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ENTC 370: Announcements

ENTC 370: Announcements. Homework assignment No.6: Assigned Problems: 6.4, 6.16, 6.21, 6.39, 6.44, 6.56, 6.62, 6.71, 6.78, 6.98, 6.129. Due Tuesday, Nov 4 th before 10:50 am For more information, go to: http://etidweb.tamu.edu/classes/entc370 Exam II Tuesday , November 25 th

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ENTC 370: Announcements

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  1. ENTC 370: Announcements • Homework assignment No.6: • Assigned Problems: • 6.4, 6.16, 6.21, 6.39, 6.44, 6.56, 6.62, 6.71, 6.78, 6.98, 6.129. • Due Tuesday, Nov 4th before 10:50 am • For more information, go to: • http://etidweb.tamu.edu/classes/entc370 • Exam II • Tuesday, November 25th • Chapters: 6, 7 and parts of 11 • HWs: 6 – 9

  2. Society of Manufacturing Engineers SME chapter S099 WE GET HIRED WHAT: 3ndGeneral Meeting WHERE: THOM 112D WHEN: 7 pm, Monday, Nov 3rd , 2014 GUEST SPEAKER:

  3. Fasten Your Seatbelts • Chapter 7: Entropy • New concepts, new challenges • First, let’s review Chapter 6 • Maximum efficiency can only be obtained by a Carnot Engine (fully reversible) • Carnot engine lacks irreversibilities (no friction nor dissipative processes) • Each process (in the cycle) is itself reversible • Can be operated in either direction • Carnot engine is an idealized engine

  4. Cycle: Reversible or Irreversible • If a cycle is reversible, it is possible to return both the system and surroundings to their initial states

  5. Key Concepts • Clausius Inequality: Reflects limits set by 2nd Law • Entropy: Quantitative expression or property based on 2nd Law • The increase of entropy principle • Entropy is thermodynamic property • Isentropic processes

  6. Clausius Inequality

  7. Clausius Inequality If the system, as well as the heat engine, is required to undergo a cycle, then: Wccannot be > 0; otherwise it would contradict the 2nd law. A heat engine can only produce work as long as it exchanges energy with two energy reservoirs.

  8. What is ? For systems connected to large energy reservoirs (sources or sinks) at constant temperature

  9. Satisfies 2nd Law ClausiusInequality (It is used to dictate if processes are possible according to the 2nd Law)

  10. The Clausius TheoremReversible Cycle

  11. The Clausius TheoremIrreversible Cycle

  12. Clausius Statement Reversible Heat Engine: TH Irreversible Heat Engine: TL

  13. ClausiusInequality & Carnot Cycle Consider the Carnot heat engine cycle For a heat engine cycle

  14. Example • A power plant works between two reservoirs, one at 440 °C and the other one at 20° C. Heat from the source is 3150 kJ and 1950 kJ is transferred to the sink. Is that Clausius inequality satisfied, and is the cycle reversible or irreversible?

  15. Development of Entropyas a Concept and Property A quantity whose cyclic integral is zero and depends on the states only, and not the process path; thus it is a property dV is differential property

  16. Development of Entropy as a Concept and Property A quantity whose cyclic integral is zero and depends on the states only ,and not the process path, thus it is a property

  17. Entropy - the definition (3)

  18. Entropy - the definition (2)

  19. Special Case: Internally Reversible Isothermal Heat Transfer Process

  20. Example • A piston-cylinder device contains a liquid-vapor mixture of water at 300 K. During the constant pressure process, 750 kJ of heat is transferred to the water. As a result, part of the water vaporizes. Determine the entropy change.

  21. The Increase of Entropy Principle

  22. The Increase of Entropy Principle Entropy Change Entropy transfer

  23. The Increase of Entropy Principle Entropy Change Entropy transfer

  24. Adiabatic Closed System The increase of Entropy Principle In the absence of heat transfer, Q, entropy change is due to irreversibilities (i.e. friction, etc.) only

  25. Heat transfer to piston Moving piston w/ friction Heat transfer to piston Moving piston w/o friction (reversible) due to irreversibilities (friction etc) Entropy Transfer (associated w/ Q)

  26. Insulated piston (Q=0) Moving piston w/ friction Insulated piston (Q=0) Moving piston w/o friction (Adiabatic + Reversible = Isentropic) → most idealized case when there is no Heat transfer (adiabatic)

  27. Sgen > 0, and is called the entropy production.

  28. Surroundings isolated system Q=0 subsystems

  29. Reversible or Irreversible • SGEN > 0 Irreversible process • SGEN = 0 Reversible process • SGEN < 0 Impossible process

  30. Conclusions about Entropy • A process will occur as long as SGEN≥ 0 • Determines direction of process • That’s why we study the 2nd Law and discuss entropy! • Entropy is a non-conserved property • Entropy is not conserved as mass or energy • Entropy of the universe is continuously increasing  • Irreversibilities generate entropy • When irreversibilities are present: Sgen > 0 • When the process is reversible: Sgen = 0

  31. The Increase of Entropy Principle Entropy Change Entropy transfer

  32. The Increase of Entropy Principle Entropy Change Entropy transfer 0 If dS > 0, it implies heat transfer (Q) alone cannot explain an increase in entropy → irreversible action (i.e. friction) leads to entropy generation (Sgen)

  33. Example • A heat source at 800 K loses 2000 kJ of heat to a sink at (a) 500 K and (b) 750K. Determine which heat transfer process is more irreversible.

  34. Entropy (s) as a Thermodynamic Property

  35. T-s diagram for water

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