外文翻译冷凝器和蒸发器的热力设计制定和应用.docx

外文翻译冷凝器和蒸发器的热力设计制定和应用.docx

《外文翻译冷凝器和蒸发器的热力设计制定和应用.docx》由会员分享，可在线阅读，更多相关《外文翻译冷凝器和蒸发器的热力设计制定和应用.docx（21页珍藏版）》请在冰豆网上搜索。

外文翻译冷凝器和蒸发器的热力设计制定和应用

附录B：

参考英文文献与译文

Thermodynamicdesignofcondensersandevaporators:

Formulationandapplications

ChristianJ.L.Hermes

abstract

Thispaperassessesthetherm-hydraulicdesignapproachintroducedinapreviouspublication（Hermes,2012）forcondensersandevaporatorsaimedatminimumentropygeneration.Analgebraicmodelwhichexpressesthedimensionlessrateofentropygenerationasafunctionofthenumberoftransferunits,thefluidproperties,thethermal-hydraulic

characteristics,andtheoperatingconditionsisderived.Casestudiesarecarriedoutwithdifferentheatexchangerconfigurgitationsforsmall-capacityrefrigerationapplications.Thetheoreticalanalysisledtotheconclusionthatahigheffectivenessheatexchangerdoesnotnecessarilyprovidethebestthermal-hydraulicdesignforcondenserandevaporatorcoils,whentheratesofentropygenerationduetoheattransferandfluidfrictionareofthesame

orderofmagnitude.Theanalysisalsoindicatedthatahighaspectratioheatexchangerproducesaloweramountofentropythanalowaspectratioone.Conceptionthermodynamiccondenseretdesse´evaporate:

formulationetapplications.

Keywords:

floatinghead;heatexchanger;design;industry

1.Introduction

CondensersandevaporatorsareheatexchangerswithfairlyuniformwalltemperatureemployedinawiderangeofHVACRproducts,spanningfromhouseholdtoindustrialapplications.Ingeneral,theyaredesignedaimingatacplishingacertainheattransferdutyatthepenaltyofpumpingpower.Therearetwowell-establishedmethodsavailableforthethermalheatexchangerdesign,thelog-meantemperaturedifference（LMTD）andtheeffectiveness/numberoftransferunits（ε-NTU）approach（Kakac¸andLiu,2002;Shahand

Siliculose,2003）.Thesecondhasbeenpreferredtotheformerforthesakeofpactheatexchangerdesignastheeffectiveness（ε）,definedastheratiobetweentheactualheat

transferrateandthemaximumamountthatcanbetransferred,providesa1st-lawcriteriontoranktheheatexchanger.performance,whereasthenumberoftransferunits（NTU）paresthethermalsizeoftheheatexchangerwithitscapacityofheatingorcoolingfluid.Furthermore,theε-NTUapproachavoidsthecumbersomeiterativesolutionrequiredbytheLMTDforoutlettemperaturecalculations.Nonetheless,neitherε-NTUorLMTDapproachesaresuitabletoaddresstheheattransfer/pumpingpowertrade-off,whichisthecruxforabalancedheatexchangerdesign.Forthispurpose,Bajan（1987）establishedtheso-calledthermodynamicdesignmethod,laterrenamedasentropygenerationminimizationmethod（Bajan,1996）,whichbalancesthethermodynamicirreversibilitiesduetotheheattransferwithafinitetemperaturedifferencetothoseassociatedwiththeviscousfluidflow,thusprovidinga2nd-lawcriterionthathasbeenwidelyusedforthesakeofheatexchangerdesignandoptimization（SanandJan,2000;Leprousetal.,2005;AchaeanandWongwises,2008;Mishapetal.,2009;Kotciogluetal.,2010;Pussolietal.,2012;Hermesetal.,2012）.However,themodelsadoptedinthosestudiesdonotprovideastraightforwardindicationofhowthedesignparameters（geometry,fluidproperties,workingconditions）affecttherateofentropy

generation.Theyalsorequireplexnumericalsolutions,beingthereforenotsuitableforback-of-the-envelopecalculationsintheindustrialenvironment.Inarecentpublication,Hermes（2012）advancedanexplicit,algebraicformulationwhichexpressesthedimensionlessrateofentropygenerationasafunctionofthenumberoftransferunits,thefluidproperties,thethermalhydrauliccharacteristics（jandfcurves）,andtheoperating.

conditions（heattransferduty,corevelocity,andcoilsurfacetemperature）forheatexchangerswithuniformwalltemperature.Anexpressionfortheoptimumheatexchangereffectiveness,basedontheworkingconditions,heatexchangergeometryandfluidproperties,wasalsopresented.ThepresentpaperisthereforeaimedatassessingtheformulationintroducedbyHermes（2012）fordesigningcondensersandevaporatorsforrefrigerationsystemsspanningfromhouse-holdapplication,whichamountsw10%oftheelectricalenergyconsumedworldwide（MaloandSilva,2010）.

2.Mathematicalformulation

Ingeneral,condensersandevaporatorsforrefrigerationapplicationsaredesignedconsideringthecoilfloodedwithtwo-phaserefrigerant,andalsoawalltemperatureequaltotherefrigeranttemperature（BarbarossaandHermes,2008）,insuchawayasthetemperatureprofilesalongthestreamsarethoserepresentedinFig.1.Inaddition,theouter（e.g.,air,water,brine）sideheattransfercoefficientandthephysicalpropertiesareassumedtobeconstant.Therefore,theheattransferrateifcalculatedfrom:

〔1〕

whereisthemassflowrate,Ti,ToandTsaretheinlet,outletandsurfacetemperatures,respectively,Q¼hAs（TseTm）istheheattransferrate,Tmisthemeanflowtemperatureovertheheattransferarea,As,andεistheheatexchangereffectiveness,calculatedfrom（KaysandLondon,1984）:

〔2〕

whereNTU¼hAs/mcpisthenumberoftransferunits.Thepressuredrop,ontheotherhand,canbecalculatedfrom（KaysandLondon,1984）:

〔3〕

wherefisthefrictionfactor,ucisthevelocityintheminimumflowpassage,Ac,andthesubscripts“i〞and“o〞refertotheheatexchangerinletandoutletports,respectively.One

shouldnotethatEqs.

