Scaling relations for the length of coaxial oxy-flames with and without swirl

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Degenève, Arthur and Vicquelin, Ronan and Mirat, Clément and Labegorre, Bernard and

Jourdaine, Paul and Caudal, Jean and Schuller, Thierry Scaling relations for the length of

coaxial oxy-flames with and without swirl. (2018) Proceedings of the Combustion Institute, 37

Scaling

relations

for

the

length

of

coaxial

oxy-flames

with

and

without

swirl

A. Degenève

,

R.

Vicquelin

,

C.

Mirat

,

B.

Labegorre

,

P.

Jourdaine

,

J.

Caudal

,

T.

Schuller

a_{LaboratoireEM2C,CNRS,CentraleSupélec,Université Paris-Saclay,3,rueJoliotCurie,Gif-sur-YvetteCedex91192,}

France

b_{AirLiquide,CentrederechercheParisSaclay,ChemindelaPortedesLoges,B.P.126,LesLogesenJosas78354,France}

c _{InstitutdeMécaniquedesFluidesdeToulouse,IMFT,Université deToulouse,CNRS,Toulouse,France}

Abstract

Anextensiveexperimentalstudyiscarriedouttoanalyzescalinglawsforthelengthofmethaneoxy-flames stabilizedonacoaxialinjector.ThecentralmethanefuelstreamisdilutedwithN2 ,CO2 orHe.Theannular airstreamisenrichedwithoxygenandcanbeimpregnatedwithswirl.Formerstudieshaveshownthatthe stoichiometricmixinglengthofrelativelyshortflames iscontrolledbythemixingprocesstakingplacein thevicinityof theinjectoroutlet.Thispropertyhasbeenusedtoderivescalinglawsatlargevaluesofthe stoichiometricmixturefraction.Itisshownherethatthesamerelationcanbeextendedtomethane oxy-flamescharacterized bysmall valuesofthestoichiometric mixturefraction.Flamelengthsaredetermined withOH∗chemiluminescencemeasurementsovermorethan1000combinationsofmomentumratio,annular swirllevelandcompositionoftheinnerandouterstreamsofthecoaxialinjector.Itisfoundthatthelengths ofalltheflamesinvestigatedwithoutswirlcollapseonasingleline,whosecoefficientscorrespond towithin 15%offlamelengthsobtained forfuelandoxidizerstreamsatmuchlargerstoichiometricmixturefractions. Thisrelationisthenextendedtothecaseofswirlingflamesbyincludingthecontributionofthetangential velocityintheflowentrainmentrateandisfoundtowell reproducethemixingdegreeofthetwo co-axial streamsaslongastheflowdoesnotexhibitavortexbreakdownbubble.Athigherswirllevels,whenthe flowfeaturesacentralrecirculationregion,theflamelengthisfoundtoalsodirectly dependontheoxygen enrichmentintheoxidizerstream.

Keywords: Coaxialjet;Oxy-combustion;Swirl;Flamelength;Turbulence

∗_{Correspondingauthorat:LaboratoireEM2C,CNRS,}

CentraleSupélec, Université Paris-Saclay, 3, rue Joliot

Curie,91192Gif-sur-YvetteCedex,France.

E-mail address: arthur.degeneve@centralesupelec.fr

(A.Degenève).

1. Introduction

Many industrial combustors are powered by

co-axial injectors in which the oxidizerand fuel streams are injected separately andexpand in a largecombustionchamber. Itiswellknownthat

thicknesse=1 mm,whereinmethane is eventu-allymixedwithadiluent(N2,CO2orHe),andan

annularinjectionchannel of outerdiameterd2 =

20 mm, with an adjustable O2 /N2 gas composi-tion.Aswirlmotioncanbeimposedtothe annu-larstreamthankstotangentialslits.Theaxialand tangentialvolumeflowrates injectedinthe annu-larchannelaredesignatedrespectivelybyQ2, zand

Q2, θ.Assumingasolid-bodyrotation,the

geomet-ricalswirlnumberS2 intheannularchannelisgiven

by S2 = π 4 Hd2 NlL 1− (d2/d1)4 1+Q2,z/Q2,θ (3)

whereHisthedistanceseparatingthetangential injectionchannelsfromtheburneraxis,lthewidth andLtheheightoftheNtangentialinjection chan-nels.Thisdevicewasdesignedtoproduce geometri-calswirlnumbersrangingfromS2 =0to1.73with

N=2slits.Thecentralfuelstreamisnotswirledin thisstudy.Thecoaxialjetexhaustsinquiescentair atambientpressure,aboveaback planeatroom temperatureT =300K,soastoavoideffectsof confinement andheat transfer to the walls. The coaxialinjectoroutletiselevated5mmabovethe backplanetoeasevisualizationandthereisno re-cessbetweenthecentralandouterinjectoroutlets. Thisaltitudedefinestheaxialoriginz=0.

