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43rd AIAA Aerospace Sciences Meeting and Exhibit AIAA-2005-1116Reno, Nevada10 - 13 Jan 2005

Characterization of Blow Out Limits for a V-Gutter Stabilized Flame

Scott M. Bush and Ephraim J. GutmarkDepartment of Aerospace Engineering and Engineering Mechanics

University of Cincinnati, Cincinnati, OH 45221-0070

Abstract

It is well known that bluff objects placed in combustible high-speed flows stabilize flames. To investigateand characterize the blow out limits for a V-gutter stabilized flame, a Combustion Wind Tunnel Facility(CWTF) was utilized in an attempt to determine a possible footprint or unique attribute of approaching ablowout condition. Characterization of these limits was achieved by running the V-Gutter at a stablecondition and approaching lean or rich blowout while investigating pressure oscillations, CH fluctuations,temperature changes, flame intensity and fluctuation, and flame structure. The main findings did result in aunique footprint of either approaching lean blow out or rich blow out. While approaching lean blow out,the mean and rms CH emission and pressure both decrease, the frequency decreases, and the flamestructure begins to shrink down from spanning the entire section towards the center of the test section untilthe flame eventually extinguishes with the intensity of these images continually diminishing untilextinction. While approaching rich blow out, mean CH emission decreases while rms CH emissionincreases, mean pressure increases with rms pressure initially decreasing, but increasing again priorextinction, frequency increases, and the flame intensity diminishes, but also has an increased fluctuation ofintensity.

Nomenclature

V VelocityP PressureT TemperatureN, D Width of bluff bodyΦ Equivalence Ratiorms root mean square

Introduction

It is well known that bluff objects placed incombustible high-speed flows stabilize flames.Typical applications for bluff body stabilizedflames include ramjet engines and turbojet orturbofan afterburners in military aircraft. Themost common object used to stabilize a flame inramjets or afterburners is a device known as a V-gutter. A V-gutter is a bluff body that is shapedjust as it sounds with the point of the “V” facingupstream. The V-gutter can be thought of as abent piece of metal forming a “V” with a typicalangle for the “V” of approximately 35 degrees.For common V-gutters used in today’s militaryaircraft, the typical width of the “V” is on theorder of 1 to 2 inches at the most downstreamsection of the “V”.

The way any bluff body object works to stabilizea flame is by creating a large recirculation zonejust downstream of the flameholding device.This recirculation zone consists of hightemperature burnt products that act as acontinuous ignition source for the fresh fuel/airmixture. A considerable amount of research hasbeen conducted on bluff body stabilization, and agood review of this topic can be found in thefollowing references [1,2,3,4,5]. Thesereferences discuss the two main zones of thebluff body stabilized flame that include theimmediate wake or recirculation zone asdiscussed above and the downstream flamespreading region. The downstream flamespreading region follows the visible recirculationzone and completes the combustion process. Inthis region, the rate that the flame spreads is ofprimary concern. The rate that the flame spreadsdetermines the amount of spacing requiredbetween bluff bodies to fully utilize the circularduct geometry and also determines the length ofthe combustion chamber or afterburner necessaryto fully complete the combustion process.

References [1,2,3,4,5] also discuss the mostimportant aspect of the bluff body stabilizedflame, which are the stability limits of the bluff

43rd AIAA Aerospace Sciences Meeting and Exhibit10 - 13 January 2005, Reno, Nevada

AIAA 2005-1116

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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body and the parameters important incharacterizing these stability limits. Twoclassical experiments done on bluff bodystabilized flames were done by DeZubay [6] andKing [7]. DeZubay’s work studied the effect ofvelocity, pressure, size, and fuel-air ratio on theability of the bluff body to successfully stabilizeflame. The results of his investigation indicatedthat the fuel-air ratio is a function of the velocity(V) at the bluff body, divided by the width (N) ofthe bluff body perpendicular to the flow and thepressure (P) at the bluff body. This led to theempirical formula known as DeZubay’sParameter and is given as follows:

85.095.0.'

