First Law of Thermodynamics

1. First Law of Thermodynamics Objective • Objective • First Law of Thermodynamics • Energy Forms and Energy Transfer • …Work, Heat and Mass • Energy Balance for Closed System • Energy Balance for Open System • Energy Balance for Steady-Flow System

2. First Law of Thermodynamics • Conservation of Energy Principle • Energy can be neither created nor destroyed ;It can only change forms. • Energy can change many forms, but its total value keeps constant. • The perpetual-motion machine of the first kind can never be made.

4. First Law of Thermodynamics In 1843, at the age of 25, James Prescott Joule did a series of careful experiments to prove the equivalence of heat and work.

5. Forms of Energy Transfer {Energy Entering CV } － {Energy Leaving CV} ={Energy Accumulating in CV} • There are three forms of energy transfer: • Work:caused by a force acting through a distance • volume work: moving boundary work • Heat: caused by temperature difference between the system and surroundings. • Mass flow: mass flow in or out of the system serves as an energy transfer.

6. Energy Transfer by Work • Definitions – Energy transfer associated with force acting through a distance – Energy crossing the boundary of a closed system that is not heat must be work • 系统与外界相互作用而传递的能量，其全部效果为使外界物体改变宏观运动状态。 • 包含两个必要条件： • 确定有力作用在边界上，即系统与外界有相互作用，且力的不平衡势差无限小； • 2) 系统边界发生位移，即外界物体改变宏观运动状态。 二者缺一不可

7. Energy Transfer by Work 例1： 拔掉销钉后，系统向真空膨胀， 系统是否做功？ p Vacuum 例2： 刚性容器，加热后系统内压力 升高，系统是否向外做功？ p Q

8. Energy Transfer by Work • Units • – or J; usually kJ in SI • • Work examples • – Boundary work (e.g., a gas working against a moving piston) closed system • – Shaft work (e.g., a rotating crankshaft on a motor) • open system

9. Energy Transfer by Work 示功图 Work－Process function 功是过程量

10. Energy Transfer by Work • Work per unit mass • Sign Convention W > 0: work done by the system W < 0: work done on the system W = 0: no work

11. 例： 如图所示，气缸初始状态下： 现对气缸加热，使气体膨胀至： 已知：初始状态下，弹簧与气缸接触但不受力，弹簧刚度 活塞面积 大气压力 求：1) 气缸内终了压力和气体做的功？ 2) 若活塞与气缸间有摩擦力 存在， 气体做的功是多少？ Q 解： 取气缸内的气体为系统，气缸内壁为边界 准静态过程 终了状态为平衡状态，活塞两侧的受力相等

12. 利用示功图求解

13. 若活塞与气缸间存在摩擦，不可逆因素出现在系统外，为外部不可逆过程，这时气体若活塞与气缸间存在摩擦，不可逆因素出现在系统外，为外部不可逆过程，这时气体 需要抵抗外力 做功。 在工程实际中，将活塞和气缸作为整体考虑，注重整套装置，即活塞的有效输出功， 这时，摩擦成为内部不可逆因素，整套装置对外做功 但是由于内部摩擦的存在，使得气体必须克服摩擦做功，因此气体必须做功

14. Energy Transfer by Heat • Definitions —Energy transfer only by a temperature difference －Heat is energy in transformation；path function. • Units •Heat per unit mass

15. Energy Transfer by Heat • Sign Convention Q > 0: heat transfer to the system Q < 0: heat transfer from the system Q = 0: adiabatic T 1(T1,s1) 2(T2,s2) S － Entropy 熵 S ds

16. Comparison Between Heat and Work Heat and work are energy transfer mechanisms between a system and its surroundings. • Similarities: • Both are recognized at boundaries of a system • as they cross the boundaries. That is ,both are • boundary phenomenon. • System processed energy, but not heat and work. • Both are associated with a process, not a state. • Heat and work have no meaning at a state. • Both are path functions. Difference:

17. Forms of Energy Although energy can take a large number of forms, we can consider only: External and Internal. Kinetic External Total Energy =Internal+External =U+Ek+Ep Potential Physical Internal (U) State Property Chemical Nuclear Note: U is state property

18. Energy Balance for Closed System {Energy Entering CV } － {Energy Leaving CV} ={Energy Accumulating in CV} For Closed System difference 注意过程量和状态量数学表达的区别！

19. Energy Balance for Open System For Open System • Difference with closed system: • Energy in mass to and from system • Mass equilibrium • Including volumetric work and flow work with surrounding 推动质量进出系统的推动功，即保证质量流动耗费的功

20. Flow Work 1 １kg : dx1 2 Total Energy of Flowing Fluid 焓－工质流动时具有的与其热力状态有关的总能量。 Enthaply焓

21. Energy Balance for Open System {Energy Entering CV } － {Energy Leaving CV} ={Energy Accumulating in CV} • Energy Entering CV: • Heat-in • energy with mass in • flow work by upfluid

22. Energy Balance for Open System • Energy Leaving CV: • Shaftwork • energy with mass out • flow work by fluid

23. Energy Balance for Open System 适用于任何工质的任何流动过程

24. Energy Balance for Open System For Steady Flow System — fluid properties remain constant during the entire process — mass equilibrium — energy in=energy out

