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Lec 19: Entropy changes, relative pressures and volumes, work

2. For next time:Prepare for Midterm 2 on Thursday, November 6thOutline:Isentropic processes for ideal gasesInternal reversible workEntropy balance equationsImportant points:Try to identify the

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Lec 19: Entropy changes, relative pressures and volumes, work

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    1. 1 Lec 19: Entropy changes, relative pressures and volumes, work

    2. 2 For next time: Prepare for Midterm 2 on Thursday, November 6th Outline: Isentropic processes for ideal gases Internal reversible work Entropy balance equations Important points: Try to identify the “governing equations” and not get bogged down in all the special cases Understand how to use the Tds relationships Don’t forget to apply the 2nd Law when working problems with entropy or internal reversible processes

    3. 3 Isentropic processes of ideal gases with constant specific heats Before we had the situation where the specific heat could be considered constant,

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    16. 16 Work in an internally reversible flow system Earlier we had This was true for a quasiequilibrium process.

    17. 17 Work for an internally reversible flow system. Quasiequilibrium process--one for which departures from equilibrium are infinitesimally small. All states through which a system passes in a quasiequilibrium process may be considered to be themselves equilibrium states.

    18. 18 Work for an internally reversible flow system A reversible process must proceed through a series of equilibrium states. Otherwise, there would be a tendency for the system to change spontaneously, which is irreversible. Therefore, a quasiequilibrium process is an (internally) reversible process

    19. 19 Work for an internally reversible flow system Consider an internally reversible steady flow system: Second law

    20. 20 Work for an internally reversible flow system The first law (not limited to internally reversible processes at this point) says

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    22. 22 Work for an internally reversible flow process Combining the laws yields Rearranging

    23. 23 Work for an internally reversible flow process Now, use a Tds relation: Tds = dh – vdp

    24. 24 Work for an internally reversible flow process

    25. 25 In the absence of KE and PE effects, On a P-v diagram, Work for an internally reversible flow process

    26. 26 Work For open systems, For closed systems

    27. 27 Compressor work By using the relationship Pvn = constant (or Pvk = constant) and solving for v = , the expression or as the book uses can be integrated to get the expressions on p. 310

    28. 28 Entropy change for a closed system

    29. 29 Entropy change of an internally reversible process; heat transfer

    30. 30 TEAMPLAY

    31. 31 Entropy production and transfer

    32. 32 Example Problem

    33. 33 Look at possibilities on a Ts diagram...

    34. 34 Adiabatic Compression of R-134a

    35. 35 Adiabatic Compression of R-134a

    36. 36 Adiabatic Compression of R-134a

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    38. 38 TEAMPLAY

    39. 39 Entropy rate balance for control volumes

    40. 40 Entropy rate balance for control volumes

    41. 41 Entropy rate balance for control volumes

    42. 42 Entropy rate balance for control volumes

    43. 43 Entropy rate balance for control volumes

    44. 44 Entropy rate balance for control volumes

    45. 45 Entropy rate balance for control volumes

    46. 46 Look at adiabatic compression processes on a Ts diagram

    47. 47 Look at adiabatic expansion processes on a Ts diagram

    48. 48 Isentropic Processes (constant entropy)

    49. 49

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