NP

VParametersDeZubay =

The various parameters making up DeZubay’sparameter have the following units: Velocity infeet per second (ft/s), Pressure in pounds persquare inch absolute (psia), and Width in inches(in).

Figure 1: Stability curve for DeZubay’sParameter [6]

A plot of DeZubay’s results is given in Figure 1and shows the stable range of fuel-air ratios forthe bluff body at a given DeZubay parameter.The upper boundary of the curve represents therich blowout limit for a particular DeZubayparameter and the lower boundary of the curverepresents the lean blowout limit.

King’s work studied the bluff bodies ability tostabilize a flame as well, but his work usedslightly different parameters than DeZubay andKing only looked on the lean blowout limit ofthe bluff body. King used afterburner inletparameters of pressure, temperature, velocity,and fuel-air ratio to perform his experiments.The results of his work showed that fuel-air ratiowas a function of incoming pressure,temperature, and 750 minus the incomingvelocity (750-V). King’s work led to anotherempirical formula that can be used in studyingthe lean blowout limit of a bluff body and isgiven as follows:

252.007.1324.0 )750(' VTPParametersKing −=

The various parameters making up King’sparameter have the following units: Pressure inpounds per square foot absolute (psfa),Temperature in degrees Rankine (oR) andVelocity in feet per second (ft/s).

The DeZubay and King parameters andcorresponding curves formed from theexperimental research offer insight into thestability limits, but work needs to be done tocharacterize the lean and rich stability limits.The work presented in this paper is an attempt tocharacterize the stabilized flame as it approaches

`‘

Figure 2: University of Cincinnati’s Combustion Wind Tunnel Facility (CWTF)

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either a rich or lean blowout limit as shown inFigure 1. While maintaining a certain DeZubayor King parameter, the fuel-air ratio will bechanged from a stable condition to a richcondition by injecting more upstream fuel orchanged from a stable condition to a leancondition by injecting less upstream fuel. Theresults of these tests will use pressureoscillations, temperature field measurements,flame intensity and fluctuation, and flamestructure to investigate a possible footprint orunique attribute of approaching a blowoutcondition.

Experimental Facility

The V-gutter stabilized flame research will takeplace in the University of Cincinnati’sCombustion Wind Tunnel Facility (CWTF)designed by the author, and shown in Figure 2.The operating parameter ranges for the CWTFare given in Table 1. The combustion windtunnel test rig is structured around a pre-mixedcan combustor from a GE J-73 turbojet engine.A schematic of the CWTF is shown in Figure 3along with the dimensions of the facility. Thepre-mixed can combustor is run off of a mixtureof air and Jet A fuel. Both the incoming air andJet A fuel are initially at room temperature. Thecombustion products from this burner go througha flow conditioning screen and then enter atransitioning section consisting of a piece toconvert the flow from a 7 inch circle to a 6 inchsquare followed by a converging nozzle andexiting into a 6 by 3 inch constant area duct. The6 by 3 inch constant area duct consists of an inlet

section, test section, and exhaust section. Theinlet and exhaust sections are instrumented tomeasure the temperature and pressure of the flowbefore and after the test section.

Table 1: CWTF Operating Conditions

Also, the inlet and exhaust sections weredesigned to allow for a traversing mechanism tobe implemented into these sections to measurethe incoming and exiting velocity andtemperature profiles. The inlet section alsohouses a fuel spray bar approximately 10 inchesupstream of the V-gutter. Jet A fuel is used forall of the V-gutter testing and is mixed with theincoming vitiated flow from the upstream pre-burner. The test section is of a modular designwith interchangeable walls to allow for variousinstrumentation. Three of the test section wallscan be replaced with quartz windows to allow foroptical access and the use of advanced laserdiagnostics. The walls can also be replaced toallow for the same traverse mechanism used inthe inlet and exhaust sections to measuretemperature and velocity profiles justdownstream of the V-gutter. The outlet of the 6by 3 inch constant area duct ends in a suddenexpansion to a 10 inch circular duct.

Figure 3: Schematic of the Combustion Wind Tunnel Facility (CWTF)

CWTF Operating ConditionsInlet stagnation pressure 15 – 18 psiaInlet stagnation temperature 65 – 1200 FAir mass flow rate Up to 1.8

lbm/secTotal/static pressure ratio Up to 1.064Inlet Mach number Up to 0.30

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Testing Conditions

As shown in Table 1, the combustion windtunnel facility has a range of conditions that itcan operate at. Therefore, a particular set ofconditions were chosen to perform the requiredtests and these conditions are listed in Table 2.As mentioned earlier, the facility will operate ata single DeZubay and King parameter whilevarying the fuel-air ratio over the entirespectrum, ranging from lean blowout (LBO) torich blowout (RBO). The tests were performedby igniting the GE J-73 preburner and bringingthe facility up to a specific velocity andtemperature. The pressure of the facility alwaysremains near atmospheric conditions. After thefreestream conditions have been set, theupstream fuel spray bar is turned on and a torchis lit just downstream of the V-gutter to ignitethe system and then the torch is extinguishedwhile the V-gutter system runs at a stablecondition. To investigate a possible footprint orunique attribute of approaching a blowoutcondition, the fuel-air ratio will be changed froma stable condition to a rich condition by injectingmore upstream fuel or changed from a stablecondition to a lean condition by injecting lessupstream fuel. The overall equivalence ratios aregiven for the lean, stable, and rich conditionstested. The equivalence ratios were determinedby knowing the air mass flowrate and the totalfuel mass flowrate used at the various conditions.Both the lean and rich equivalence ratios weredetermined in a similar manner by lowering orraising the fuel injection until the V-gutter flamewas extinguished. The values obtained for thedifferent equivalence ratios may seem rather low,but this can be explained by the mixing of the JetA fuel and the vitiated flow upstream of the V-Gutter. A single spray bar is used with 0.018”diameter holes spaced a half an inch apartspanning the facility from the bottom wall to the

Table 2: CWTF Test Conditions

top wall and centered on the same axis as the V-gutter as shown in Figure 3. There are two rowsof holes 180 degrees apart that are alignedperpendicular to the oncoming flow. This singlespray bar method of upstream fuel injection doesnot allow for a homogeneous mixture of the fueland air. The values obtained for the equivalenceratios, especially the rich condition which shouldhave an equivalence ratio greater than 1, showthat the fuel/air mixture is not homogenous andall of the air is not used in the process ofcombustion. This is the reason for thediscrepancy in the equivalence ratio values. Onepossible solution to this problem would requirethe measurement of the oxygen content beforeand after the V-gutter combustor to determinethe actual amount of air being used in thecombustion process. This solution has not beenimplemented at this time, but the results areobtained with confidence because the flame isextinguished from either too little or too muchfuel being injected upstream of the V-gutter.

Measurement Techniques

Temperature MeasurementsTo analyze the temperature field behind the V-gutter at a lean, stable, and rich case, athermocouple probe was used to map out thetemperature field at three various downstreamlocations. The thermocouple probe used was aType K Super OmegaClad XL probe. A one-dimensional traverse was used to scan the probealong the centerline of the facility. Thetemperature field was only done for thecenterline plane at three various locationsdownstream of the V-gutter.

Pressure MeasurementsThree different pressure sensors were used toanalyze the pressure response at the variousoperating conditions. All three of these sensorswere identical, but placed at various locationsalong the facility. One sensor was justdownstream of the pre-burner combustionchamber, the second was located at the inletsection of the facility, and the third sensor wasdownstream of the V-gutter. The sensor type is aDruck PMP 4065-A276 static pressure sensorand was used to measure the static pressure asthe fuel injection was varied from stable to richand stable to lean. These pressure signals wereused to analyze the various conditions in anattempt to characterize to blowout limits.

Test ConditionsPressure 14.7 psiaTemperature 750 oFVelocity 340 ft/secDeZubay Parameter 22King Parameter 108,270Lean Condition Φ 0.36Stable Condition Φ 0.47Rich Condition Φ 0.68

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CH MeasurementsTwo separate sources were used to measure theCH content in the flame. Both sources used theoptical access available from the quartzwindows. The first method measured a pointsource using an optical fiber and a CH filter. Asecond method used a Princeton Instruments PI-MAX ICCD camera and a CH optical filter tomeasure the CH content of the entire flame.Each method was used to determine if anoticeable unique characteristic could beobserved near the blowout limits.

High-Speed ImagesTwo separate cameras were used to take high-speed images of the combustion flame. Thehigh-speed images can give insight into theflame structure as well as the flame intensity andfluctuation as the V-gutter is operated at variousconditions. The first camera is a Phantom high-speed camera capable of taking up to 4000frames per second. The second camera is theICCD camera mentioned before. Taking directimages of the flame without the CH filter allowsfor a much smaller gate width to be used and abetter resolution of the flame structure.

Results and Discussion

The results of these experiments will be brokendown into two categories of quantitative resultsand qualitative results. The temperature,pressure, and CH measurements will lead toquantitative results while the high-speed imagingwill lead to the qualitative results.

Quantitative Results

The temperature measurements were made atthree axial positions (X/D – 0.8, X/D – 4.4, X/D8.4) downstream of the V-Gutter. At eachlocation, three separate conditions (near LBO,stable, near RBO) were run to determine thedifference in the temperature field behind the V-gutter. The temperature profiles were comparedwith a few references [8,9,10] and showed goodcomparison at the various downstream distances.Figure 4 shows the temperature profiles at thethree locations for the various operatingconditions and Figure 5 shows a side-by-sidecomparison of the contour plots. At X/D – 0.8downstream of the V-gutter, the profile showsthe cooler vitiated freestream profile followed bya small dip in temperature just at the edge of the

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Figure 4: Temperature Profiles for threeaxial positions downstream of the V-gutterX/D (a) 8.4, (b) 4.4, (c) 0.8.

Figure 5: Side-by-Side Temperature contourplots for Stable, LBO and RBO conditions.

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flameholder. This small temperature drop can beattributed to the Jet A fuel being introduced atroom temperature and not being fully heated tothe freestream temperature. The drop becomesmore pronounced as more fuel is added goingfrom the lean case to the rich case. After thesmall drop in temperature, there is a steepincrease in the profile due to the flames presenceand ending in the recirculation zone along thecenterline axis of the V-gutter. The most notabledifference at this location is that the fuel richcase is at a higher temperature than the othersand the stable and lean case are at nearlyidentical temperatures. At X/D – 4.4 and 8.8downstream, the freestream flow remainssimilar, but the flame spread can clearly be seenin these profiles. The slope of the profiles in thereacting shear layer begins to flatten out as youmove further downstream from the V-gutter.The total flame spread angle for these cases isapproximately 13 degrees. It can clearly be seenthat the lean case results in the lowesttemperature, but the stable case has increasedabove the rich case at X/D – 4.4 downstream andthe two profiles nearly fall on top of each otherat X/D – 8.4 downstream of the V-gutter.

