a similar aerodynamic loss to that of the optimized tip but had a much worse mechanical performance. The mechanisms leading to the reduction of the tip leakage loss were identified. Using this knowledge, a simple method for designing the tip geometry of a shroudless turbine rotor is proposed. Numerical calculations indicated that the optimized low-speed winglet-squealer geometry maintained its aerodynamic superiority over the flat tip blade with the exit Mach number increased from 0.1 to 0.8.

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Fig. 4

Front part of winglet tip (CAD model)

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Fig. 2

General features of cascade

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Fig. 1

Flow through the tip gap for an unshrouded blade [8]

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Fig. 5

Computational domain

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Fig. 7

Winglet shape parameterization

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Fig. 8

Winglet size range

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Fig. 6

Example geometry from the optimization process

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Fig. 9

Optimizer result: loss (τ = 1.84% of chord) versus radial deflection

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Fig. 10

Optimizer result: loss (τ = 1.84% of chord) versus area

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Fig. 11

Distribution of radial deflection on the optimum geometry

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Fig. 12

Static pressure coefficient in the tip section of the optimum winglet plotted against tangential coordinate

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Fig. 13

Positions of line plots from Fig. 12

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Fig. 14

Predicted Yp contours on tangential plane at 0.5Cy for the optimum at tip gap of 2% chord

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Fig. 15

Deviation in the pitchwise flow angle between the leakage flow leaving the tip gap and main-stream flow

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Fig. 16

Measured and predicted Yp on axial plane 0.5Cx downstream of the cascade TE for the optimum design at tip gap of 2% chord: (a) experiment, (b) CFD

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Fig. 17

Pitchwise mass-averaged profiles at 0.5Cx downstream of the cascade TE for the optimum and flat tip at tip gap of 2% chord: (a) loss coefficient, (b) deviation from midspan exit flow angle

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Fig. 18

Measured and predicted tip region mixed-out total pressure loss coefficient - optimum tip

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Fig. 19

Winglet shape comparison: optimum versus Opt19

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Fig. 20

Derivative of optimum design with leant (30 deg from vertical) pressure side squealer

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Fig. 21

Measured tip region mixed-out total pressure loss coefficient – leant PSS

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Fig. 22

Derivative of optimum design with straight squealers and open cavity inlet

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Fig. 23

Measured tip region mixed-out total pressure loss coefficient – open cavity inlet

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Fig. 24

Ma number for the flat tip at cut plane positioned at half tangential chord, Ma2 = 0.8, tip gap 2% of chord

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Fig. 25

Ma number for the optimum tip at cut plane positioned at half tangential chord, Ma2 = 0.8, tip gap 2% of chord



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