工程热力学第二章热力学第一定律

1、2.1Macroscopic and microscopic energy of system 系统的宏观和微观储存能系统的宏观和微观储存能 2.2Energy transfer by Heat, Work and Mass 热量、功量及质量引起的能量传递热量、功量及质量引起的能量传递-传递中的能量传递中的能量 2.3 The first law of thermodynamics and Energy balance equation for closed system 热力学第一定律与闭口系统的能量平衡方程热力学第一定律与闭口系统的能量平衡方程 2.4 Energy balance equ

2、ation of open system 开口系统的能量平衡方程开口系统的能量平衡方程2.5Energy balance for steady-flow systems and some applications 稳态稳定流动的能量平衡及其应用稳态稳定流动的能量平衡及其应用 Chapter 2. The first law of thermodynamics第二章 热力学第一定律Essence of the First Law of Thermodynamics热力学第一定律的本质 In 1909，C. Caratheodory finally established the first la

3、w. 最后完善热一律最后完善热一律本质：本质：能量能量转换转换及及守恒守恒定律定律在热过程中的应用在热过程中的应用Early 1700s，Industrial Revolution，thermal efficiency was only 1% In 1842，J.R. Mayer put forward the first law，However, it did not attract a lot attention.From 1840 to 1849，Joule investigated the first law by the consistency of several experime

4、nts. And his publication in 1850 was well accepted. 用多种实验的一致性证明热一律，于用多种实验的一致性证明热一律，于1850年发表年发表并得到公认并得到公认 能量守恒定律阐明能量既不能被创造，也不能消灭，它只能量守恒定律阐明能量既不能被创造，也不能消灭，它只 能从一种形式转换成另一种形式，或从一个系统转移到另一能从一种形式转换成另一种形式，或从一个系统转移到另一个系统，而其总量保持恒定。个系统，而其总量保持恒定。 Conservation of energy principle states that energy can be neithe

5、r created nor destroyed; it can only change from one form to another but the total amount of energy remains constant. Energy Conservation Principle能量守恒定律能量守恒定律热力学第一定律主要说明热能与机械能在转换过程中的热力学第一定律主要说明热能与机械能在转换过程中的能量守恒能量守恒 The first law of thermodynamics is viewed as the Conservation of energy principle wh

6、ich governs the energy transfer process from thermal energy to mechanical ones. 热力学第一定律热力学第一定律The First Law of Thermodynamics 系统能量的变化量等于加给的热量与系统对外所作功系统能量的变化量等于加给的热量与系统对外所作功量之差。量之差。 The change in energy of a system is equal to the difference between the heat added to the system and the work done by t

7、he system.l E = Q - W “第一类永动机是不可能制成的第一类永动机是不可能制成的”Perpetual motion machine of the first kindPerpetual motion machine of the first kindQ锅锅炉炉汽轮机汽轮机发电机发电机给水泵给水泵凝凝汽汽器器WnetQout电电加加热热器器2.1 Macroscopic and microscopic energy of system 系统的宏观和微观储存能Energy is a property of every system. It is denoted as E for

8、a system, or e for a system with a unit mass. E = internal energy + kinetic energy + potential energy 1.热力学能热力学能U Microscopic energy-Internal energy U 热力学能指系统所有微观形式的能量之和热力学能指系统所有微观形式的能量之和. Internal energy is defined as the sum of all the microscopic forms of energy of a system. 热力学能的微观组成分子动能分子动能分子位能

9、分子位能 binding forces化学能化学能 chemical energy核能核能 nuclear energy热力学能热力学能microscopic forms of internal energy 移动移动 translation转动转动 rotation振动振动 vibration（1）Kinetic and potential energies of the molecules 分子的动能和势能分子的动能和势能 热力学能热力学能内动能内动能内位能内位能T, vTInternal Energy internal kinetic energyinternal potential e

10、nergy(2) Physical meaning of internal energy of IDEAL GAS 理想气体热力学能的物理解释热力学能热力学能内动能内动能内位能内位能T, v理想气体无分子间作用力，理想气体无分子间作用力，热力学能热力学能只决定于只决定于内内动能动能? 如何求理想气体的如何求理想气体的热力学能热力学能 u( )uf TT理想气体理想气体u只与只与T有关有关Internal energy 热力学能l 热力学能热力学能是状态量是状态量 state propertyl U : : 广延参数广延参数 kJ l u : : 比参数比参数 kJ/kg l The chang

11、e in internal energy is frequently concerned. 热力学能热力学能总以变化量出现，总以变化量出现，热力学能热力学能零点人为定义零点人为定义2.2.宏观储存能：动能和势能宏观储存能：动能和势能Macroscopic energy-kinetic energy potential energy They are related to some outside reference frame。Kinetic energy is the energy a system possesses as a result of its motion relative t

12、o some reference frame. It is denoted as Eke.Potential energy is the energy a system possesses as a result of its elevation in a gravitational field. 22mcEkemgzEpOn a unit mass basisgzcueeuepke221mgzmcUEEUEpke223. Total energy(系统的总能量系统的总能量 )1. Heat transfer and heat（传热和热量）（传热和热量） 热量是以温差为推动力时，系统与外界之间

13、传递的能量热量是以温差为推动力时，系统与外界之间传递的能量 Heat is energy interaction between a system and its surroundings if its driving force is temperature difference only.(1) Heat is energy in transition. （热量是传递中的能量）（热量是传递中的能量） Eg. The baked potato contains energy, this energy is heat transfer only as it passes through the

14、 skin of potato to reach the air. Once in the surroundings, the transferred heat becomes part of the internal energy of the surroundings, or vise versa.(2) Heat is directional quantity. (热量是有方向的量热量是有方向的量) Heat addition (加热）加热） is the transfer of heat into a system. Heat rejection（放热）（放热） is the tran

15、sfer of heat out of a system. Formal sign convention: heat addition is positive, +. heat rejection is negative, -.(3)“Adiabatic ” means no heat is transferred （“ “绝热绝热” ”意味着没有热量传递。）意味着没有热量传递。）. (4) Heat transfer can change the state of the system. （传热可以引起系统状态的变化。）（传热可以引起系统状态的变化。） 2.功功 ( Energy trans

16、fer by Work ）Work is the energy transfer associated with a force acting through a distance. (1)功量也是传递中的能量功量也是传递中的能量 Work is also energy in transition. We can have push-pull work (e.g. in a piston-cylinder, lifting a weight), electric and magnetic work (e.g. an electric motor), chemical work, surface

17、 tension work, elastic work, a rotating shaft.(2)功量是有方向的量功量是有方向的量Work is also directional quantity. (It is stipulated: work done by a system is positive, +. That is, energy leaves the system. ( 系统作功，系统作功，功为正，也即能量离开系统。功为正，也即能量离开系统。 work done on a system is negative, -. That is, energy added to the sy

18、stem, （外界对系（外界对系统作功，功为负，即能量加入系统。）统作功，功为负，即能量加入系统。） (In defining work, we focus on the effects that the system (e.g. an engine) has on its surroundings. (3) Similarities of heat and work（热量和功量的相似之处）（热量和功量的相似之处） l 热量和功量是系统与外界能量交换的机理。热量和功量是系统与外界能量交换的机理。 Heat and work are energy transfer mechanisms betw

19、een a system and its surroundings. l 功量和热量都要穿越边界。功量和热量都要穿越边界。 Both heat and work are boundary phenomena.l 系统具有能量，但不是功量或热量系统具有能量，但不是功量或热量 System possess energy, but not heat or work. Example. Work vs. heat transfer - which is which? Can have one, the other, or both?It depends on what crosses the syst

20、em boundary. For example consider a resistor that is heating a volume of water. Both are associated with a process, not a state. 两者都是过程量，而不是状态量两者都是过程量，而不是状态量Both are path functions.（两者都是路径的函数）（两者都是路径的函数） If the change in a function is dependent on the route taken, then the function is known as a pat

