Detection and Control of Instabilities and Blowoff for Low ... · Propul. Controls & Diag. Workshop, 8 Dec 2009 Detection and Control of Instabilities Combustion Control • Control

Detection and Control of InstabilitiesPropul. Controls & Diag. Workshop, 8 Dec 2009

Propulsion Controls and Diagnostic WorkshopDec. 8-10, 2009

Detection and Control of Instabilities and Blowoff for Low Emissions Combustors

J. Seitzman, T. LieuwenJ. Crawford, R. Kiran

Georgia Institute of Technology

SUP-06-0030 NNX07AC92A

Technical Monitor: J. DeLaat NASA GRC


MotivationMotivation

Adiabatic Flame Temperature (°F)

NO

x (p

pm) a

t 15%

O2

2400 2600 2800 3000 3200 3400 36000

20

40

60

80

100

Fuel Lean

McVey et al., ASME 92-GT-133

• Combustor control can provide optimal tradeoff between low emissions and engine reliability/ operability– static and dynamic instability

(blowout and oscillations)

-5-4-3-2-1012345

0.1 0.15 0.2 0.25 0.3

Time (s)

Aco

ustic

Pre

ssur

e

0

0.050.1

0.15

0.2

0.250.3

0.35

0.4

Che

milu

min

esce

nce

PressureChemiluminescence

OperatingLines

φ

Flow

rate

Safe

LBO

LBOlimit

Margin=φop- φLBO

Heat Release

Hea

t Rel

ease


Combustion Control Combustion Control • Control Needs: Control systems that can 1) provide low emissions

(lean operation) and 2) prevent or recover from loss of dynamic stabilityand 3) prevent loss of static stability

Engine Power Control(Fuel Rate and Placement)

Emissions DynamicStability

StaticStability

Prevention Recovery Prevention Recovery

MarginSensing

MarginSensing

RelightSensing

ω,ϕSensing


NeedsNeeds• Goal: fundamental understanding needed to convert signals

from simple sensors to stability margin estimates and active control inputs

• Tasks

Active DynamicStability Control

Static StabilityMargin Sensing

Local Fuel-AirSensing

Reliable Combustor Operation at Low

Emissions Conditions

Dynamic StabilityMargin SensingAvoiding

Instability

ReducingInstabilityManaging

Fuel Placement


Active Control of Combustion DynamicsActive Control of Combustion Dynamics• State of the art

– widely demonstrated that active control can reduce instability amplitude

– poorly understood variability in effectiveness

• SPL reductionsrange from 3 to 40 dB

– controlled combustors can exhibit significant amplitude modulation

• Goal– model factors that limit

control effectiveness


Modeling ApproachModeling Approach( ) ( ) ( ) ( ) ( ) ( )ttptptptptp ccvv σξτετεωζω +−′+−=+′+′′ 22

• Use Culick’s formulation to describe time evolution of combustor modes with time delays and noise• include both “internal” and control induced delays

• Solve for statistical and spectral characteristics of pressure as function of control time delays and gain


AccomplishmentsAccomplishments

• Developed statistical theory of controlled combustion dynamics including time delays and noise– closed form solutions for statistics and spectra of combustor pressure

• Reproduced common experimental observations of controlled combustor dynamics– e.g., one controller can have significantly different influences on instability,

depending upon “nominal” combustor dynamics


Interplay between Interplay between ““NominalNominal”” Combustor Combustor Dynamics and ControlDynamics and Control

• Increasing control time delays decreases control effectiveness and window of controllability

• Controller impact varies with internal time delays

•Plots show inverse limit cycle amplitude of controlled combustor


Quantifying Controller PerformanceQuantifying Controller Performance• Model captures experimentally

observed– increase of minimum limit cycle

amplitude with controller time delay

– variation of controller performance with internal delays

• Future work - add physics– model predicts much smaller

amplitude modulation than experimentally observed

– currently working on incorporating observer dynamics


Static Stability : LBO Precursors

0 100 200 300 400 500 600 700 800

-0.5

0

0.5

Ban

dpas

sou

tput

0 100 200 300 400 500 600 700 8000

0.5

1

time [msec]

OH

sig

nal

Aco

ustic

Se

nsor

Opt

ical

(OH

*)

Sens

or

Precursor Events

• Goal: Estimate LBO margin in-situ• Approach: Occurrence of local

extinction/re-ignition events = LBO precursors– optical/acoustic detection– e.g., double thresholding of signals

127

mm

70 mm Dia.

