1. ENGR 2213 Thermodynamics F. C. Lai School of Aerospace and Mechanical Engineering University of Oklahoma

2. Second Law of Thermodynamics Any device that violates either the first or the second law of thermodynamics is called a perpetual-motion machine. Perpetual-Motion Machines Perpetual-Motion Machines of the First Kind Devices that violates the first law of thermodynamics. Perpetual-Motion Machines of the Second Kind Devices that violates the second law of thermodynamics.

3. Second Law of Thermodynamics Perpetual-Motion Machines of the First Kind Turbine Boiler Condenser Pump Generator QLQL W

4. Second Law of Thermodynamics Perpetual-Motion Machines of the Second Kind Turbine Boiler Condenser Pump Q OUT

5. Second Law of Thermodynamics Perpetual-Motion Machines of the Second Kind Turbine Boiler Pump W Q IN

6. Second Law of Thermodynamics Cold Fusion

7. Second Law of Thermodynamics Cold Fusion http://www.ncas.org/erab/index.html

8. Second Law of Thermodynamics Reversible Process A process which can be reversed without leaving any trace on the surroundings. Reversible processes actually do not occur in nature. They are merely idealizations of actual processes. Both the system and the surroundings are returned to their initial states at the end of the reversed process.

9. Second Law of Thermodynamics Irreversibilities The factors that cause a process to be irreversible. ● Friction ● Unrestrained Expansion ● Mixing of Two Gases ● Heat Transfer across a Finite Temperature Difference ● Electrical Resistance ● Chemical Reactions

10. Second Law of Thermodynamics Cycles that consist entirely of reversible processes. Carnot Cycle ► the best known reversible cycle. Reversible Cycles ► proposed in 1824 by French engineer Sadi Carnot. ► composed of four reversible processes, two isothermal and two adiabatic.

11. Second Law of Thermodynamics Carnot Power Cycle 1. Reversible isothermal expansion (1-2). 2. Reversible adiabatic expansion (2-3). 3. Reversible isothermal compression (3-4). 4. Reversible adiabatic compression (4-1). p v 1 4 3 2 QHQH QLQL W net

12. Second Law of Thermodynamics Carnot Refrigeration Cycle 1. Reversible adiabatic expansion (1-2). 2. Reversible isothermal expansion (2-3). 3. Reversible adiabatic compression (3-4). 4. Reversible isothermal compression (4-1). p v 1 2 3 4 QLQL QHQH W net

13. Second Law of Thermodynamics Carnot Principles 1.The efficiency of an irreversible heat engine is always less than that of a reversible one operating between the same two reservoirs. 2.The efficiencies of all reversible heat engines operating between the same two reservoirs are the same. A violation of either statement results in the Violation of the second law of thermodynamics.

14. Proof of the First Carnot Principle High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH Q L, Rev W Irr Rev HE Irrev HE QHQH Q L, Irr W Rev W Irr > W Rev Q L, Irr < Q L, Rev Assume η Irr > η Rev

15. Proof of the First Carnot Principle High-temperature Reservoir at T H Low-temperature Reservoir at T L QHQH Q L, Rev W Irr Rev Ref Irrev HE QHQH Q L,Rev – Q L, Irr Irrev Rev HE Ref Q L, Irr W Irr - W Rev

16. Second Law of Thermodynamics Carnot Principles 2.The efficiencies of all reversible heat engines operating between the same two reservoirs are the same. The efficiency of a reversible engine is independent of the working fluid employed and its properties, the way the cycle is executed, or the type of reversible engine used.

17. Second Law of Thermodynamics η = f(Q H, Q L ) Since the energy reservoirs are characterized by their temperatures, η = f(T H, T L ) Reservoir T 1 Reservoir T 3 Reservoir T 2 A B C A: B: C:

18. Second Law of Thermodynamics f(T 1, T 3 ) = f(T 1, T 2 ) f(T 2, T 3 ) What kind of function f would be?

19. Second Law of Thermodynamics What kind of function Φ would be? Several functions are possible, and the choice is completely arbitrary. ► Lord Kevin first propose that Φ(T) = T. This equation partially defines the absolute temperature.

20. Second Law of Thermodynamics Efficiency of a Heat Engine The efficiency of Carnot heat engine is the highest efficiency a heat engine operating between the two reservoirs at temperature T H and T L can have. Carnot Heat Engines

21. Second Law of Thermodynamics Efficiency of a Heat Engine Maximize the Efficiency of a Real Engine η th < η th, rev = η th, rev > η th, rev Irreversible heat engines Reversible heat engines Impossible heat engines ► Supply heat at the highest possible temperature ► Reject heat at the lowest possible temperature

22. Example 1 0.5 kg of air undergoes a Carnot cycle with η = 0.5. Given the initial pressure p 1 = 700 kPa, initial volume V 1 = 0.12 m 3 and heat transfer during the isothermal expansion process Q 12 = 40 kJ, Find (a) the highest and the lowest temperatures in the cycle. (b) the amount of heat rejection. (c) work in each process. 1 2 3 4 p v

23. Example 1 (continued) T L = 292.7 K Carnot cycle

24. Example 1 (continued) Q L = 20 kJ Carnot cycle Process 1-2T 1 = T 2 ΔU 12 = Q 12 – W 12 W 12 = Q 12 = 40 kJ Process 2-3Q 23 = 0 ΔU 23 = Q 23 – W 23

25. Example 1 (continued) Process 3-4T 3 = T 4 ΔU 34 = Q 34 – W 34 W 34 = Q 34 = -20 kJ Process 4-1Q 41 = 0 ΔU 41 = Q 41 – W 41 W 23 = m(u 2 – u 3 ) W 41 = m(u 4 – u 1 ) = 0.5(423.7 – 208.8) = 107.5 kJ = 0.5(208.8 – 423.7) = -107.5 kJ