ASME Turbo Expo 2010 / GT2010-23024
Numerical Study on Unsteadiness of Tip Clearance Flow
Induced by Downstream Stator Row in Axial Compressor
Yoojun Hwang* ∙ Shin-Hyoung Kang* ∙ Sungryoung Lee**
June 17, 2010
* Mechanical and Aerospace Engineering, Seoul National University, Korea
** Doosan Heavy Industries and Construction Co., Ltd, Korea
Contents
 Introduction
 Calculation Models and Methods
 Results
– Unsteady Flow Structure
– Effect of Downstream Stator Row
– Non-Synchronous Vibration
 Concluding Remarks
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Introduction (1/2)
 Previous Studies
– Unsteady Tip Clearance Flow in Axial Compressors near Stall
 Periodically fluctuating tip leakage vortex was investigated by Mailach et al.
(2001)1, Marz et al. (2002)2, Kielb et al. (2003)3, Bae et al. (2004)4, Hah et al.
(2008)5, etc.
 The unsteady flow was referred to as self-induced unsteadiness.
 The origin or the role of the flow has been studied by Du et al. (2010)6,
Thomassin et al. (2009)7, Drolet et al. (2009)8, etc.
Mailach, R., Lehmann, I., and Vogeler, K., 2001, “Rotating Instabilities in an Axial Compressor Originating From the Fluctuating Blade Tip Vortex,” Journal of
Turbomachinery, Vol. 123, pp. 453-463.
2 März, J., Hah, C., and Neise, W., 2002, “An Experimental and Numerical Investigating Into the Mechanisms of Rotating Instability,” Journal of Turbomachinery,
Vol. 124, pp. 367-375.
3 Kielb, R. E., Barter, J. W., Thomas, J. P., and Hall, K. C., 2003, “Blade Excitation by Aerodynamics Instatbilites — A Compressor Blade Study,” ASME Turbo Expo
2003, GT2003-38634.
4 Bae, J., Breuer, K. S., and Tan, C. S., 2004, “Periodic Unsteadiness of Compressor Tip Clearance Vortex,” ASME Turbo Expo 2004, GT2004-53015.
5 Hah, C., Bergner, J., and Schiffer, H.-P., 2008, “Tip Clearance Vortex Oscillation, Vortex Shedding and Rotating Instability in an Axial Transonic Compressor
Rotor,” ASME Turbo Expo 2008, GT2008-50105.
6 Du, J., Lin, F., Zhang, H., and Chen, J., 2010, “Numerical Investigation on the Self-Induced Unsteadiness in Tip Leakage Flow for a Transonic Fan Rotor,”
Journal of Turbomachinery, Vol. 132, pp. 021017.
7 Thomassin, J., Vo, H. D., and Mureithi, N. W., 2009, “Blade Tip Clearance Flow and Compressor Nonsynchronous Vibrations: The Jet Core Feedback Theory as
the Coupling Mechanism,” Journal of Turbomachinery, Vol. 132, pp. 011013.
8 Drolet, M., Thomassin, J., Vo, H. D., and Mureithi, N. W., 2009, “Numerical Investigation into Non-Synchronous Vibrations of Axial Flow Compressors by the
Resonant Tip Clearance Flow,” ASME Turbo Expo 2009, GT2009-59074
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Introduction (2/2)
 Motivation
– In the previous studies, the unsteady tip leakage flow has been found to be
inherent.
– Most of the numerical investigations have been done only for a rotor row or for
single blade passages.
 Objective
– Investigate the influence of the downstream stator row on the unsteady flow
– Conduct time-accurate numerical calculations for a stage
– Performance characteristic, unsteady flow structure, tip leakage flow vibration
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Calculation Models and Methods (1/4)
 Compressor Model
–
–
–
–
–
Low speed research axial compressor (LSRC)
4 stages
Number of blades: IGV(53), Rotor(54), Stator(74)
Hub-to-tip ratio: 0.85
Tip clearance size to blade height: 2.8%
 Experimentally Measured Data
– Performance measured by Wisler (1981)1
– 1st stage has a casing treatment with circumferential grooves.
