空调专业毕业设计外文翻译工程热力学和制冷循环Word格式.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Word格式.docx
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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)
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
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