Optical fiber

AcousticSensor

CH4

Air

CH4/air mixture

Pilot fuel


LBO Control

0 0.01 0.02 0.03 0.04 0.05 0.06 0.070

0.5

1

1.5

2

2.5

3

φOp -φ LBO

# of

eve

nts

/ sec

Premixed gas combustor - OpticPremixed gas combustor - AcousticLiquid fueled combustor - OpticJet-fuel aviation burner

Num

ber E

vent

s/se

c

φop-φLBO

0 0.01 0.02 0.03 0.04 0.05 0.06 0.070

0.5

1

1.5

2

2.5

3

φOp -φ LBO

# of

eve

nts

/ sec


0 0.01 0.02 0.03 0.04 0.05 0.06 0.070

0.5

1

1.5

2

2.5

3

φOp -φ LBO

# of

eve

nts

/ sec


Num

ber E

vent

s/se

c

φop-φLBO

• LBO proximity parameter : event rate (avg number of events/sec)– more frequent events as LBO is approached

• Can use as input to LBO avoidance controller– e.g., fuel distribution control

• Static stability can be linked to dynamic instabilityLBO margin sensing in the presence of combustion dynamics?

0 20 40 60 80 100 120 1400.75

0.8

φ

0 20 40 60 80 100 120 1400

5Ev

ent c

ount

0 20 40 60 80 100 120 1400

10

20

time [sec]

Pilo

t %

Unpiloted LBO Limit


Precursor Events: Combustion Dynamics• Two kinds of instability mechanisms

– Equivalence ratio oscillations (w/ Φ' )– Flame-acoustic interactions (w/o Φ')

• Low pass filtering (~50Hz) for improved precursor detection

• Same event signatures as dynamically stable conditions

Fuel Plenum

Choked Valve

Fuel

Optical Fiber

Fuel

AcousticSensor

Time (ms)

0 50 100 150 200 250 3000

0.4

0.8

1.2

1.6Un-filtered

0 50 100 150 200 250 300

0.2

0.4

Low Pass Filtered

Sign

al (a

.u.)

Sign

al (a

.u.)

Low pass filtered

Un-filteredw/ Φ'

Sign

al (a

.u.)

Sign

al (

a.u.

)

0 50 100 150 200 250 3000

0.5

1

1.5

Un-filtered OH*

0 50 100 150 200 250 3000

0.20.40.60.8

Filtered OH*

Time (ms)

Un-filtered

Low pass filtered

w/o Φ'


Precursor Events: Acoustic Signal• Precursors not evident in raw acoustic signal• LP filtering reveals precursor event signature• Same event characteristic shape as dynamically

stable conditions– can use same event sensing algorithm (e.g.,

identification with wavelet filter)

0 50 100 150 200 250 300-4

-2

0

2

4

Unfiltered

0 50 100 150 200 250 300-0.2

-0.1

0

0.1

0.2Low Pass Filtered

Time (ms)

Sig

nal (

a.u.

)S

igna

l (a.

u.)

Un-filtered

Low pass filtered0 50 100 150 200 250 300

-5

0

5Un-filtered Acoustic

0 50 100 150 200 250 300-0.2

0

0.2Filtered Acoustic

Time (ms)

Sig

nal (

a.u.

)S

igna

l (a.

u.)

Low pass filtered

Un-filteredw/o Φ'w/ Φ'

Wavelet


LBO Sensing• Robust optical signal

stability parameter : Stability Index (SI)

SI

0 0.02 0.04 0.06 0.080

0.5

1

1.5

2

WithoutWith Φ'

Φ'

Num

ber o

f eve

nts/

sec

Stability Margin (Φ- ΦLBO)

Acoustic

0 0.02 0.04 0.06 0.080

1

2

3

4

5

6

7

8

WithoutWith Φ'

Φ'


Stab

ility

Inde

x (S

I) %

Optical

Num

ber o

f eve

nts/

sec


0 0.02 0.04 0.06 0.080

0.5

1

1.5

2

2.5

WithoutWith

Φ'Φ'

Optical


Single Nozzle LDI System

Inlet

Single nozzle Injector Quartz flame

tube

Quartz optical windowsCooling flow

Water coolingPressure Vessel

Exhaust

• Objective: Examine LBO margin sensing in advanced, low NOx, elevated pressure combustor


LDI Tests – Jet A• Initial shakedown at

atmospheric pressure• Flame zone has lean

signature (blue flame)

T3=740 F (667K), Φ=0.46,Vref = 46 ft/s (14 m/s)

T3=751 F (672K), Φ=0.7, Vref=28 ft/s (8.5 m/s)

Movie

T3=740 F, Vref = 46 ft/s, Φ=0.51-0.37


Summary

• Developed statistical theory of controlled combustion dynamics including time delays and noise– closed form solutions for statistics and spectra of

combustor pressure• Reproduced common experimental observations of

controlled combustor dynamics– can use to study limits of control effectiveness and

improve controller design• Demonstrated robust LBO precursor sensing

– robust event sensing algorithms: multiple combustors, with and without dynamics instability

• Currently extending work to NASA single-element LDI combustor

Documents

Detection and Control of Instabilities and Blowoff for Low ... · Propul. Controls & Diag. Workshop, 8 Dec 2009 Detection and Control of Instabilities Combustion Control • Control