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Research Papers: Gas Turbines: Turbomachinery

The Effect of the Degree of Reaction on the Leakage Loss in Steam Turbines

[+] Author and Article Information
Sungho Yoon

Alstom Power,
Rugby, U.K.
e-mail: sungho.yoon@cantab.net

This section includes overlap with the author's previous paper, Yoon et al [1]. However, this is necessary as a basis for further studies presented in the following sections.

Contributed by International Gas Turbine Institute (IGTI) division of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received July 2, 2012; final manuscript received August 17, 2012; published online January 10, 2013. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(2), 022602 (Jan 10, 2013) (9 pages) Paper No: GTP-12-1247; doi: 10.1115/1.4007772 History: Received July 02, 2012; Revised August 17, 2012

The degree of reaction selected in designing steam turbines is of paramount importance. There has been competition between 50% reaction and impulse turbines over a century. It is, therefore, important to understand the effect of the degree of reaction on aerodynamic performance. In particular, a change in the degree of reaction affects the leakage flow substantially in both the stationary and rotating blades due to a change in the blade loading. The effect of the degree of reaction on the efficiency loss due to leakage flows is systematically investigated in this paper using analytical models. It is shown that the appropriate way to understand the efficiency loss due to leakage flows is to estimate the kinetic energy dissipation rather than the leakage mass flow rate, as demonstrated by Yoon et al. (Yoon, S., Curtis, E., Denton, J., and Longley, J., 2010, “The Effect of Clearance on Shrouded and Unshrouded Turbine at Two Different Levels of Reaction,” ASME Paper No. GT2010-22541). In order to estimate the efficiency loss due to leakage flows, the well-known Denton model (Denton, J. D., 1993, “Loss Mechanisms in Turbomachinery,” ASME J. Turbomach., 115, pp. 621–656) is extended by considering the velocity triangles in a repeating turbine stage. The extended model is compared with experimental data, at different degrees of reaction, and shows good agreement with measurements. It is shown that a reduction in the degree of reaction, at a fixed flow coefficient and a fixed work coefficient, results in an increase in the efficiency loss across the stationary blade but a decrease in that across the rotating blade. However, the efficiency loss across the stationary blade hub is estimated to be smaller than the efficiency loss across the rotating blade tip. A stationary blade can be better sealed than a rotating blade by applying multiple seals and using a leakage path with a low radius. The efficiency loss due to the tip leakage flow is substantially influenced by the choice of the tip configuration. Shrouded blades show several aerodynamic advantages over unshrouded blades in reducing the tip leakage efficiency loss. Employing multiple seals over the shroud decreases the tip leakage mass flow rate significantly. Moreover, as the degree of reaction approaches zero, the tip leakage mass flow rate over the shroud becomes small since the axial pressure drop across the rotating blade becomes small. In unshrouded blades, a reduction in the degree of reaction is shown to increase the leakage mass flow rate over the tip because the circumferential pressure difference between the blade pressure side and blade suction side generally increases when the pitch-to-chord ratio remains unchanged.

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References

Yoon, S., Curtis, E., Denton, J., and Longley, J., 2010, “The Effect of Clearance on Shrouded and Unshrouded Turbine at Two Different Levels of Reaction,” ASME Paper No. GT2010-22541. [CrossRef]
Harris, F. R., 1984, “The Parsons Centenary—A Hundred Years of Steam Turbine,” P. I. Mech. Eng. A, 53, pp. 193–224. [CrossRef]
Walker, P. J., and Hesketh, J. A., 1999, “Design of Low-Reaction Steam Turbine Blades,” P. I. Mech. Eng. C-J. Mec., 213, pp. 151–171. [CrossRef]
Havakechian, S., and Greim, R., 1999, “Aerodynamic Design of 50 Percent Reaction Steam Turbines,” P. I. Mech. Eng. C-J.Mec., 213, pp. 1–25. [CrossRef]
Harvey, N. W., 2004, “Turbine Blade Tip Design and Tip Clearance Treatment,” VKI Lecture Series 2004-02.
Denton, J. D., 1993, “Loss Mechanisms in Turbomachinery,” ASME J. Turbomach., 115, pp. 621–656. [CrossRef]
Cordes, G., 1963, Strömungstechnik der Gasbeaufschlagten Axialturbine, Springer-Verlag, Berlin.
AbiancV. H., 1953, “Teorija aviacionnyh gazovyh turbin, Oborongiz.”
Stechkin, B. S., Kazandzhan, P. K., Alekseev, L. P., GovorovA. N., Nechaev JuN. I., and FjodorovR. M., 1956, “Teorija Reaktivnyh Dvigatelej, Lopatochnye mashiny. Moskva, Oborongiz.”
Hong, Y. S., and Groh, F. G., 1996, “Axial Turbine Loss Analysis and Efficiency Prediction Method,” Boeing Report D4-320.
Glassman, A. J., 1973, “Turbine Design and Application,” Vol. 2, NASA SP 290.
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Yoon, S., 2009, “Advanced Aerodynamic Design of the Intermediate Pressure Turbine for Aero-Engines,” Ph.D. thesis, Cambridge University, Cambridge, UK.
Wheeler, A. P. S., Korankiantis, T., and Banneheke, S., 2001, “Tip Leakage Losses in Subsonic and Transonic Blade Rows,” ASME Paper No. GT2011-45798. [CrossRef] [CrossRef]
Simon, V., Stephan, I., Bell, R. M., Capelle, U., Deckers, M., Schnaus, J., and Simkine, M., 1997, “Axial Steam Turbines With Variable-Reaction Blading,” Proceedings of the 4th Int. Charles Parsons Turbine Conference, Newcastle Upon Tyne, UK, November 4–6, pp. 46–60.
Denton, J. D., and Xu, L., 1999, “The Exploitation of Three-Dimensional Flow in Turbomachinery Design,” P. I. Mech. Eng. C-J. Mec., 112, pp. 125–137. [CrossRef]
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Curtis, E. M., 2005, private communication.

Figures

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Fig. 1

Impulse turbine versus reaction turbine (based on Harris, [2])

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Fig. 2

Driving pressure for the leakage flow

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Fig. 3

Correlated efficiency loss due to the tip leakage flow for the range of the degree of reaction from zero to 70% (Hong and Groh, [10])

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Fig. 4

Velocity triangles in a repeating stage

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Fig. 5

Change in measured efficiencies, with respect to the tip clearance, at two degrees of reaction, Yoon et al. [1]

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Fig. 6

Contours of normalized relative kinetic energy at the exit of the rotating blade, (W3/U)2

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Fig. 7

Contours of loss coefficients (ζR) due to the tip leakage flow for shrouded and unshrouded rotating blades at g/h = 0.01

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Fig. 8

Contours of efficiency loss (Δη, %) due to the tip leakage flow for shrouded and unshrouded rotating blades at g/h = 0.01

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Fig. 9

Effect of multiple seals on the contours of efficiency loss (Δη, %) due to the tip leakage flow in shrouded blades at g/h = 0.01

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Fig. 10

Contours of efficiency loss (Δη, %) due to the hub leakage flow across the stationary blade at g/h = 0.01

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Fig. 11

Contours of efficiency loss (Δη, %) due to the hub and tip leakage flow, in a repeating stage, at g/h = 0.01. The rotating blade is shrouded with three seals and the stationary blade has either (a) three seals or (b) six seals.

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Fig. 12

Contours of efficiency loss (Δη, %) due to the hub and tip leakage flow in a repeating stage, at g/h = 0.01. The rotating blade is unshrouded and the stationary blade has either (a) three seals or (b) six seals.

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