1. Combustion Humming (Instabilities) Overview
Tim Lieuwen
Assistant Professor
Georgia Institute of Technology ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

2. 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

3. Basic Feedback Cycle
Heat release
Pressure
Data showing growth in amplitude of pressure oscillations due to feedback loop
Oscillations due to resonant coupling between flames and acoustic waves ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

4. Fourier Transform of Combustor Pressure
During an instability, combustion process generally excites one or more of the natural acoustic modes of the combustor ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

5. Key Problem: Flame is sensitive to acoustic perturbations
From Ducruix et al., Proc. Comb. Inst., Vol. 28, 2000, pp , used with permission of S. Ducruix ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

6. 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? ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

7. 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 ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

8. 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 ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

9. Predicting Dynamics Amplitude 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

10. 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

11. Monitoring Dynamics Standoff 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!!!
Sound dissipation in 1/4” tube ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

12. Humming is not unique to gas turbines!
Historical Overview
Humming is not unique to gas turbines!
From Liquid Propellant Rocket Combustion Instability, Ed. Harrje and Reardon, NASA Publication SP-194 ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

13. 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

14. 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

15. 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

16. 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

17. 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 # ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

18. 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 ,
Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited