工程熱力學第二章熱力學第一定律

1、2.1Macroscopic and microscopic energy of system 系統(tǒng)系統(tǒng)的宏觀和微觀儲存能的宏觀和微觀儲存能 2.2Energy transfer by Heat, Work and Mass 熱量熱量、功量及質(zhì)量引起的能量傳遞、功量及質(zhì)量引起的能量傳遞-傳遞中的能量傳遞中的能量 2.3 The first law of thermodynamics and Energy balance equation for closed system 熱力學第一定律熱力學第一定律與閉口系統(tǒng)的能量平衡方程與閉口系統(tǒng)的能量平衡方程 2.4 Energy balance equ

2、ation of open system 開口開口系統(tǒng)的能量平衡系統(tǒng)的能量平衡方程方程2.5Energy balance for steady-flow systems and some applications 穩(wěn)態(tài)穩(wěn)態(tài)穩(wěn)定流動的穩(wěn)定流動的能量平衡及其應用能量平衡及其應用 Chapter 2. The first law of thermodynamics第二章 熱力學第一定律Essence of the First Law of Thermodynamics熱力學第一定律的本質(zhì) In 1909，C. Caratheodory finally established the first la

3、w. 最后最后完善熱一律完善熱一律本質(zhì)：本質(zhì)：能量能量轉(zhuǎn)換轉(zhuǎn)換及及守恒守恒定律定律在熱過程中的應用在熱過程中的應用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. 用多種實驗的一致性證明熱一律，于用多種實驗的一致性證明熱一律，于1850年發(fā)表年發(fā)表并得到公認并得到公認 能量守恒定律闡明能量既不能被創(chuàng)造，也不能消滅，它只能量守恒定律闡明能量既不能被創(chuàng)造，也不能消滅，它只 能從一種形式轉(zhuǎn)換成另一種形式，或從一個系統(tǒng)轉(zhuǎn)移到另一能從一種形式轉(zhuǎn)換成另一種形式，或從一個系統(tǒng)轉(zhuǎn)移到另一個系統(tǒng)，而其總量保持恒定。個系統(tǒng)，而其總量保持恒定。 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熱力學第一定律主要說明熱能與機械能在轉(zhuǎn)換過程中的熱力學第一定律主要說明熱能與機械能在轉(zhuǎn)換過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. 熱力學第一定律熱力學第一定律The First Law of Thermodynamics 系統(tǒng)能量的變化量等于加給的熱量與系統(tǒng)對外所作功系統(tǒng)能量的變化量等于加給的熱量與系統(tǒng)對外所作功量之差。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 “第一類永動機是不可能制成的第一類永動機是不可能制成的”Perpetual motion machine of the first kindPerpetual motion machine of the first kindQ鍋鍋爐爐汽輪機汽輪機發(fā)電機發(fā)電機給水泵給水泵凝凝汽汽器器WnetQout電電加加熱熱器器2.1 Macroscopic and microscopic energy of system 系統(tǒng)的宏觀和微觀儲存能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.熱力學能熱力學能U Microscopic energy-Internal energy U 熱力學能指系統(tǒng)所有微觀形式的能量之和熱力學能指系統(tǒng)所有微觀形式的能量之和. Internal energy is defined as the sum of all the microscopic forms of energy of a system. 熱力學能的微觀組成分子動能分子動能分子位能

9、分子位能 binding forces化學能化學能 chemical energy核能核能 nuclear energy熱力學能熱力學能microscopic forms of internal energy 移動移動 translation轉(zhuǎn)動轉(zhuǎn)動 rotation振動振動 vibration（1）Kinetic and potential energies of the molecules 分子的動能和勢能分子的動能和勢能 熱力學能熱力學能內(nèi)動能內(nèi)動能內(nèi)位能內(nèi)位能T, vTInternal Energy internal kinetic energyinternal potential e

10、nergy(2) Physical meaning of internal energy of IDEAL GAS 理想氣體熱力學能的物理解釋熱力學能熱力學能內(nèi)動能內(nèi)動能內(nèi)位能內(nèi)位能T, v理想氣體無分子間作用力，理想氣體無分子間作用力，熱力學能熱力學能只決定于只決定于內(nèi)內(nèi)動能動能? 如何求理想氣體的如何求理想氣體的熱力學能熱力學能 u( )uf TT理想氣體理想氣體u只與只與T有關(guān)有關(guān)Internal energy 熱力學能l 熱力學能熱力學能是狀態(tài)量是狀態(tài)量 state propertyl U : : 廣延參數(shù)廣延參數(shù) kJ l u : : 比參數(shù)比參數(shù) kJ/kg l The chang