Pressure and CH measurements wereinvestigated to see if they offer a uniquecharacteristic for approaching either a leanblowout condition or a rich blowout condition.The CH measurements were taken with anoptical fiber at a single point, but numerous testswere run with the fiber located at differentpositions throughout the flame. All CHmeasurements at different locations resulted in asimilar trend and therefore only one specificlocation is discussed. Figure 6 is a combinationof mean and rms CH measurements and meanand rms pressure measurements. Figures 6 (a)and (c) show results of reducing the equivalenceratio from an initial stable condition until LBOoccurs, and (b) and (d) show the results ofstarting at a stable condition and increasing theequivalence ratio until RBO occurs. If acomparison is made between CH measurements,(a) and (b), the mean light intensity shows thatthe most CH emission occurs at the initial stablecondition and drops off as you approach eitherlean blowout or rich blowout. If you comparethe rms of the light intensity it is evident thatthere is a clear difference in approaching LBO orRBO. If the equivalence ratio is decreased fromthe stable condition towards LBO the rms light

Figure 6: Mean and RMS single point CH emisson (a), (b) and Mean and RMS pressure (c), (d) forvarying equivalence ratio from a stable condition to a lean condition (a), (c) and from a stablecondition to a rich condition (b), (d).

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intensity decreases with the equivalence ratio allthe way through LBO. However, if theequivalence ratio is increased from the stablecondition towards RBO the rms light intensitybegins to drop off slightly but has a significantincrease just before the flame extinguishes. Theresults of the CH measurement do offer a uniquecharacteristic that separates the two events ofblow out. Looking into the pressuremeasurements helps solidify this clear distinctionbetween RBO and LBO. Again, starting at astable condition and decreasing the equivalenceratio to LBO results in a decrease in both themean and rms pressure. There is a significantdifference in starting at a stable condition andincreasing the equivalence ratio to RBO. Thisresults in the mean pressure increasing all theway to RBO and the rms pressure initiallybeginning to decrease but as RBO is approachedthe rms value begins to increase again. Thepressure and CH give a quantitative uniquefootprint for approaching a blowout conditionand allowing for the recognition of which limit isbeing approached.

Further analysis of the pressure signal wascarried out by computing a moving FFT of thesignal as the conditions were changed fromstable to lean or rich blowout. Figure 7 showsthe results of these tests and plots the frequencyas a function of time. When time is equal to zeroseconds the V-gutter is operating at a stablecondition. As time progresses, the equivalenceratio is either decreasing (a) or increasing (b) toapproach either lean blowout or rich blowoutrespectively. In Figure 7(a) the general trend ofthe frequency is decreasing over time or as theequivalence ratio is lowered. This trend can bedescribed because of the direct relation betweenthe frequency and speed of sound. When theequivalence ratio is decreased to LBO, thetemperature field is decreased which results in adecrease in the speed of sound and an overalldecrease in the frequency signal. The exactopposite is true in Figure 7(b) when going fromthe stable condition to RBO. The equivalenceratio increases resulting in a slight increase in theoverall temperature and an increase in the speedof sound that results in an overall increase in thefrequency signal as RBO is approached. Onedistinct difference in between Figures 7 (a) and(b), other than the frequency either decreasing orincreasing, is that the frequency just less than100 Hz seems to go away as lean blowout is

Figure 7: Moving FFT of pressure signal asequivalence ratio is changed from stable toLBO (a) or RBO (b)

approached, but seems to intensify as richblowout is approached. This feature has beencircled in the figure to point out the distinction.Further work needs to be done to determine thereason for this phenomenon.

Qualitative Results

To further characterize the blow out limits of a Vgutter stabilized flame, qualitative experimentswere run to be able to visualize the difference inflame structure, intensity, or fluctuation. Thefirst distinct difference presented involves boththe flame structure and also the CH intensity ofthe flame as the conditions are changed fromeither stable to LBO or stable to RBO. The CHintensity captured by the ICCD camera and a CHoptical filter correspond very nicely with the

