Numericalsimulationsofpremixedcoolflamesofdimethylether／oxygenmixtures.docx

Article history:Received 6 April 2015Revised 18 June 2015Accepted 19 June 2015 Availableonline 21 July 2015Keywvak:Cool amcsFlame speedExtinction limitDinieihyl etherOzoneNumericalsimulationsofpremixedcoolflamesofdimethylether/oxygenmixturesYiguangJu,ChristopherB.Reuter,SangHeeWonDepartmentOfMechanicalandAerospaceEngineering,PfiMegnUnNersiiy,PrbWeion.Nl08X4,USAarticleinfoThCformationanddynamicsofpremixedCoOlIlamcsarenumericallyinvestigatedbyusingadetailedkineticmechanismofdimethylethermixturesinbothfreely-propagatingandstretchedcounterflowflameswithandwithoutozonesensitization.ThepresentstudyfocusesonthedynamicsandtransitionsbetweenCOOInanleSandhightemperatureflames.Theimpactsofmixturetemperature,inertgastemperature,andozoneconcentrationonlowtemperatureignition.COOl11ameformation,and11amnubleregionsofdiHerent11ameregimesareinvestigated.Forthefreely-propagating11ames,threedifYerentflameSiniciures(hightemperature11anes,doubleflames,andCOOlflames)arefound.ThepresentstudyshowsthattheflammabilitylimitofdimethyletherisSigniflCanllyextendedbytheappearanceofcoolflamesandthattheconventionalconceptofthe(Iamniabilitylimitofahightemperatureflameoughttobereconsidered.Furthermore,theresultsdemonstratethatthecoolamepropagationspeedcanbesignificantlyhigherthanthatofncar-limithightemperatureflamesandthatozoneadditiondramaticallyacceleratesCheformationofcool11anesatlowtemperaturesandextendstheflammabilitylimit.Aschematicofamodified11ammabilitylimitdiagramincludingbothhightemperatureflamesandCOOInameSisproposed.Forstretchedcounterflowflanes.the*sulsalsoshowthatmultipleameregimesexistwithandwithoutozoneaddition.Itisdemonstratedthatatthesamemixtureenthalpy,ozoneadditionkineticallyextendsthecoolflameextinctionlimit(oahigherstretchrale.Moreover,withozoneaddition,twodierenicoolflametransitionregimes:alowtemperatureignitiontransitionandadirectcool11amctransitionwithoutanignitionlimitathighertemperature,arcpredicted.ThepresentresultssuggestthatCoOltlamescanbeanimportantcombustionprocessinaffectingIlammabilitylimitsandflameregimesasthemixturetemperature,turbulentmixing,andradicalProductioiVrecirculationareincreased.Theresultsalsoprovideguidanceinobservingself-sustainingpremixedcl11anesinexperiments.©2015TheCombustionInstitute.PublishedbyElscvicrInc.Allrightsrcser,cd.IUre/pressuredomainwereobtained112.I4.Nevertheless,fewstudieshavebeencarriedoutonCOOlflamedynamics,extinction,andabstractCooIflameshavelongbeenconsideredakeyprocessresponsibleforengineknockandarealsoanimportantphenomenonforfiresafetyl-6.Sincethefirstdiscoveryofclflamestwocenturiesago7.8,extensiveeilbrtshavebeenmadetoobservecoolflamesbyusingvariousflamegeometriesincludingheatedsurfacesandheatedburners8-11,stirredreactors112-14,heatedflowreactors1.8,I5-17J,rapidCOmPreSS沁nmachinesI8,counterflowflames119,droplets16.20.211.andplasma-assistedflames22.23.Despitethatmanyoftheobservedclflameswereoscillatory,transient,andstronglyaffectedbyflame-surfaceinieraciion.insightsintocoolIlamespectroscopy,negativetemperaturecoefficient(NTC)chemistry,heatrelease,andthecoolflamepeninsulainthetempera-BCocrespondingauthor.Fax:+16092586233.E-mailaddress：yjuprirKeion.edu(Y.Ju).h(ipdx.doi.org10.1016',j.conbusflanfe.2015.06.014flammabilitylimits.Muchofourknowledgetodayaboutamepropagationspeeds,amestructures,extinctionlimits,andflammabilitylimitsisstilllimitedtohightemperatureflames24.Duetothedifficultyinestablishingself-sustainingclflames,adetailedandfundamentalunderstandingofclflamedynamicbehaviorshasnotbeenwellestablished.Recently,aself-sustainingn-heptanecooldiffusionflamewasexperimentallyobservedforthefirsttimebyusingozonesensitizationinacounterflowflameoverabroadrangeofflowresidencetimes19).Theflamestructuresandextinctionlimitswerequantitativelymeasuredinthisstudy.Theresultsshowedthatozoneadditionextendedtheflammableregionofcoolflamesandallowedselfsustainingclflamestobeobservedonalaboratoryflamescale.Inotherstudies.LawandZhaonumericallymodeledtheNTCeffectonn-heptane/airignitioninadiffusioncounterflowflame25.ThcirresultsdemonstratedtheexistenceofNTC-affectedignition.Inn-heptanedropletcombustionexperiments.Nayagametal.6reportedthedualmodesofcombustionandextinctionandpresentedahypothesisofclflameformationinamicrogravityenvironment.Inarelatedn-heptanedropletstudy,Farouketal.211numericallymodeledOSCiHalOryCoOlflamesgovernedbyradiationheatlossandconductiveheatlossfromflametodroplet.However,theabovestudieswerealllimitedtodiffusionflames.Forpremixedcoolflames.Marutaandcoworkersrecently16,17Jstudiedthestabilizationlimitsofclflamesbyusingabumerteatedmicrochannel.Unfortunately,inthisgeometrythewalltemperaturegradientandflame-wallcouplingstronglydictatetheflamestability,makingitditYiculttoaddresshowfastaclflamecanpropagateandwhattherelationisbetweentheflammabilitylimitsofhightemperaturetlamesandcoolflames.Withtheexpectedincreasesinpressureandinitialmixturetemperatureinfutureengines,(hereisa