Page 66 - konishipaper

JL.iJqlUlJi.dCOl1lvectlol1l Ul1ldeJrl1lemHntlhleFueliS1UlrfmcerluetoEvmpOJratloil TadashiKONISHI* andAkihikoITo**
Department 01 Mechanical Engine ering， Oita National Col lege 01 Technology， Oitα 料 Department01ProductionsystemsEnginering，Oita University，Oita
Liquidconvectionbeneaththefuelsurfacecausedbyevaporationwasexperimentalyexamined. Thesurfacetemperatureandthetemperβtureprofilesinthesub-surfacelayerofmethanol weremeasuredbyusinganinfraredcamera(IR)and holographicinterferometry(HI)， respectively.Therapidchangeofvaporconcenirationinthegasphasewasalsomeasuredby HI.Visualizationofthefluidmotionsinthesub-surfacelayerwasacomplishedbyparticle trackvelocimetry(PTV)withthesuspensionofaluminumpowder.Theresultsshowthat:1) liquidconvectionistrigeredbysurfacetensionforcesduetotemperaturechangeafterabout 0.2secondsfromtheonsetofevaporation，2)fuelbuoyancyafectsthesizeandpaternofthe Benardcel1sand3)thevaporconcentrationattheinterfacebetweengasandliquidrapidly decreasesduetosurfacetemperaturedrop，thengradual1yincreasesasthesurfacetemperature israisedbyliquidconvection.
1.INTRODUCTION
Fueltemperahlreisaveryimportantparameteririliquidfuelcombustion，especialy，inflamespreadover liquidsurface1).Thefuelevaporates'fromthe1iquidsurfaceanddifusestothesuroundingairtoformaflammable layer.Whentheinitialfueltemperatureexcedstheflashpoin七asubstantialflammablelayerisformedoverthe liquidsurfaceandflamepropagatesinthislayer2).Theflashpointofasubstanceisthetemperatureatwhichthe vaporpresureinair，atequilibriumconditions，equalsthelowerflammablelimitforthatsubstance3).Thethicknes oftheflammablelayerisdeterminedbyconcentrationprofiles，variesdrasticalybyfueltemperature，especialy neartheflashpoin.tThereforefueItemperatureafectsthepropagationfeaturessuchasflamespeedandspread mechanism.
Previously，vaporconcentrationprofilesformedovermethanoIaremeasuredusingholographic interferometry(HI).Itwasfoundthatthev'aporconcentrationprofi1eisdeterminedbysurfacetemperatureand theflowofsuroundingair2). Whenmethanolevaporatesatthetemperaturebeyondtheflashpointof1oC，the heatofevaporationmaychangethetemperatureoffuelintheliquidphase.Ifthetemperaturediferencebetween thefuelsurfaceandbulkbodyreachesthecrit，icalconditionofeithertheMarangoninumberortheRayleigh number，corespondingconvectivemotionocursandmodifiesthesurfacetemperature.
Theliquidconvectionbeneaththefuelsurfacecausedbyevaporationwasexperimentalyinvestigated. Thesurfacetemperatureandthetemperatureprofilesinthesub-surfac~ layerofmethanolweremeasured-qsing aninfraredcamera(IR)4)5)andHI，respectively.Therapidchangesofvaporconcentrationinthegasphasewere alsomeasuredbyHI.Visualizationofthefluidmotionsinthesub-surfacelayerwereacomplishedbyparticle trackvelocimetry(PTV)withthesuspensionofaluminumpowder6)7).
2. EXPERHv1ENTALAPPARATUSANDPROCEDURES
Fig.lshowsthehighspedshuUersystemusedformeasuringthetransientstateoftheevaporationproces. The~hutter wasmadeoftwoaluminumplateseach100mmwide，300mmIongand1mmthick.Theplateswere movedhorizontalybytheaircylindersandtheentireliquidsurfacewasexposedwithin30msec.Theclearance betweenthe1iquidsurfaceandplateswasmaintainedat1.5mm.Puremethanol(purity9.9%)wasusedasafuel
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