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ENGR 2213 Thermodynamics

ENGR 2213 Thermodynamics. F. C. Lai School of Aerospace and Mechanical Engineering University of Oklahoma. Second Law of Thermodynamics. Perpetual-Motion Machines. Any device that violates either the first or the second law of thermodynamics is called a perpetual-motion machine.

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ENGR 2213 Thermodynamics

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

  2. Second Law of Thermodynamics Perpetual-Motion Machines Any device that violates either the first or the second law of thermodynamics is called a perpetual-motion machine. 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. Boiler Generator Turbine Pump Condenser Second Law of Thermodynamics Perpetual-Motion Machines of the First Kind W QL

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

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

  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. Both the system and the surroundings are returned to their initial states at the end of the reversed process. Reversible processes actually do not occur in nature. They are merely idealizations of actual processes.

  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 Reversible Cycles Cycles that consist entirely of reversible processes. Carnot Cycle ► the best known reversible cycle. ► 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). 1 QH p 2 Wnet QL 4 3 v

  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). 1 QH p 4 Wnet QL 2 3 v

  13. Second Law of Thermodynamics Carnot Principles • The efficiency of an irreversible heat engine is • always less than that of a reversible one • operating between the same two reservoirs. • 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 TH QH QH Assume ηIrr > ηRev WIrr > WRev Rev HE Irrev HE QL, Irr < QL, Rev WIrr WRev QL, Irr QL, Rev Low-temperature Reservoir at TL

  15. Proof of the First Carnot Principle High-temperature Reservoir at TH QH QH WIrr - WRev Rev Ref Irrev HE Irrev Rev HE Ref WIrr QL, Irr QL, Rev QL,Rev – QL, Irr Low-temperature Reservoir at TL

  16. Second Law of Thermodynamics Carnot Principles • 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. Reservoir T1 A Reservoir T2 C B Reservoir T3 Second Law of Thermodynamics η = f(QH, QL) Since the energy reservoirs are characterized by their temperatures, η = f(TH, TL) A: B: C:

  18. Second Law of Thermodynamics f(T1, T3) = f(T1, T2) f(T2, T3) 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 Carnot Heat Engines The efficiency of Carnot heat engine is the highest efficiency a heat engine operating between the two reservoirs at temperature TH and TL can have.

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

  22. p 1 2 4 3 v Example 1 • 0.5 kg of air undergoes a Carnot cycle with η = 0.5. • Given the initial pressure p1 = 700 kPa, initial • volume V1 = 0.12 m3 and heat transfer during the • isothermal expansion process Q12 = 40 kJ, Find • the highest and the lowest temperatures in the • cycle. • (b) the amount of heat rejection. • (c) work in each process.

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

  24. Example 1 (continued) Carnot cycle QL = 20 kJ Process 1-2 T1 = T2 ΔU12 = Q12 – W12 W12 = Q12 = 40 kJ Process 2-3 Q23 = 0 ΔU23 = Q23 – W23

  25. Example 1 (continued) W23 = m(u2 – u3) = 0.5(423.7 – 208.8) = 107.5 kJ Process 3-4 T3 = T4 ΔU34 = Q34 – W34 W34 = Q34 = -20 kJ Process 4-1 Q41 = 0 ΔU41 = Q41 – W41 W41 = m(u4 – u1) = 0.5(208.8 – 423.7) = -107.5 kJ

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