Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

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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).

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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

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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

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Motivation and Objectives
Compressor instabilities can cause fatigue and
damage to entire engine
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What is Surge?
Mild Surge
Deep Surge
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How to Alleviate Surge

• 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 (CalTech)
• Weigl, Paduano, Bright (NASA Glenn)
• Fleeter, Lawless (Purdue)

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Numerical Formulation (Flow Solver)
Reynolds-averaged Navier-Stokes equations in
finite volume representation

• 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.

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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
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Outflow Boundary Condition (Flow Solver)
Conservation of mass and isentropic expression
for speed of sound
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DLR High-Speed Centrifugal Compressor

• Designed and tested by DLR
• High pressure ratio
• AGARD test case

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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

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Validation Results (Design Conditions) Static
Pressure Along Shroud

• Excellent agreement between CFD and experiment
• Results indicate grid insensitivity gt Baseline
  Grid is used subsequently

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Off-Design Results Performance Characteristic Map
Computational and experimental data are within 5
Fluctuations at 3.2 kg/sec are 23 times larger
than at 4.6 kg/sec
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Off-Design Results (High-Speed) Performance
Characteristic Map
Large limit cycle oscillations develop near surge
line
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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)
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Air Injection Setup
Systematic study injection rate and yaw angle
were identified as the most sensitive
parameters. Related work Rolls Royce, Cal Tech,
NASA Glenn /MIT,
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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)
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Air Injection Results (Parametric Study)

• An optimum yaw angle exists.
• A reasonable amount (3) of injected air is
  sufficient to suppress surge.

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Air Injection Results (Pulsed Jets)

• Surge fluctuations decrease as long as the
  injection phase was lagged 180 deg. relative to
  the flow gt suggests feedback control
• reduction in external air requirements by 50
  (compared to steady jets)

Nondim. Surge Fluctuations ()
Rotor Revolutions, wt/2p
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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
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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

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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.

Jet
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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.

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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.