1. Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection
Alexander Stein, Saeid Niazi and Lakshmi 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
High Performance Computer Time was Provided by the Major Shared Resource Center of the U.S. Army Engineer Research and Development Center (ERDC MSRC).

2. Outline of Presentation
Objectives and motivation
Background of compressor control
Introduction of numerical tools
Configuration and validation results
DLR high-speed centrifugal compressor (DLRCC)
Off-design results without control
Surge analysis
Off-design results with air injection control
Steady jets
Pulsed jets
Conclusions and recommendations

3. Motivation and Objectives
Use CFD to explore and understand compressor stall and surge
Develop and test control strategies (air injection) for centrifugal compressors
Apply CFD to compare low-speed and high-speed configurations
Lines of Constant
Rotational Speed
Efficiency
Choke Limit
Surge Limit
Flow Rate
Total Pressure Rise
Desired Extension of Operating Range

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

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

6. How to Alleviate Surge Diffuser Bleed Valves Movable Plenum Wall
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 (CalTech)
Weigl, Paduano, Bright (NASA Glenn)
Fleeter, Lawless (Purdue)
Movable Plenum Walls
Guide Vanes
Air-Injection

7. Numerical Formulation (Flow Solver)
Reynolds-averaged Navier-Stokes equations in finite volume
representation:   t q
 dV  E ˆ i F j G k      n dS  R S T
q is the state vector.
E, F, and G are the inviscid fluxes (3rd order accurate). R, S, and T are the viscous fluxes (2nd order accurate).
A one-equation Spalart-Allmaras model is used.
Code can handle multiple computational blocks and rotor-stator-interaction.

8. Boundary Conditions (Flow Solver)
Periodic boundary
at clearance gap
Solid wall boundary
at impeller blades
Solid wall boundary
at compressor casing
Inflow
boundary
Periodic
boundary
at diffuser
Solid wall boundary
at compressor hub
Outflow boundary
(coupling with plenum)
Periodic boundary
at compressor inlet

9. Outflow Boundary Condition (Flow Solver)
Plenum chamber
up(x,y,z) = 0
pp(x,y,z) = const.
isentropy
ap, Vp mc . mt
Outflow boundary
Conservation of mass and isentropic expression for speed of sound:

10. DLR High-Speed Centrifugal Compressor
40cm
Designed and tested by DLR
High pressure ratio
AGARD test case

11. DLR High-Speed Centrifugal Compressor
24 Main blades
30 Backsweep
Grid 141 x 49 x 33 (230,000 nodes)
A grid sensitivity study was done with up to 1.8 Million nodes.
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

12. Validation Results (Design Conditions) Static Pressure Along Shroud
Excellent agreement between CFD and experiment
Results indicate grid insensitivity => Baseline Grid is used subsequently

13. Off-Design Results Performance Characteristic MapB C D
Computational and experimental data are within 5%
Fluctuations at 3.2 kg/sec are 23 times larger than at 4.6 kg/sec

14. Off-Design Results (High-Speed) Performance Characteristic Map
-30
-20
-10 10 20 30
Mass Flow Fluctuations (%)
A: 4.6 kg/sec
Pressure Rise Fluctuations (%)
-30
-20
-10 10 20 30
Mass Flow Fluctuations (%)
B: 3.8 kg/sec
Pressure Rise Fluctuations (%)
Large limit cycle oscillations develop near surge line
-30
-20
-10 10 20 30
Mass Flow Fluctuations (%)
C: 3.4 kg/sec
Pressure Rise Fluctuations (%)
-30
-20
-10 10 20 30
Mass Flow Fluctuations (%)
D: 3.2 kg/sec
Pressure Rise Fluctuations (%)

15. Off-Design Results (High-Speed) Mass Flow Fluctuations
Mild surge cycles develop
Surge amplitude grows to 60% of mean flow rate
Surge frequency = 90 Hz
(1/100 of blade passing frequency)

16. Air Injection Setup Systematic study:
0.04RInlet
Casing 5°
Rotation Axis
Impeller
RInlet
Yaw Angle b
Main Flow
Injected Fluid Sheet
Compressor Face
Compressor Casing
Systematic study:
injection rate and yaw angle were identified as the most sensitive parameters.
Related work: Rolls Royce,
Cal Tech, NASA Glenn /MIT,

17. Air Injection Results (Steady Jets) Different Yaw Angles, 3% Injected Mass Flow Rate
Optimum yaw angle of 7.5deg. yields best result
Mass Flow (kg/sec)
Rotor Revolutions, wt/2p
Reduction in Surge Amplitude (%)
Positive yaw angle is measured in opposite direction of impeller rotation
Yaw Angle (Degree)

18. Air Injection Results (Parametric Study)
Optimum:
Surge amplitude/main flow = 8 %
Injected flow/main flow = 3.2 %
Yaw angle = 7.5 degrees
An optimum yaw angle exists.
A reasonable amount (~3%) of injected air is sufficient to suppress surge.

19. Air Injection Results (Pulsed Jets)
Surge fluctuations decrease as long as the injection phase was lagged 180 deg. relative to the flow => suggests feedback control
reduction in external air requirements by 50% (compared to steady jets)
With Phase Angle Adjustments
Without Phase Angle Adjustments
Nondim. Surge Fluctuations (%)
Rotor Revolutions, wt/2p

20. Air Injection Results (Pulsed Jets)
1.5% injected mass is sufficient to suppress surge
High-frequency jets (winj = 4wsurge) perform better than low-frequency jets (winj = wsurge)
Nondim. Surge Fluctuations (%)
Rotor Revolutions, wt/2p

21. Air Injection Results (Pulsed Jets) Vorticity Magnitudes Near Leading Edge Tip
Increased amounts of mixing enhance the momentum transfer from the injected fluid to the low-kinetic energy particles in the separation zone
Numerical
Probe
Jet

22. Air Injection Results (Pulsed Jets) Shear Stresses Near Leading Edge Tip
High-frequency actuation leads to significantly larger shear stress levels.
Produce smaller but intense turbulent eddies.
Enhances the mixing at small length scales.
Area of Interest
Jet

23. Conclusions A Viscous flow solver has been developed to
obtain a detailed understanding of instabilities in centrifugal compressors.
determine fluid dynamic factors that lead to stall onset.
Steady jets are effective means of controlling surge:
Alter local incidence angles and suppress boundary layer separation.
Yawed jets are more effective than parallel jets.
An optimum yaw angle exists for each configuration.
Pulsed jets yield additional performance enhancements:
Lead to a reduction in external air requirements.
Jets pulsed at higher frequencies perform better than low-frequency jets due to enhanced mixing at small length scales.

24. Recommendations
Perform studies that link air injection rates to surge amplitude via a feedback control law.
Use flow solver to analyze and optimize other control strategies, e.g. inlet guide vanes, synthetic jets, casing treatments.
Employ multi-passage flow simulations to study rotating stall and appropriate control strategies.
Study inflow distortion and its effects on stall inception.
Improve turbulence modeling of current generation turbomachinery solvers. Analyze the feasibility of LES methods.