CUED Publications database

The effect of clearance on shrouded and unshrouded turbines at two levels of reaction

Yoon, S and Curtis, E and Denton, J and Longley, J (2010) The effect of clearance on shrouded and unshrouded turbines at two levels of reaction. Proceedings of the ASME Turbo Expo, 7. pp. 1231-1241.

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Abstract

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. © 2010 by ASME.

Item Type: Article
Subjects: UNSPECIFIED
Divisions: Div A > Turbomachinery
Depositing User: Cron Job
Date Deposited: 07 Mar 2014 12:04
Last Modified: 28 Nov 2014 19:05
DOI: 10.1115/GT2010-22541