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10th Asian International Conference on Fluid Machinery
Flow and Performance Calculations of Axial Compressor
near Stall Margin
Yoojun Hwang ∙ Shin-Hyoung Kang
Oct. 22 2009
Turbo System and Control Laboratory
Seoul National University
Contents
 Introduction
 Method
 Result: Steady-State Calculation
 Result: Unsteady Calculation
 Conclusion
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Introduction
 Motivation and Objective
– Numerical calculations have been conducted to investigate the flow and the
performance of compressors
– Steady-state assumptions have been widely used in many fields owing to the
time and cost
– Steady-state calculations are inappropriate to examine unstable phenomena
– Unsteady calculations have been conducted to inspect near-stall behaviors
• 2D Euler equation was solved by Saxer-Felici et al. (2000)
• Aeroelastic behavior with the changeable area boundary concept was studied by
Vahdati et al. (2008)
• Full-annulus simulation was conducted by Chen et al. (2008)
 The difference between the unsteady calculation and the steady-state
calculation should be investigated
 In particular, the calculation near stall margin should be studied
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Method (1/2)
 Compressor for Numerical Calculations
– Low Speed Research Compressor at Seoul National University (SNU LSRC)
• 4 stages (identical)
• 2/3 down-scaled version of GE LSRC
• Rotate at 800 rpm
Parameter
SNU LSRC
GE LSRC
Casing Diameter
1.0 m
1.5 m
Hub to Tip Ratio
0.85
0.85
Airfoil Chord
6 cm
9 cm
2.8 %
1.36 %
IGV
53
53
Rotor
54
54
Stator
74
74
Tip Clearance
(relative to blade height)
Blade
Number
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Method (2/2)
 CFD Commercial Code
– Mesh Generator: ANSYS-TurboGrid 11.0
– Solver: ANSYS-CFX 11.0
 Turbulence Model
– Standard k-ε Model with wall function was applied
• Coarse grid is available rather than k-ω model, etc
• Coarse grid is needed since unsteady calculations
require a large amount of time and money
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 Grid Dependency
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– 80,000 cells / blade passage
– 40,000 cells / blade passage were selected
cx [m/s]
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• Validated by Kang (2007)*
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• Conduct steady-state calculation
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• Validated by comparing pressure rise and velocity
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distribution
0
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* Kang, Y. S., 2007, “Characteristic Analysis and Prediction of Alfords Force in an Axial Compressor,”
Ph. D. Thesis Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul, Korea
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PS
SS
80,000 cells/passage
40,000 cells/passage
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25
26
27
28
theta [deg]
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Result: Steady-State Calculation (1/3)
 Computational Model (blade number)
– Stationary Domain: IGV (1), Stator (1)
– Rotating Domain: Rotor (1)
 Boundary Conditions
– Inlet: Atmospheric Total Conditions
• Total Pressure (101,325 Pa)
• Total Temperature (288.15 K)
Single Passage Model
– Exit: Mass Flow Rate
• Adjust operating conditions
– Circumferential Planes: Periodic Conditions
– Blade Surfaces, Hub, Casing: No-Slip Conditions
 Interfaces
– Between Domains (Rotating to Stationary Domain and vice versa)
– Frozen-Rotor Scheme: Assume blades are fixed relatively
– Mixing-Plane Scheme: Average circumferential variations
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Result: Steady-State Calculation (2/3)
 Performance Characteristics
– GE LSRC: Calculation, Measured Data by Experiment
– SNU LSRC: Calculation
0.7
cx
, 
Ut
p
1
2
U t2
 Tip Clearance Size
 No difference at high flow rates
 Difference grows as the flow rate
reduces (7.3 % at design point)
 Operating range is shorter for the
large tip clearance
0.6
0.5

0.4
design point
0.3
 Explain by flow structures
 Compared with Measured Data
 Calculation underestimates
performance by 7% at the design
point
0.2
t = 1.36% CFD (GE model)
t = 2.80% CFD (SNU model)
t = 1.36% EXPERIMENT *
0.1
0
0.3

