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LiquidConvectIonUnderneaththeFuelSurfaceduetoEvaporation calculatedbythefolowequation:
235
qz=o=Jz=odHvap=ðHvap
盟
CA()~I事 (3) R T . ." V π t
Thetheoreticalresultsoftheheatofevaporation calculated by Eq.(3) are compared with the experimentaldataobtainedbyHIinFig.6(a).Atthe beginingofevaporation，thecalculatedvaluesarein goodagreementwiththeexperimentalresults.
Thetempera旬 rechangein且quidphasedueto evaporationissolvedbythefollowingl-Dheat conductionequationwiththeinitialandboundary conditions:
(4)
oT e)2T 一 一 =α 一一-
e)t e)z2 at t=O， B.C.l: at z=O，
.rc・-
B.C.2: at z=αコf
22 T=30.C
whereαisthethermaldifusivityofmethanotTois
the initial fuel temperature and λis thermal conductivityo_fmethanol.Equation(4)issolvedby usingMAR00，ageneralfiniteelementmethod program.Thecalculatedresultsoftemperaturedrop andtheresultingCIGLareshowninFig.6(b)，which areconsistentwiththemeasuredvaluesbeforethe
ocurenceoftheconvectivemotion. O123
Fromtheabovediscusion，itwasconcluded血at
themechanismofconvectionanditsefectonthe
concentrationprofilesoverthesurfaceisasfolows.
Whentheshuterplatesareopened，avertical
temperaturedlstributionisformedintheliquidphase，
becausetheliquidsurfaceiscoledbyevaporation.If theverticaltemperaturediferencebetweenthesurfaceandbulkbodyreachesthecorespondingcriticalMarangoni number，surface-tension-driven-Benardconvectionocurs.Theconcentrationattheinterfacebetween'gasand liquid(CIGL)decreaseswithdecreasingsurfacetemperatureasaresultofevaporation.AsBenardcelsdevelop andchangetheirp"aternandsize，theaveragesurfacetemperature andcorespondingC1GLincreases.Ifthe verticaltemperaturediferencereachesthecorespondingcriticalRayleighnumber，buoyancy-driven-Rayleigh- BenardconvectioncommenceswhichmodifiesthesizeandpaternofBenardcels，andconsequent1ychanges surfacetemperatureandC1GL.
5.SUMMARY
Liquidconvectivemotionunderneaththefuelsurfacecausedbyevaporationwasexperimentalyinvestigated bymeasuringthesurfacetemperatureandthetemperatureprofileintheliquidphaseandtheconcentration profileinthegasphase.1Rcameracanmeasurethetemperatureprofilesoftheliquidsurfaceandthesizeand pat ternofthecel ls. TheHIwasaveryef fectivemethodtoacquirethe ac curateconcentrationprofilesingas phase and consequent1y the concentration at the interface betwe er r gas and liquid (C1GL). However it is not apropriatetodeterminethelocaltemperaturesfromtheinterferogramsbecauseofrefractionoflightduetothe meniscusnearthePyrexwal.l
Atfirstil-definedsmalscalecelsoflmmwavelengthemergedafter0.2secondfromtheonsetof evaporation.Secondthesecel1sdeveloptoformpolygonalcel1sandthenincreaseinthesize.Finalyafter2 seconds，theyreachedaquasi-steadystatekeepingtheirsizeof4mmwavelength.CIGLquicklydecreasesasa resultofsurfacetemperaturedescentuntil0.2secondsandthengradualyincreasesasthesurfacetemperature increasesduetoconvectivemotionintheliquidphase.1ntheviewofliquidtemperatureprofile，temperature profilesarebui1tbyconformingwithl-Dheatconductionequationuntiltheliquidconvectiontakesplace.After
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27
21
18
(a)
Time[s] Fig.6.ExperimentalandtheoreticalresuItsof
theheatofevaporation(a)andsurface temperaturechangecalculatedbyFEM(b).
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