空调专业毕业设计外文翻译工程热力学和制冷循环.docx

空调专业毕业设计外文翻译工程热力学和制冷循环.docx

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空调专业毕业设计外文翻译工程热力学和制冷循环

附录B英文翻译

THERMODYNAMICSANDREFRIGERATIONCYCLES

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.Onemethodusestheconceptofentropyflowinanopensystemandtheirreversibilityassociatedwiththeprocess.Theconceptofirreversibilityprovidesaddedinsightintotheoperationofcycles.Forexample,thelargertheirreversibilityinarefrigerationcycleoperatingwithagivenrefrigerationloadbetweentwofixedtemperaturelevels,thelargertheamountofworkrequiredtooperatethecycle.Irreversibilitiesincludepressuredropsinlinesand

heatexchangers,heattransferbetweenfluidsofdifferenttemperature,andmechanicalfriction.Reducingtotalirreversibilityinacycleimprovescycleperformance.Inthelimitofnoirreversibilities,acycleattainsitsmaximumidealefficiency.Inanopensystem,thesecondlawofthermodynamicscanbedescribedintermsofentropyas

（8）

where

dS=totalchangewithinsystemintimedtduringprocesssystem

δms=entropyincreasecausedbymassentering（incoming）

δms=entropydecreasecausedbymassleaving（exiting）

δQ/T=entropychangecausedbyreversibleheattransferbetweensystemandsurroundingsattemperatureT

dI=entropycausedbyirreversibilities（alwayspositive）

Equation（8）accountsforallentropychangesinthesystem.Rearranged,thisequationbecomes

（9）

Inintegratedform,ifinletandoutletproperties,massflow,andinteractionswiththesurroundingsdonotvarywithtime,thegeneralequationforthesecondlawis

（10）

Inmanyapplications,theprocesscanbeconsideredtooperatesteadilywithnochangeintime.Thechangeinentropyofthesystemisthereforezero.Theirreversibilityrate,whichistherateofentropyproductioncausedbyirreversibilitiesintheprocess,canbedeterminedbyrearrangingEquation（10）:

（11）

Equation（6）canbeusedtoreplacetheheattransferquantity.Notethattheabsolutetemperatureofthesurroundingswithwhichthesystemisexchangingheatisusedinthelastterm.Ifthetemper-

atureofthesurroundingsisequaltothesystemtemperature,heatistransferredreversiblyandthelastterminEquation（11）equalszero.

Equation（11）iscommonlyappliedtoasystemwithonemassflowin,thesamemassflowout,nowork,andnegligiblekineticorpotentialenergyflows.CombiningEquations（6）and（11）yields

（12）

Inacycle,thereductionofworkproducedbyapowercycle（ortheincreaseinworkrequiredbyarefrigerationcycle）equalstheabsoluteambienttemperaturemultipliedbythesumofirreversibilitiesinallprocessesinthecycle.Thus,thedifferenceinreversibleandactualworkforanyrefrigerationcycle,theoreticalorreal,operatingunderthesameconditions,becomes

（13）

THERMODYNAMICANAL