, copyright t.lieuwen, 2003, unauthorized reproduction prohibited combustion humming (instabilities)...

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Copyright T.Lieuwen, 2003, Unauthorized reproduction prohibited

Combustion Humming (Instabilities) Overview

Tim LieuwenAssistant Professor

Georgia Institute of [email protected]

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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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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

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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

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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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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

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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

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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

)

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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

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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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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

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Historical Overview

From Liquid Propellant Rocket Combustion Instability, Ed. Harrje and Reardon, NASA Publication SP-194

Humming is not unique to gas turbines!

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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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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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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

• Adverse effects –thrust oscillations, mean pressure changes, changes in burning rates

From Blomshield, AIAA Paper #2001-3875

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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

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combustor slide

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