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Research Papers

The Reduction of Over Tip Leakage Loss in Unshrouded Axial Turbines Using Winglets and Squealers

[+] Author and Article Information
Howard Hodson

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, 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 April 8, 2013; published online September 26, 2013. Assoc. Editor: David Wisler.

J. Turbomach 136(4), 041001 (Sep 26, 2013) (11 pages) Paper No: TURBO-12-1247; doi: 10.1115/1.4024677 History: Received December 19, 2012; Revised April 08, 2013

The possibilities of reducing the over tip leakage loss of unshrouded rotors have been investigated using a linear cascade of turbine blades and computational fluid dynamics (CFD). The large-scale blade profile is the same as that of the tip profile of a low-speed high-pressure research turbine facility. The impact of various combinations of squealer and winglet geometries on the turbine performance has been investigated. The influence of the thickness of the squealers has also been assessed. It was found that a 22% reduction in loss slope was possible, when compared to the flat tip blade, using simple tip modifications. The results obtained with the suction side squealer and cavity tip agreed well with the work of other researchers. Three winglet-based tip geometries were tested. One was a plain winglet, the other two had squealers applied. A significant impact of the squealers and their shape on the tip gap flow pattern and loss generation was found. The physical processes occurring within the tip gap region for the tested geometries are explained using both numerical and experimental results. The impact of the flow pattern within the tip gap on the loss generation is described. Good agreement between CFD and the experimental data was found. This shows that CFD can be used with confidence in the design process of shroudless turbines.

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References

Figures

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Fig. 1

Flow through the tip gap for an unshrouded blade [2]

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Fig. 2

General features of cascade

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Fig. 4

Squealer and winglet geometries: (a) WING1, (b) WING2, (c) WING3, (d) cavity tip, (e) suction side squealer

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Fig. 5

WING1 front part detail

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Fig. 6

Computational domain

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Fig. 7

Blade loading at midspan

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Fig. 8

Predicted flow pattern on flat tip. Color represents velocity magnitude (blue–low, red–high).

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Fig. 9

Total pressure loss coefficient contours on tangential plane at 0.5Cy (flat tip–CFD)

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Fig. 10

Tip surface Cp distribution (flat tip–CFD)

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Fig. 11

Line Cp profile on casing and blade tip at 0.5Cx (flat tip)

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Fig. 12

Total pressure loss coefficient contours on tangential plane at 0.5Cy (cavity tip–CFD)

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Fig. 13

Mixed-out total pressure loss coefficient comparison

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Fig. 14

Predicted flow pattern for WING1. Color represents velocity magnitude (blue–low, red–high).

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Fig. 15

Total pressure loss coefficient contours on tangential plane at 0.5Cy (WING1–CFD)

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Fig. 16

Tip surface Cp distribution (WING1–CFD)

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Fig. 17

Line Cp profile on casing and blade tip at 0.5Cx (WING1)

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Fig. 18

Calculated tip region driving pressure for flat tip and WING1

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Fig. 19

Total pressure loss coefficient contours on tangential plane at 0.5Cy (WING2–CFD)

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Fig. 20

Experimental loss coefficient distribution on axial plane 0.5Cx downstream of the trailing edges. (a) Flat tip, (b) WING1.

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Fig. 21

Pitchwise mass-averaged Yp at plane 0.5Cx downstream of the trailing edges

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Fig. 22

Mixed-out total pressure loss coefficient comparison

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