11、e in internal energy is frequently concerned. 熱力學能熱力學能總以變化量出現(xiàn)，總以變化量出現(xiàn)，熱力學能熱力學能零點人為定義零點人為定義2.2.宏觀儲存能：動能和勢能宏觀儲存能：動能和勢能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(系統(tǒng)的總能量系統(tǒng)的總能量 )1. Heat transfer and heat（傳熱和熱量）（傳熱和熱量） 熱量是以溫差為推動力時，系統(tǒng)與外界之間

13、傳遞的能量熱量是以溫差為推動力時，系統(tǒng)與外界之間傳遞的能量 Heat is energy interaction between a system and its surroundings if its driving force is temperature difference only.(1) Heat is energy in transition. （熱量是傳遞中的能量）（熱量是傳遞中的能量） 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. (熱量是有方向的量熱量是有方向的量) Heat addition (加熱）加熱） is the transfer of heat into a system. Heat rejection（放熱）（放熱） 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 （“ “絕熱絕熱” ”意味著沒有熱量傳遞。）意味著沒有熱量傳遞。）. (4) Heat transfer can change the state of the system. （傳熱可以引起系統(tǒng)狀態(tài)的變化。）傳熱可以引起系統(tǒng)狀態(tài)的變化。） 2.功功 ( Energy transf

16、er by Work ）Work is the energy transfer associated with a force acting through a distance. (1)功量也是傳遞中的能量功量也是傳遞中的能量 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. ( 系統(tǒng)作功，系統(tǒng)作功，功為正，也即能量離開系統(tǒng)。功為正，也即能量離開系統(tǒng)。 work done on a system is negative, -. That is, energy added to the sys

18、tem, （外界對系（外界對系統(tǒng)作功，功為負，即能量加入系統(tǒng)。）統(tǒng)作功，功為負，即能量加入系統(tǒng)。） (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（熱量（熱量和功量的和功量的相似之處）相似之處） l 熱量和功量是系統(tǒng)與外界能量交換的機理。熱量和功量是系統(tǒng)與外界能量交換的機理。 Heat and work are energy transfer mechanisms betwe

19、en a system and its surroundings. l 功量和熱量都要穿越邊界。功量和熱量都要穿越邊界。 Both heat and work are boundary phenomena.l 系統(tǒng)具有能量，但不是功量或熱量系統(tǒng)具有能量，但不是功量或熱量 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 syste

20、m boundary. For example consider a resistor that is heating a volume of water. Both are associated with a process, not a state. 兩者都是過程量，而不是狀態(tài)量兩者都是過程量，而不是狀態(tài)量Both are path functions.（兩者都是路徑的函數(shù)）（兩者都是路徑的函數(shù)） If the change in a function is dependent on the route taken, then the function is known as a path

21、 function. therefore: = or in terms of the specific volume, v: =21vvsysdvpmuIf system volume expands against a force, work is done by the system. u若系統(tǒng)克服一個力體積膨脹，則系統(tǒng)對外作功u If system volume contracts under a force, work is done on the system.u若系統(tǒng)受力而體積收縮，則外界對系統(tǒng)作功）21VVsysdVpC. Work is a path dependent pro

22、cess 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 (軸功）軸功）Energy transmission with a rotating shaft.A force F acting through a moment arm r generate a torque TThis force acts through a dis

23、tance sThen the shaft work is determined from(6) 彈簧力功彈簧力功 (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 force

24、 applied. Substitute F into the previous equation and integrating yield 3. Conservation of Mass Principle(質(zhì)量守恒定律質(zhì)量守恒定律）質(zhì)量守恒定律可表述為：在一個過程中，傳遞給系統(tǒng)的凈質(zhì)量質(zhì)量守恒定律可表述為：在一個過程中，傳遞給系統(tǒng)的凈質(zhì)量等于系統(tǒng)總質(zhì)量的凈變化量（增加或減?。?。等于系統(tǒng)總質(zhì)量的凈變化量（增加或減小）。 Net mass transfer to or from a system during a process is equal to the net change (inc

25、rease 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. 流動功與流動工質(zhì)所攜帶的能量流動功與流動工質(zhì)所攜帶的

26、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 control

27、 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

28、 the mass of fluid across the system boundary. 開口系統(tǒng)中開口系統(tǒng)中,焓指流動工質(zhì)所攜帶能量的一部分，這部分焓指流動工質(zhì)所攜帶能量的一部分，這部分能量取決于熱力狀態(tài)）能量取決于熱力狀態(tài)） 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 a

