翻译车辆空调系统的设计和暂态仿真aWord文档格式.docx

翻译车辆空调系统的设计和暂态仿真aWord文档格式.docx

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RTechnologies,Inc.

TerryJ.Hendricks

NationalRenewableEnergyLaboratory

ABSTRACT

Thispaperdescribestheneedfordynamic（transient）simulationofautomotiveairconditioningsystems,thereasonswhysuchsimulationsarechallenging,andtheapplicabilityofageneralpurposeoff-the-shelfthermohydraulicanalyzertoanswersuchchallengesAnoverviewofmodelingmethodsforthebasiccomponentsarepresented,alongwithrelevantapproximationsandtheireffectonspeedandaccuracyoftheresults.

THEMOTIVATION:

THENEEDFORDYNAMICMODELING

MajorDepartmentofEnergy（DoE）objectivesincludedevelopinginnovativetransportationtechnologiesandsystemsthatdecreasevehiclefuelconsumptionandemissionsacrossthenation,therebyreducingthenation'

srelianceonforeignoilconsumption.RecentchangestotheFederalTestProcedurehaveaddedSC03andUS06drivecyclestoformtheSupplementalFederalTestProcedure（STFP）,withcorrespondingrequirementsforevaluating

vehicleemissionsduringadditionaldrivingconditions.Inparticular,theSC03drivecycleisspecificallyintendedtoevaluatevehicleemissionswhiletheairconditioning（A/C）

systemisoperatingintypicalhigh-temperature,highsolarloadconditions.TheUS06drivecycleisintendedtoevaluatevehicleemissionsduringmorehighspeed,highacceleration

conditions.

TheadditionoftheSC03drivecyclecreatesasignificantneedforbetterunderstandingtheimpactofdynamicconditions（i.e.,vehicleexternalenvironments,passengercompartment

environments,etc.）onthevehicleA/Csystemsandtheirdynamicresponsetotheseconditions.SincevehicleA/Csystemsrepresentthemajorauxiliaryloadontheengineoflight-dutypassengervehicles,sport-utilityvehicles（SUV）,andheavy-dutyvehicles,theA/Csystemperformancehasadramaticeffectonfuelconsumptionandexhaustemissions.Recentstudies（Ref1）haveshownthat,duringtheSC03drivecycle,theaverageimpactoftheA/Csystemoverarangeoflight-dutyvehicleswastoincrease1）fuelconsumptionby28%,2）carbonmonoxideemissionsby71%,3）nitrogenoxideemissionsby81%,and4）non-methanehydrocarbonsby30%.

TheA/CsystemexperiencestransientconditionsthroughouttheSFTPdrivecyclesandduringtypicalcity/highwaydrivingpatternsaroundthecountry.Inparticular,theevaporator

load,compressorspeed,refrigerantflowrate,andheatexchangerairflowratescanbequitevariable.KnowledgeandbetterunderstandingofthetransientA/Csystembehavior,especiallytheintegratedinterdependenciesandstrongcouplingbetweensystemcomponents,iscriticaltounderstandingA/Csystemperformancerequirementsduringthesedrivecycles.TheremustbeincreasedemphasisonoptimizingtheintegratedA/Csystemdesignandperformance

underthesetransientconditions,ratherthansimplyfocusingonpeaksteady-stateconditions,tominimizeitsimpactonvehiclefueleconomyandemissionsacrossthespectrumofthenation'

svehiclefleet.

THEPROBLEM:

TRANSIENTSELF-DETERMINATIONOFPRESSURE

Rankinecyclesaretaughtineveryintroductoryundergraduatethermodynamiccourse,andthebasicvaporcompressioncycleusedinmostA/CsystemsisessentiallyareverseRankinecycle.Insuchsimpletreatises,pressuresarespecifiedandnoconsiderationisgiventoconservingworkingfluidmass.Inarealapplication,ofcourse,theA/Cunitischargedwithafixedmassofrefrigerant,andthehighandlowpressureswillvaryaswillthecoefficientofperformance（COP）oftheunit.Theaccuratepredictionofthesepressuresturnsouttoberathercomplicated.

Obviously,analyticmodelsofcompressorsandthrottlingdevicesmustpredictpressurerisesanddropsaccurately.Butitmaynotbeasobviousthatcomparativelyisobaricdevicessuchascondensers,evaporators,andtransportlineshaveaninfluenceontheresultingpressurelevels,because,withtheexceptionofthereceiver/drier,itisinthosecomponentsthattheamountofworkingfluidchargevariesthemost.

Atanyinstantaneousoperatingpoint,theenergyflowsthroughtheloopmustbalance（neglectingtransientthermalandthermodynamicstorageterms）.Thismeansthattheheattransfercoefficients（anddegreeofsingle-phase“blockage”）inthecondensersandevaporatorsmustbecalculatedaccurately.Thisinturnmeansthattheregimesandthermodynamicqualitieswithinthecondensersandevaporatorsmustbecalculatedaccurately,conservingtotalchargemassinthesystem.