21、h function. therefore: = or in terms of the specific volume, v: =21vvsysdvpmuIf system volume expands against a force, work is done by the system. u若系统克服一个力体积膨胀，则系统对外作功u If system volume contracts under a force, work is done on the system.u若系统受力而体积收缩，则外界对系统作功）21VVsysdVpC. Work is a path dependent pr

22、ocess Work depends on pathMust specify path if we need to determine work Along Path a: W = 2p0(2V0 - V0) = 2p0V0Along Path b: W = p0(2V0 - V0) = p0V0(5) Shaft work (轴功）轴功）Energy transmission with a rotating shaft.A force F acting through a moment arm r generate a torque TThis force acts through a di

23、stance sThen the shaft work is determined from(6) 弹簧力功弹簧力功 (Spring Work ） The length of a spring will change by a differential amount dx , on which a force F is applied. The work done is rTF rFTnrs)2(nTrnrTsFWsh2)2)(dxFWspringFor linear elastic springs, the displacement x is proportional to the forc

24、e applied. Substitute F into the previous equation and integrating yield 3. Conservation of Mass Principle(质量守恒定律质量守恒定律）质量守恒定律可表述为：在一个过程中，传递给系统的净质量质量守恒定律可表述为：在一个过程中，传递给系统的净质量等于系统总质量的净变化量（增加或减小）。等于系统总质量的净变化量（增加或减小）。 Net mass transfer to or from a system during a process is equal to the net change (in

25、crease or decrease) in the total mass of the system during the process.kxF )(212122xxkWspring（Total mass entering the system)-(Total mass leaving the system)=(Net change in mass with the system)systemoutinmmmddmmmsystemoutin/ 4 . Flow work and the energy of a flowing fluid. 流动功与流动工质所携带的能量流动功与流动工质所携带

26、的能量 (1) Flow work For open systems, some work is required to push the mass into or out of the control volume, the work is known as the flow work. (2) The energy of a flowing fluid When fluid enters or leaves a control volume (masses flow across their boundaries), energies are brought into the contro

27、l volume.pvgzcupveeupke221vpwflow )21(2gzcpvummEmass21()2massEmm upvcgz&5. A Useful New State Function Enthalpy（焓焓 ）Enthalpy, a state function, is defined as follows, h= u + pvThis energy is composed of two parts: the internal energy of the fluid (u) and the flow work (pv) associated with pushing th

28、e mass of fluid across the system boundary. 开口系统中开口系统中,焓指流动工质所携带能量的一部分，这部分焓指流动工质所携带能量的一部分，这部分能量取决于热力状态）能量取决于热力状态） In an open flow system, enthalpy is part of energy that is transferred across a system boundary by a moving flow, this part depends on the state. 2.3 The First Law of Thermodynamics and

29、Energy Balance equation for closed system热力学第一定律与闭口系统的热力学定律表达式热力学第一定律与闭口系统的热力学定律表达式 1.The Statement of the first Law of Thermodynamics The change in energy of a system is equal to the difference between thetotal energy entering system and that leaving the system. Expression for the first law热一律的文字表达

30、式热一律热一律: 能量守恒与转换定律能量守恒与转换定律=进入进入系统的系统的能量能量离开离开系统的系统的能量能量系统系统内部储存内部储存能量能量的的变化变化-Total energy entering the systemTotal energy leaving the systemChange in the total energy of the system=-Energy balance for closed system闭口系能量方程 W Q Q = U + Wq = u + w单位工质单位工质 The signs are important (正负号规定很重要正负号规定很重要) U

31、is the internal energy of the system Q is positive (+), if the heat transferred to the system Q is negative (-), if it is transferred from the system. W is positive (+), if the work is done by the system, W is negative (-), if work is done on the system. (2) Q and W are path dependent, U depends onl

32、y on the state of the system. Company Logo W Q一般式一般式 Q = dU + W Q = U + W q = du + w q = u + w单位工质单位工质适用条件：适用条件： 1）任何工质）任何工质 2) 任何过程任何过程Point function-Exact differentials- dPath function-Inexact differentials- 闭口系能量方程中的功功功 （ w） 是广义功是广义功 Generalized Work 闭口系与外界交换的功量闭口系与外界交换的功量 q = du + w准静态容积变化功准静态容积