 Numerically Calculated Data
–
–
–
–
1
Code: ANSYS-CFX 11.0
Standard k-ε model with the wall function
Structured H-mesh with as a coarse grid as 40,000 cells/blade passage
No casing treatment
Wisler, D. C., 1981, “Core Compressor Exit Stage Study Volume IV—Data and Performance Report
for the Best Stage Configuration,” NASA CR-165357.
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Calculation Models and Methods (2/4)
 Performance Map

– Averaging 4 stages
H s
, 
2
1U
2 t
cx
Ut
 Calculation Data
• Steady-state assumption at the frame
interfaces
• Single blade passage
 Casing Treatment Effect - Wisler (1981)
• No change in pressure rise
• 8.2% improvement in stall margin
 Similar trend around the design point
[Wisler (1981)]
 The calculation underestimated the
pressure rise by 7% at the design point.
 The operating range from the
calculation is shorter.
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Calculation Models and Methods (3/4)
 Effect of the Wake from the Upstream Blade
– Total temperature in the wake is higher than that in the core flow.
– Not captured in steady-state calculations
 Effect of the Wake
• Up to 5.2% of pressure rise
 Need Unsteady Calculations
Negative Jet Effect - Mailach et al. (2008)1
1
Beneficial Effect of Wake
- Sirakov et al. (2003)2
Mailach, R., Lehmann, I. and Vogeler, K., 2008, “Periodic Unsteady Flow Within a Rotor Row of an Axial Compressor—
Part II: Wake-Tip Clearance Vortex Interaction,” ASME J. Turbomachinery, Vol. 130, 041005.
2
Sirakov, B. T. and Tan, C. S., 2003, “Effect of Unsteady Stator Wake—Rotor Double-Leakage Tip Clearance Flow Interaction
on Time-Average Compressor Performance,” ASME J. Turbomachinery, Vol. 125, pp. 465-474
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Calculation Models and Methods (4/4)
 Unsteady Calculation Method
– Modified 1/8 annulus
 Number of Blades for Each Row
• Rotor: 54  56
• Stator: 74  72
 IGV + Additional Domain
• Single blade + Mixing-Plane
• Provide circumferentially uniform flow to the inlet
of the rotor row
Numerical monitor
 Boundary Conditions
• Inlet: Atmospheric conditions (Pt, Tt)
• Exit: Mass flow rate  Adjust operating conditions
 Calculation Process
• Reducing mass flow rate from the design point
• One rotor revolution at each point
 Numerical Monitor
• Between rotor and stator in the stationary frame
• Static pressure
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (1/5)
 Entropy Distribution
– Contours at 50% and 90% span height
– Design point
T
p
s  C p ln(
)  R ln(
) , ref : inletof rotor
Tref
pref
 50% Span
• Wakes from the rotor blades
• The structures are identical at
every blade passage.
 90% Span
• The tip leakage flow interacts with
the wakes.
• The structures are not identical at
every blade passage.
• The tip leakage flow varies with
time.
50% span
90% span
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (2/5)
 Entropy Distribution
– Contours at 50% and 90% span height
– Design point
– Without stator row
 Without Stator
• The tip leakage flow structures
are identical at every blade
passage.
• Tip leakage flow varies with time.
 Potential effect of the downstream
stator row on the rotor tip leakage
flow behavior
50% span
90% span
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (3/5)
 Performance Characteristics
– Unsteady and Steady-state calculations
– Modified 1/8 annulus model
 Pressure Rise
• The unsteady calculation improves the
underestimation of the steady-state
calculation.
• The difference increases as the flow
rate decreases.
 Operating Range
• The steady-state calculation predicts
the limit earlier.
