Transcript Combustion Dynamics Short Course

Combustion Humming
(Instabilities) Overview
Tim Lieuwen
Assistant Professor
Georgia Institute of Technology
[email protected]
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
What is Humming?
• Combustion humming referred to by a variety of terms:
– Combustion instabilities
– Combustion dynamics
– Rumble, screech, growl, buzz, howl, …
• All of them refer to essentially the same phenomenon:
– Large amplitude pressure oscillations in combustion chamber,
driven by heat release oscillations
– Oscillations are destructive to engine hardware (damage is
measured in billions of dollars)
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Basic Feedback Cycle
Heat
release
Pressure
•Oscillations due to
resonant coupling
between flames and
acoustic waves
Normalized Pressure (p'/p)
Data showing growth in amplitude of
pressure oscillations due to feedback
loop
0.015
0.01
0.005
0
-0.005
-0.01
-0.015
0
500
1000
1500
2000
2500
Number of Cycles
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Fourier Transform of Combustor
Pressure
Fourier Transform
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100
0
0
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Frequency (Hz)
•
During an instability, combustion process generally excites one or more of the
natural acoustic modes of the combustor
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Key Problem: Flame is sensitive
to acoustic perturbations
From Ducruix et al., Proc. Comb.
Inst., Vol. 28, 2000, pp.765-773,
used with permission of S. Ducruix
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
How Well Can We Predict
Dynamic Characteristics of
Combustor?
Increasing difficulty
• Three basic issues:
– What is frequency of oscillations?
– Under what conditions will oscillations
occur?
– What is the amplitude of oscillations?
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting Dynamics
Frequency Predictions
•
•
Reasonable predictive capabilities occur
Typical frequency predictions accurate
to within 5-20% with no calibration
Most OEM’s have developed models of
varying sophistication with good
success
Fourier Transform
•
500
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100
0
0
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800
900 1000
Frequency (Hz)
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting Dynamics
Conditions of Occurrence
•
Mechanisms reasonably well
understood
Complexity of flame region renders
predictive capabilities difficult
– existing codes have difficulty with
steady flame characteristics
– Can “post-dict” characteristics
– We know the key parameters, how to
correlate the data
50
Inlet Velocity (m/s)
•
40
30
20
10
0
0
100 200 300 400 500 600 700 800
Frequency (Hz)
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting Dynamics
Amplitude of Oscillations
•
Neither predictive nor “postdictive” capabilities exist
•
Don’t even know key parameters
with which to correlate data
•
Subject of intensive investigation
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
How Well can We Monitor
these Oscillations?
•
Availability of high temp pressure
instrumentation has increased
dramatically in last 5 years
•
Most are piezo-electric based
– Be careful about depolarization
– Be careful about claims about high
temperature capabilities, they may
degrade substantially with time
– If your dynamics amplitude is gradually
decreasing with time, you should check
your transducer!
From Kistler product literature
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Monitoring Dynamics
Standoff Tubes
• High temperature environments
often necessitate physical
separation between combustor
and transducer
1
• Need to understand acoustics of
coil arrangement
L=24"
0.95
Transfer Function
– Bends in pipe, very slight area
changes, valves can have
MAJOR affects!!!
Sound dissipation in 1/4” tube
0.9
0.85
L=120"
0.8
0.75
0.7
0
100
200
300
Frequency, Hz
400
500
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Historical Overview
Humming is not unique to gas turbines!
From Liquid Propellant Rocket
Combustion Instability, Ed. Harrje and
Reardon, NASA Publication SP-194
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Thermo-acoustics
• Related phenomenon see in
non-combusting systems with
temperature gradients:
– Rijke Tube (heated gauze in
tube)
– Self-excited oscillations in
cryogenic tubes
– Thermo-acoustic
refrigerators/heat pumps
Purdue’s Thermoacoustic Refrigerator
Los Alamos NL’s Thermoacoustic Engine
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Industrial Systems
(see Putnam’s book)
• Oil fired heating units
• Scrap melting burners
• Boilers
• Pulse combustion
From Thring et al., ed. , Pulsating
Combustion: The Collected Works
of F.H. Reynst, Pergamon Press,
1961
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Liquid Rockets
• BIG OSCILLATIONS (>1000
psi)!!!
• e.g., F-1 Engine
– used on Saturn V
– largest thrust engine
developed by U.S
– Problem overcome with over
2000 (out of 3200) full scale tests
From Liquid Propellant Rocket
Combustion Instability, Ed. Harrje and
Reardon, NASA Publication SP-194
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Ramjets and Afterburners
• Vortex-flame interactions
generated large oscillations
• Ramjets: Caused un-starting of
inlet shock
• Afterburners: Lightweight
construction causes damage,
loss of flameholders
From D. Smith, Ph.D. thesis
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Solid Rockets
•
Examples:
–
–
–
–
–
•
SERGEANT Theater ballistic missile – tangential
instabilities generated roll torques so strong that
outside of motor case was scored due to rotation in
restraints
Minuteman missile –USAF experienced 5 flight
failures in 1968 during test due to loss of flight
control because of severe vibrations
Sidewinder missile
Space shuttle booster- 1-3 psi oscillations (1 psi =
33,000 pounds of thrust)
Mars pathfinder descent motor
From Blomshield, AIAA Paper #2001-3875
Adverse effects –thrust oscillations, mean
pressure changes, changes in burning rates
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Gas Turbines
•
Dry low NOx systems have huge
dynamics problems!
–
•
Introduced by low emissions designs
Some reasons:
–
Operate near lean blowout:
•
–
Minimal combustor cooling air (to
minimize CO) as in aero combustors:
•
–
acoustic damping substantially reduced
High velocity premixer for flashback:
•
–
system already right on stability line,
small perturbations give very large
effects
Pressure maximum at flame
Compact reaction zone for CO
•
Heat release concentrated at pressure
maximum
From “Flamebeat: Predicting Combustion Problems from Pressure
Signals”, by Adriaan Verhage, in Turbomachinery, Vol. 43(2),
2002
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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited