A Mechanism of Combustion Instability in Lean, Premixed Gas Turbine Combustors*

1
A Mechanism of Combustion Instability in Lean,
Premixed Gas Turbine Combustors

• Tim Lieuwen, Hector Torres, Cliff Johnson, Ben T.
  Zinn
• Schools of Aerospace and Mechanical Engineering
• Georgia Institute of Technology
• Atlanta, GA
• __________________________________________________
  __________________
• Research supported by AGTSR Dr. Dan Fant,
  contract monitor

2
Combustion Instabilities in Lean Premixed (LP)
Gas Turbines

• Combustion instabilities are an important problem
  hindering the development of LP industrial gas
  turbines
• Flame Flashback or blowoff - constrains regions
  of operability
• Fatigue Cracking of Liners - reduces combustor
  life
• Development of approaches to suppress these
  instabilities requires understanding the key
  physical processes responsible for their
  initiation.

3
Occurrence of Combustion Instabilities

• Combustion Instabilities occur when
• Energy Addition by Combustion Process (Rayleigh
  Criterion)
• Damping Mechanisms
• Radiation through boundaries
• Viscosity, heat conduction

Energy addition to the acoustic field by unsteady
combustion process
gt
Energy losses due to dissipation
dE/dt ltpqgt
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Mechanisms of Combustion Instability
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Identifying the Mechanism Responsible for
Instability Initiation

• A number of processes are simultaneously present
  in combustors that cause heat release
  fluctuations
• Pulsations in fuel supply rate
• Pressure and/or velocity dependent burning rate
• Vortex shedding
• Changes in flame area
• Periodic changes in mixture composition

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Flow and Reaction Processes in Gas Turbine
Combustors
Flame Stabilization
Fuel Inflow
Heat Losses
Convection
Swirling
Mixing
Flame and Flow Instabilities
Air Inflow
Exhausting
Chemical Reaction
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Sensitivity of Lean Combustion Systems to
Disturbances in Mixture Composition

• Combustion process becomes very sensitive to
  disturbances in composition under lean conditions

Characteristic Chemical time, s
Zukoski's Experimental Data
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Sensitivity of Lean Combustion Systems to
Disturbances in Mixture Composition

• Combustion process becomes very sensitive to
  disturbances in composition under lean conditions

0.0015
0.001
Characteristic Chemical Time, s
0.0005
0
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
Equivalence Ratio
Zukoski's Experimental Data
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A Possible Mechanism for Combustion Instabilities

• Recent combustor stability models that capture
  this mechanism
• Lieuwen and Zinn, AIAA Paper 98-0641, 27th
  Intl Symposium on Combustion
• Peraccio and Proscia, ASME Paper 98-GT-269

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Time evolution of disturbances responsible for
the onset of instability
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Time evolution of disturbances responsible for
the onset of instability (continued)

• Combustion process adds energy to the acoustic
  field when
• (tci tpv tf tconvect teq)/T 1, 2, 3,
  ...
• Simplifications
• Typical geometries tci/T 0
• Choked Fuel Injector tf/T1/2
• teq essentially an additional convective time
• discussed extensively in paper

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Time evolution of disturbances responsible for
the onset of instability (continued)

• Combustion process adds energy to the acoustic
  field when
• (tpv tconvect)/T 1/2, 3/2,
• Implies that significant combustors parameters
  are
• Natural modes of combustor - T 1/f
• Inlet velocity - tconvect
• Fuel injector location - tconvect
• Upstream boundary condition of inlet - tpv
• Length of inlet - tpv

Linj/u
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Dependence of Instability Region on Combustor
Configuration
Penn State Facility -
GT Facility -
DOE Facility -

• Conclusion For fixed geometry, instabilities can
  occur when tconvect/T fLinj/u? constant
• i.e., when uinletconstant x f

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Combustor Section-Front View
Fuel
Hot Products
Air
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Effect of Inlet Velocity on Instability Amplitude
and Frequency

• In both cases, peak amplitude occurs where
  tconvect/T ?1.1-1.2

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Effect of Inlet Velocity on Instability Frequency
uinlet36 m/s
uinlet18 m/s
uinlet25 m/s
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Inter-dependence of Instability Frequency and
Inlet Velocity
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Comparisons of Theory with Georgia Tech Data
(Rigid Upstream Boundary)
Conditions u10- 50 m/s f 0.65-0.9 p1-9 atm
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Comparisons of Theory with Georgia Tech Data
(Rigid Upstream Boundary)
Theoretical Predictions
Conditions u10- 50 m/s f 0.65-0.9 p1-9 atm
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Comparisons of Theory with DOE Data(Pressure
Release Upstream Boundary)
Theoretical Predictions
Conditions u30- 60 m/s f 0.6-0.8 p5-10 atm 3
Fuel injector locations
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Comparisons of Theory with Penn State
Data(Anechoic Upstream Boundary)
Theoretical Predictions
Conditions Tin600-740 K f 0.4-0.7 p2.5-7
atm 3 Fuel injector locations
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Conclusions

• Experimental observed stability limits consistent
  with theoretical predictions
• Suggests that instabilities are initiated by a
  feedback loop between the combustion process,
  combustor acoustics, and fluctuations in
  reactants composition
• Characteristic time analysis illustrates key
  processes in this mechanism and suggests methods
  for passive control

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Supporting Slides
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Characteristic Times Associated with Combustor
Processes
Acoustic Period 100 - 500 Hz Oscillations
Chemical Kinetics
Heat Loss, Diffusion
Convection
Mixing
Swirling Flow Turnover Time
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Convective Processes in Gas Turbine Combustors

• Reactive mixture composition
• Flame Dynamics - Response of flame to flow
  disturbances
• Flow Instabilities - Distortion of flame by
  convecting vortex structure

• tLinjector/u
• tLflame/u
• tLflame/u

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Recent Measurements at U. Cal -Berkeley R.
Mongia, R. Dibble, J. Lovett, ASME Paper
98-GT-304
Combustor Pressure Spectrum
Methane Mole Fraction Spectrum
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Results of a well-stirred reactor (WSR) model

• Unsteady WSR model subjected to perturbations in
  the inlet f.
• Response of the unsteady rate of reaction
  increased as much as 200 times as f was decreased
  from stoichiometric to lean mixtures
• Conclusion f oscillations induce strong heat
  release oscillations that can drive combustion
  instabilities under lean conditions

From Lieuwen, T., Neumeier, Y., Zinn, B.T., Comb.
Sci. and Tech, Vol. 135, 1-6, 1998.
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Phased Locked Images of Combustion Instability

• Line of sight (top half of picture) and Abel
  inverted (bottom half of picture) images of CH
  Chemiluminescence of 200 Hz. instability

Flow