Numerical simulations have been carried out to define the loss generation mechanisms associated with tip leakage in unshrouded axial turbines. Tip clearance vortex dynamics are a dominant feature of two mechanisms important in determining this loss: (i) decreased swirl velocity due to vortex line contraction in regions of decreasing axial velocity, i.e., adverse pressure gradient, and (ii) vortex breakdown and reverse flow in the vortex core. The mixing losses behave differently from the conventional view of flow exiting a turbine tip clearance. More specifically, it is shown through control volume arguments and computations that as a swirling leakage flow passes through a pressure rise, such as in the aft portion of the suction side of a turbine blade, the mixed-out loss can either decrease or increase. For turbines, the latter typically occurs if the deceleration is large enough to initiate vortex breakdown, and it is demonstrated that this can occur in modern turbines. The effect of blade pressure distribution on clearance losses is illustrated through computational examination of turbine blades with forward loading at the tip and with aft loading. A 15% difference in leakage loss is found between the two due to lower clearance vortex deceleration (lower core static pressure rise) with forward loading and, hence, lower vortex breakdown loss. Additional computational experiments, carried out to define the effects of blade loading, incidence, and solidity, are found to be consistent with the proposed ideas linking blade pressure distribution, vortex breakdown, and turbine tip leakage loss.

References

References
1.
Booth
,
T. C.
,
1985
, “
Importance of Tip Clearance Flows in Turbine Design
,”
Tip Clearance Effects in Axial Turbomachines
, (VKI Lecture Series, 1985-05),
C. H.
Sieverding
, ed.,
VKI, Rhode-St-Genese
,
Belgium
.
2.
Moore
,
J.
, and
Tilton
,
J. S.
,
1988
, “
Tip Leakage Flow in a Linear Turbine Cascade
,”
ASME J. Turbomach.
,
110
(
1
), pp.
18
26
.10.1115/1.3262162
3.
Harvey
,
N. W.
,
2004
, “
Aerothermal Implications of Shroudless and Shrouded Blades
,”
Turbine Blade Tip Design and Tip Clearance Treatment
, (VKI Lecture Series, 2004-02),
VKI, Rhode-St-Genese
,
Belgium
.
4.
Bindon
,
J. P.
,
1989
, “
The Measurement and Formation of Tip Clearance Loss
,”
ASME J. Turbomach.
,
111
(
3
), pp.
257
263
.10.1115/1.3262264
5.
Krishnababu
,
S. K.
,
Hodson
,
H. P.
,
Dawes
,
W. N.
,
Newton
,
P. J.
, and
Lock
,
G. D.
,
2009
, “
Numerical and Experimental Investigation of Tip Leakage Flow and Heat Transfer Using Idealised Rotor-Tip Models at Transonic Conditions
,”
Aeronaut. J.
,
113
(
1141
), pp.
165
175
.
6.
Denton
,
J. D.
,
1993
, “
Loss Mechanisms in Turbomachines
,”
ASME J. Turbomach.
,
115
(
4
), pp.
621
656
.10.1115/1.2929299
7.
Heyes
,
F. J. G.
, and
Hodson
,
H. P.
,
1993
, “
Measurement and Prediction of Tip Clearance Flow in Linear Turbine Cascades
,”
ASME J. Turbomach.
,
115
(
3
), pp.
376
382
.10.1115/1.2929264
8.
Yaras
,
M. I.
, and
Sjolander
,
S. A.
,
1992
, “
Prediction of Tip-Leakage Losses in Axial Turbines
,”
ASME J. Turbomach.
,
114
(
1
), pp.
204
210
.10.1115/1.2927987
9.
Kacker
,
S. C.
, and
Okapuu
,
U.
,
1982
, “
A Mean Line Prediction Method for Axial Flow Turbine Efficiency
,”
ASME J. Eng. Power
,
104
(
1
), pp.
111
119
.10.1115/1.3227240
10.
Spalart
,
P. R.
, and
Allmaras
,
S. R.
,
1992
, “
A One-Equation Turbulence Model for Aerodynamic Flows
,”
AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV
, January 6–9, Paper No. AIAA 92-0439.
