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Research Papers

The Effect of Airfoil Thickness on the Efficiency of Low-Pressure Turbines

[+] Author and Article Information
Diego Torre, Guillermo García-Valdecasas

Industria de Turbopropulsores S.A.,
Madrid 28830, Spain

Raúl Vázquez

Industria de Turbopropulsores S.A.,
Madrid 28830, Spain;
Universidad Politécnica,
Madrid 28040, Spain

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 14, 2012; final manuscript received July 22, 2013; published online October 23, 2013. Editor: Ronald Bunker.

J. Turbomach 136(5), 051014 (Oct 23, 2013) (9 pages) Paper No: TURBO-12-1218; doi: 10.1115/1.4025163 History: Received November 14, 2012; Revised July 22, 2013

The effect of airfoil thickness on the efficiency of low-pressure (LP) turbines has been investigated experimentally in a multistage turbine high-speed rig. The rig consists of three stages of a state of the art LP turbine. The stages are characterized by a very high hade angle, reverse cut-off design, very high lift, and very high aspect ratio airfoils. Two different sets of stators have been designed and tested. The first set of stators is made of airfoils with a thickness to chord ratio around 10% along the span with the exception of a small areas close to the end walls. In those areas, the thickness has been increased above the previous value to reduce the secondary flows. These types of airfoils have been referred to in the literature as “spoon” airfoils. The second set of stators has been designed to have the same spanwise distribution of pressure coefficient (Cp) on the suction surface than the first set. However, the thickness to chord ratio was increased along the span up to values around 20% to raise the velocity of the flow and to remove any separation bubble on the pressure side. The resulting shape of the profiles is representative of “hollow” airfoils. The velocity triangles, chord distribution, leading and trailing edge locations, and flowpath have been maintained between both sets. They have been tested with the same blades and at the same operating conditions with the intention of determining the impact of the profile thickness on the overall efficiency. The turbine characteristics: sensitivity to speed, specific work, Reynolds number, and purge flows have been obtained for both sets. The comparison of the results suggests that the efficiency of both types of airfoils exhibit the same behavior; no significant differences in the results can be distinguished.

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Figures

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Fig. 6

Predicted streamlines and negative velocity contours at pressure side of NGVs of rig-E

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Fig. 5

Predicted streamlines and negative velocity contours at pressure side of NGV2 of rig-D

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Fig. 4

Predicted contours of isentropic velocity around the leading edge for profile A (left) and profile B (right)

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Fig. 3

Predicted streamlines profile A (left) and profile B (right)

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Fig. 2

Geometry comparison of two thin solid airfoils with different design style

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Fig. 1

Predicted streamlines at midspan section of NGV2 of rig-D (left) and rig-E (right)

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Fig. 11

Measured and predicted midspan Cp distributions of NGV2 (a) and NGV3 (b) at design conditions

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Fig. 12

Measured and predicted midspan Cp distributions of NGV2 (a) and NGV3 (b) at nominal shaft speed and 70% specific work

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Fig. 13

Measured and predicted midspan Cp distributions of NGV2 (a) and NGV3 (b) at nominal shaft speed and 40% specific work

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Fig. 8

Efficiency versus specific work at nominal shaft speed

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Fig. 9

Radial distribution of efficiency at design conditions

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Fig. 10

Measured and predicted Cp distributions for 25% (a), 50% (b), and 75% (c) span locations of NGV3 at design conditions

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Fig. 14

Efficiency versus specific work at 60%, 80%, 100%, and 120% of the nominal shaft speed

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Fig. 15

Measured midspan Cp distributions of NGV3 at 60% shaft speed and 40% (a) and 70% (b) specific work

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Fig. 16

Delta efficiency with design conditions versus Reynolds number

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Fig. 17

Delta efficiency with purge flow injection

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