热工基础第二章热力学第一定律

Thermodynamics---Chapter2

TheFirstLawofThermodynamics---Chapter2

TheFirstLaw

ofThermodynamicsContentsTheinternalenergy,thetotalenergyThefirstlawofthermodynamicsFirstlawofthermodynamicsforaclosedsystem,andanopensystemMovingboundarywork,

FlowworkEnthalpy2-1TheTotalEnergyofaSystem

Thetotalenergyofasystem（系统的总能/储存能）（1）Microscopicforms—internalenergy

（2）macroscopicforms—KE，PE1.InternalEnergy（热力学能）

isdefinedasthesumofallthemicroscopicformsofenergyofasystem.Itisrelatedtothemolecularstructureandthedegreeofmolecularactivity.

2-1-2InternalEnergy（热力学能）（3）Chemicalenergy（化学能）

Nuclearenergy（核能）

Magneticenergy（磁能）Including:（1）Molecular

kineticenergy（分子动能/内动能）

Translationalenergy,rotationalkineticenergy,vibrationalkineticenergyIncreasesasthetemperatureofthesubstanceincreases（2）Molecularpotentialenergy（分子势能/内势能）

AssociatedwiththeattractiveforcesbetweenmoleculesIncreasesasthedistancebetweenmoleculesincreasesItishigherforgases,andlowerforsolidsinternalenergyMolecular

kineticenergyMolecularPotentialenergyChemicalenergyNuclearenergymagneticenergy++++Molecular

kineticenergyMolecularPotentialenergy+negligibleUnit：J，denotedbyU.Specificinternalenergy（比热力学能）

Theinternalenergyonamassbasis.u,J/kg.2.Internalenergyisaproperty,itisafunctionoftemperatureandspecificvolumeofthesubstance.3.Thermodynamicsprovidesnoinformationastoabsolutevaluesofinternalenergy.

Itisonlythechangeininternalenergy,However,whichisimportantinengineeringproblem.AvalueofU=0canbeassignedtoanyparticularstateofasystem.itisusualtochoosetheoriginas0ºC,atwhichtemperaturetheinternalenergyissuggestedasbeingzero.

2-1-2KineticEnergyandPotentialEnergyKineticEnergy（宏观动能）

Theenergythatasystempossessesasaresultofitsmotionrelativetosomereferenceframe.EK2.PotentialEnergy（宏观位能）

Theenergythatasystempossessesasaresultofitselevationinagravitationalfield.EP

Expressedas:

2-1-3TheTotalEnergy（总储存能）

Thetotalenergy：E=U+EK+EPJ，KJ

Onpermassbasis:e=u+ek+epJ/kg，KJ/kg

等量空气从相同的初态出发，分别经过可逆绝热过程A和不可逆绝热过程B到达相同的终态，分析空气的热力学能变化：ΔUA,ΔUB关系。解：ΔUA=ΔUB（热力学能是状态参数，热力学能的变化只与初终状态有关）。2-2TheFirstLawofThermodynamics

2-2-1SomeStatementsStatement2（陈述2）

Theperpetualmotionmachineofthefirstkind

（第一类永动机）canneverbemanufactured.Statement1（陈述1）

Energycanbeneithercreatednordestroyed.itcanonlychangeforms.TheFirstLawofThermodynamics,knownastheconservationofenergyprinciple,providesasoundbasisforstudyingtherelationshipsamongthevariousformsofenergyandenergyinteractions.

2-2-2EnergyBalance（方程表达式）

ForanykindofsystemundergoinganykindofprocessTheenergywhichleavethesystemThechangeinthetotalenergyofthesystem-Theenergywhichenterthesystem=

Thefirstlawofthermodynamicsisafar-reachingprincipleofnaturewhichisinducedfromtheresultsofmanyexperiments.Itcannotbededucedorprovedfromanyotherprincipleofnature,Itisentirelyempirical.Energytransferbyheat,work,andmassChangeininternal,kinetic,potential.etc.energies

自然界中发生的一切过程都必须遵守能量守恒定律，反之，遵守能量守恒与转换定律的一切过程都可以自发进行。（x）2-3EnergyBalanceforClosedSystem（闭口系统的热力学第一定律表达式）