Topredicttheupperandloweroperatingpressuresatanysteadyoperatingpoint,ortotrackchangesinthosepressuresduringdynamiccycleoperation,requiresthattheanalyticmodelbeabletotrackandconservechargemass,andtodetermineitsdistribution.Becausetheresultingpressuresinturninfluencetheoperatingconditionswiththeevaporatorandcondenser,asurprisinglytightlycoupledanddetailedsolutionisrequiredtocorrectlypredict

theperformance。

SINDA/FLUINTOVERVIEW

UnderstandingsomeofthemodelingchoicespresentedinthispaperrequiresabriefoverviewofthenomenclatureandconceptsintheSINDA/FLUINTthermohydraulicanalyzer

（Ref1）.

SINDA/FLUINTisusedtodesignandsimulatethermal/fluidsystemsthatcanberepresentedinnetworkscorrespondingtofinitedifference,finiteelement,and/orlumped

parameterequations.Inadditiontoconduction,convection,andradiationheattransfer,theprogramcanmodelsteadyorunsteadysingle-andtwo-phaseflownetworks,including

nonreactingmixturesandnonequilibriumphenomena.

Table1presentstheoverallorganizationofavailablemodelingtools.

SINDA（ThermalNetworks）–SINDAusesathermalnetworkapproach,breakingaproblemdownintopointsatwhichenergyisconserved（nodes）,andintothepaths（conductors）throughwhichthesepointsexchangeenergyviaradiationandconduction.Whileoftenappliedasalumped-parametermodelingtool,theprogramcanalsobeusedtosolvethefinitedifference（FDM）orfiniteelement（FEM）equationsforconductioninappropriatelymeshed

shellsorsolids.Onecanemployfinitedifference,finiteelement,andarbitrary（lumpedparameter）nodesallwithinthesamemodel.

FLUINT（FluidNetworks）–FLUINTusesadifferenttypeofnetworkcomposedoflumpsandpaths,whichareanalogoustothermalnodesandconductors,butwhicharemuchmoresuitedtofluidsystemmodeling.Unlikethermalnetworks,fluidnetworksareabletosimultaneouslyconservemassandmomentumaswellasenergy.

Lumpsaresubdividedintotanks（finite-volumecontrolvolumes）,junctions（zero-volumecontrolvolumes:

conservationpoints,instantaneouscontrolvolumes）,andplena（boundarystates）.Pathsaresubdividedintotubes（inertialducts）,orconnectors（instantaneousflowpassagesincludingshort[zeroinertia]ducts,valves,etc.）.

Inadditiontolumpsandpaths,therearethreeadditionalfluidnetworkelements:

ties,fties,andifaces.Tiesrepresentheattransferbetweenthefluidandthewall（i.e.,betweenFLUINTandSINDA）.Ftiesor“fluidties”representheattransferwithinthefluiditself.Ifacesor“interfaceele-ments”representmovingboundariesbetweenadjacentcontrolvolumes.

FLUINTmodelscanbeconstructedthatemployfullytransientthermohydraulicsolutions（usingtanks）,orthatperformpseudo-steadytransientsolutions（neglectingperhapsinertialeffectsandothermassandenergystoragetermsusingjunctions）,orthatemploybothtechniquesatonce.Inotherwords,theengineerhastheabilitytoapproximateoridealizewherepossible,andtofocuscomputationalresourceswherenecessary.Aswillbedescribedlater,thesechoicesarecriticalforsuccessfulmodelingofvaporcompressioncycles.

Built-inSpreadsheetandUserLogic–Abuilt-inspreadsheetenablesuserstodefinecustom（andperhapsinterrelated）variablescalledregisters（Figure2）.Userscanalsodefinecomplexself-resolvinginterrelationshipsbetweeninputs,andalsobetweeninputsandoutputs.Thisspreadsheetallowsrapidandconsistentmodelchanges,minimizestheneedforuserlogic,andmakesparametricandsensitivitystudieseasytoperform.

Duringprogramoperation,concurrentlyexecutedlogicblocksarealsoavailable,parallelingthespreadsheetsystem.Inboththespreadsheetandthelogicblocks,full

accessisprovidednotonlytothebasicmodelingparameters（dimensions,properties,lossfactors,etc.）,butalsotoprogramcontrolparametersandtounderlyingcorrelations

forheattransfer,pressuredrop,fluidproperties,etc.

WORKINGFLUIDPROPERTIES

Becauseoftherangeofpressuresinvolvedandthepresenceoftwo-phaseflow,vaporcompressioncycleanalysesrequireafull-rangesetofpropertieswiththevaporphase

treatedasareal（notperfect）gas.ForR134a,severalsuchsetsofpropertydataexist,buttheonemostcommonlyemployedisatabulardescriptioncreatedfromNIST’sREFPROPdatabase（Ref3）.

PropertiesforotherfluidsofinteresttoA/CsystemsareavailableincludingHFCs,HCFCs,supercriticalcarbondioxide,andmoistair（forpassengercompartmentorenvironmental

psychrometricanalyses）.Also,noncondensiblegasesandnonvolatileliquids（e.g.,oils）canbeaddedtothemixture.

However,forthepurposesofthispaper,pureR134aisassumedunlessotherwisenoted.

VAPORCOMPRESSIONCYCLECOMPONENTS

Thissectiondescribesthemaincomponentswithinatypicalvaporcompressioncycle.Abuilding-blockapproachallowsboththearrangementofthecomponentsandthemethodsofmodelingthemtobevariable.

COMPRESSOR

Aswithalldevices,therearemanywaystomodelacompressordependingontheinformationavailableandthedetaildesi