33、变化功 pdv拉伸功拉伸功 w拉伸拉伸= - dl表面张力功表面张力功 w表面张力表面张力= - dA w = pdv - dl - dA +.闭口系能量方程的通式 q = du + w若在地球上研究飞行器若在地球上研究飞行器 q = de + w = du + dek + dep + w 工程热力学用此式较少工程热力学用此式较少准静态和可逆闭口系能量方程简单可压缩系简单可压缩系准静态过程准静态过程 w = pdv简单可压缩系简单可压缩系可逆过程可逆过程(For reversible process in simple compressible system) q = Tds q = du +

34、 pdv q = u + pdv热一律解析式之一热一律解析式之一Tds = du + pdv Tds = u + pdv热力学恒等式热力学恒等式In an open flow system, enthalpy is part of energy that is transferred across a system boundary by a moving flow, this part depends on the state. （开口系统，焓指流动工质所携带能量的一部分，这部分能（开口系统，焓指流动工质所携带能量的一部分，这部分能量取决于热力状态）量取决于热力状态） 对于闭口系统而言对于闭

35、口系统而言,焓没有物理意义焓没有物理意义,但它依旧是系统的一个但它依旧是系统的一个状态参数状态参数.门窗紧闭房间用电冰箱降温Take a room as the system 房间为房间为系统系统 An adiabatic closed system 绝热闭口系绝热闭口系Energy balance equation 闭口系能量方程闭口系能量方程QUW 0Q 0UW 0W T电电冰冰箱箱RefrigeratorIcebox门窗紧闭房间用空调降温Take the room as the system 以房间为以房间为系统系统 Closed System闭口系闭口系闭口系能量方程闭口系能量方程QU

36、W 0Q UQ W0W T空空调调 QQWAir-conditioner2. Corollaries of the First Law （热力学第一定律的推论）（热力学第一定律的推论）(1) Work done in any adiabatic (Q=0) process is path independent. (2) For a cyclic process heat and work transfers are numerically equal or 内能是状态参数，因此内能的变化量与过程的路径无关。内能是状态参数，因此内能的变化量与过程的路径无关。 Internal energy i

37、s a function of state, therefore the change in internal energy is independent of the path of processes.理想气体在任意过程中热力学能的改变量都等于相同温度范围理想气体在任意过程中热力学能的改变量都等于相同温度范围内定容过程中吸收的热量。若比热为常数，则内定容过程中吸收的热量。若比热为常数，则u=cvt Enthalpy is also a state property, its change is independent of process path.焓也是一个状态参数，它的改变量与过程的路

38、径无关焓也是一个状态参数，它的改变量与过程的路径无关 In an open flow system, enthalpy is part of energy that is transferred across a system boundary by a moving flow, this part depends on the state. （开口系统，焓指流动工质所携带能量的一部分，这部分能（开口系统，焓指流动工质所携带能量的一部分，这部分能量取决于热力状态）量取决于热力状态） 对于闭口系统而言对于闭口系统而言,焓没有物理意义焓没有物理意义,但它依旧是系统的一个但它依旧是系统的一个状态参数

39、状态参数. As shown in the figure, for ideal gas, if point 2、2、2” are on the same isothermal line（等温线）（等温线） 1-2 is constant volume process, 1-2 is constant pressure process and 1-2 is a random process. For reason that 2,2,2 is of the same temperature, u1-2=uu1-21-2=u=u1-2” 1-2” hh1-21-2=h=h1-21-2=h=h1-2”

40、1-2” Example:2.4 Energy Balance For Open System 开口系统的能量方程开口系统的能量方程1. Mass Balance for an Open System (Control Volume)开口系统的质量守恒方程开口系统的质量守恒方程systemoutinmmm 在一个过程中，传递给系统的净质量等于系统总质量的净变化量在一个过程中，传递给系统的净质量等于系统总质量的净变化量（增加或减小（增加或减小)。 Net mass transfer to or from a system during a process is equal to the net

