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Research Papers

Secondary Flow Control in Low Aspect Ratio Vanes Using Splitters

[+] Author and Article Information
Christopher J. Clark

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK
e-mail: cjc95@cam.ac.uk

Graham Pullan, Eric Curtis

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK

Frederic Goenaga

Rolls Royce,
Bristol BS34 7QE, UK

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received December 2, 2016; final manuscript received January 11, 2017; published online April 11, 2017. Editor: Kenneth Hall.

J. Turbomach 139(9), 091003 (Apr 11, 2017) (11 pages) Paper No: TURBO-16-1304; doi: 10.1115/1.4036190 History: Received December 02, 2016; Revised January 11, 2017

Low aspect ratio vanes, often the result of overall engine architecture constraints, create strong secondary flows and high end-wall loss. In this paper, a splitter concept is demonstrated that reduces secondary flow strength and improves stage performance. An analytic conceptual study, corroborated by inviscid computations, shows that the total secondary kinetic energy (SKE) of the secondary flow vortices is reduced when the number of passages is increased and, for a given number of vanes, when the inlet end-wall boundary layer is evenly distributed between the passages. Viscous computations show that, for this to be achieved in a splitter configuration, the pressure-side leg of the low aspect ratio vane horseshoe vortex, must enter the adjacent passage (and not “jump” in front of the splitter leading edge). For a target turbine application, four vane designs were produced using a multi-objective optimization approach. These designs represent current practice for a low aspect ratio vane, a design exempt from thickness constraints, and two designs incorporating splitter vanes. Each geometry is tested experimentally, as a sector, within a low-speed turbine stage. The vane designs with splitter geometries were found to reduce the measured secondary kinetic energy, by up to 85%, to a value similar to the design exempt from thickness constraints. The resulting flow field was also more uniform in both the circumferential and radial directions. One splitter design was selected for a full annulus test where a mixed-out loss reduction, compared to the current practice design, of 15.3% was measured and the stage efficiency increased by 0.88%.

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Figures

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Fig. 1

Comparison of a uniform low aspect ratio vane row and splitter vane concept highlighting the reduced secondary flow penetration achieved by the splitter concept: (a) uniform low aspect ratio vanes and (b) splitter concept (one splitter per low aspect ratio vane)

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Fig. 2

Results for an inviscid blade row computation. Exit secondary kinetic energy, CSKE, reduces as aspect ratio (AR) increases.

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Fig. 3

Streamlines in the inlet end-wall boundary layer to illustrate traditional secondary flow (left) and splitter secondary flows with (middle) and without (right) a vortex jump

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Fig. 4

Meridional view of the test facility showing traverse planes

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Fig. 5

Schematic demonstrating how a changing a conventional variable (stagger) causes a different effect for a nonuniform row compared to a uniform row, in this case causing some throats to open and others to close rather than a uniform closing of throats

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Fig. 6

Blade definition parameters

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Fig. 7

NGV midspan vane profiles: (a) NGV1, (b) NGV2, (c) NGV3, and (d) NGV4

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Fig. 8

NGV midspan surface pressure distributions: (a) NGV2, (b) NGV3, and (c) NGV4

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Fig. 9

Flow visualization on the vane suction surface: (a) NGV1, (b) NGV2, (c) NGV3, and (d) NGV4

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Fig. 10

Flow visualization on the hub end wall for NGV3 (upper) and NGV4 (lower)

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Fig. 11

NGV exit yaw angle; αm=tan−1(Vt/Vm): (a) NGV1, (b) NGV2, (c) NGV3, and (d) NGV4

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Fig. 12

NGV exit spanwise yaw angle distribution

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Fig. 13

NGV exit spanwise flow coefficient distribution

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Fig. 14

NGV exit secondary kinetic energy coefficient distribution

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Fig. 15

Total pressure coefficient; Yp=(P0,1¯¯−P0)/((1/2)ρ(Vm¯¯2+Vt¯¯2)): (a) NGV1, (b) NGV2, (c) NGV3, and (d) NGV4

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Fig. 16

NGV exit total pressure distribution

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Fig. 17

NGV exit mixing loss distribution

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Fig. 18

Stage efficiency characteristics

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Fig. 19

Casing surface streamlines showing the formation of a vortex jump as the chord of the first splitter is reduced: (a) NGV4, (b) −5% chord, and (c) −10% chord

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Fig. 20

Total pressure coefficient; Yp=(P0,1¯¯−P0)/((1/2)ρ(Vm¯¯2+Vt¯¯2)): (a) NGV4, (b) −5% chord, and (c) −10% chord

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Fig. 21

Yaw angle: αm=tan−1(Vt/Vm): (a) NGV4, (b) −5% chord, and (c) −10% chord

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Fig. 22

NGV exit secondary kinetic energy coefficient distribution

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