Upload
abril-wing
View
221
Download
2
Tags:
Embed Size (px)
Citation preview
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Combustion Humming (Instabilities) Overview
Tim LieuwenAssistant Professor
Georgia Institute of [email protected]
,
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)
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Basic Feedback Cycle
Heat release Pressure
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0 500 1000 1500 2000 2500
Number of Cycles
Nor
mal
ized
Pre
ssur
e (p
'/p)
•Oscillations due to resonant coupling between flames and acoustic waves
Data showing growth in amplitude of pressure oscillations due to feedback
loop
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Fourier Transform of Combustor Pressure
• During an instability, combustion process generally excites one or more of the natural acoustic modes of the combustor
0
100
200
300
400
500
0 100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)
Fou
rier
Tra
nsfo
rm
,
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
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
How Well Can We Predict Dynamic Characteristics of Combustor?
• Three basic issues:– What is frequency of oscillations?
– Under what conditions will oscillations occur?
– What is the amplitude of oscillations?
Increasing difficulty
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting DynamicsFrequency 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
0
100
200
300
400
500
0 100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)Fo
urie
r Tra
nsfo
rm
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting DynamicsConditions 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 0
10
20
30
40
50
0 100 200 300 400 500 600 700 800
Frequency (Hz)
Inle
t Vel
ocity
(m/s
)
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Predicting DynamicsAmplitude of Oscillations
• Neither predictive nor “post-dictive” capabilities exist
• Don’t even know key parameters with which to correlate data
• Subject of intensive investigation
,
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
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Monitoring DynamicsStandoff Tubes
• High temperature environments often necessitate physical separation between combustor and transducer
• Need to understand acoustics of coil arrangement– Bends in pipe, very slight area
changes, valves can have MAJOR affects!!!
0 100 200 300 400 5000.7
0.75
0.8
0.85
0.9
0.95
1
Frequency, Hz
Tra
nsf
er
Fu
nct
ion
L=24"
L=120"
Sound dissipation in 1/4” tube
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
Historical Overview
From Liquid Propellant Rocket Combustion Instability, Ed. Harrje and Reardon, NASA Publication SP-194
Humming is not unique to gas turbines!
,
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
,
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
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
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
,
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
,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited
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
• Adverse effects –thrust oscillations, mean pressure changes, changes in burning rates
From Blomshield, AIAA Paper #2001-3875
,
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:
• system already right on stability line, small perturbations give very large effects
– Minimal combustor cooling air (to minimize CO) as in aero combustors:
• acoustic damping substantially reduced
– High velocity premixer for flashback:• 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