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.

References

References
1.
Denton
,
J. D.
,
1993
, “
Loss Mechanisms in Turbomachinery
,”
ASME J. Turbomach.
,
115
, pp
621
656
.10.1115/1.2929299
2.
Harvey
,
N. W.
,
2004
, “
Turbine Blade Tip Design and Tip Clearance Treatment
,” VKI Lecture Series 2004-02, Von Karman Institute for Fluid Dynamics, Sint-Genesius-Rode, Belgium.
3.
Harvey
,
N. W.
,
2004
, personal communication.
4.
Denton
,
J. D.
,
2004
, Whittle Lab Internal Seminar.
5.
Abianc
,
V. H.
,
1953
, “
Teorija Aviacionnyh Gazovyh Turbin, Oborongiz
.”
6.
Stechkin
,
B. S.
,
Kazandzhan
,
P. K.
,
Alekseev
,
L. P.
,
Govorov
A. N.
,
Nechaev
Yu. N.
, and
Fjodorov
R. M.
,
1956
, “Teorija Reaktivnyh Dvigatelej, Lopatochnye Mashiny. Moskva, Oborongiz.”
7.
Cordes
,
G.
,
1963
, “
Strömungstechnik der Gasbeaufschlagten Axialturbine
,” Springer-Verlag, Berlin.
8.
Hong
,
Y. S.
, and
Groh
,
F. G.
,
1966
, “
Axial Turbine Loss Analysis and Efficiency Prediction Method
,” Boeing Report D4-320.
9.
Glassman
,
A. J.
,
1973
, “
Turbine Design and Application
,” Vol. 2, NASA SP 290.
10.
Booth
,
T. C.
,
1985
, “
Importance of Tip Clearance Flows in Turbine Design—Tip Clearance Effects in Axial Turbomachines
,” VKI Lecture Series 1985-05, Von Karman Institute for Fluid Dynamics, Sint-Genesius-Rode, Belgium.
11.
Haas
,
J. E.
, and
Kofskey
,
M. G.
,
1979
, “
Effect of Rotor Tip Clearance and Configuration on Overall Performance of a 12.77 Centimeter Tip Diameter Axial-Flow Turbine
,” ASME Paper No. 79-GT-42.
12.
Pullan
,
G.
,
Denton
,
J. D.
, and
Dunkley
,
M.
,
2003
, “
An Experimental and Computational Study of the Formation of a Streamwise Shed Vortex in a Turbine Stage
,”
ASME J. Turbomach.
,
125
, pp.
291
297
.10.1115/1.1545766
13.
Harris
,
F. R.
,
1984
, “
The Parsons Centenary—A Hundred Years of Steam Turbine
”,
P. I. Mech. Eng. A
,
53
, pp.
193
224
.10.1243/PIME_PROC_1984_198_024_02
14.
Yoon
,
S.
,
2009
, “
Advanced Aerodynamic Design of the Intermediate Pressure Turbine for Aero-Engines
,” Ph.D. thesis, Cambridge University, Cambridge, UK.
15.
Farokhi
,
S.
,
1988
, “
Analysis of Rotor Tip Clearance in Axial-Flow Turbines
,”
AIAA J. Prop. Power
, pp.
452
457
.10.2514/3.23087
You do not currently have access to this content.