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2824 FIRE-IgnitionandFlameSpread
forma世onofthegas-phasecirculationcel也aten- trainsevaporatedfuelvaporinfrontoftheleading edge of the flame. Recent ex.periments in micro- gravity，however，revealedthatinspiteofthegas- phasecirculaせoncelth批 existsinfrontoftheflame edge，nopulsa世onocurs.Instead，slowpseudo-uni- formspreadhasbeenobservedusingadeepn七u- tanolpol[9，]indicatinglitleefectofthecircula- tioncelonthecauseofthepulsation.Bycomparing oure守 erimentalresultswithUCImodelcalcula- tionsbothingasandliquidphases，wefou~d that thegascirculationcelmeasuredin也eex.pmiments waslargerthanthatofthenumericalprediction.By studyingourthre-rumensionalvelocitydata[13，] wefoundth批 thethermocapilaryforceintheliquid phasea妊ected也eairflowfieldwi白inaheightof ap proximately l.5 m m above the fuel surface. The buoyantairflowoverl.5m mabovethefuelsmface wascoupledtotheshapeoftleflamefront.How- ever，thenumericalcalculationsshowedthatthe thermocapilarアforceconstrictedtheairmovement over吐leliq凶 dsu市 ceon砂wi也inaheightof0.2 mm.Thepenetrationofthethermocapilaryflow 加 totheliquiddepthclearlyafected出esizeofthe gascirculationcel;thatis，thelargerpenetration depthcausedthelargergascirculationcel.Howwas thisdiferencecreated?Clearly，viscosityandinten- sityoftheinducedairflowcanbesugested.Inthe liquidphase，thepenetrationdepthforthenumeri- calresultswassmalerthanfortheex.perimentalre- sults.Although也eex.perimentshowsthattheliquid undertheflameleadingedgecameupfromthebot- tomofthefueltray，tlenumericalcalculationcon- sideredonlythe2m mdepthforthecomputational domainoftheliquidphase.Onthecontr紅 y，the占s- tance for the surface tension flow showed good agreementbetweenthenumericalandthee却 eri- mentalresults.
InStep[b]，thecoldtemperaturevaleywasgen- eratedaproximately5m maheadoftheflameedge intheexperiment，whilethesurfacetemperatureof thenumericalcalculationdereasedmonotonicaly alongthelongiturunalaxisofthefueltray.Although thesurfacetensionflowvelocitydecreasedmono-
.tonicalyinthee守 eriment，thenumericalresultof thevelocityprofilehadafl此 region5m maheadof theflameedge.Noteth抗 thenumericalmodelpre- dictedtlattheflamablelayerisproducedjust aheadoftheflameedge;however，DWHIresults in占cate也at"thereisalmostzerofi
theliquidphase.Thebuoyancyefectseemednotto workwithaveryshalowliquidcontainerinthis mode.l Thus，modification is sugested for the modeltoextendthecomputationaldomainofthe liquidphasetoarealisticdepthofroughly10m m toimprovetheagreement.
WhyPulsationDoesNotOcωγ 的 Microgravity
A series ofNASKs microgravityexperiments aboardasoundingrocketofersvaluableinsightto us[7，9，18].Researchersusedn-butanolasafueland performed 也e micrograviザ e守 eriments with a forcedairflowof30cm/s.Thee守 erimentrevealed thatinspiteofthegascirculationcel，thecharacter ofthemicrogravityspreadwasalmostste吋 thatis， withoutpulsation，atavelocityofaboutl.5cm/s. Fmthermore，aliquid-phasevortextraveledsteadily withtheflame，extendedthepreheatrustancemuch farther ahead of the flame， car ried warm liquid deperintothepol，andhadhigherrotational羽 田 locitiesthaninnormalgravity.ThetwoIRimages， 也esurfacetemperatureofn-butanolaheadofthe flameinmicrogravityandsteadyspreadwithalarge regionofas戸nmetricalpreheatedfluid[7]andthe surfacetemperatureofn-propanolin也epseudo- uniformregimeat19，werefoundtobeverysimilar. Themicrogravi，勺 flamehadasimilarliquidsurface temperaturestructureasa19pseudo-uniformflame， composedoflargeasymetricalcurlsaheadofthe flame. Notethatthelow-temperatureregionjust aheadoftheflameedgecouldbeobservedinboth IRpictures.Wefoundinoursepar此eexperiment thatthiscoldtemperaturevaleyalwayspreceded 也eflameedgeinthepseudo-uniformregimeand causedslowsteadyflamespread.Sim辻紅t，句 ofthe surfacetemperature rustributionformicrogravity and 19 flame sug gests that the same mech紅 白 m causestlemicrogravi，句 spreadphenomenon.We sug gest th批， when the fuel temperature is low enoughasin出epseudo-uniformregion，theheat transferprocescanotincreasethefueltempera- turebeyondtheflashpoint.Ontheotherhand，the initialfuel匂 mperatureofthemicrogravityexperi- mentsisnominalyin也e19pulsatingregime.We postulatethatthecontinuousexistenceofthelow- temperatureregioniscausedbythelackofabuoy- ancyforceandastemperaturestrati宣cation泊 the liquidphasecanbereducedbybuoyancy，thesur- facetemperaturedistributionwilremaininhomo- geneous.
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l.Liquidconvectionisakeycontributortothe mechanismofpulsatingspread.Themeasurement ofthesurfacetemperaturedistributionprovedthe existenceofthecoldtemperaturevaley也批 always
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