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268 FIRE-IgnitionandFlameSpread
tostep1.Oneofthemajorconclusionsobtained fromourpreviousstudies[8，9]wasthattherewasa cychc ap pearance and disap pearance of a cold (liq- uid)temperaturevaleywhosecyclefrequencywas welcorelatedwiththefrequencyofthemainflame pulsation.Wealsodiscusedaposiblereasonwhy therewasnopulsatingflamespreadundermicro- gravity.Ourthoughtwasthatconvectiveheattrans- feristhemajormodeofheattransferfromtheflame totheliquid，andundermicrogravitythereismuch weakerconvectionthaninnormalgravity，sothe convec世onefectisnotstrongenoughtocancelout 出e cold-temperature valey profiles once 出ey formedinthehquid，a1lowingthecoldvaleytocon- tinuouslyexist(therefore，nopulsatingspread).
Itmaybeinterestingtoreviewhownumerical modelscandealwithliquidtemperatureprofiles. Two-dimensionalnumericalmodehng[1]showeda lineardecayinthetemperatureoftheliquidahead oftheflameleadingedgeanddidnotpredictthe existenceofthecold-temperaturevaley.Atthebe- gining，wethoughtthiswasduetotwo-dimensional modelasumptionsbeingapphedtoactual也re-品目 mensionalphenomena.Later，出islineofthought wasprovedtobewrong，whentheNASAgroup[6] showed也attherewasa'pulsatingflamespreadover a出inliquidlayerpoledinasha1lowcircularpan wherenocold-temperaturevaleywasthoughttoex-
Toexplain出isdisagrement，wesearchedfora clueinthetemperaturestructurecreatedina_shal- lowliquidpolduringpulsa世ngflamespread.We conducted a flame-spread e守 eriment using the samerectangulartrayandanenclosurebox(50m m l o n g x 3 0 0 m m w i d e .x 4 0 0 m m h i g h ) ， d e s c r i b e d inourpreviouse守 eriments[8-10]，andmeasured thedetailedtemperaturestructureintheliquidus- ingHI，shadowgraph(SG)，andIRtechniques.In particular，HIisveryefectiveinmeasuringtransient tempera旬 reprofileswi出 highspecialresolution (ourHIsystemcanoferamicrosecondtimere- sponseandabe仕er出an0.1m mspecialresolution). Interestingly，wefoundtherewasasmal-scaleliquid surfacewave(theheightofthecrestwasmeasured tobe0.9m mbyHI[1]forthe20m mdeeppol) justaheadofthespreadingflameedge.Inthispaper， wewilexplainthenatureof出isnewlyobserved sma1l-scalesurfacewaveanditsroleonpulsa也 19 flamespread.
ExperimentalMethods
Thefueltray(madefrompyrex)usedfor白isex- perimentissimilartothatusedpreviously[8，9]，hav- ing dimensions of 30 0 m m long x 20 m m wide X sevendiferentheights(0.2，0.5，1.0，2.0，5.0，10，and 20mm).Thesevendiferenttrayheights.wereused toprovidesevendiferentliquiddepthsbykeping
altheothertraydimensionsandtheinitialiquid surfaceleveltothetraywalrimheightthesame. The bottom of the 廿 ay was made of Pyrex for SG measurement.A.smalpuotflamethatwaslocated atoneendofthetrayinitiatedtheflamespread出at propagatedtotheotherend.Thebehaviorofflame spreadwasrecordedbyahigh-sped_digitalvideo camera(120frames/sand41 X 10ーる m mspatial resolu世on).Theflame-spreadrateandpulsationfre- quencyweremeasuredfromtherecordeddigitalim- age.'Yith!hehighestsutpulsationfrequency_ob- served to be 12 Hz and its minimum traveling distancetobe0.5m m，theresultingmaximumeror isestimatedtobelesthanafewpercent.Tempera目 turestructurescreatedin也eliquidandliquidsur- faceweremeasuredbyHIandSGtechniques.The HItechniquewaspreviouslyapliedtoflame-spread e守 eriments [10]也ataresimilartothecurent stud)ん anditsproceduresarewelexpl瓜nedelse- where[10，12，13].TheSGtechniquewasemployed tomeasurere宜ectionofthebeamwhenitpases throughtheliquidverticaly.Apieceofwhitepaper onwhich450 linesweredrawnevery1m mwas placedontheoutersurfaceofthePyrex廿aybotom. AHe-Nelaser，asthelightsource，wasplacedabove 吐letraytoprovideaverticalbeamthroughtheliq- uid.Amiror(surfacearea:5cmwideX 5cmlong) wasplacedabovetheliquidsurfaceata450 angleto recordthebeamreflec世ononthepape工 OurSG technique[14]hashigh~r sensi世.vi守than也eordinal onethatwasapliedbyAkita[15].AnIRcamera witha8-13μmwavelengthdetectorwasalsoplaced abovetheliquidtraytomeasuretheliquidsurface temperatureduringtheflamespread.Ourprevious tests[16，17]connrmedth武也eemisivityofbutanol was0.95.andtheefectofthebluebutanolflame hadnegligibleefectsontheIRliquidsurfacetem- peraturemeasurement.A75μmdiameterChromel- Alumelthermocouplewasplacedat吐lecenterand justbelowthesurfaceofn-butanoltomeasurethe initialiquidtemperature.
ResultsandDiscusion Fiοe-StepProcesesηiPulsatingFlameSpread
Figure1showsthedetailedschema世csofstep[b] (sebelow)inthenve.，stepprocesofpulsa世ng flamespread[9].Theuppercolumnshowsthetop view，andthebottomcolumnshowsthesideview. Wemeasuredthefolowingparametersduringboth 也ecurentandpreviousstudies[8-10，12J.:theflame frontspreadrate，theexistenceandthespreadrate ofasmal-scalesurfacewave，thethermalstructure ofacold-temperaturevaley，transientveloci句Tpro- filesintheliquidandgasphases，andthefuelvapor concentrationabovethefuelsurface.Tohelpread- erstobeterunderstandwhatwehavedoneinthis