School of Aerospace Engineering
MITE
Computational Analysis of Stall and Computational Analysis of Stall and Separation Control in Separation Control in
CompressorsCompressors
Lakshmi SankarSaeid Niazi, Alexander Stein
School of Aerospace EngineeringGeorgia Institute of Technology
Supported by the U.S. Army Research Office Under the Multidisciplinary University Research Initiative (MURI) on Intelligent Turbine Engines
School of Aerospace Engineering
MITE
Motivation and ObjectivesMotivation and Objectives• Use CFD to explore and
understand compressor stall and surge
• Develop and test flow control strategies (air-injection, bleeding) for compressors
• Apply CFD to compare low-speed and high-speed configurations
Compressor instabilities can cause fatigue and damage to entire engine
School of Aerospace Engineering
MITE
Summary of Earlier AccomplishmentsSummary of Earlier Accomplishments
• 2-D rotating stall was numerically modeled, and the underlying physical phenomena studied
• A 3-D flow solver capable of modeling unsteady viscous flow through axial and centrifugal compressors was developed and validated
• The mechanisms behind the onset and growth of surge in NASA Low Speed Centrifugal Compressor was studied
• Control of Surge through diffuser bleed was simulated
School of Aerospace Engineering
MITE
• Diffuser bleed valves•Pinsley, Greitzer, Epstein (MIT)•Prasad, Neumeier, Haddad (GT)
• Movable plenum wall•Gysling, Greitzer, Epstein (MIT)
• Guide vanes•Dussourd (Ingersoll-Rand Research Inc.)
• Air-injection•Murray (Cal Tech)•Fleeter, Lawless (Purdue)•Weigl, Paduano, Bright (MIT & NASA Lewis)
How to Control Surge (Passive Control)How to Control Surge (Passive Control)
Bleed Valves
Movable Plenum Walls
Guide Vanes
Air-Injection
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Boundary Conditions (GTTURBO3D)Boundary Conditions (GTTURBO3D)
Outflow boundary(coupling with plenum)
Periodic Boundaryat compressor inlet
Solid Wall Boundaryat compressor casing
Periodic Boundaryat diffuser
Solid Wall Boundaryat impeller blades
Periodic Boundaryat clearance gap
Solid Wall Boundaryat compressor hub
Inflow Boundary
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Outflow BC (GTTURBO3D)Outflow BC (GTTURBO3D)
Plenum Chamber•u(x,y,z) = 0 •pp(x,y,z) = const.•isentropic
ap, Vp
mc
.
mt
.
Outflow Boundary
)mm(Va
dtdp
tcp
2pp
Conservation of mass:
School of Aerospace Engineering
MITE
DLR High-Speed Centrifugal CompressorDLR High-Speed Centrifugal CompressorAGARD Test CaseAGARD Test Case
•24 main blades•30 backsweep•CFD-grid 141 x 49 x 33 (230,000 grid-points)
Design Conditions:•22360 RPM•Mass flow = 4.0 kg/s•Total pressure ratio = 4.7•Adiab. efficiency = 83%•Exit tip speed = 468 m/s•Inlet Mrel = 0.92
School of Aerospace Engineering
MITE
DLRCC-Results (Off-Design Conditions)DLRCC-Results (Off-Design Conditions) Performance Characteristic MapPerformance Characteristic Map
Unsteady fluctuations are denoted by size of circles
Fluctuations at 3.1 kg/sec are 30 times larger than at 4.6 kg/sec
33.23.43.63.8
44.24.44.64.8
55.2
2 2.5 3 3.5 4 4.5 5
Mass Flow (kg/s)
Experiment
CFDTota
l Pre
ssur
e R
atio
School of Aerospace Engineering
MITE
DLRCC-Results (Surge Conditions)DLRCC-Results (Surge Conditions)
Mild surge develops.
Surge amplitude grows to 60% of mean flow rate.
