Page 102 - konishipaper

totheleftdirection.1nfrontofflameleadingedge，airflowis generatedbythebuoyancyoftrailingflame.1nliquid-phase，the thermocapilaryflowisinducedduetotemperaturediference betwenhotliquidnearflameleadingedgeandlowtemperature liquidaheadofflame.Atstep[b]，theflamespreadsfasterthan theoneatstep[a]andgeneratesabigerthermocapilaryflowin thesamedirectionoftheflame.Tobedeservedspecialatention
istheexisteneoftemperαturevaleyaheadofflameleadingedge.
，
Thelowtemperaturefluidcomesupfromthebouomofliquid fuel.Thiscoldliquidisharmfultoformtheflamabilityvapor layerinfrontofflameleadingedgeandc1earlyobstructscrawl- ingflamefromshiftingtojumpingstage，becausethesurface temperatureinthis紅 eaisbelowtheflashpoin.tInthegas-phase， thesituationisthesamea~ step[a]，thereisabuoyancy-induced airflowwhichmovesagainsttheflamespreaddirection.Thebuoy- ancy-inducedairflowandthethermocapilaryflowdirectagainst eachothercreatingasmal1circulationinthegas-phasejustahead oftheflameleadingedge.Atstep[c]，astheregionofcoldsurface 1iquiddisapearsduetoheat-transferfromliquid-phaseandgas- phase，theflamablefuelvaporisacumulatedaheadoftheflame
Step[a]
Step[b]
Step[c]
Step[d]
Step[e]
Figure2Schematicdiagramsofpulsating flamespreadmechanism.
leadingedge.Ingas-phase，therecirculationcel1isdevelopedin itssizeandmightchangetheconcentrationfieldsubstantialy.At step[d]，theflamemightjump，becausetheconditionofflam- mablevaporconcentrationisfulfi1led.
MEASUREMENTOFSURFACE TEMPERATUREAND CONCENTRATIONPROFILESINEACHPULSATING STEPS
Figure3throughFigure6show(a)thermographicimages， (b)surfacetemperatureprofilesmeasuredalongthelongitudinal centerlineoffueltray，and(c)concentrationprofilesmeasured bydualwavelengthholographicinterferometry(DWHI).The DWH1isoneoftheholographictechniquewhichusesthedepen- denceofGladstone-Daleconstantsonwavelengthtodetermine bothconc.entrationandtemperaturechange.WeadaptedtheDWHI tomeasurementofconcentrationprofilesoverspreadingflame indepn-propanolpool.Thedetailsaredescribedinourprevi- ous paper (lto et a，.1 19 97b).
Steoral:Onsetofpulsation Figures3(a)through(c)showtheinitiationprocesofpul-
sation，inwhichourpreviousstudiesrevealedthattheliquidflow wassmal，whereasairflowwasinducedbybuoyancyaheadof theflameleadingedge.Althoughtheflamefronthasatwo-di- mensionalstructure，thesurfacetemperaturehasinherentlythre- dimensionalnatureasshowninFig.3(a).Thefigure3(b)shows thatthesurfacetemperaturerapidlydecreasewiththedistance fromtheflameleadingedge.官 le旬 mperaturerangeoftheinfra- redcameraisnarowlyadjustedsothatthesensitivechangeof surfacetemperatureaheadofflameleadingedgecanbedetected. Therefore，althoughthepositionofflameleadingedgeisdeter- minedto血.atwherethesurfacetemperat町 eissaturated，thisflame positionisnotsameastherealflameone.At3mminfrontofthe flameleadingedge，thesurfacetemperaturedecreasesto24.5 C， whichisneartheflashJ>ointofn-propano.lThetemperatureIn- creasesagainupto27 Cat5mmaheadofflameedgeandrap- idlydecreasestoinitialiquidtemperature.AsshQwninFig.3(c)， thevaporconcentrationprofi1esisalmostconstantoverthefuel surfaceandthepositionofleanflammablelimit，whichis2.2 vol%forn-propanol，is1m mabovethefuelsurface.百lequench- ingdistanceofspreadingflameoverliquidisdetermined0.8- lmminourpreviousstudies(Itoeta，.l197b).Evennearthe flameleadingedge，theleanflammablelimitexistswithinthe quenchingdistance.百lerefore，theflamecanotpropagateingas- phase.
Step[b]:Formationoftemperaturevall~~ Figures4(a)through(c)showtheformationptocesoftem-
peraturevaley.Thepreviousstudies(1toeta，.l197b)disclosed 出at;asmalrecirculationcel1atseveralmil1imeteraheadofflame leadingedgewasobservedingas-phase.1nliquid-phase， 出ermocapilaη，flowwasinducedbythetemperaturediference betweenthehotliquidunderflameandthecoldliquidfar合om flame.百lebuoyancyinducedflowwasgeneratedinsidetheliq- uidfuelat1-4mmunderthefuelsurface.Thetemperatureofthis liquidmightbesametotheinitialfueltemperature.Thesurface temperatureprofi1esconfirmedthattemperaturevaleyexistsat about5mmaheadofflameleadingedgeasshowninFig.4(b).
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