Research Papers

The Effect of Clearance on Shrouded and Unshrouded Turbines at Two Levels of Reaction

[+] Author and Article Information
Sungho Yoon

e-mail: sungho.yoon@cantab.net

John Longley

Whittle Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge, UK

Clearance refers to both the shrouded and unshrouded configurations.

Assumed to be measured but not explicitly stated in the source text.

Early steam turbine designers often used impulse turbines in order to minimize the tip leakage flow over the shroud, Harris [13].

At the smallest clearance.

The performance was also measured at the fixed design work coefficient. Since the results and the conclusions drawn from the fixed design work coefficient are very similar to those from the fixed flow coefficient, they are not included.

The 24% reaction turbine has a higher flow velocity across the upstream cavity than the 50% reaction turbine but it has a lower flow velocity across the downstream cavity.

Estimated using the best-fit curves through the data.

The equations can be directly applied with the flow angles at the rotor inlet and the exit, which can be obtained from traverse measurements. Alternatively, the flow angles can be expressed as a function of the flow coefficient, the work coefficient, and the level of reaction as demonstrated in Yoon [14]. Then the data in Table 1 are sufficient to predict the efficiency penalty.

The reduced mass flow through the rotor will reduce the rotor pressure drop and hence the turbine exit pressure will increase.

1Currently at GE Global Research Center, Munich, Germany.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received February 10, 2013; final manuscript received February 22, 2013; published online September 26, 2013. Editor: David Wisler.

J. Turbomach 136(2), 021013 (Sep 26, 2013) (9 pages) Paper No: TURBO-13-1022; doi: 10.1115/1.4023942 History: Received February 10, 2013; Revised February 22, 2013

In this paper, the effect of seal clearance on the efficiency of a turbine with a shrouded rotor is compared with the effect of the tip clearance when the same turbine has an unshrouded rotor. The shrouded versus unshrouded comparison was undertaken for two turbine stage designs one having 50% reaction, the other having 24% reaction. Measurements for a range of clearances, including very small clearances, showed three important phenomena. Firstly, as the clearance is reduced, there is a “break-even clearance” at which both the shrouded turbine and the unshrouded turbine have the same efficiency. If the clearance is reduced further, the unshrouded turbine performs better than the shrouded turbine, with the difference at zero clearance termed the “offset loss.” This is contrary to the traditional assumption that both shrouded and unshrouded turbines have the same efficiency at zero clearance. The physics of the break-even clearance and the offset loss are discussed. Secondly, the use of a lower reaction had the effect of reducing the tip leakage efficiency penalty for both the shrouded and the unshrouded turbines. In order to understand the effect of reaction on the tip leakage, an analytical model was used and it was found that the tip leakage efficiency penalty should be understood as the dissipated kinetic energy rather than either the tip leakage mass flow rate or the tip leakage loss coefficient. Thirdly, it was also observed that, at a fixed flow coefficient, the fractional change in the output power with clearance was approximately twice the fractional change in efficiency with clearance. This was explained by using an analytical model.

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Fig. 1

The break-even clearance and the offset loss (reproduced from Harvey [3])

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Fig. 2

The effect of reaction on the efficiency of unshrouded turbines (Abianc [5])

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Fig. 3

Intermediate pressure (IP) turbine rig

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Fig. 4

Shrouded and unshrouded rotor configuration

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Fig. 5

Axial-radial movement of the rotor blade between static and running conditions

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Fig. 6

Effect of clearance on the 50% reaction shrouded and unshrouded turbines

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Fig. 7

Effect of clearance on the 24% reaction shrouded and unshrouded turbines

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Fig. 8

Fraction of the measured shroud pressure drop that occurred across the balsa seal for the shrouded turbines

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Fig. 9

Measured efficiency versus clearance for the four turbines tested (with best-fit curves)

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Fig. 10

Measured stage loading coefficient versus clearance (at the design flow coefficient)




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