0.35
0.4

0.45
0.5
 Treated with unsteady calculations
* GE report by Wisler (1977)
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Result: Steady-State Calculation (3/3)
 Effects of Large Tip Clearance on Flow Structures
– Compare velocities at exit of rotor at 90% of design mass flow rate
0.8
0.8
0.6
0.6
Span
1
Span
1
0.4
0.4
t = 1.36%
t = 2.80%
0.2
0
-0.1
0
0.1
0.2
t = 1.36%
t = 2.80%
0.2
0.3
0.4
0.5
0
0.6
0
0.1
0.2
0.3
0.4
0.5
0.6
cx / U t
w / U t
Axial velocity
Relative tangential velocity
0.7
 Tip clearance flow invades the main flow
 Axial flow distribution is less uniform
 Relative tangential velocity is higher
 Flow turning is smaller in the rotor passage
 The pressure rise is lower
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Result: Unsteady Calculation (1/10)
 Computational Model
– 1.5 Stage
– Modified 1/8 Annulus
Interfaces - Unsteady: Sliding
- Steady: frozen-rotor
Mixing-plane
- Unsteady: Sliding
- Steady: frozen-rotor
• IGV (1) + Rotor (7) + Stator (9)
 1:1 pitch ratio is required
for sliding interface
– Number of Blades
• Rotor: 54  56
• Stator: 74  72
Monitoring point
IGV
– 1 IGV + Additional Domain
• Maintain spanwise variations
• Circumferentially uniform inlet
conditions to the rotor row
Rotor
– Blade Interaction
• Between rotor row and stator row
Additional domain
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Stator
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Result: Unsteady Calculation (2/10)
 Pressure Rise at Design Point
– Time-averaged unsteady calculation result with steady-state calculations
101600
unsteady (time-averaged)
steady / mixing-plane
steady / frozen-rotor
101500
5%
2%
101400
p [Pa]
IGV
Rotor
Stator
 Steady-state Assumption
 Unsteady result is higher than any
other steady-state result
 Unsteady calculation is needed
to obtain better prediction
101300
101200
 Difference
 From the rotor trailing edge to
stator exit
 How to simulate blade interaction
determines the difference
101100
101000
0
0.1
0.2
0.3
 Steady-state Interface Schemes
 Mixing-plane result is higher than
frozen-rotor by 5%
 Frozen-rotor scheme more
underestimates the performance
0.4
x [m]
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Result: Unsteady Calculation (3/10)
 Flow Structure
T
p
s  C p ln(
)  R ln(
) , ref : inletof rotor
Tref
pref
– Meridional plane
– Instantaneous distribution
 Entropy Contour
 represents the losses with
respect to the inlet flow
 wake, vortex, boundary layer,
etc
Rotor tip leakage flow
Stator wake
Rotor wake
Rotor
 Rotor Tip Leakage Flow
 flows downstream
 affects the flow from casing to
70% span height
Stator
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Result: Unsteady Calculation (4/10)
 Flow Structure
– 50% and 80% span plane
– Instantaneous distribution
T
p
s  C p ln(
)  R ln(
) , ref : inletof rotor
Tref
pref
Rotor tip clearance flow
 Rotor Wake
 flows into stator passages and
mixes out gradually
 occurs at every blade with the
same pattern
Rotor wake
 Rotor Tip Clearance Flow
 flows far downstream
 not identical at every blade
(phase difference)
 Formation varies over time
(fluctuation)
 Need to consider the interaction
between the rotor and the stator
50% span
80% span
 Additional calculation without
stator row was conducted
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Result: Unsteady Calculation (5/10)
 Flow Structure Without Stator Row
T
p
s  C p ln(
)  R ln(
) , ref : inletof rotor
Tref
pref
– 50% and 80% span plane
– Instantaneous distribution
 Rotor Tip Clearance Flow
 Formation has no difference
between rotor blades
 Affected by blade interaction
 Explain by frequency analysis
50% span
80% span
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Result: Unsteady Calculation (6/10)
 Frequency Spectrum
– Fourier Transformation of pressure signal at monitoring point
2
2
1.5
1.5
Blade passing frequency
P*
P*
Blade passing frequency
1
71% of bpf
0.5
0
0
1
0.5
1000
2000
0
3000
0
f [Hz]
1000
2000
3000
f [Hz]
With Stator Row
Without Stator Row
 Tip Leakage Flow Fluctuation Frequency
 Induced by the rotor-stator interaction
 Characteristic of compressor with large tip clearance: Mailach (2001)*, Bae (2004)**
* Mailach, R., Sauer, H., Vogeler, K., “The Periodical Interaction of the Tip Clearance Flow in the Blade Rows of Axial
Compressors,” ASME paper 2001-GT-0299
** Bae, J., Breuer, K. S., Tan, C. S., “Periodic Unsteadiness of Compressor Tip Clearance Vortex,” ASME paper GT2004-53015
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Result: Unsteady Calculation (7/10)
 Performance Characteristic

