熱工基礎(chǔ)第二章熱力學(xué)第一定律

Thermodynamics---Chapter2

TheFirstLawofThermodynamics---Chapter2

TheFirstLaw

ofThermodynamicsContentsTheinternalenergy,thetotalenergyThefirstlawofthermodynamicsFirstlawofthermodynamicsforaclosedsystem,andanopensystemMovingboundarywork,

FlowworkEnthalpy2-1TheTotalEnergyofaSystem

Thetotalenergyofasystem（系統(tǒng)的總能/儲存能）（1）Microscopicforms—internalenergy

（2）macroscopicforms—KE，PE1.InternalEnergy（熱力學(xué)能）

isdefinedasthesumofallthemicroscopicformsofenergyofasystem.Itisrelatedtothemolecularstructureandthedegreeofmolecularactivity.

2-1-2InternalEnergy（熱力學(xué)能）（3）Chemicalenergy（化學(xué)能）

Nuclearenergy（核能）

Magneticenergy（磁能）Including:（1）Molecular

kineticenergy（分子動能/內(nèi)動能）

Translationalenergy,rotationalkineticenergy,vibrationalkineticenergyIncreasesasthetemperatureofthesubstanceincreases（2）Molecularpotentialenergy（分子勢能/內(nèi)勢能）

AssociatedwiththeattractiveforcesbetweenmoleculesIncreasesasthedistancebetweenmoleculesincreasesItishigherforgases,andlowerforsolidsinternalenergyMolecular

kineticenergyMolecularPotentialenergyChemicalenergyNuclearenergymagneticenergy++++Molecular

kineticenergyMolecularPotentialenergy+negligibleUnit：J，denotedbyU.Specificinternalenergy（比熱力學(xué)能）

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

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

2-1-2KineticEnergyandPotentialEnergyKineticEnergy（宏觀動能）

Theenergythatasystempossessesasaresultofitsmotionrelativetosomereferenceframe.EK2.PotentialEnergy（宏觀位能）

Theenergythatasystempossessesasaresultofitselevationinagravitationalfield.EP

Expressedas:

2-1-3TheTotalEnergy（總儲存能）

Thetotalenergy：E=U+EK+EPJ，KJ

Onpermassbasis:e=u+ek+epJ/kg，KJ/kg

等量空氣從相同的初態(tài)出發(fā)，分別經(jīng)過可逆絕熱過程A和不可逆絕熱過程B到達相同的終態(tài)，分析空氣的熱力學(xué)能變化：ΔUA,ΔUB關(guān)系。解：ΔUA=ΔUB（熱力學(xué)能是狀態(tài)參數(shù)，熱力學(xué)能的變化只與初終狀態(tài)有關(guān)）。2-2TheFirstLawofThermodynamics

2-2-1SomeStatementsStatement2（陳述2）

Theperpetualmotionmachineofthefirstkind

（第一類永動機）canneverbemanufactured.Statement1（陳述1）

Energycanbeneithercreatednordestroyed.itcanonlychangeforms.TheFirstLawofThermodynamics,knownastheconservationofenergyprinciple,providesasoundbasisforstudyingtherelationshipsamongthevariousformsofenergyandenergyinteractions.

2-2-2EnergyBalance（方程表達式）

ForanykindofsystemundergoinganykindofprocessTheenergywhichleavethesystemThechangeinthetotalenergyofthesystem-Theenergywhichenterthesystem=

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

自然界中發(fā)生的一切過程都必須遵守能量守恒定律，反之，遵守能量守恒與轉(zhuǎn)換定律的一切過程都可以自發(fā)進行。（x）2-3EnergyBalanceforClosedSystem（閉口系統(tǒng)的熱力學(xué)第一定律表達式）

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（說明）

Signification（意義）:

Theheatabsorbedbyasystem:

Onaunitmassbasis:q=w+u

Applicationconditions（適用條件）: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

（可逆過程）q=u+12pdvDifferentialreversible

Q=dU+pdV

（可逆微元過程）

q=du+pdv

Cycle

（循環(huán)）

Qnet=wnet

q:微元熱量

w:微元功量du:熱力學(xué)能的微元變化量1.系統(tǒng)中工質(zhì)經(jīng)歷一個可逆定溫過程，由于沒有溫度變化，故該系統(tǒng)中工質(zhì)不能與外界交換熱量。2.封閉熱力系內(nèi)發(fā)生可逆定容過程時，系統(tǒng)一定不對外作容積變化功。3.封閉熱力系中，不作膨脹功的過程一定是定容過程。4.氣體膨脹時一定對外作功。5.工質(zhì)吸熱后一定會膨脹。

q=w+u（x）（x）（x）（）（x）例題例１：一個裝有２kg工質(zhì)的閉口系統(tǒng)經(jīng)歷了如下過程：過程中系統(tǒng)散熱25kJ，外界對系統(tǒng)作功100kJ，比熱力學(xué)能減少15kJ/kg，而且整個系統(tǒng)被舉高1000m。試確定過程中系統(tǒng)動能的變化。簡單求解過程：系統(tǒng)為閉口系統(tǒng)，并考慮閉口系動能，位能的變化。應(yīng)用熱力學(xué)第一定律：說明：系統(tǒng)動能增加，增加了85.4kJ

