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.