The effect of a boiling additive on R123 condensation on a vertical integral finsurface.docx
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The effect of a boiling additive on R123 condensation on a vertical integral finsurface.docx
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TheeffectofaboilingadditiveonR123condensationonaverticalintegralfinsurface
TheeffectofaboilingadditiveonR123condensationonaverticalintegralfinsurface
Abstract
Thispaperexaminestheeffectoftheadditionof0.5%massisopentanetoR123onthevapor-spacecondensationheattransferofR123.Inapreviousstudy,thepoolboilingperformanceofR123wasimprovedbyadding0.5%massisopentane.Consequently,theimpetusofthepresentstudywasadesiretoquantifytheconsequenceoftheboilingadditiveonthecondensationheattransferperformanceofpureR123.Inthisway,theneteffectoftheadditiveonthecycleperformanceofpureR123canbeestimated.
ThedataconsistedoftheheatfluxandthewalltemperaturediferencemeasurementforpureR123andR123/isopentane(99.5/0.5)onanintegral-trapezoidal-finsurface.Thetem-peratureofthesaturatedvaporwasheldconstantat313.15Kforallofthetests.Onaverage,theR123/isopentanemixtureexhibiteda4%smallerheatfluxthanthatofpureR123.
Presumably,thedegradationwascausedbythezeotropicbehaviorofthemixture,whichledtoalossofavailabledrivingtemperaturediferenceforheattransferacrosstheliquidfilm.Consideringthattheboilingperformancewasenhancedonaverageby10%withtheadditionof0.5%massisopentane,isopentanemaystillbeaviablemeansofimprovingthecycleperformanceofR123despitethe4%condensationheattransferdegradation.#2000ElsevierScienceLtdandIIR.Allrightsreserved.
kyewords:
Heattransfer;Masstransfer;Condensation;Refrigerant;R123;Additive;Heattransfercoefficient;Surface;Finnedtube.
Introduction
Fortherefrigerationandair-conditioningindustry,aliquidadditivewouldbeaneconomicalmeanstoreducemanufacturingand/oroperatingcosts.Forexample,aliquidadditivefor1,1-dichloro-2,2,2-tri-uoroethane(R123)wouldenableexistingwaterchillerstooperatemoreeffcientlyorenablenewwaterchillerstomeetthesamedutywithfewertubes.
However,theeconomicbenefitofadditivesthatenhanceboilingheattransfercanberealizedonlywhentheadditivedoesnotsignificantlydegradethecondensationheattransfer.Kedzierski[1]measuredasignificantenhancementofR123poolboilingwiththeadditionof1and2%hexanebymasstoR123.HeusedtheGibbsadsorptionequa-tionandtheYoungandDupreequationtospeculatethattheboilingheattransferenhancementofR123bytheadditionofhexanewascausedbyanaccumulationofhydrocarbonattheboilingsurface.Inessence,thegreaterconcentrationofhydrocarbonor``excesslayer''attheheattransfersurfacecausedareductionofthesurfaceenergybetweenthesolidsurfaceandtheliquid.
Theexistenceofanexcesslayerattheliquidsolidinterfaceisanalogoustotheexistenceofasurfactantinducedexcesslayerataliquidvaporinterface.Conse-quently,thehydrocarbonisnotatypicalsurfactantbecauseitaccumulatesatthesolidliquidinterfaceratherthantheliquidvaporinterface.
However,thereductionintheliquidsolidsurfaceenergyresultsinasimilarreductioninbubbledeparturediameterthatoccurswithaconventionalsurfactant.Asaconsequenceofthebubblesizereduction,theactivesitedensityincreases.Aboilingheattransferenhancementexistedwhenafavorablebalancebetweenanincreaseinsitedensityandareductioninbubblesizeoccurred.
Inanotherboilingadditivestudy,Kedzierski[2]speculatedthatfoulingcausedamoremodestimprove-mentintheheatfuxofR123withtheadditionofiso-pentaneandhexane.Overall,theR123/isopentane(99.5/0.5)bymassmixtureexhibiteda10%heatenhancementforheatwithintherangeof10to90kW/m2.Similarly,theR123/hexane(99.5/0.5)mixtureshowedanoverall4%andamaximumof13%heatenhancementoverthatofpureR123.
ThepurposeofthepresentstudyistodeterminetheeffectofaboilingadditiveonthecondensationheattransferperformanceofR123.Aboilingadditiveisunlikelytobecommerciallyviableifitcausesaheattransferdegradationinthecondenserthatmorethanofsetstheheattransferenhancementintheevaporator.Assumingthatisopentaneisabetteradditivethanhex-anefortheenhancementofR123boilingonallsurfaces,isopentanemaypotentiallyproducethegreatestnetheattransferimprovementbetweenthecondenserandtheevaporator.Basedonthatpremise,thevapor-spacecondensationheattransferperformanceofpureR123andanR123/isopentane(99.5/0.5)bymassmixtureweremeasuredonavertical,trapezoidalansurface.
Apparatus
Fig.1showsaschematicoftheapparatusthatwasusedtomeasurethevapor-spacecondensationheattransferdataofthisstudy.Specifically,theapparatuswasusedtomeasurethevaporsaturationtemperature(Tv),theaveragecondensationheat(q00),andthewalltemperature(Tw)ofthetestsurfaceattherootofthefin.Thethreeprincipalcomponentsoftheapparatusweretestchamber,postcondenser,andboiler.Theinternaldimensionsofthetestchamberwereapproximately254200130mm.