Theflamelengthisdefinedasthefurthestpoint ontheaxiswithacombustionreaction[12,17].In manycases,thelengthofdiffusionflamesis deter-minedbyrecordingitsglobalemissioninthe visi-bleband[12,19,20]ortheemissionofselected in-termediateradicals[9,17,18].Inthesestudies,the flame lengthiscommonly correlated to the stoi-chiometricmixinglengthLS .Giventhelargerange

ofoperatingconditionstargetedinthiswork,itis firstworthexploringiftheOH∗emissionsignalisa goodtracerforthelengthofoxy-flamesandifitis wellcorrelatedtothestoichiometricmixinglength

LS.

Numerical simulations of steady

one-dimensional counterflow methane oxy-flames

werecarriedoutwiththeGrimech3.0mechanism

[21] in [22] and completed with OH∗ chemistry

in[23].TheseauthorsverifiedthattheOH∗peak emissionfromdiffusionoxy-flamesliesveryclose to stoichiometry at z=zst for a large range of

oxygen enrichments and strain rates. Numerical

simulationshavealsobeencarriedoutinthisstudy toverifythispropertyforselectedoperatingpoints

withthesameGrimech3.0mechanismcompleted

withthechemistryofOH∗[23].Theyareprovided assupplementarymaterialandconfirmthatboth

peaks of temperature and OH∗ lie close to the

flame front at z=zst , when compared to the

flamethickness.Self-similarcounterflowequations were solvedwithanin-housecodewith identical

transport and thermodynamics properties as in

the CHEMKIN package and similar numerical

algorithms.

Fig.2.OH∗intensitydistributionofselectedflamesfrom

datasetD2inTable1andincreasingm values.

Num-bersrefertoFig.4(b).WhitecontoursindicatetheOtsu

thresholdusedtodetermineLS.

Fig.3.OH∗intensitydistributionofselectedflamesfrom

datasetsD5andD6inTable1withhighannularswirl

lev-elsS₂ .LatinlettersrefertodatashowninFig.7.Greek

lettersmakereferencestodatashowninFig.8.White

con-toursindicatetheOtsuthresholdusedtodetermineL_S .

The OH∗ images are recorded with a 16 bit

intensified CCD camera (ICCD, Princeton,

PI-MAX4,1024 × 1024pixels)equippedwithanUV

objective (Nikkor 105 mm) and a 10 nm

band-passfiltercentered at310 nm (Asahi

XBPA310-Bandpass).Lineofsightintegratedaveragedfields ofOH∗emissionaredeterminedfrom30snapshots eachwithanexposuretimeof 200ms.Statistical convergenceofthemeanOH∗intensityis system-aticallyverified.

The Otsu thresholding method [24]is chosen

heretoinfertheflamefrontlocationfromtheOH∗ imagesas in [25,26].Exampleswithoutswirl are showninFig.2andeffectsofswirlareillustrated inFig.3.ThestoichiometricmixinglengthLS is de-terminedasthehighestpointoftheOtsucontour, whichisfoundtoliealongtheburnercenterline, ex-ceptforthehighlyswirledflamesfeaturingacentral recirculationzone.Asensitivityanalysishasbeen carriedouttoexaminechangesof LS when

vary-ingtheimagebrightnessandcontrast.Different ex-posuretimesandcameraaperturesweretested.It appearsthatthelargestvariations of LS are

lim-itedtowithin5%.Themethodisalsovalidatedin

Section 4bycomparingtheresults withthe

mea-Acknowledgments

ThisworkissupportedbytheAirLiquide,

Cen-traleSupélecandCNRSChaironoxy-combustion

andheattransferforenergyandenvironmentand

by the OXYTEC project (ANR-12-CHIN-0001)

froml’AgenceNationaledelaRecherche.

Supplementary material

Supplementarymaterialassociatedwiththis ar-ticlecanbefound,intheonlineversion,atdoi:10. 1016/j.proci.2018.06.032.

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