Surge frequency = 94 Hz (1/100 of blade passing frequency)
School of Aerospace Engineering
MITE
DLRCC-Results (Surge Conditions)DLRCC-Results (Surge Conditions)
Flow field vectors show two separation zones:• near leading edge• in the diffuser
Mild surge cycle colored by Mrel
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DLRCC-Results (Surge Conditions)DLRCC-Results (Surge Conditions) Stagnation pressure contoursStagnation pressure contours
•Vortex shedding causes reversed flow•Origin of separation occurs at leading edge pressure side
Direction of rotation
School of Aerospace Engineering
MITE
LSCC-Results (Air-Injection)LSCC-Results (Air-Injection)
Injection angle, = 5º3 to 10% injected mass flow rate
0.04RInletCasing
5°
Rotation Axis
Impeller
RInlet
School of Aerospace Engineering
MITE
DLRCC-Results (Air-Injection)DLRCC-Results (Air-Injection) Different yaw angles, 3% injected mass flow rateDifferent yaw angles, 3% injected mass flow rate
Yaw angle directly affects the unsteady leading edge vortex shedding
Positive yaw angle is measured in positive direction of impeller rotation
School of Aerospace Engineering
MITE
DLRCC-Results (Air-Injection)DLRCC-Results (Air-Injection)
Leading edge separation suppressed due to injection
Velocity vectors colored by Mrel
School of Aerospace Engineering
MITE
DLRCC-Results (Air-Injection)DLRCC-Results (Air-Injection) Different yaw angles, 3% injected mass flow rateDifferent yaw angles, 3% injected mass flow rate
-25
0
25
50
75
100-20 0 20 40 60
Yaw Angle (in Degrees)
Red
uctio
n in
Sur
ge A
mpl
itude
(in
%)
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MITE
Axial Compressor (NASA Rotor 67)Axial Compressor (NASA Rotor 67)• 22 Full Blades• Inlet Tip Diameter 0.514 m• Exit Tip Diameter 0.485 m• Tip Clearance 0.61 mm• 22 Full Blades• Design Conditions:– Mass Flow Rate 33.25 kg/sec– Rotational Speed 16043 RPM– Rotor Tip Speed 429 m/sec– Inlet Tip Relative Mach
Number 1.38– Total Pressure Ratio 1.63– Adiabatic Efficiency 0.93
Multi-flow-passage-grid for rotating stall modeling
School of Aerospace Engineering
MITE
Performance Map (NASA Rotor 67)Performance Map (NASA Rotor 67) measured
mass flow rate at choke: 34.96 kg/s
CFD choke mass flow rate: 34.76 kg/s
1.3
1.4
1.5
1.6
1.7
1.8
0.88 0.9 0.92 0.94 0.96 0.98 1
Tota
l Pre
ssur
e ra
tio
Turb
Experiment
laminar
Choke mm
School of Aerospace Engineering
MITE
Mach Contours at MidspanMach Contours at Midspan
Spatially uniform flow at design conditions
School of Aerospace Engineering
MITE
Summary of Current Year WorkSummary of Current Year Work• The CFD compressor modeling capability was extended to:
• Higher speed, higher pressure compression systems• Turbulence model• Shock capturing capability • Boundary conditions
• Development of surge mechanism in centrifugal compressors was studied. Surge Control through upstream injection was optimized
• In preparation for rotating stall simulations, a multi-blade passage version of the solver was developed and validated
School of Aerospace Engineering
MITE
Future and Planned ActivitiesFuture and Planned Activities• 3-D rotating stall phenomenon and efficient stall control in axial compressors (bleeding, vortex generators) will be modeled
• Develop a criterion for efficient injection control of centrifugal compressors
• Examine the effectiveness of control laws developed by Drs. Haddad, Prasad and Neumeier through CFD-simulations
School of Aerospace Engineering
MITE
Technology Transition• The suite of codes may be used by industry partners for pilot studies of promising concepts:
• Compact size of the code• Optimized for turbomachinery applications• Advanced analysis features (fifth order Roe solver, implicit
time marching algorithm, Spalart-Allmaras model) • Documentation is available
• Optimized injection control scheme may be implemented in real engines:
• Injection location• Injection rates• Injection angles
School of Aerospace Engineering
MITE
DLRCC-Results (Design Conditions)DLRCC-Results (Design Conditions) Static Pressure Along ShroudStatic Pressure Along Shroud
Excellent agreement between CFD and experiment
0
0.5
1
1.5
2
2.5
3
0 0.2 0.4 0.6 0.8 1Meridional Chord, S/Smax
Experiment
CFD
Loca
l Sta
tic P
ress
ure,
p/p
std