高等工程热力学英文课件：Chapter-3-the-second-law-of-thermodyna

1Chapter3Thesecondlawofthermodynamics3-1Statementsofthesecondlaw(directionandextentofirreversibility)3-2CarnotcycleandCarnottheorems3–3Entropyandthemathematicalexpressionofthesecondlaw3-4Entropyequationandprincipleofentropyincrease3-5ExergyandAnergy3-6Exergyequilibriumequationandprincipleofenergydepreciation2a.Frommechanicalenergytothermalenergyb.Fromelectricenergytothermalenergyd.Freeexpansionofgase.BlendingQQ'?3–1Statementsofthesecondlaw1.Thedirectionalityofaspontaneousprocess(irreversibility)c.HeattransferfromahotobjecttoacoldobjectDirectionalityofenergytransformationortransferhasbeenobservedinpractice.3Finitepotentialdifference(temperaturedifference,pressuredifference,concentrationdifference)Dissipationeffect(friction,electricalresistance)processaandbNotinquasi-equilibrium(processc,dande)Irreversibleprocess•Irreversibilityisanimportantcharacteristicofaspontaneousprocess.ABspontaneousnon-spontaneous4•Thedirectionalityofenergytransformationisessentiallyattributedtothedifferenceinthequalityofenergy.•Thetransformationfromhigh-qualityenergytolow-qualityenergyisspontaneous,suchasfrommechanicalenergytothermalenergy.Theinverseprocesscanoccuronlyundertheconditionthatanotherspontaneousprocessoccurs(asacompensation).•Thefirstlawishelplessinfiguringoutthedirectionalityofaprocess.•Tofindoutthedirectionalityofaprocessandtheextentofirreversibilityiswhatthesecondlawstudies.512netqqwattheexpenseof12TT2q21TT2qnet12wqqattheexpenseofThermalenergytoworkLowtemperaturetohightemperatureSpontaneousprocess62.ThestatementsofthesecondlawClausiusstatement:Heatcanflowspontaneouslyfromahotobjecttoacoldobject;heatwillnotflowspontaneouslyfromacoldobjecttoahotobject.Kelvin-Plankstatement:Nodeviceispossiblewhosewholeeffectistotransformagivenamountofheatcompletelyintowork.Thesestatementsareequivalenttoeachother.Allhaveaqualitativeratherthanaquantitativecharacter.Otherstatement:perpetual-motionengineofthesecondkindisimpossibleOtherstatement:twoheatsourceswithtemperaturedifferencehasthepotentialtoproducepower.73-2CarnotcycleandCarnottheoremsab:isothermal(T1),endothermicbc:adiabaticexpansioncd:isothermal(T2),exothermicda:adiabaticcompressionAssumptions:idealgas,reversiblecycle,twoconstanttemperatureheatsourcesThederivationprocessisomitted.Thewnet,q1,q2areabsolutevalueratherthanalgebraicvalue1.Carnotcycle𝑣𝑐𝑣𝑏=𝑣𝑑𝑣𝑎8Discussion:ThethermalefficiencyoftheCarnotcycleonlydependsonthetemperaturesofheatsources(ifT1orT2approachtoenvironmentT,).Thethermalefficiencyisalwayssmallerthan100%.Itisimpossibletocompletelyconvertthermalenergytomechanicalenergy(AbsolutezeroKisnotpossible).thetemperaturedifferencemakesthetransformationofthermalenergytomechanicalenergypossible(ifT1=T2,then=0).Adiabaticcompressionisagoodmethodtoincreasethetemperatureofworkingmedium,widespreadapplicationinthermalpowerplants.TheoveralleffectoftheCarnotcycle9•Carnotenginegivestheupperlimitofaheatengine.•NoengineoperatingbetweenthesametwotemperaturescanbemoreefficientthanaCarnotengineduetoirreversibility.•Realenginesthatarewelldesignedreach60%to80%oftheefficiencyoftheCarnotengine.•Itisimportanttoknowhowtotakeheatfromasourcetoconvertasmuchofitaspossibleintowork.10TheimportanceoftheCarnotcycle:•Itisthetheoreticalfoundationofthesecondlaw.•Itdirectshowtoincreasethethermalefficiency•Itisanidealmodeloftherealthermalengine.•ItisnoteasytoachievetheCarnotcycleas(1)isothermalprocess,(2)reversibleprocess,(2)smallamountofnetwork112.TheinverseCarnotcycleThecycleproceedsinaninversedirection(anti-clockwise).Thecalculationofheatandworkisthesamebutthetransferdirectionisopposite.•Theenvironmentdoesworktosystem•TheoveralleffectistotransportheatfromlowTheatsourceandhighTsourceassistedbyinputtingwork.•Thisistheworkingprincipleofrefrigerationandheatpump.EconomicefficiencyrefrigerationHeatpump123.GeneralizedCarnotcycle•Twoconstanttemperatureheatsources.•ab,cd:isothermal,reversiblebc,da:otherreversibleprocess,Thesamepolytropicexponentn.•Thequasi-equilibriumtemperaturechangefromT1toT2(fromT2toT1)isachievedbyanumberofheataccumulators.Theheattotheaccumulator(areabcmnb)isequaltotheheatprovidedbytheaccumulator(areaadhga).Themethodofusingtheheatreleasedfromthesystemtoheatupthesystemistermedasheatrecovery(regeneration),whichplaysanimportantroleinincreasingthesystemperformance.Note:heataccumulatorisnotaheatsource.AppliedinGasturbineSteampowerplant,Sterlingengine134.ReversiblecyclewithmultipleheatsourcesThenumberofheatsourcesismorethantwo.Thethermalefficiencyissmallerthanthatwithonlytwoheatsourcesifthetemperaturelimitisthesame.AreaTwoheatsourcesIftheareaofabcmnaisequaltotheareaofobpmno,TmHistheaveragetemperatureofheatabsorption(theprocessofabc)Iftheareaofadcmnaisequaltotheareaofqdrmnq,TmListheaveragetemperatureofheatrelease(theprocessofcda)η𝑡=1−𝑇𝑚𝐿𝑇𝑚𝐻145.CarnottheoremsTheoremone:thethermalefficiencyofareversiblecyclebetweentwoheatsourcesonlydependsonthetemperaturesofheatsourcesandisindependentoftheworkingmediumandthetypeofcycleprocess.Theoremtwo:thethermalefficiencyofanir