1. Numerical Studies of Stall and Surge Alleviation in Compressors
Alex Stein, Saeid Niazi, and Lakshmi N. Sankar
School of Aerospace Engineering
Georgia Institute of Technology
Supported by the U.S. Army Research Office Under the Multidisciplinary University Research Initiative (MURI) on Intelligent Turbine Engines

2. Overview Objectives and Motivation Rotating Stall and Surge
Flow Solver and Boundary Conditions
DLR High-Speed Centrifugal Compressor
Unsteady Surge Simulations
Surge Control Using Air-Injection
NASA Axial Rotor 67 Results
Peak Efficiency Conditions
Off-design Conditions
Bleed Valve Control
Conclusions

3. Objectives and Motivation
Lines of Constant
Rotational Speed
Efficiency
Choke Limit
Surge Limit
Flow Rate
Total Pressure Rise
Desired Extension of Operating Range
Develop a numerical scheme to model and understand compressor stall and surge.
Explore active and passive control strategies (Bleed Valve, Air-Injection) to extend useful operating range of compressors.

4. Motivation and Objectives
Compressor instabilities can cause fatigue and damage to entire engine

5. Rotating Stall Rotating stall is a 2-D unsteady local phenomenon
Types of rotating stall:
Part-span
Full-span 2 2 2 1 1 1
Blade 1 sees a high a.
Blade 1 recovers.
Blase 2 stalls.
Blade 1 stalls.

6. Surge Mild Surge Deep Surge Mean Operating Point Pressure Rise
Flow Rate
Mean
Operating Point
Pressure Rise
Flow Rate
Peak
Performance
Limit Cycle
Oscillations
Time
Flow Rate
Period of
Mild Surge Cycle
Time
Flow Rate
Period of Deep Surge Cycle
Flow Reversal

7. How to Control Instabilities
Bleed Valves
Diffuser bleed valves
Pinsley, Greitzer, Epstein (MIT)
Prasad, Neumeier, Haddad (GT)
Movable plenum walls
Gysling, Greitzer, Epstein (MIT)
Guide vanes
Dussourd (Ingersoll-Rand Research Inc.)
Air-injection
Murray (CalTech)
Fleeter, Lawless (Purdue)
Weigl, Paduano, Bright (MIT & NASA Lewis)
Movable Plenum Walls
Guide Vanes
Air-Injection

8. GTTURBO3D Flow Solver
Reynolds averaged Navier-Stokes equations in finite volume representation.
A Four Point Stencil is used to compute the inviscid flux terms at the cell faces according to Roe’s formulation (Third-order accurate in space, first- or second-order accurate in time)
The viscous fluxes are computed to second order spatial accuracy.
Turbulence is modeled by one-equation Spalart-Allmaras model
Code can handle multiple computational blocks and inlet distortions

9. Boundary Conditions (GTTURBO3D)
Outflow boundary
(coupling with plenum)
Periodic Boundary
at compressor inlet
Solid Wall Boundary
at compressor casing
Periodic
Boundary
at diffuser
at impeller blades
at clearance gap
at compressor hub
Inflow
Zonal Boundary

10. Outflow BC (GTTURBO3D) Conservation of mass: CFD Outflow Boundary
Plenum Chamber
u(x,y,z) = 0
pp(x,y,z) = const.
isentropic
ap, Vp mc . mt
CFD Outflow Boundary
Conservation of mass:

11. DLR High-Speed Centrifugal Compressor
40cm
Designed and tested by DLR (Germany)
High pressure ratio
AGARD test case

12. DLR High-Speed Centrifugal Compressor
24 main blades
30 backsweep
CFD-grid 141 x 49 x 33 (230,000 grid-points)
Design Conditions:
22,360 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

13. DLR-High-Speed-Results (Design Conditions) Static Pressure Along Shroud
Excellent agreement between CFD and experiment

14. DLR-High-Speed-Results (Off-Design Conditions) Performance Characteristic Map
Unsteady fluctuations are denoted by size of circles
Design
Fluctuations at 3.1 kg/sec are 30 times larger than at 4.6 kg/sec
Surge
Choke

15. DLR-High-Speed-Results (Surge Conditions)
wt/2p,
Mild surge cycles develop
Surge amplitude grows to 60% of mean flow rate
Surge frequency = 94 Hz (1/100 of blade passing frequency)

16. DLR-High-Speed-Results (Air-Injection-Setup)
0.04RInlet
Casing 5°
Rotation Axis
Impeller
RInlet
Injection angle,  = 5º
3 to 6% injected mass flow rate

17. DLR-High-Speed-Results (Air-Injection) Different yaw angles, 3% injected mass flow rate
Yaw angle directly affects the unsteady leading edge vortex shedding

