科技论文写作作业xjtu.docx
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科技论文写作作业xjtu.docx
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科技论文写作作业xjtu
英语科技论文实用
写作方法作业
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AnExperimentalstudyonEvaporation/BoilingHeat
TransferinSinteredPorousWick
SUNZhenHONGFang-JunZHENGPing
(SchoolofMechanicalEngineering,ShanghaiJiaoTongUniversity,Shanghai200240,China)
AbstractWehasdesignedandestablishedanexperimentalapparatustostudytheevaporation/boilingheattransferinparticlesinteredporouswickundertheworkingconditionofheatpipe.Theeffectsofparticlesizeaswellasthethicknessandvolumetricporosityofporouswickonevaporation/boilingheattransferwereinvestigatedbyusingdistilledwaterasworkingfluid.Itwasfoundthatforthefixedparticlesizeandporosity,thereexistsanoptimalwickthicknesscorrespondingtothemaximumheattransfercoefficient.Forthefixedthicknessofporouswick,thereisanoptimalporositywhentheheatfluxisrelativelylow,however,thesmallertheporosityis,andthehighertheheattransfercoefficientiswhenheatfluxisrelativelyhigh.Anevaporation/boilingheattransfermodelwasalsosummarizedtogiveplausibleexplanationsofexperimentalresults.
Keywordsparticlesintered;porouswick;evaporation/boiling;heatpipe
0Introduction
Theflatheatpipeisahigh-performancethermaldiffusiontechnologywhichconnectstheelectronicchip(heatsauce)andtheheatsink.Itcanbeconsideredasatwo–dimensionheatpipefromthedirectionofsteamflow.Theparticlesinteredporouswickisanimportantinternalstructureoftheflatheatpipe.Whentheflatheatpipeisonthestablerunningperiodandtheliquidlevelislowerthantheuppersurfaceoftheporouswick,thephasetransitionoccurs.Todistinguishitfromthepoolboiling,thisphasetransitioniscalledevaporation/boilingheattransfer.Theevaporationandboilingheattransferofrefrigerantinandatthesurfaceofporouswickisoneoftheimportantfactorsinfluencingheatpipe’sproperty.Althoughmuchliteratureaboutporouswickboilingheattransferhasbeenpublished(seepaper[1],[2]andtheirreferences),muchoftheresearchwasaboutpoolboilingorone-dimensionheatpipe,andlittleresearchwasaimedatevaporation/boilingheattransferinsintered[3-5]porouswickinflatheatpipe.
HanlonandMal[3]examinedtheevaporation/boilingheattransferofthickcooper(>1.9mm)particlesinteredporouswick,andtheyfoundthatboilingwouldoccurandbubblesappearedinwickwhichledtoadeclineofheattransferwhentheheatfluxwasbetween5-20w/cm2.Theperformanceofheattransferwoulddeclinewiththeincreaseofwick’sthickness.Li,Petersonetal.[4,5]studiedthe0.37-1mm(bubble’sdeparturediameter)cooperwick’sevaporation/boilingheattransfer.Theyfoundthatthecriticalheatfluxincreasedwiththeincreaseofwick’sthickness,whiletheheattransfercoefficientchangedlittle.Theyconsideredthattheevaporationofliquidfilmonporouswick’ssurfacewasthemainwayofheattransfer,andalsothereexistedanoptimalporositycorrespondingtothemaximalcriticalheatflux.Theoptimalporositywasrelatedtothethicknessofporouswickandthesizeofthemesh:
whenthediameterofmesh’scooperwirewasfixed,theheattransfercoefficientdecreasedwiththeincreaseofporosity.
Fromtheresearch[3-5]above,theliquidlevelevenedthesurfaceofporouswickaccordingtotheoverflowprinciple;meanwhilethoseexperimentswereconductedattheatmospherepressurewhichdidn’tcorrespondtotheheatpipeworkingcondition.Inthisstudy,anexperimentinvestigationinheatpipe(vacuumdecompression)conditionwasdonetocharacterizetheeffectsfromwick’sdiameter,typeandporouswick’sthickness(<1mm)andalsotodiscoverthemechanismofevaporation/boilingheattransfer.
1Experimentalsystemanddataprocessing
Inthispaper,weestablishedasetofexperimentalplatformwithrefrigerantcycleabilitywhichcouldsimulatetheheatpipe’sworkingcondition(vacuumdecompression)andcouldstudytheevaporation/boilingheattransferofallstructuralparameters’particlesinteredporouswick.Fig.1showstheexperimentalapparatus,wecanfindthesystemworkslikealoopheatpipe,butinthissystemweaddedgravity-assisteddevice(usingthedifferentlevelofcondensatetrapandvaportrap)inordertocondensatefortheadditionallosscausedbyfluidflowingthroughthejunctionline.Thecycleofworkingfluid(water)wasasfollow:
theheat-leadblockimportedheat;workingfluidevaporated/boiledinparticlesinteredporouswicks;thevaporgeneratedbyevaporation/boilingflowedinthecondensatetrap(thedrivenforcecamefromthepressuredifferencebetweencondensatetrapandvaportrap);thenundertheinfluenceofwatercoolerinthecondensatetrap,thevaporcondensatedonthesurfaceoftheporouswick;thecondensateaccumulatedatthebottomofthecondensatetrapandwenttothevaportrapthroughthejunctionline(thedrivenforcecamefromthepressuredifferencecausedbygravity.Allabovedidacompletecycle.
