工程热力学和制冷循环外文翻译本科学位论文Word文档格式.docx
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工程热力学和制冷循环外文翻译本科学位论文Word文档格式.docx
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240112608
班级:
K暖通111
Class:
K-Nuantong111
所在学院:
康尼学院
College:
KangniCollege
专业:
建筑环境与设备工程
Profession:
BuildingEnvironmentandEquipmentEngineering
指导教师:
Tutor:
LiuMinghui
2015年3月7日
CHAPTER1
THERMODYNAMICSANDREFRIGERATIONCYCLES
THERMODYNAMICS...................................................................1.1
FirstLawofThermodynamics.........................................................1.2
SecondLawofThermodynamics.....................................................1.2
ThermodynamicAnalysisofRefrigerationCycles...........................1.3
EquationsofState..............................................................................1.3
CalculatingThermodynamicProperties.............................................1.4
COMPRESSIONREFRIGERATIONCYCLES................................1.6
CarnotCycle........................................................................................1.6
TheoreticalSingle-StageCycleUsingaPureRefrigerant
orAzeotropicMixture..................................................................1.8
LorenzRefrigerationCycle...................................................................1.9
TheoreticalSingle-StageCycleUsingZeotropic
RefrigerantMixture.....................................................................1.10
MultistageVaporCompressionRefrigerationCycles........................1.10
ActualRefrigerationSystems................................................................1.12
ABSORPTIONREFRIGERATIONCYCLES.....................................1.14
IdealThermalCycle..............................................................................1.14
WorkingFluidPhaseChangeConstraints...............................................1.14
WorkingFluids.......................................................................................1.15
AbsorptionCycleRepresentations..........................................................1.16
ConceptualizingtheCycle.......................................................................1.16
AbsorptionCycleModeling.....................................................................1.17
Ammonia-WaterAbsorptionCycles........................................................1.19
NomenclatureforExamples....................................................................1.20
THERMODYNAMICSisthestudyofenergy,itstransformations,anditsrelationtostatesofmatter.Thischaptercoverstheapplicationofthermodynamicstorefrigerationcycles.Thefirstpartreviewsthefirstandsecondlawsofthermodynamicsandpresentsmethodsforcalculatingthermodynamicproperties.Thesecondandthirdpartsaddresscompressionandabsorptionrefrigerationcycles,twocommonmethodsofthermalenergytransfer.
THERMODYNAMICS
Athermodynamicsystemisaregioninspaceoraquantityofmatterboundedbyaclosedsurface.Thesurroundingsincludeeverythingexternaltothesystem,andthesystemisseparatedfrom
thesurroundingsbythesystemboundaries.Theseboundariescanbemovableorfixed,realorimaginary.Entropyandenergyareimportantinanythermodynamicsystem.Entropymeasuresthemoleculardisorderofasystem.Themoremixedasystem,thegreateritsentropy;
anorderlyorunmixedconfigurationisoneoflowentropy.Energyhasthecapacityforproducinganeffectandcanbecategorizedintoeitherstoredortransientforms.
StoredEnergy
Thermal(internal)energyiscausedbythemotionofmoleculesand/orintermolecularforces.
Potentialenergy(PE)iscausedbyattractiveforcesexistingbetweenmolecules,ortheelevationofthesystem.
(1)
where
m=mass
g=localaccelerationofgravity
z=elevationabovehorizontalreferenceplane
Kineticenergy(KE)istheenergycausedbythevelocityofmoleculesandisexpressedas
(2)
where
Visthevelocityofafluidstreamcrossingthesystemboundary.
Chemicalenergyiscausedbythearrangementofatomscomposingthemolecules.
Nuclear(atomic)energyderivesfromthecohesiveforcesholdingprotonsandneutronstogetherastheatom’snucleus.
EnergyinTransition
HeatQisthemechanismthattransfersenergyacrosstheboundariesofsystemswithdifferingtemperatures,alwaystowardthelowertemperature.Heatispositivewhenenergyisaddedtothesystem(seeFigure1).
Workisthemechanismthattransfersenergyacrosstheboundariesofsystemswithdifferingpressures(orforceofanykind),alwaystowardthelowerpressure.Ifthetotaleffectproducedinthesystemcanbereducedtotheraisingofaweight,thennothingbutworkhascrossedtheboundary.Workispositivewhenenergyisremovedfromthesystem(seeFigure1).
MechanicalorshaftworkWistheenergydeliveredorabsorbedbyamechanism,suchasaturbine,aircompressor,orinternalcombustionengine.
