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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
a,b,∗,
R.
Vicquelin
a,
C.
Mirat
a,
B.
Labegorre
b,
P.
Jourdaine
b,
J.
Caudal
b,
T.
Schuller
a,caLaboratoireEM2C,CNRS,CentraleSupélec,Université Paris-Saclay,3,rueJoliotCurie,Gif-sur-YvetteCedex91192,
France
bAirLiquide,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-toursindicatetheOtsuthresholdusedtodetermineLS .
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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