29、nd Energy Balance equation for closed system熱力學第一定律熱力學第一定律與閉口系統(tǒng)的熱力學與閉口系統(tǒng)的熱力學定律表達式定律表達式 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熱一律的文

30、字表達式熱一律熱一律: 能量守恒與轉(zhuǎn)換定律能量守恒與轉(zhuǎn)換定律=進入進入系統(tǒng)的系統(tǒng)的能量能量離開離開系統(tǒng)的系統(tǒng)的能量能量系統(tǒng)系統(tǒng)內(nèi)部儲存內(nèi)部儲存能量能量的的變化變化-Total energy entering the systemTotal energy leaving the systemChange in the total energy of the system=-2.Energy balance for closed system閉口系能量方程 W Q Q = U + Wq = u + w單位工質(zhì)單位工質(zhì) (1) The signs are important (正負號規(guī)定很重要正負號

31、規(guī)定很重要) U 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 de

32、pends only 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單位工質(zhì)單位工質(zhì)適用條件：適用條件： 1）任何工質(zhì)）任何工質(zhì) 2) 任何過程任何過程Point function-Exact differentials- dPath function-Inexact differentials- 閉口系能量方程中的功功功 （ w） 是廣義功是廣義功 Generalized Work 閉口系與外界交換的功量閉口系與外界交換的功量 q = du + w準靜態(tài)容

33、積變化功準靜態(tài)容積變化功 pdv拉伸功拉伸功 w拉伸拉伸= - dl表面張力功表面張力功 w表面張力表面張力= - dA w = pdv - dl - dA +.閉口系能量方程的通式 q = du + w若在地球上研究飛行器若在地球上研究飛行器 q = de + w = du + dek + dep + w 工程熱力學用此式較少工程熱力學用此式較少準靜態(tài)和可逆閉口系能量方程簡單可壓縮系簡單可壓縮系準靜態(tài)過程準靜態(tài)過程 w = pdv簡單可壓縮系簡單可壓縮系可逆過程可逆過程(For reversible process in simple compressible system) q = Tds

34、 q = du + pdv q = u + pdv熱一律解析式之一熱一律解析式之一Tds = du + pdv Tds = u + pdv熱力學恒等式熱力學恒等式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. （開口系統(tǒng)，焓指流動工質(zhì)所攜帶能量的一部分，這部分能（開口系統(tǒng)，焓指流動工質(zhì)所攜帶能量的一部分，這部分能量取決于熱力狀態(tài)）量取決于熱力狀態(tài)） 對于

35、閉口系統(tǒng)而言對于閉口系統(tǒng)而言,焓沒有物理意義焓沒有物理意義,但它依舊是系統(tǒng)的一個但它依舊是系統(tǒng)的一個狀態(tài)參數(shù)狀態(tài)參數(shù).門窗緊閉房間用電冰箱降溫Take a room as the system 房間為房間為系統(tǒng)系統(tǒng) An adiabatic closed system 絕熱閉口系絕熱閉口系Energy balance equation 閉口系能量方程閉口系能量方程QUW 0Q 0UW 0W T電電冰冰箱箱RefrigeratorIcebox門窗緊閉房間用空調(diào)降溫Take the room as the system 以房間為以房間為系統(tǒng)系統(tǒng) Closed System閉口系閉口系閉口系能量方程

36、閉口系能量方程QUW 0Q UQ W0W T空空調(diào)調(diào) QQWAir-conditioner2. Corollaries of the First Law （熱力學第一定律的推論）（熱力學第一定律的推論）(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 內(nèi)能是狀態(tài)參數(shù)，因此內(nèi)能的變化量與過程的路徑無關(guān)。內(nèi)能是狀態(tài)參數(shù)，因此內(nèi)能的變化量與過程的路徑無關(guān)。 Internal

37、 energy is a function of state, therefore the change in internal energy is independent of the path of processes.理想氣體在任意過程中熱力學能的改變量都等于相同溫度范圍理想氣體在任意過程中熱力學能的改變量都等于相同溫度范圍內(nèi)定容過程中吸收的熱量。若比熱為常數(shù)，則內(nèi)定容過程中吸收的熱量。若比熱為常數(shù)，則u=cvt Enthalpy is also a state property, its change is independent of process path.焓也是一個狀態(tài)參數(shù)，它