Flowworkisenergycarriedintoortransmittedacrossthesystemboundarybecauseapumpingprocessoccurssomewhereoutsidethesystem,causingfluidtoenterthesystem.Itcanbe
moreeasilyunderstoodastheworkdonebythefluidjustoutsidethesystemontheadjacentfluidenteringthesystemtoforceorpushitintothesystem.Flowworkalsooccursasfluidleavesthe
system.
Flowwork=pv(3)
wherepisthepressureandvisthespecificvolume,orthevolumedisplacedperunitmassevaluatedattheinletorexit.
Apropertyofasystemisanyobservablecharacteristicofthesystem.Thestateofasystemisdefinedbyspecifyingtheminimumsetofindependentproperties.ThemostcommonthermodynamicpropertiesaretemperatureT,pressurep,andspecificvolumevordensityρ.Additionalthermodynamicpropertiesincludeentropy,storedformsofenergy,andenthalpy.
Frequently,thermodynamicpropertiescombinetoformotherproperties.Enthalpyhisanimportantpropertythatincludesinternalenergyandflowworkandisdefinedas
(4)
whereuistheinternalenergyperunitmass.
Eachpropertyinagivenstatehasonlyonedefinitevalue,andanypropertyalwayshasthesamevalueforagivenstate,regardlessofhowthesubstancearrivedatthatstate.
Aprocessisachangeinstatethatcanbedefinedasanychangeinthepropertiesofasystem.Aprocessisdescribedbyspecifyingtheinitialandfinalequilibriumstates,thepath(ifidentifiable),andtheinteractionsthattakeplaceacrosssystemboundariesduringthe
process.
Acycleisaprocessoraseriesofprocesseswhereintheinitialandfinalstatesofthesystemareidentical.Therefore,attheconclusionofacycle,allthepropertieshavethesamevaluetheyhadatthebeginning.Refrigerantcirculatinginaclosedsystemundergoesa
cycle.
Apuresubstancehasahomogeneousandinvariablechemicalcomposition.Itcanexistinmorethanonephase,butthechemicalcompositionisthesameinallphases.
Ifasubstanceisliquidatthesaturationtemperatureandpressure,itiscalledasaturatedliquid.Ifthetemperatureoftheliquidislowerthanthesaturationtemperaturefortheexistingpressure,itiscalledeitherasubcooledliquid(thetemperatureislowerthanthesaturationtemperatureforthegivenpressure)oracompressedliquid(thepressureisgreaterthanthesaturationpressureforthegiventemperature).
Whenasubstanceexistsaspartliquidandpartvaporatthesaturationtemperature,itsqualityisdefinedastheratioofthemassofvaportothetotalmass.Qualityhasmeaningonlywhenthesubstanceissaturated(i.e.,atsaturationpressureandtemperature).Pressureandtemperatureofsaturatedsubstancesarenotindependentproperties.
Ifasubstanceexistsasavaporatsaturationtemperatureandpressure,itiscalledasaturatedvapor.(Sometimesthetermdrysaturatedvaporisusedtoemphasizethatthequalityis100%.)
Whenthevaporisatatemperaturegreaterthanthesaturationtemperature,itisasuperheatedvapor.Pressureandtemperatureofasuperheatedvaporareindependentproperties,becausethetemperaturecanincreasewhilepressureremainsconstant.Gasessuchasairatroomtemperatureandpressurearehighlysuperheatedvapors.
FIRSTLAWOFTHERMODYNAMICS
Thefirstlawofthermodynamicsisoftencalledthelawofconservationofenergy.Thefollowingformofthefirst-lawequationisvalidonlyintheabsenceofanuclearorchemicalreaction.
Basedonthefirstlaworthelawofconservationofenergyforanysystem,openorclosed,thereisanenergybalanceas
NetamountofenergyNetincreaseofstored
=
addedtosystemenergyinsystem
or
[Energyin]–[Energyout]=[Increaseofstoredenergyinsystem]
Figure1illustratesenergyflowsintoandoutofathermodynamicsystem.Forthegeneralcaseofmultiplemassflowswithuniformpropertiesinandoutofthesystem,theenergybalancecanbewritten
(5)
wheresubscriptsiandfrefertotheinitialandfinalstates,respectively.
Nearlyallimportantengineeringprocessesarecommonlymodeledassteady-flowprocesses.Steadyflowsignifiesthatallquantitiesassociatedwiththesystemdonotvarywithtime.Consequently,
(6)
whereh=u+pvasdescribedinEquation(4).
Asecondcommonapplicationistheclosedstationarysystemforwhichthefirstlawequationreducesto
(7)
SECONDLAWOFTHERMODYNAMICS
Thesecondlawofthermodynamicsdifferentiatesandquantifiesprocessesthatonlyproceedinacertaindirection(irreversible)fromthosethatarereversible.Thesecondlawmaybedescribedinseveralways.Onemethodusestheconceptofentropyflowinan
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