（1）and（3）canbelinkedtoeachotherthroughthefollowingapproximationfortheGibbsrelation,

〔4〕

whereTmz（TiþTo）/2,andtheentropyvariation,soesi,iscalculatedfromthe2nd-lawofThermodynamics,

〔5〕

wherethefirsttermintheright-handsideaccountsforthereversibleentropytransportwithheat（_Q=Ts）,whereas_Sgistheirreversibleentropygenerationduetoboththeheat

transferwithfinitetemperaturedifferenceandtheviscousow.SubstitutingEqs.

（1）,（3）and（5）intoEq.（4）,itfollowsthat:

NS¼

〔6〕

whereNSisthedimensionlessrateofentropygeneration.TheerrorsassociatedtotheapproximationusedinEq.（4）aremarginal:

notingthatDTm<20Kinmostsmall-capacity

refrigerationapplications,itfollowsthatthedifferencebetweentheexactandapproximatedmeantemperatureneverexceeds1K,whichinturnaffectsthedimensionlessentropygenerationbylessthan1%.NownotingthatbothcondensersandevaporatorsaredesignedtoprovideaheattransferdutysubjectedtoflowrateandfaceareaconstraintsEq.（6）canbere-writtenasfollows（Hermes,2012）:

〔7〕

AndQ¼（ToeTi）/TsadimensionlesstemperaturedifferencewithbothToandTiknownfromtheapplication.Oneshouldnotethatthefirstandsecondtermsoftheright-handsideofEq.（7）standforthedimensionlessentropygenerationratesassociatedwiththeheattransferwithfinitetemperaturedifferenceandtheviscousflow,respectively.TheoptimumheatexchangerdesignNTUoptthatminimizestherateofentropygenerationisobtainedfrom（Hermes,2012）:

〔8〕

dropeffects,whichruletheentropygenerationforthelowaspectratiodesigns,areattenuatedforlowNTUvalueswheretheentropygenerationduetofinitetemperaturedifferenceis

Dominant.

4.Casestudies

Forthesakeofheatexchangerdesign,Eq.（8）hastobesolvedconcurrentlywithε¼（ToeTi）/（TseTi）asthecoilsurfacetemperature,Ts,mustbefreetovarythusensuringthatQ（andso_Qand_m）isconstrained.However,thesolutionisimplicit.forTs,thusrequiringaniterativecalculationprocedure:

aguessedTsvalueisneededtocalculatetheeffectivenessandNTU¼eln（1eε）,whichisusedinEq.（8）withj¼j（Re）andf¼f（Re）curves,andalsowiththedimensionlesscorevelocitytoeoutwithQ,whichinturnisusedtorecalculateTsuntilconvergenceisachieved.Firstlyconsideranair-suppliedtube-fincondenserforsmall-capacityrefrigerationappliancesrunningunderthefollowingworkingconditions:

_Q¼1kW,_V¼1000m3h1Ti¼300K（Waltrichetal.,2011;Hermesetal.,2012）.Letsassumetwoheatexchangerconfigurations:

（i）circulartubeswithflatfins（i.e.,KaysandLondon’ssurface8.0-3/8T）,whosethermal-hydrauliccharacteristicsarej¼0.16$Re0.4

tubesandfins（KaysandLondon’ssurfaceCF-8.72）,whosethermal-hydrauliccharacteristicsarej¼0.22$Re0.4f¼0.20$Re0.2,s¼0.524andDh¼3.93mm.AlsonotethatPrz0.7forair.Fig.4parestheperformancecharacteristics（jandfcurves）ofsurfaces8.0-3/8TandCF-8.72asfunctionsofRe¼rucDh/m.Fig.5paresthedimensionlessentropygeneration

ObservedforbothsurfacesasafunctionofNTU.Acurveofε¼ε（NTU）,whichthesameforbothsurfaces,isalsoplottedtobeusedasareference.Itcanbeclearlyseenthatthe（ε,NTU）designwhichminimizestherateofentropygenerationis（0.61,0.95）forsurface8.0-3/8Tand（0.57,0.81）forsurfaceCF-8.72.Itcanalsobenotedthatthecircular-finsurface

showedahigherrateofentropygenerationforallNTUspan,whichismostlyduetotheviscousfluidfloweffectassurfaceCF-8.72hasahigherfrictionfactorthansurface8.0-

3/8TforthesameReynoldsnumber（seeFig.4）.ForlowNTUvalues,wheretheentropygenerationisruledbyNS,DT,bothsurfacesshowedsimilarNSvaluesastheirj-curvesareclose（seeFig.4）.Fig.6paresthreedifferentcondenserdesignsconsid-eringsurface8.0-3/8Tandfaceareasvaryingfrom0.025to0.1m2runningunderthesameworkingconditions.Theheatexchangerlengthwasalsovariedinordertoacmodatetheheattransfersurfaceareafordifferentfaceareas.Foravertical,constantNTUline（i.e.sameheattransferarea）,itcanbeclearlyobservedthataheatexchangerdesignwithhighaspectratio（higherfacearea,smallerlengthintheflowdirection）producesasignificantlyloweramountofentropyinparisont