1.Vaportrap2.Vacuummeter3.CCDcamera
4.Vaporline5.Valve6.Vacuuminterface7.Condensatetrap8.Levelmeter9.Returnline
10.Fluidlevel11.Testsample12.Heatingblock
13.Sleeve14.Drainvalve15.Inletvalve
Fig.1schematicofexperimentalapparatus
Theheatedcooperwasfixedontheinsulationsupportwhichwasatthebottomofvaportrap,thesampleassemblyandtemperaturetestpointswereshowninFig.2.Theporouswicksweresinteredonthesupportplat,andthereexistedineluctablethermalresistancebetweenheatingblockandsupportplat.
Inordertoaccuratelyevaluateheattransferperformanceoftheporouswick,wedefinitethecoefficientasbelow:
(1)
InEq.
(1),ΦistheheatwhichcomesdirectlyfromDCpowdersuppliedheater;Aisthesurfaceofcopperblock’scross-sectionalarea;Twisthetemperatureofporouswicklayer’ssubsurfacewhichcouldn’tbemeasureddirectly,andTeisthesaturatedtemperatureofthevaportrapwhichismeasuredbythearmoredthermocouple.
Inourstudy,thetemperaturesT1-T6weremeasuredbytheapparatusshowninFig.2.Wecoulddetermineheattransfercoefficient’sdistributionoftheporouswicklayerandthethermalcontactresistancebetweenheatingsurfaceandsupportplatthroughheattransferreverseproblemalgorithm,thuswecanobtainTw.Becauseofspacelimitationthedetailswillnotbedealtwithhere.Inourexperiment,thesizeofsampleandtheheatingareawere30mm*90mmand30mm*30mm,thethicknessofsupportplatwas3mm,thethermocoupleT3,T2andT1were16mm,26mmand36mm’sdistancefromheatingpoint.T4,T5,T6’sdistancetoheatingsurfaceranged12mm,7mmand2mm.
Fig.2Schematicofheatingapparatusandsampleinstallation
2ResultsandAnalysis
2.1ParametersoftheSamples
OurexperimentalsamplenumbersareshowninTable1.DeltaElectronicsLimitedCompany(Shanghai)cooperatedwithusandprovidedalltheexperimentalsamplesandtheirparameters.Inthetablebelowtheroughcooperpowderrepresents100-120’smesh
whichcorrespondstheparticlediameterof120-150μm,andthefinecooperpowderrepresents160-200’smeshwhichcorrespondstheparticlediameterof75-96μm.Theexperimentalsamplesaresinteredatthesamecondition,soweassumethatsamples’porosityonlyconcernstothetypeandsizeofthepower.
Table1Experimentalsamplenumbersandsize
SampleNumber
Porosity
ThicknessofPorousWick/mm
KindofCooperPowder
PM1
52.85%
0.35
Fineelectrolysispowder
PM2
45.38%
0.36
Finespraypowder
PM3
55.50%
0.37
Roughspraypowder
PM4
52.85%
0.42
Fineelectrolysispowder
PM5
45.38%
0.48
Finespraypowder
PM6
55.50%
0.67
Roughspraypowder
PM7
52.85%
0.45
Fineelectrolysispowder
PM8
45.38%
0.59
Finespraypowder
PM9
55.50%
0.81
Roughspraypowder
2.2Porouswickevaporation/boilingheattransfermodel
InFig.3,wecanseethatwhentheflatheatpipeisworkingatthestablestate,fluidcomesfrombothsidesofthepipeandflowstotheheatingareaincentre.Althoughthefluidgoesintotheheatingplatevenly,thepracticalheatfluxofporouswick’ssurfacepresentsa highmiddleandtwolowsidesduetothediffusion.Withtheexistingofevaporationalonetheflowdirection,thefluidlevelisalmostparalleltotheporouswick’slayeratthedistalendfromheatingarea,whileclosetothecentrethefluidleveldecreases.
Fig.3Distributionofheatfluxandliquidlevelinporouswick
Inordertomakeaclearexplanationoftheexperimentalresult,weconcludeaccordingtotheexperimentalphenomenon,resultandthefindingsin[3]-[5]thattheporouswickevaporation/boilingheattransfermodelworksasbelow:
1)whentheheatfluxisrelativelylow,theworkingfluidvaporslittle,andthedrivingforceneededisalsoverysmall.AsisshowninFig.4(a),fluidlevelisalmostparalleltotheporouswick’slayer,thusthemainwaysofheattransferaresingle-phasefluid’sconvectiveheattransfer,solidparticle’sheatconductionandfilmevaporationtransferonthesurfaceofporouswick.2)Whentheheattransferincreases,theevaporationofworkingfluidalsoincreases.AsshowninFig.4(b),toensurethesupplyoffluid,thefluidleveldescendsgreatlytoobtainthedrivenforce,andthesurfaceofporouswickinthecentreheatingareabecomesgasprovince.Meanwhile,theevaporationthinliquidfilmbeginstospreadinhorizontalandverticaldirectionthusincreasestheevaporationareaanddecreasesthethicknessofliquidregioninporouswick.Finallytheheattransferisenforced.3)Whentheheatfluxcontinuestoincrease,theevaporationofworkingfluidkeepsonincreasing.Thefluidle
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