2-3-1EnergyBalanceforClosedSystemSystem:gasclosedinapiston-cylinderAssumptions：

Process:fromequilibriumstate1tostate2Heatinput:Qworkoutput:WNeglect:ΔEK,ΔEP21QWTheenergywhichleavethesystemThechangeinthetotalenergyofthesystem-Theenergywhichenterthesystem=Energybalance:——Energyequationforclosedsystem

Q–W=UQ=W+U=W+U2-U1

2-3-2Notes（说明）

Signification（意义）:

Theheatabsorbedbyasystem:

Onaunitmassbasis:q=w+u

Applicationconditions（适用条件）:a:Reversibleprocess,non-reversibleprocessb:Idealgas,actualgasc:Theendstatesisinequilibriumstate.QWu2-3-3VariousFormsoftheFirstLawRelationforClosedSystem（不同形式）GeneralQ=U+W

（一般形式）q=

u+wDifferentialform

Q=dU+

W

（微分形式）

q=du+

wReversibleprocessQ=U+12pdV

（可逆过程）q=u+12pdvDifferentialreversible

Q=dU+pdV

（可逆微元过程）

q=du+pdv

Cycle

（循环）

Qnet=wnet

q:微元热量

w:微元功量du:热力学能的微元变化量1.系统中工质经历一个可逆定温过程，由于没有温度变化，故该系统中工质不能与外界交换热量。2.封闭热力系内发生可逆定容过程时，系统一定不对外作容积变化功。3.封闭热力系中，不作膨胀功的过程一定是定容过程。4.气体膨胀时一定对外作功。5.工质吸热后一定会膨胀。

q=w+u（x）（x）（x）（）（x）例题例１：一个装有２kg工质的闭口系统经历了如下过程：过程中系统散热25kJ，外界对系统作功100kJ，比热力学能减少15kJ/kg，而且整个系统被举高1000m。试确定过程中系统动能的变化。简单求解过程：系统为闭口系统，并考虑闭口系动能，位能的变化。应用热力学第一定律：说明：系统动能增加，增加了85.4kJ

。注意：注意求解过程中热量，功的正负。例2：一活塞汽缸中的气体经历了两个过程，从状态1到状态2，气体吸热500kJ，活塞对外做功800kJ。从状态2到状态3是一个定压的压缩过程，压力为400kPa，气体向外散热450kJ。并且已知U1=2000kJ，U3=3500kJ，试计算2-3过程中气体体积的变化。简单求解过程：系统为闭口系统，考虑气体经历两个过程：过程1-2和过程2-3。对过程1-2：Q12=W12+U2-U1（1）对过程2-3：Q23=W23+U3-U2（2）过程2-3为可逆定压过程，根据可逆过程体积变化功的计算公式：将（1）+（2）得：

Q12+Q23=W12+W23+U3-U1代入数据：

500+（-450）=800+400（V3-V2）+（3500-2000）解得：V3-V2=-5.625m3说明：压缩过程中，体积减小。

2-4EnergyBalanceforSteady-FlowSystems（开口系统稳定流动的能量方程式）（1）Thefluidpropertiesatdifferentpositionisdifferent（2）Energycanbetransferredtoorfromasysteminthreeforms:heat,work,massflow（3）Consideringbothenergybalanceandmass

balance（4）Work:Movingboundarywork,flowwork.etc.notes

2-4-1SteadyFlow（稳定流动）1.SteadyFlow（稳定流动）

Aprocessduringwhichafluidflowsthroughacontrolvolumesteadily.thatis,thefluidpropertiescanchangefrompointtopointwithinthecontrolvolume,butatanypoint,theyremainconstantduringtheentireprocess.