38、的改變量與過程的路徑無關(guān)焓也是一個狀態(tài)參數(shù)，它的改變量與過程的路徑無關(guān) 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. （開口系統(tǒng)，焓指流動工質(zhì)所攜帶能量的一部分，這部分能（開口系統(tǒng)，焓指流動工質(zhì)所攜帶能量的一部分，這部分能量取決于熱力狀態(tài)）量取決于熱力狀態(tài)） 對于閉口系統(tǒng)而言對于閉口系統(tǒng)而言,焓沒有物理意義焓沒有物理意義,但它依舊是系統(tǒng)的一個但它依舊是

39、系統(tǒng)的一個狀態(tài)參數(shù)狀態(tài)參數(shù). As shown in the figure, for ideal gas, if point 2、2、2” are on the same isothermal line（等溫線）（等溫線） 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-

40、2=h=h1-2”1-2” Example:2.4 Energy Balance For Open System 開口開口系統(tǒng)的能量方程系統(tǒng)的能量方程1. Mass Balance for an Open System (Control Volume)開口系統(tǒng)的質(zhì)量守恒方程開口系統(tǒng)的質(zhì)量守恒方程systemoutinmmm 在一個過程中，傳遞給系統(tǒng)的凈質(zhì)量等于系統(tǒng)總質(zhì)量的凈變化量在一個過程中，傳遞給系統(tǒng)的凈質(zhì)量等于系統(tǒng)總質(zhì)量的凈變化量（增加或減?。ㄔ黾踊驕p小)。 Net mass transfer to or from a system during a process is equal to

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

42、tinexCVsCV)2()2(22221211,.gzchmgzchmWWQdtdEoutinexCVsVCVC 2.5 Energy Equation For Steady Flow and Some Applications 穩(wěn)定穩(wěn)定流動的流動的能量方程及其應用能量方程及其應用1. 穩(wěn)態(tài)穩(wěn)定流動的定義穩(wěn)態(tài)穩(wěn)定流動的定義Definition of Steady-flow Steady-flow is a process during which a fluid flows through a control volume steadily.穩(wěn)定就指流動空間中各點的狀態(tài)不隨時間而變化穩(wěn)定就指

43、流動空間中各點的狀態(tài)不隨時間而變化 Steady means no change with time. (1) Steady and uniform (穩(wěn)定和均勻穩(wěn)定和均勻 ) Steady implies no change with time. Uniform implies no change with location over specific region.(2) Steady-flow process (穩(wěn)定流動過程穩(wěn)定流動過程) A process during which a fluid flows through a control volume steadily. Tha

44、t is, the properties remain the same at a fixed point during the entire process.2. Characteristics of steady-flow process 穩(wěn)態(tài)穩(wěn)定流動過程的特點穩(wěn)態(tài)穩(wěn)定流動過程的特點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. 狀態(tài)參數(shù)可沿流動空間中的各點

45、而變化狀態(tài)參數(shù)可沿流動空間中的各點而變化,但對于其中任意一點但對于其中任意一點,狀態(tài)狀態(tài)參數(shù)在整個過程中保持不變參數(shù)在整個過程中保持不變.) lThe total mass or energy entering the control volume must be equal to the total mass or energy leaving it.流入控制體積的總質(zhì)量和能量一定等于流出系統(tǒng)的總質(zhì)量和能量流入控制體積的總質(zhì)量和能量一定等于流出系統(tǒng)的總質(zhì)量和能量. 0outinsystemmmm0CVEmmmoutin)21()21(22221211,gzchmgzchmWWQEoutine

46、xCVSCV)()(2112212212zzgcchhwqs 穩(wěn)態(tài)穩(wěn)定流動系統(tǒng)中不涉及移動邊界功。穩(wěn)態(tài)穩(wěn)定流動系統(tǒng)中不涉及移動邊界功。 No moving boundary work is involved.SWgzchmgzchmQ)21()21(12112222swdzgdcdhq221() Derivation of the Energy Balance Equation 方程的推導方程的推導（）The physical meaning of each item 每每一項的物理意義一項的物理意義 Q and W means the heat added to the system and

47、 work done by the system respectively. No moving boundary work is involved. (穩(wěn)態(tài)穩(wěn)定流動系統(tǒng)中不涉及移動邊界功。）穩(wěn)態(tài)穩(wěn)定流動系統(tǒng)中不涉及移動邊界功。） 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 negativ

48、e (-), if the work is done on the system. Physical interpretation for enthalpy. 方程直觀地顯示出焓的物理意義 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

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

50、d potential energy is negligible.如果動能和勢能可以忽略不計，那么技術(shù)功就等于軸功Then, the energy balance equation can be rewritten astwdhqswdzgdcdhq221zgcwwst22wduqwduwdht)(pvdwwt并非所有的膨脹功在技術(shù)上都是可用的并非所有的膨脹功在技術(shù)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 stea