thermodynamics · for non‐ flow calculaons • enthalpy is a property that consists of two...
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Thermodynamics
Lecture5Thermodynamics
Objec6ves
• Energy,Work,andHeat• ThermodynamicSystems
• WorkingCycles
• FirstandSecondLawofThermodynamics
ThermodynamicProper6es
• Inthermodynamics,aworkingsubstanceisanyfluid(includinggases)whichreceives,transports,andtransfersenergyinasystem.Thestateorcondi6onofaworkingsubstanceatanyloca6oninanysystemmaybedeterminedifthethermodynamicproper6esofthesubstanceattheloca6onofinterestareknown.Theseproper6esinclude:– Pressure– Temperature– Density(andSpecificVolume)– Internalenergy– Enthalpy– Entropy
Phases
• Solid,liquid,andvapor–fluidmayexistinanyphaseorcombina6onofphases
Proper6es
• Anintensivepropertyisindependentofthemass;thevalueofanextensivepropertyvariesdirectlywiththemass.Thus,ifaquan6tyofmaTerinagivenstateisdividedintotwoequalparts,eachpartwillhavethesamevalueofintensiveproper6esastheoriginal,andhalfthevalueoftheextensiveproper6es.
• Pressure,temperature,anddensityareexamplesofintensiveproper6es.Massandtotalvolumeareexamplesofextensiveproper6es.Extensiveproper6esperunitmass,suchasspecificvolume,areintensiveproper6es.
Proper6es
• Massandweight• Volume
• Velocity• Pressure• Temperature
SpecificHeat• Heat(Q)isanenergyintransi6on.Ifthisheatissuppliedtoamass,m,itcausesthemasstoriseintemperaturefromT1toT2.ThismaybeexpressedbytheSpecificHeatEqua7on:
PhaseChange• Thermalenergysuppliedtoasubstancecausesits
temperaturetorisewithanincreaseininternalenergyun6litreachesthetemperatureoffusionorevapora6on.– Whenthetemperatureoffusionisreached,theheatoffusion
causesthesubstancetochangefromasolidtoaliquidatthetemperatureoffusion.
– Whenthetemperatureofvaporiza6onorboiling(calledsatura6ontemperature)isreached,theheatofvaporiza6on(alsocalledlatentheatorenthalpyofvaporiza6on)causesthesubstancetochangefromaliquidtoavapor(gaseousform)atthevaporiza6ontemperature.
• Sensibleheataddi6onresultsinachangeoftemperaturebutnotachangeofphase
• Latentheataddi6onresultsinachangeofphasebutnochangeintemperature
LawsofThermodynamics
• TheFirstLawofThermodynamicssimplystated:Energycannotbecreatedordestroyed;thisisalsocalledtheLawofConserva6onofEnergy.
• TheSecondLawofThermodynamicssimplystated:Allenergyreceivedasheatbyaheat‐enginecyclecannotbeconvertedintomechanicalwork(someheatmustberejected).
DerivedProper6es
• Internalenergyisamo6onenergystoredintheindividualmoleculesofasubstance.Internalenergydependsontemperatureandthephaseofthesubstancesuchassolid,liquidorgas.Usedfornon‐flowcalcula6ons
• Enthalpyisapropertythatconsistsoftwoparts:(1)internalenergy(U)storedintheindividualmoleculesasheatenergyand(2)flowenergypossessedbythefluidbecauseitisflowingwithapressure(P)andavolume(V).
• Entropyisamathema6calquan6tythatincreasesifheatisdegradedbyallowingittoflowfromahightemperaturetolowtemperaturewithoutproducingmechanicalwork.
FormsofEnergy
• Poten6alEnergy• Kine6cEnergy• InternalEnergy• FlowEnergy
FormsofEnergy
• Kine6cenergy,poten6alenergy,internalenergyandPVenergyareformsofenergythatareallproper6esofthesystem.
• Workandheatalsoareformsofenergy,buttheyareenergyintransit.Theyarenotproper6esofthesystem.– Workisdonebyoronasystem,butasystemcontainsnowork.
– Heatistransferredtoorfromasystem,butasystemcontainsnoheat.
ThermodynamicSystem
• Asystemisapar6cularpor6onoftheuniverse,normallyapar6cularquan6tyofmaTerorapar6cularspace,whichweintendtostudydirectly
• Thestateofaworkingfluidinasystemisdefinedintermsofitsthermodynamicproper6es:pressure,temperature,specificvolume,enthalpy,internalenergy,andentropy.Onlytwoindependentthermodynamicproper6esarerequiredtocompletelydefinethestateoftheworkingfluid.
• Workingfluid• Systemboundary
– Closedvs.Open• Equilibrium,process,andcycle
ThermodynamicSystem
• Areversibleprocessforasystemisdefinedasaprocessthat,oncehavingtakenplace,canbereversed,andinsodoingleavesnochangeineitherthesystemorsurroundings.
• Anirreversibleprocessisaprocessthatcannotreturnboththesystemandthesurroundingstotheiroriginalcondi6ons.
• Anadiaba6cprocessisoneinwhichthereisnoheattransferintooroutofthesystem.Thesystemcanbeconsideredtobeperfectlyinsulated.
• Anisentropicprocessisoneinwhichtheentropyofthefluidremainsconstant.
• AthroTlingprocessisdefinedasaprocessinwhichthereisnochangeinenthalpy
CarnotCycle
• 1‐2:adiaba6ccompressionfromTCtoTHduetoworkperformedonfluid.
• 2‐3:isothermalexpansionasfluidexpandswhenheatisaddedtothefluidattemperatureTH.
• 3‐4:adiaba6cexpansionasthefluidperformsworkduringtheexpansionprocessandtemperaturedropsfromTHtoTC.
• 4‐1:isothermalcompressionasthefluidcontractswhenheatisremovedfromthefluidattemperatureTC.
CarnotCycle
• ThemostefficientheatenginecycleistheCarnotcycle,consis6ngoftwoisothermalprocessesandtwoadiaba6cprocesses.TheCarnotcyclecanbethoughtofasthemostefficientheatenginecycleallowedbyphysicallaws.Whenthesecondlawofthermodynamicsstatesthatnotallthesuppliedheatinaheatenginecanbeusedtodowork,theCarnotefficiencysetsthelimi6ngvalueonthefrac6onoftheheatwhichcanbesoused.
RankineCycle• TherearefourprocessesintheRankinecycle,thesestatesareiden6fied
bynumberinthediagramtotheright.• Process1‐2:Theworkingfluidispumpedfromlowtohighpressure,asthe
fluidisaliquidatthisstagethepumprequiresliTleinputenergy.• Process2‐3:Thehighpressureliquidentersaboilerwhereitisheatedat
constantpressurebyanexternalheatsourcetobecomeadrysaturatedvapor.
• Process3‐4:Thedrysaturatedvaporexpandsthroughaturbine,genera6ngpower.Thisdecreasesthetemperatureandpressureofthevapor,andsomecondensa6onmayoccur.
• Process4‐1:Thewetvaporthenentersacondenserwhereitiscondensedataconstantpressureandtemperaturetobecomeasaturatedliquid.Thepressureandtemperatureofthecondenserisfixedbythetemperatureofthecoolingcoilsasthefluidisundergoingaphase‐change.