Exergy--Chapter08_精品文档PPT文档格式.ppt
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Exergy--Chapter08_精品文档PPT文档格式.ppt
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Chapter8Exergy:
@#@AMeasureofWorkPotentialStudyGuideinPowerPointtoaccompanyThermodynamics:
@#@AnEngineeringApproach,5theditionbyYunusA.engelandMichaelA.BolesTheenergycontentoftheuniverseisconstant,justasitsmasscontentis.Yetattimesofcrisiswearebombardedwithspeechesandarticlesonhowto“conserve”energy.Asengineers,weknowthatenergyisalreadyconserved.Whatisnotconservedisexergy,whichistheusefulworkpotentialoftheenergy.Oncetheexergyiswasted,itcanneverberecovered.Whenweuseenergy(toheatourhomes,forexample),wearenotdestroyinganyenergy;@#@wearemerelyconvertingittoalessusefulform,aformoflessexergy.ExergyandtheDeadStateTheusefulworkpotentialofasystemistheamountofenergyweextractasusefulwork.Theusefulworkpotentialofasystematthespecifiedstateiscalledexergy.Exergyisapropertyandisassociatedwiththestateofthesystemandtheenvironment.Asystemthatisinequilibriumwithitssurroundingshaszeroexergyandissaidtobeatthedeadstate.TheexergyofthethermalenergyofthermalreservoirsisequivalenttotheworkoutputofaCarnotheatengineoperatingbetweenthereservoirandtheenvironment.ExergyFormsNowletsdeterminetheexergyofvariousformsofenergy.2ExergyofkineticenergyKineticenergyisaformofmechanicalenergyandcanbeconverteddirectlyintowork.Kineticenergyitselfistheworkpotentialorexergyofkineticenergyindependentofthetemperatureandpressureoftheenvironment.Exergyofkineticenergy:
@#@ExergyofpotentialenergyPotentialenergyisaformofmechanicalenergyandcanbeconverteddirectlyintowork.Potentialenergyitselfistheworkpotentialorexergyofpotentialenergyindependentofthetemperatureandpressureoftheenvironment.3Exergyofpotentialenergy:
@#@UsefulWorkTheworkdonebyworkproducingdevicesisnotalwaysentirelyinauseableform.Considerthepistoncylinderdeviceshowninthefollowingfigure.TheworkdonebythegasexpandinginthepistoncylinderdeviceistheboundaryworkandcanbewrittenasTheactualworkdonebythegasis4TheworddoneonthesurroundingsisAnyusefulworkdeliveredbyapistoncylinderdeviceisduetothepressureabovetheatmosphericlevel.ReversibleWorkReversibleworkWrevisdefinedasthemaximumamountofusefulworkthatcanbeproduced(ortheminimumworkthatneedstobesupplied)asasystemundergoesaprocessbetweenthespecifiedinitialandfinalstates.Thisistheusefulworkoutput(orinput)obtainedwhentheprocessbetweentheinitialandfinalstatesisexecutedinatotallyreversiblemanner.IrreversibilityThedifferencebetweenthereversibleworkWrevandtheusefulworkWuisduetotheirreversibilitiespresentduringtheprocessandiscalledtheirreversibilityI.Itisequivalenttotheexergydestroyedandisexpressedas5whereSgenistheentropygeneratedduringtheprocess.Foratotallyreversibleprocess,theusefulandreversibleworktermsareidenticalandthusirreversibilityiszero.Irreversibilitycanbeviewedasthewastedworkpotentialorthelostopportunitytodowork.Itrepresentstheenergythatcouldhavebeenconvertedtoworkbutwasnot.Exergydestroyedrepresentsthelostworkpotentialandisalsocalledthewastedworkorlostwork.Second-LawEfficiencyThesecond-lawefficiencyisameasureoftheperformanceofadevicerelativetotheperformanceunderreversibleconditionsforthesameendstatesandisgivenbyforheatenginesandotherworkproducingdevicesand6forrefrigerators,heatpumps,andotherworkconsumingdevices.Ingeneral,thesecondlawefficiencyisexpressedasExergyofchangeofasystemConsiderheattransferredtoorfromaclosedsystemwheneverthereisatemperaturedifferenceacrossthesystemboundary.Theexergyforasystemmaybedeterminedbyconsideringhowmuchofthisheattransferisconvertedtoworkentirely.Letstakeasecondlookatthefollowingfigure.7Takingtheheattransfertobefromthesystemtoitssurroundings,theconservationofenergyisTheworkistheboundaryworkandcanbewrittenasAnyusefulworkdeliveredbyapistoncylinderdeviceisduetothepressureabovetheatmosphericlevel.Toassurethereversibilityoftheprocess,theheattransferoccursthroughareversibleheatengine.8Integratingfromthegivenstate(nosubscript)tothedeadstate(0subscript),wehaveThisisthetotalusefulworkduetoasystemundergoingareversibleprocessfromagivenstatetothedeadstate,whichisthedefinitionofexergy.Includingthekineticenergyandpotentialenergy,theexergyofaclosedsystemisonaunitmassbasis,theclosedsystem(ornonflow)exergyis9Here,u0,v0,ands0arethepropertiesofthesystemevaluatedatthedeadstate.Notethattheexergyoftheinternalenergyofasystemiszeroatthedeadstateiszerosinceu=u0,v=v0,ands=s0atthatstate.Theexergychangeofaclosedsystemdurin
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