2.CharacteristicsofSteady-FlowProcess（特性）（2）Energybalance(perunittime):

Ein=Eout,Ecv=constant（3）Nopropertieswithinthecontrolvolumechangewithtime（1）Massbalance(perunittime):

min=mout,

mcv=constant

2-4-2FlowWork（流动功）

1.FlowWork

Requiredtopushthemassintooroutofthecontrolvolume

2.FlowWorkRelation（关系式）

2112Perunitmass:Theproductpvrepresentsenergyforflowingfluidsonlyanddoesnotrepresentanyformofenergyfornon-flow(closed)system.notes

2-4-3Enthalpy（焓）Definition（定义）：H=U+pVJkJperunitmass:h=u+pvJ/kgkJ/kg（1）Enthalpyisaproperty.（2）Wecannotobtainabsolutevaluesofenthalpy.Internalenergyandflowworkusuallytransferredatthesametimeinflowingfluid.notes

2-4-4EnergyBalanceforSteady-FlowSystems

（开口系统稳定流动能量方程式）

System:Considerasteady-flowsystemasshown.thesystemhasasingleinletandasingleoutlet2112WsQ1.EnergyBalanceforSteady-Flow

Systems

Assumption:forperunittimein:m1，cf1out:m2，cf2

energy:input:Q，

output:WS（shaftwork）

totalenergychangeECV

TheenergywhichenterthesystemTheenergywhichleavethesystemTheenergywhichleavethesystemThechangeinthetotalenergyofthesystem-Theenergywhichenterthesystem=Energybalance:ThechangeinthetotalenergyofthesystemForsteadyflowΔECV=0So：——EnergyBalanceforSteady-FlowSystems

Applicationconditions（适用条件）:

（1）Steadyflow

（2）Reversibleprocess,non-reversibleprocess

2.Analysis（分析）HeattransferChangeofinternalenergyChangeofmechanic-alenergyFlowworkShaftwork

EnergyBalanceforClosedSystem：

EnergyBalanceforSteady-FlowSystems：notesSameprinciple:expansion----workoutputDifferentforms:

2-4-5TechnicalWork（技术功）

1.Definition

Inthermodynamics,thesummationofenergywhichincreasesinkineticenergy,potentialenergyandworkofshaftiscalledtechnicalwork.

2.TherelationshipofW，WS，Wt，

(pv)

3.TechnicalWorkinReversibleProcess

（可逆过程中技术功表示）pv12０vdpIfdp<0，wt>0，workdonebythesystemIfdp>0，wt<0，workdoneonthesystemIfdp=0，wt=0Notes

2-4-6VariousFormsoftheFirstLawRelationforSteady-FlowSystem（不同形式）Differentialform

Q=dH+Wt

（微分形式）

q=dh+wtReversibleprocessQ=H-12Vdp

（可逆过程）

q=h-12vdp

DifferentialreversibleprocessQ=dH-Vdp

（可逆微元过程）

q=dh-vdpGeneralQ=H+Wt

（一般形式）q=h+wt根据热力学第一定律，任何循环的净热量等于该循环的净功量。热力过程中，工质向外界放热，其温度必然降低。工质从同一初态出发，分别经历可逆过程和不可逆过程达到相同的终态，则两过程中工质与外界交换的热量相同。工质所作的膨胀功与技术功，在某种条件下，两者的数值会相等。Q=H+Wt，q=h+wt

（）（）（x）（x）功不是状态参数，热力学能与推动功之和也不是状态参数。流动功的改变量仅取决于系统进出口状态，而与工质经历的过程无关。（x）（）2-5SomeSteady-flowEngineeringDevices（稳定流动能量方程式的应用）

Energybalanceforsteady-flowsystems

Somecommonsteady-flowdevicesTurbinesandcompressors(pump,fan)（汽轮机和压缩机）

Heatexchanges（热交换器）

Nozzlesanddiffusers（喷嘴和扩压器）

Throttlingvalues（节流阀）

2-5-1HeatExchanges（热交换器）EnergyBalanceNotes:Theheattransferassociatedwithaheatexchangemaybezeroornonzerodependingonhowthesystemisselected.Heatexchangesaredeviceswheretwomovingfluidstreamsexchangeheatwithoutmixing.

Characteristics

Noworkinteractions,ws=0Negligiblekineticandpotentialenergychanges,cf2=0,z=0h2h1

2-5-2Turbines（涡轮，汽轮机）Characteristics

Wellinsulated,Negligible,q=0Negligiblekineticandpotentialenergychanges,cf2=0,z=0EnergyBalanceNotes:Ws=Wt.Thedevicethatdrivestheelectricgeneratorinsteam,gas,orhydroelectricplants.

2-5-3Compressors（压缩机）

Compressors,aswellaspumpsandfans,aredevicesusedtoincreasethepressureofafluid.Charac