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a b c d e f

g h i j k lFigure 8: Side view CH measurements of flame going from stable conditions (a) to LBO (k)

a b c d e f

g h i j k lFigure 9: Side view CH measurements of flame going from stable conditions (a) to RBO (k)

single point analysis discussed in the quantitativeresults section. Figures 8 and 9 are side viewimages with the flow from left to right with theV-gutter edge slightly in the left side of theimage. The V-gutter does span most of the testsection, but there is a small clearance at the topwall to allow for thermal growth of the V-gutter.The impact of this gap can be visualized inFigures 8 and 9. Figure 8 shows the CH in theflame as the equivalence ratio is varied from aninitial stable condition to LBO. At the initialstable condition the flame spans the entiresection as shown in the first few figures with themaximum intensity fluctuating about the span.As the equivalence ratio is decreased towardsLBO, the impact on the flame structure andintensity becomes evident. The flame structurebegins to shrink down from spanning the entiresection towards the center of the test section untilthe flame eventually extinguishes and theintensity of these images continually diminishesuntil extinction. Figure 9 shows the CH in theflame as the equivalence ratio is varied from astable condition to RBO. The first few imagesagain are at a stable condition with the followingimages showing the footprint of approachingRBO. The major observation that can be madeis that the flame intensity diminishes, but it also

has an increased fluctuation of intensity thatagain corresponds with the quantitative results.

The ICCD camera was used again but withoutthe CH filter to investigate the flame structure atlean, stable, and rich conditions. Figure 10 is atop view image of the V-gutter stabilized flamewith an exposure time of 50 µs. The flow fieldin Figure 10 goes from top to bottom in theimage. The small exposure time allows fordetails of the flame to be captured andcharacterized for the various conditions. It isclear in Figures 10 (a), (b), and (c), that vortexshedding and roll up occurs, but the size andintensity of these structures changes as theequivalence ratio is changed. For a detaileddescription of the process of vortex shedding in areactive environment and combustion instabilityrefer to references [11,12]. For the stable case inFigure 10 (b), it seems that symmetric large-scale vortices dominate the flow field justdownstream of the V-gutter. The center core onthese vortices has the highest intensity thatcorresponds to the highest heat release or mostintense burning at the center of the vortex thatthen propagates out to the edge of the flame. Forthe lean case in Figure 10 (a), there seems to bemore vortices of smaller scale that increase in

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(a) (b) (c)Figure 10: High Speed visualization of flame structure near LBO (a), stable (b), and RBO (c)

size as they propagate further downstream.Again, the two sides seem fairly symmetric withapproximately five individual vortices sheddingfrom each side of the stabilized flame. In theshear layer between the recirculation zone andthe flame front there seems to be even smallerscale structures present in this mixing region.Comparing this image to the stable case, a majordifference can be observed between the size ofthe vortices, amount of vortices present, intensityof these structures, and the small scale structuresobserved in the mixing layer between the flamefront and the recirculation zone. The stable casevortices are more intense as would be expectedwith more fuel in the flow. For the rich case or10 (c), there again seems to be more vortices ofsmaller scale that increase as they propagatedownstream. The overall size, structure, andsmall scale mixing in the region between therecirculation zone and flame front of this flameis very similar to the lean case with the exceptionof the increase in intensity of the stabilizedflame. The same comparison holds true for thestable flame because of the similar nature to thelean case.

Further analysis of flame intensity andfluctuation was carried out with the Phantomhigh-speed camera by direct imaging of thestabilized flame. The high-speed camera allowsfor a small time interval between figures tocapture a better characterization of the flamefluctuation as either lean blowout or richblowout is approached. The high-speed camerais capable of over 4000 frames per second, butfor this analysis a much larger interval of 200frames per second was used to capture distinctcharacteristics of the blowout limits. The samplerate results in a 5 ms time step between images.Figures 11, 12, and 13 are top view images of theV-gutter stabilized flame with flow going fromtop to bottom in the images. Figure 11 is a

