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Lec 11: Flow work, energy transport by mass, first law of thermodynamics

2. For next time:Read:

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Lec 11: Flow work, energy transport by mass, first law of thermodynamics

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    1. 1 Lec 11: Flow work, energy transport by mass, first law of thermodynamics

    2. 2 For next time: Read: § 5-2 to 5-3 HW 6 due Wednesday, October 8, 2003 Outline: Flow work Conservation of energy Open and closed systems Important points: Know how to calculate start and end states Know how to find the path between states Know when to apply the various forms of the conservation of energy equation

    3. 3 Flow work For open systems, obviously work must be done to move the fluid into and out of the control volume. It is a form of boundary work.

    4. 4 Flow work The force on the fluid element F=PA But AL is V, so W=PV or

    5. 5 Energy of fluid in closed system Energy of a simple, compressible system is

    6. 6 Energy of fluid in open system Now there is a fourth term: the flow energy Where Pv + u is just h, so

    7. 7 Energy transport by mass Now that we know the energy per unit mass ?=h+ke+pe for an open system, the energy transported in and out with the mass is just the product of the mass and the energy:

    8. 8 Conservation of Energy First Law of Thermodynamics The net change (increase or decrease) in the total energy (?Esystem) of a system during a process is equal to the difference between the total energy transferred in or entering (Ein) and the total energy transferred out or leaving the system during that process.

    9. 9 For a simple, compressible system, and a change in E is Conservation of Energy First Law of Thermodynamics

    10. 10 The kinetic energy is given by:

    11. 11 Energy has units of force times distance. The energy change in accelerating a mass of 10 kg from Vi= 0 to Vf = 10 m/s is?

    12. 12 NOTE THE CONVERSION TO GET FROM m2/s2 to kJ/kg

    13. 13 TEAMPLAY

    14. 14 Gravity is another force acting on our system. It shows up in the potential energy change.

    15. 15 TEAMPLAY

    16. 16 Internal energy…..

    17. 17 Energy can be transferred into (or out of) the system in three ways: Heat interactions (called heat transfer) Work interactions Mass flow (carrying energy with the mass). Conservation of Energy First Law of Thermodynamics

    18. 18 Conservation of Energy First Law of Thermodynamics

    19. 19 Adiabatic means simply that there is no heat transfer, or Q=0. Consider a closed, adiabatic system. Conservation of Energy First Law of Thermodynamics

    20. 20 Now we have or in differential form Conservation of Energy First Law of Thermodynamics

    21. 21 So for an adiabatic closed system the work is equal to a path independent quantity, which means adiabatic work is independent of path. This is another form of the first law; “For all adiabatic processes between two specified states of a closed system, the work is the same, regardless of the nature of the closed system and the details of the process.” Conservation of Energy First Law of Thermodynamics

    22. 22 TEAMPLAY

    23. 23 Let us replace Qin- Qout by a single Q that represents the sum of all heat interactions during the process. Remember--if heat is transferred in it is positive--if transferred out it is negative. Conservation of Energy First Law of Thermodynamics

    24. 24

    25. 25 The first law is now simply Note that heat transferred to the system (+) or work done on the system (-) both will raise the energy of the system. Conservation of Energy First Law of Thermodynamics

    26. 26

    27. 27 Stationary means not moving--so ?PE and ?KE are zero and the first law becomes Conservation of Energy First Law of Thermodynamics

    28. 28 TEAMPLAY Solve problem 5-15.

    29. 29

    30. 30

    31. 31 TEAMPLAY A closed system undergoes a cycle consisting of two processes. During the first process, 40 Btu of heat is transferred to the system while the system does 60 Btu of work. During the second process, 45 Btu of work is done on the system. Determine the heat transfer during the second process. Determine the net work and net heat transfer of the cycle.

    32. 32 TEAMPLAY Solve problem 5-47

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