18. DLR-High-Speed-Results (Air-Injection) Different yaw angles, 3% injected mass flow rate
Optimum:
Surge amplitude/main flow = 8 %
Injected flow/main flow = 3.2 %
Yaw angle = 7.5 degrees

19. DLR-High-Speed-Results (Air-Injection) Yaw angle vs
DLR-High-Speed-Results (Air-Injection) Yaw angle vs. angle of attack, 3% injected mass flow rate
Injection yaw angle directly affects leading edge angle of attack
=> maximum control for designer

20. DLR-High-Speed-Results (Air-Injection) Angle of attack is directly altered by injection
Optimum injection yaw angle of 7.5 degrees yields best result

21. Axial Compressor (NASA Rotor 67)
22 Full Blades
Inlet Tip Diameter m
Exit Tip Diameter m
Tip Clearance mm
Design Conditions:
Mass Flow Rate kg/sec
Rotational Speed RPM (267.4 Hz)
Rotor Tip Speed 429 m/sec
Inlet Tip Relative Mach Number 1.38
Total Pressure Ratio 1.63
Adiabatic Efficiency 0.93
Hub
4 Blocks
73X32X21
Total of 196,224 cells

22. Literature Survey of NASA Rotor 67
Computation of the stable part of the design speed operating line:
NASA Glenn Research Center (Chima, Wood, Adamczyk, Reid, and Hah)
MIT (Greitzer, and Tan)
U.S. Army Propulsion Laboratory (Pierzga)
Alison Gas Turbine Division (Crook)
University of Florence, Italy (Arnone )
Honda R&D Co., Japan (Arima)
Effects of tip clearance gap:
NASA Glenn Research Center (Chima and Adamczyk)
MIT (Greitzer)
Shock boundary layer interaction and wake development:
NASA Glenn Research Center (Hah and Reid).
End-wall and casing treatment:
NASA Glenn Research Center (Adamczyk)

23. Relative Mach Contours at Mid-Span (Peak Efficiency) IV
III II I LE TE
Spatially uniform flow at design conditions

24. Relative Mach Number at 90% Radius (Peak Efficiency)
30% Pitch
50% Pitch TE LE TE LE

25. Shock-Boundary Layer Interaction (Peak Efficiency)LE TE
Shock
Near Suction Side

26. Velocity Profile at Mid-Passage (Peak efficiency)TE
Shock
% Mass Flow rate Fluctuations
% Pressure
Fluctuations
Fluctuations are
very small (2%)
Flow is well aligned.
Very small regions of separation observed in the tip clearance gap(Enlarged view)

27. Enlarged View of Velocity Profile in the Clearance Gap (Peak efficiency)TE
Clearance Gap
The reversed flow in the gap and the leading edge vorticity grow in size and magnitude as the compressor operates at off-design conditions

28. Performance Map (NASA Rotor 67)
measured mass flow rate at choke: kg/s
CFD choke mass flow rate: kg/s C B D
Unstable Conditions A
Near Stall
Controlled
Peak
Efficiency

29. Location of the Probes for Observing the Pressure and Velocity Fluctuations
The probes are located at 30% chord upstream of the rotor and 90% span They are fixed in space. LE TE  I II
III IV I II
III IV

30. Onset of the Stall (Clean Inlet)
Wt/2p I II
III IV I II
III IV 
Probes show
identical
fluctuations.
Flow while
unsteady, is still
symmetric from
blade to blade.

31. Onset of the Stall (Disturbed Inlet)II
III IV 
Wt/2p
Inlet distortion
simulated by dropping
the stagnation pressure
in one block by 20%.
Flow is no longer symmetric from blade to blade.
Frequency of rotating stall
is NW/3.5, where
NW : blade passage frequency.

32. Bleed Valve Control Un = mb/(rAb) Pressure, density and tangential
One Tip Chord
Pressure, density
and tangential
velocities are
extrapolated from
interior. .
Un = mb/(rAb)
Shroud
Hub

33. % Mass Flow Rate Fluctuations
Bleed Valve Control
Without Bleed Valve
With Bleed Valve
% Total
Pressure
Fluctuations
3% bleed air reduces
the total pressure fluctuations by 75%
% Mass Flow Rate Fluctuations

34. Bleed Valve Control Axial Velocity Near LE
After 1.5 Rev.
% From Hub
After 0.5 Rev.
Bleed
Valve.

35. Conclusions
A 3-D numerical flow solver has been developed to investigate compressor instabilities.
The flow solver has been applied to obtain a detailed understanding of surge and rotating stall phenomena in axial and centrifugal compressors.
Air-injection and bleeding have been numerically analyzed as compressor control schemes.
Surge margin extension was achieved for both compression systems.
The proper application of air-injection is sensitive to the injection-parameters (e.g. yaw angle).