Research Papers

The Effect of Surface Roughness on Efficiency of Low Pressure Turbines

[+] Author and Article Information
Raúl Vázquez

UPM, Universidad Politécnica,
ITP, Industria de Turbo Propulsores S.A.,
Madrid 28830, Spain
e-mail: raul.vazquez@itp.es

Diego Torre

ITP, Industria de Turbo Propulsores S.A.
Madrid 28830, Spain
e-mail: diego.torre@itp.es

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 21, 2013; final manuscript received August 8, 2013; published online November 19, 2013. Editor: Ronald Bunker.

J. Turbomach 136(6), 061008 (Nov 19, 2013) (7 pages) Paper No: TURBO-13-1105; doi: 10.1115/1.4025571 History: Received June 21, 2013; Revised August 08, 2013

The effect of surface roughness on the efficiency of low pressure turbines (LPTs) was experimentally investigated in a multistage turbine high-speed rig. The rig consisted of three stages of a state-of-the-art LPT. The stages were characterized by a very high wall-slope angle, reverse cut-off design, very high lift, and very high aspect ratio airfoils. Two sets of airfoils (both stators and rotors) were tested. The first set was made of airfoils with a roughness size of 0.7 μm Ra (25–35 × 10−5 ks/Cm), which was representative of LPT polished airfoils. The surface finish for the second set of airfoils was 1.8 μm Ra for blades and 2.5 μm Ra for stators (approximately 90 × 10−5 in terms of ks/Cm for both stators and blades). The resulting roughness of this set was representative of “as-cast” airfoils of low pressure turbines. The airfoil geometries, velocity triangles, leading and trailing edge locations, and flowpath were maintained between both sets. They were tested with the same instrumentation and at the same operating conditions with the intention of determining the isolated impact of the surface roughness on the overall efficiency. The turbine characteristics: sensitivity to speed, specific work, Reynolds number, and purge flows, were obtained for both sets. The comparison of the results suggests that the efficiency and capacity of both types of airfoils exhibit the same behavior. No significant differences in the results can be distinguished for the range of operating conditions in this study. The results agree with previous studies of distributed roughness in turbines: the use of as-cast rough airfoils in some low pressure turbines at high altitude does not introduce additional pressure losses.

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Fig. 1

Variation of the roughness Reynolds number over the critical Reynolds number with altitude

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Fig. 2

View of the rig installed in the facility

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Fig. 4

Measured and calculated Cp distributions for (a) 25%, (b) 50%, and (c) 75% (c) span locations of the third stator at design conditions

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Fig. 5

LPT efficiency variation with Reynolds number or Reynolds lapse with altitude

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Fig. 6

Radial distribution of efficiency at design conditions

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Fig. 7

LPT efficiency versus specific work at the nominal shaft speed (top) and at 60% of the nominal speed (bottom)

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Fig. 8

Measured and predicted midspan Cp distributions of (a) stator 2, and (b) stator 3 at the nominal shaft speed and 40% of the nominal specific work

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Fig. 9

Measured and predicted midspan Cp distributions of stator 3 at 60% of the nominal shaft speed and (a) 40% of specific work, and (b) 70% of the nominal specific work

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Fig. 10

LPT efficiency variation with the amount of injected purge/cooling flow




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