– Modified Compressor Model
0.5
1
2
U t2
 Operating Range
 Steady-state result has short
operating range
• Peak point is around 94% of the
design point
• The lowest point seems to be
near stall margin
 By unsteady calculation, the stall
margin has not been reached
 Unsteady calculation estimates
wide operating range
1.05
0.4

p
 Prediction of Performance
 Unsteady result is higher than
steady-state result
0.6
0.3
0.2
unsteady (time-averaged)
steady / frozen-rotor
0.1
0
0.85
cx
, 
Ut
0.9
0.95


1
 Explain by flow structures
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Result: Unsteady Calculation (8/10)
 Flow Structure
– Design Point and 92% of Design Point
– Axial Velocity Contour at Exit of Stator
Design point
92% of design point
Unsteady
Steady-state
 Core Flow
 Unsteady result has more uniform flow
 Tip Leakage Flow
 As reducing the flow rate, steady-state assumption makes larger separations
 Uniformity needs to be quantified
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Result: Unsteady Calculation (9/10)
 Blockage Factor
B  1

AE
m
 1
A
Ac x ,max
AE : effective area
Cx,max : maximum axial velocity at the area
0.4
 Blockage Factor
 Analogous to displacement thickness in
2-D analysis
 Represents the non-uniformity of the
internal flow
B
0.3
0.2
 Steady-State Calculation
 Results in more blockage
 Pressure rise is lower
0.1
Unsteady
Steady / Frozen-Rotor
0
0.85
0.9
0.95


Exit of stator row
1
1.05
 Low Flow Rate
 Blockage increases as reducing the flow
rate
 Stall seems to be approached
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Result: Unsteady Calculation (10/10)
 Flow Structure
– Tip Leakage Flow Near Casing (90% span height)
Unsteady
95% of design point
Steady-state
92% of design point
95% of design point
92% of design point
 Steady-state Calculation
 More blockage at the leading edge plane than the unsteady result
 Flow Rate
 As reducing the flow rate, blockage at the leading edge plane increases in
both cases
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Conclusion
 Steady-state Calculations
– The computational method was validated with measured data
– The influence of the tip clearance size to the performance was investigated
 Unsteady Calculations
– Unsteadiness was compared with steady-state assuming interfaces
– The tip leakage fluctuation occurred due to blade interactions
– Near stall margin, the performance predicted by the unsteady calculation was
more reasonable than the steady-state calculation
– The difference of the performance and the operating range between unsteady
and steady-state calculations was able to be explained by flow uniformity and
tip clearance flow affected by unsteadiness
 Acknowledgement

This study has been supported by Doosan Heavy Industries and Construction Co., Ltd
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Thank you
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