representation of the stabilized flame operatingat a stable condition. The flame intensity ofFigure 11 is much greater than Figures 12 and13, which corresponds to LBO and RBO,respectively. Again, this reiterates the resultsobtained by the single point CH measurementdiscussed in the quantitative analysis. The mainpoint of Figure 11 is just to show the largedifference in intensity of the flame whenoperating at either a stable, rich, or leancondition. Figure 12 captures the flamefluctuation and intensity when nearing leanblowout and actually extinguishing the flame.The images start at time equal to 0 ms with theequivalence ratio slightly higher than LBO andshow sequential 5 ms images as the equivalenceratio is being decreased until LBO occurs. Themajor observation from these images is that theflame intensity continues to decrease with lessfluctuation as LBO occurs. Figure 13 capturesthe flame fluctuation and intensity when nearingrich blowout and extinguishing the flame. Againthe images start at a time of 0 ms with theequivalence ratio slightly less than RBO andshow sequential 5 ms images as the equivalenceratio is increased until RBO occurs. The processinvolved in RBO offers a more interestingphenomenon than LBO. The maincharacteristics of this process involve the flameinitially at a certain intensity which begins todrop off as RBO is approached. However, as theintensity decreases, the maximum intensitybegins to fluctuate from one side of the V-gutterto the other shown at a time interval between 15and 45 ms. After this process completes, theentire flame drops off in intensity almost all theway to extinction followed by a large burst inflame intensity where the flame seems to bereigniting itself in an attempt to stay lit. Theprocess seems to repeat twice for this case shownat a time interval between 50 and 70 ms andagain between 75 and 85 ms. This is a last

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attempt of the flame to stay lit before it drops offin intensity and extinguishes. Again, results of

these images correspond nicely with thequantitative single point CH measurements.

0 5 10 15 20 25 30Figure 11: High speed visualization of the flame at a stable condition, time in ms

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35 40 45 50 55 60 65Figure 12: High speed visualization of the flame approaching LBO, time in ms

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35 40 45 50 55 60 65

70 75 80 85 90 95 100Figure 13: High speed visualization of the flame approaching RBO, time in ms

Conclusions

The main goal of this work was to distinguishbetween approaching a lean blowout condition ora rich blowout condition and determine a uniquefootprint or attribute of approaching either limit.This goal was obtained and the majordistinguishing features between LBO and RBOwill be summarized.

Lean Blow Out

Approaching LBO results in decreased mean andrms CH emission and pressure.

Decrease in frequency as LBO is approached.Further analysis of the frequency signal needs tobe carried out

As LBO is approached, the flame structurebegins to shrink down from spanning the entiresection towards the center of the test section untilthe flame eventually extinguishes with theintensity of these images continually diminishinguntil extinction.

For the lean case, a major difference from thestable condition is the smaller size of thevortices, increased amount of vortices present,

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decreased intensity of these structures, and thesmall scale structures observed in the mixinglayer between the flame front and therecirculation zone.

Rich Blow Out

Approaching RBO results in decreased mean CHemission but increased rms CH emission.

Approaching RBO results in increased meanpressure, with rms pressure initially decreasing,but increasing again prior to RBO.

Increase in frequency as RBO is approached.Further analysis of the frequency signal needs tobe carried out.

As RBO is approached the major observationfrom the images that can be made is that theflame intensity diminishes, but also has anincreased fluctuation of intensity.

For the rich case, the overall size, structure, andsmall scale mixing in the region between therecirculation zone and flame front of this flameis very similar to the lean case with the exceptionof the increase in intensity of the stabilizedflame.

Additional Conclusions

The slope of the temperature profiles in thereacting shear layer, for all cases, begins toflatten out as you move further downstream fromthe V-gutter representing the flame spread.

The lean case results in the lowest temperature,and the stable case increases above the rich caseat X/D – 4.4 downstream, but the two profilesnearly fall on top of each other at X/D – 8.4downstream of the V-gutter.

Acknowledgements

The financial support from GE Aircraft enginesis greatly appreciated. The authors would alsolike to thank Mr. R. DiMicco for aid in facilitydesign and setup, and Dr. G. Li for help inexperimental testing.

References

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