11.
Lighthill
,
M. J.
,
1963
, “
Introduction: Boundary-Layer Theory
,”
Laminar Boundary Layers
,
L.
Rosenhead
, ed.,
Oxford University Press
,
New York
.
12.
Young
,
J. B.
, and
Wilcock
,
R. C.
,
2002
, “
Modeling the Air-Cooled Gas Turbine: Part 2—Coolant Flows and Losses
,”
ASME J. Turbomach.
,
124
(
2
), pp.
214
221
.10.1115/1.1415038
13.
Shapiro
,
A. H.
,
1953
,
The Dynamics and Thermodynamics of Compressible Flow
, Vol. I,
The Ronald Press Company
,
New York
.
14.
Greitzer
,
E. M.
,
Tan
,
C. S.
, and
Graf
,
M. B.
,
2004
,
Internal Flow
,
Cambridge University Press
,
Cambridge, UK
.
15.
Darmofal
,
D. L.
,
Khan
,
R.
,
Greitzer
,
E. M.
, and
Tan
,
C. S.
,
2001
, “
Vortex Core Behaviour in Confined and Unconfined Geometries: A Quasi-One-Dimensional Model
,”
J. Fluid Mech.
,
449
, pp.
61
84
.10.1017/S0022112001006103
16.
Yakhot
,
V.
,
Orszag
,
S. A.
,
Thangam
,
S.
,
Gatski
,
T. B.
, and
Speziale
,
C. G.
,
1992
, “
Development of Turbulence Models for Shear Flows by a Double Expansion Technique
,”
Phys. Fluids A
,
4
(
7
), pp.
1510
1520
.10.1063/1.858424
17.
Khalid
,
S. A.
,
Khalsa
,
A. S.
,
Waitz
,
I. A.
,
Tan
,
C. S.
,
Greitzer
,
E. M.
,
Cumpsty
,
N. A.
,
Adamczyk
,
J. J.
, and
Marble
,
F. E.
,
1999
, “
Endwall Blockage in Axial Compressors
,”
ASME J. Turbomach.
,
121
(
3
), pp.
499
509
.10.1115/1.2841344
18.
Khalid
,
S. A.
,
1995
, “
The Effects of Tip Clearance on Axial Compressor Pressure Rise
,” Ph.D. thesis,
Massachusetts Institute of Technology
,
Cambridge, MA
.
19.
Kundu
,
P. K.
, and
Cohen
,
I. M.
,
2008
,
Fluid Mechanics
,
Elsevier Academic Press
,
New York
.
20.
Fritz
,
W.
, and
Cummings
,
R. M.
,
2009
, “
Lessons Learned From the Numerical Investigations on the VFE-2 Configuration
,”
Report: Understanding and Modeling Vortical Flows to Improve the Technology Readiness Level for Military Aircraft
, NASA Center for AeroSpace Information, Hanover, MD, Chap. 34.
21.
Wells
,
J.
,
2009
, “
Effects of Turbulence Modeling on RANS Simulations of Tip Vortices
,” M.S. thesis,
Virginia Polytechnic and State University
,
Blackburg, VA
.
22.
Escue
,
A.
, and
Cui
,
J.
,
2010
, “
Comparison of Turbulence Models in Simulating Swirling Pipe Flows
,”
Appl. Math. Model.
,
34
(
10
), pp.
2840
2849
.10.1016/j.apm.2009.12.018
23.
Revell
,
A.
,
Iaccarino
,
G.
, and
Wu
,
X.
,
2006
, “
Advanced RANS Modeling of Wingtip Vortex Flows
,”
Center for Turbulence Research, Proceedings of the 2006 Summer Program
,
Stanford, CA, July 9–August 4,
pp.
73
86
.
24.
Menter
,
F. R.
,
1994
, “
Two-Equation Eddy-Viscosity Turbulence Models for Engineering Applications
,”
AIAA J.
,
32
(
8
), pp.
1598
1605
.10.2514/3.12149
You do not currently have access to this content.