Theboilerwaschargedwithapproximately10kgofR123.Hotcitywaterflowedinsidethetubesoftheboilertoheatthetestrefrigerantontheshell-sideoftheboiler.Thetestsectionwasvisi-blethroughthree,flatquartzwindows.Theopposingsideofthefinnedcondensingtestsurfacewascooledwithhighvelocity(2.5m/s)waterflow.Varyingthetemperatureofthecoolingwatervariedtheheatfluxofthetestsection.
Thevaporproducedbytheboilerwascondensedbythepostcondenserandthetestsectionandreturnedbygravitytotheliquidpool.Thepostcondenserwasidenticaltotheshell-and-tubeboiler;however,chilledwaterflowedinsidethetubeswhilethevaporcondensedontheoutsideofthetubes.Thedutyoftheboilerandthepostcondenserweresignificantlylargesothatawidevariationinthedutyofthetestsurfacewouldnotafectthesaturationpressureofthetestapparatus.Thepurgerandthedesiccantfilterremovednon-condensiblegasesandwater,respectively,fromthetestrefrigerantafterchargingandbeforetesting.
Toreducetheerrorsassociatedwiththesaturationtemperaturemeasurement,thesaturationtemperatureofthevaporwasmeasuredwithtwo450mmlong1.6mmdiameterstainlesssteelsheathedthermocouples.Thesmalldiameterprovidedforarelativelyrapidresponsetime.Approximately180mmofeachthermo-couplelengthwasexposedtothevaporofthetestchamber.Theportionofeachthermocouplethatwasinthetestchamberwasshieldedwitha6mmdiameterstainlesssteeltubeandwasincontactwiththesaturatedrefrigerantvapor.Thetipsofthetwothermocoupleswereplacedneartheloweredgeofthetestplateandapproximately60and95mm,respectively,fromthefrontofit.
Testsurface
Fig.2showstheoxygen-freehigh-conductivity(OFHC)copperintegral-trapezoidal-fintestplateusedinthisstudy.Theintegral-trapezoidal-finsurfaceinthisstudywasmachineddirectlyontothetopofthetestplatebyelectricdischargemachining(EDM).
Fig.3showsadrawingofthefincrosssection.Thefinpitchwas1.36mm.Thesurfacehadnominally746finspermeterorientedalongthelongaxisoftheplate.Theratioofthesurfaceareatotheprojectedareaofthesurfacewas2.87.Theratioofthefinarea(Af)tothetotalarea(Ao)was0.74.Thefin-tipwidthandthefin-heightwere0.24and1.53mm,respectively.
Measurementsanduncertainties
Thestandarduncertainty(ui)isthepositivesquarerootoftheestimatedvarianceu2.Theindividualstandarduncertaintiesarecombinedtoobtaintheexpandeduncertainty(U).Theexpandeduncertaintyiscommonlyreferredtoasthelawofpropagationofuncertaintywithacoveragefactor.Allmeasurementuncertaintiesarereportedfora95%confidenceinterval.
Thecopper-constantanthermocouplesandthedataacquisitionsystemwerecalibratedagainstaglass-rodstandardplatinumresistancethermometer(SPRT)andareferencevoltagetoaresidualstandarddeviationof0.013K.TheNISTthermometrygroupcalibratedthefixedSPRTtotwofixedpointshavingexpandeduncer-taintiesof0.06mKand0.38mK.
Aquartzthermo-meter,whichwascalibratedwithadistilledicebath,agreedwiththeSPRTtemperaturetowithinapproxi-mately0.003K.Nocorrelationwasfoundtoexistbetweenthemeasuredthermocoupleelectromotiveforce(EMF)andameasured1mVreference.Consequently,therewasnomeasurabledriftintheacquisitionvoltagemeasurementoveramonthperiod.Beforeeachtestrun,
themeasurementsofathermocoupleinthebathwerecomparedwiththeSPRT.ThemedianabsolutediferencebetweenthethermocoupleandtheSPRTwas0.02Koverthedurationoftheentirestudy.Consideringtheᆵuctuationsinthesaturationtemperatureduringthetestandthestandarduncertaintiesinthecalibration,theexpandeduncertaintyoftheaveragesaturationtem-peraturewasnogreaterthan0.04K.
Consequently,itisbelievedthattheexpandeduncertaintyofthetempera-turemeasurementswaslessthan0.1K.Thesaturationtemperaturewasalsoobtainedfromapressuretrans-ducermeasurementwithanexpandeduncertaintyoflessthan0.03kPa.Theexpandeduncertaintyofthesaturationtemperaturefromaregression(witharesi-dualstandarddeviationof0.6mK)ofequilibriumdata[3]forR123was0.17K.Thesaturationtemperatureobtainedfromthethermocoupleandthepressuremea-surementnearlyalwaysagreedwithinヒ0.17KforthepureR123data.
Fig.2showsthecoordinatesystemforthe20wellswhereindividualthermocoupleswereforce-fittedintothesideofthetestplate.Thewellswere16mmdeeptoreduceconductionerrors.UsingamethodgivenbyEckerta
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