25. Energy Balance for Open System =0 适用于任何工质的任何稳定流动过程

26. Analysis of Energy Balance Shaft Work w 技术功

27. Analysis of Energy Balance 稳定流动系统能量方程 适用于任何工质的任何稳定流动过程

28. p 1 vdp 2 v Analysis of Energy Balance 对可逆过程 dp > 0,wt<0 work done on the system dp < 0,wt>0 work done by the system dp = 0, wt=0

29. Mechanical Energy Conservation For Reversible Process For Quasi-Equilibrium Process with Friction =0 Bonulli Equation

30. q 3 换热器 4 2 喷管 压缩机 气轮机 5 空气 1 Example 例2-2 绝热压缩 换热器吸热 绝热膨胀 空气流量 过程中忽略位能变化 求:1)压缩机功率；2)喷管出口流速；3)气轮机功率；4)整套装置功率 解： 工质在整套装置内的流动为稳定流动，应用稳定流动能量方程求解。

31. q 3 换热器 4 2 喷管 压缩机 气轮机 5 空气 1 1)压缩机功率 2)喷管出口流速,流经换热器和喷管 3)气轮机功率

32. 4)整套装置功率 或 将整套装置取为系统

33. End of 1st Law of Thermodynamics Well done!

34. 2.6 Second Law of Thermodynamics Natural process is directive. 2nd Law is used to determine the direction condition limitation of thermal process. *** We will use the 2nd Law as a tool to evaluate whether a process is possible. ***

35. Mechanical Heat Spontaneous Process • Heat Transfer Transferring heat to a paddle wheel will notcause it to rotate. A cup of hot coffee does not get hotter in a cooler room. • Others: • Gas Free Expansion • Mixture Process • Combustion and Reaction Process

36. 2.6 Second Law of Thermodynamics 一切实际的热力过程都具有方向性，只能单独自动地朝一个方向进行，这类过程称为自发过程；而其逆方向地过程不能单独自动进行，这类过程称为非自发过程。若要非自发过程得以实现，必须附加某些补充条件，付出一定的代价。

37. Statement of 2nd Law • Clausius: • It is impossible to transfer energy from a cooler to a hotter body as a sole effect (requires a heat pump or other device, which needs energy input) • 2. Kelvin-Planck: • It is impossible to operate a thermodynamic cycle to produce work with only heat transfer from a single reservoir (requires both heat addition and heat rejection)

38. High-Temperature Reservoir TH Low-Temperature Reservoir TL Statement of 2nd Law • Clausius: Reservoir: a “large” body that can absorb or supply heat without a “noticeable” temperature change

39. High-Temperature Reservoir TH Heat Engine Statement of 2nd Law 2. Kelvin-Planck

40. Statement of 2nd Law 3. 2nd perpetual-motion machine NEVER be made

41. Second Law of Thermodynamics 热力学第二定律的实质: 自发过程是不可逆的； 若要非自发过程得以实现，必须伴随一个适当的 自发过程作为补充条件。

42. Spontaneous Process • Heat Transfer 若要实现热量由低温向高温的非自发过程，必须消耗功， 即配合功变热这个自发过程作为补充条件。

43. Mechanical Heat Mechanical Heat 自发过程 非自发过程 1.若要实现热转化功的非自发过程，必须配合向低温热源放热 这一自发过程，因此热机的效率必定小于1。 2.“功可以全部转化成热，但热不能完全变为功” 理想气体的等温膨胀 3.热变功的最高效率－Carnot Cycle

44. T TH TL S S1 S2 Carnot Cycle(Reversible) Ⅰ Isothermal Expansion Ⅱ Adiabatic Expansion Ⅲ Isothermal Compression Ⅳ Adiabatic Compression

45. Desired Output Thermal Efficiency = Required Input Net Work Output WO Qout = = =1－ Qin Qin Total Heat Input Thermal Efficiency Definition: Q1—heat transfer between cycle device and high-temerature medium at TH Q2—heat transfer between cycle device and Low-temerature medium at TL

46. 区别？ Thermal Efficiency • For Heat Engine(卡诺热机) High-Temperature Reservoir TH Heat Engine Low-Temperature Reservoir TL

47. Thermal Efficiency • For Refrigerator(卡诺制冷机) High-Temperature Reservoir TH Refrig- erator 制冷系数 Low-Temperature Reservoir TL

48. Thermal Efficiency • For Heat Pump(卡诺热泵) High-Temperature Reservoir TH Heat Pump 供暖系数 Low-Temperature Reservoir TL

49. Carnot Principles 1. All reversible cycles operating between TL and TH have the same efficiency (ηrev) . 2. For same TL and TH, reversible cycles have higher efficiencies than any irreversible cycles.

50. Conclusions of Carnot Principles • Carnot 循环热效率只取决于高温热源和低温热源的温度，即工质吸热和放热的温度。提高T1或降低T2均可提高热效率。 • 任何循环的热效率均小于1。 • T1=T2时，循环热效率为零，说明只从单一热源吸热是不可能把热转变成功的。 • 要提高实际装置的热效率，必须尽可能减少摩擦等不可逆功损失。