The effect of a boiling additive on R123 condensation on a vertical integral finsurface.docx

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