 Low Flow Rate
• Plateau on the curve
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (4/5)
 Axial Velocity Distribution
– Contours at the exit of the rotor row
– 86% of the design point
 Blockage
• The steady-state calculation predicts
more blockage near the casing.
• In the unsteady calculation result, the
tip leakage vortex blocks less area.
Unsteady
 Predicted blockage may have caused
the pressure rise difference.
Steady-state
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Unsteady Flow Structure (5/5)
 Velocity Distribution
– Axial & tangential velocities at the exit of the rotor row
– Circumferentially averaged
 Axial Velocity
• Higher at 72% than that
at 80% near the casing
 Tangential Velocity
• Flow turning at 72% is not
much smaller than that at 80%.
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Plateau
 Pressure rise does not
significantly decrease at
the low flow rates.
Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Effect of Downstream Stator Row (1/2)
 Formation of Tip Leakage Vortex
– Contours of pressure and streamlines at 90% span
– At 80% of the design mass flow rate
 Role of Pressure Field
• Pressure gradient between the
rotor and the stator pushes the tip
leakage flow.
• Pressure difference variation
across the blade tip makes the
leakage flows different for the two
adjacent blade passages.
 The leakage flow forms
Rotating instability
Static pressure
Streamlines
Velocity vectors
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Effect of Downstream Stator Row (2/2)
 Rotating Instability
 Circumferential mode order is nearly half
the blade number - Mailach et al. (2001)
 Interactions of the upstream wake and the
tip leakage flow - Mailach et al. (2008)
Experiment
 Affected by the axial gap between blade
rows - Deng et al. (2005)1
Propagation of Rotating Instabilities
- Mailach et al. (2001)
 Resonant tip clearance flow
- Drolet et al. (2009)
CFD
Single blade
 Self-induced unsteadiness
- Du et al. (2010)
1
Deng, X., Zhang, H., Chen, J. and Huang, W., 2005, “Unsteady Tip Clearance Flow in a Low-Speed Axial Compressor Rotor
with Upstream and Downstream Stators,” ASME paper GT2005-68571.
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (1/3)
 Tip Leakage Flow
– Time-variation at the rotor row
– Design mass flow rate
 Rotating Instability
• Not correspond to the blade periodicity
• Rotates at 47% of the rotor speed
• NSV frequency: 498Hz
• Blade passing frequency: 747Hz
Negative Axial Velocity
Pressure Signal
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (2/3)
 Tip Leakage Flow
– Time-variation at the rotor row
– 80% of the design mass flow rate
 Rotating Instability
• 4 discrete vortices
• Rotates at 67% of the rotor speed
• NSV frequency: 285Hz
Negative Axial Velocity
Pressure Signal
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Non-Synchronous Vibration (3/3)
 Tip Leakage Flow
– At the rotor row
– 72% of the design mass flow rate
 Rotating Instability
• 3 discrete vortices
• Rotates at 72% of the rotor speed
• NSV frequency: 231Hz
Negative Axial Velocity
Pressure Signal
Operating point
(Mass flow rate)
100%
80%
72%
Circumferential mode
order per annulus
80
32
24
Rotating speed
47%
67%
72%
NSV frequency
498Hz
285Hz
231Hz
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Concluding Remarks
 Unsteady tip leakage flow was influenced by the potential effect of the
downstream stator row.
 Rotating instability developed as the flow rate was reduced.
 The speed and the circumferential mode order of the rotating instability
varied with the flow rate, which corresponded to unsteady tip leakage flow
frequency.
 In future work, further calculations towards or beyond stall is needed to
investigate the behavior of the unsteadiness.
 Acknowledgement
Supported by R&D Research Fund from the Korea Institute of Energy Technology Evaluation and
Planning in the Ministry of Knowledge Economy, Korea.
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024
Thank you for your attention.
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Turbo Expo 2010, Glasgow, UK, June 14-18, 2010 / GT2010-23024