。注意：注意求解過程中熱量，功的正負。例2：一活塞汽缸中的氣體經(jīng)歷了兩個過程，從狀態(tài)1到狀態(tài)2，氣體吸熱500kJ，活塞對外做功800kJ。從狀態(tài)2到狀態(tài)3是一個定壓的壓縮過程，壓力為400kPa，氣體向外散熱450kJ。并且已知U1=2000kJ，U3=3500kJ，試計算2-3過程中氣體體積的變化。簡單求解過程：系統(tǒng)為閉口系統(tǒng)，考慮氣體經(jīng)歷兩個過程：過程1-2和過程2-3。對過程1-2：Q12=W12+U2-U1（1）對過程2-3：Q23=W23+U3-U2（2）過程2-3為可逆定壓過程，根據(jù)可逆過程體積變化功的計算公式：將（1）+（2）得：

Q12+Q23=W12+W23+U3-U1代入數(shù)據(jù)：

500+（-450）=800+400（V3-V2）+（3500-2000）解得：V3-V2=-5.625m3說明：壓縮過程中，體積減小。

2-4EnergyBalanceforSteady-FlowSystems（開口系統(tǒng)穩(wěn)定流動的能量方程式）（1）Thefluidpropertiesatdifferentpositionisdifferent（2）Energycanbetransferredtoorfromasysteminthreeforms:heat,work,massflow（3）Consideringbothenergybalanceandmass

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

2-4-1SteadyFlow（穩(wěn)定流動）1.SteadyFlow（穩(wěn)定流動）

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（流動功）

1.FlowWork

Requiredtopushthemassintooroutofthecontrolvolume

2.FlowWorkRelation（關(guān)系式）

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

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

2-4-4EnergyBalanceforSteady-FlowSystems

（開口系統(tǒng)穩(wěn)定流動能量方程式）

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（適用條件）:

（1）Steadyflow

（2）Reversibleprocess,non-reversibleprocess

2.Analysis（分析）HeattransferChangeofinternalenergyChangeofmechanic-alenergyFlowworkShaftwork

EnergyBalanceforClosedSystem：

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

2-4-5TechnicalWork（技術(shù)功）

1.Definition

Inthermodynamics,thesummationofenergywhichincreasesinkineticenergy,potentialenergyandworkofshaftiscalledtechnicalwork.

2.TherelationshipofW，WS，Wt，

(pv)

3.TechnicalWorkinReversibleProcess

（可逆過程中技術(shù)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

（可逆過程）

q=h-12vdp

DifferentialreversibleprocessQ=dH-Vdp

（可逆微元過程）

q=dh-vdpGeneralQ=H+Wt

（一般形式）q=h+wt根據(jù)熱力學(xué)第一定律，任何循環(huán)的凈熱量等于該循環(huán)的凈功量。熱力過程中，工質(zhì)向外界放熱，其溫度必然降低。工質(zhì)從同一初態(tài)出發(fā)，分別經(jīng)歷可逆過程和不可逆過程達到相同的終態(tài)，則兩過程中工質(zhì)與外界交換的熱量相同。工質(zhì)所作的膨脹功與技術(shù)功，在某種條件下，兩者的數(shù)值會相等。Q=H+Wt，q=h+wt

（）（）（x）（x）功不是狀態(tài)參數(shù)，熱力學(xué)能與推動功之和也不是狀態(tài)參數(shù)。流動功的改變量僅取決于系統(tǒng)進出口狀態(tài)，而與工質(zhì)經(jīng)歷的過程無關(guān)。（x）（）2-5SomeSteady-flowEngineeringDevices（穩(wěn)定流動能量方程式的應(yīng)用）

Energybalanceforsteady-flowsystems

Somecommonsteady-flowdevicesTurbinesandcompressors(pump,fan)（汽輪機和壓縮機）

Heatexchanges（熱交換器）

Nozzlesanddiffusers（噴嘴和擴壓器）

Throttlingvalues（節(jié)流閥）

2-5-1HeatExchanges（熱交換器）EnergyBalanceNotes:Theheattransferassociatedwithaheatexchangemaybezeroornonzerodependingonhowthesystemisselected.Heatexchangesaredeviceswheretwomovingfluidstreamsexchangeheatwithoutmixing.

Characteristics

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

2-5-2Turbines（渦輪，汽輪機）Characteristics

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

2-5-3Compressors（壓縮機）

Compressors,aswellaspumpsandfans,aredevicesusedtoincreasethepressureofafluid.Charac