41、change (increase or decrease) in the total mass of the system during the process.2. Energy Balance for an Open System 开口系统的能量方程开口系统的能量方程outinCVEEEQ)21(2gzcpvummE)21(12111gzchmEinQWscv,ex：inE:outEexCVsWW,)21(22222gzchmEoutexCVW,is called moving boundary work.)21()21(22221211,gzchmgzchmWWQEoutinexCVsC

42、V)2()2(22221211,.gzchmgzchmWWQdtdEoutinexCVsVCVC 2.5 Energy Equation For Steady Flow and Some Applications 稳定流动的能量方程及其应用稳定流动的能量方程及其应用1. 稳态稳定流动的定义稳态稳定流动的定义Definition of Steady-flow Steady-flow is a process during which a fluid flows through a control volume steadily.稳定就指流动空间中各点的状态不随时间而变化稳定就指流动空间中各点的状

43、态不随时间而变化 Steady means no change with time. (1) Steady and uniform (稳定和均匀稳定和均匀 ) Steady implies no change with time. Uniform implies no change with location over specific region.(2) Steady-flow process (稳定流动过程稳定流动过程) A process during which a fluid flows through a control volume steadily. That is, the

44、 properties remain the same at a fixed point during the entire process.2. Characteristics of steady-flow process 稳态稳定流动过程的特点稳态稳定流动过程的特点l In the control volume, the properties can only change from point to point, but at any point they remain constant during the entire process. 状态参数可沿流动空间中的各点而变化状态参数可沿

45、流动空间中的各点而变化,但对于其中任意一点但对于其中任意一点,状态状态参数在整个过程中保持不变参数在整个过程中保持不变.) lThe total mass or energy entering the control volume must be equal to the total mass or energy leaving it.流入控制体积的总质量和能量一定等于流出系统的总质量和能量流入控制体积的总质量和能量一定等于流出系统的总质量和能量. 0outinsystemmmm0CVEmmmoutin)21()21(22221211,gzchmgzchmWWQEoutinexCVSCV)()

46、(2112212212zzgcchhwqs 稳态稳定流动系统中不涉及移动边界功。稳态稳定流动系统中不涉及移动边界功。 No moving boundary work is involved.SWgzchmgzchmQ)21()21(12112222swdzgdcdhq221() Derivation of the Energy Balance Equation 方程的推导方程的推导（）（）The physical meaning of each item 每一项的物理意义每一项的物理意义 Q and W means the heat added to the system and work d

47、one by the system respectively. No moving boundary work is involved. (稳态稳定流动系统中不涉及移动边界功。）稳态稳定流动系统中不涉及移动边界功。） Q is positive (+), if the heat is transferred to the system. Q is negative (-), if it is transferred from the system. W is positive (+), if the work is done by the system, W is negative (-),

48、if the work is done on the system. Physical interpretation for enthalpy. 方程直观地显示出焓的物理意义 In an open flow system, enthalpy is the amount of energy that is transferred across a system boundary by a moving flow which depends on the system states. This energy is composed of two parts: the internal energy

49、 of the fluid u the flow work pv associated with pushing the mass of fluid across the system boundary. vpwflowswzzgcchhq)()(2112212212(3) Technical work 技术功技术功 The kinetic energy, potential energy and shaft work can be used easily. we define Technical work is equal to shaft work if kinetic and poten

50、tial energy is negligible.如果动能和势能可以忽略不计，那么技术功就等于轴功Then, the energy balance equation can be rewritten astwdhqswdzgdcdhq221zgcwwst22wduqwduwdht)(pvdwwt并非所有的膨胀功在技术上都是可用的并非所有的膨胀功在技术上都是可用的Not all moving boundary work can be usedtechnically.For example.For reversible processpdvw )(pvdwwt)(pvdpdvwtvdppdvpdvwtvdpwtpvwt12Substitute it into We get4. Some ste