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Chapter 4 The First Law of Thermodynamics: Control Volumes

Chapter 4 The First Law of Thermodynamics: Control Volumes. 4-1 Thermodynamic analysis of Control Volume. 4-1-1 Conservation of Mass principle. 4-1-2 Conservation of Energy principle for Control Volume. Total energy of mass entering CV. Total energy of mass leaving CV.

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Chapter 4 The First Law of Thermodynamics: Control Volumes

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  1. Chapter 4 The First Law of Thermodynamics:Control Volumes

  2. 4-1 Thermodynamic analysis of Control Volume 4-1-1 Conservation of Mass principle 4-1-2 Conservation of Energy principle for Control Volume

  3. Total energy of mass entering CV Total energy of mass leaving CV Total energy crossing boundary + - The net energy change in energy of CV =

  4. 4-1-3 Flow Work

  5. 4-1-4 Total Energy of a Flowing Fluid On a unit-mass basis, the energy stored in fluid: Considering the flow work, we have The total energy of a flowing fluid (denoted θ):

  6. 4-1-5 Enthalpy Internal energy and flow work usually transferred at the same time in flowing fluid, so we define Professor J. Kestin proposed in 1966 that the term θ be called methalpy (from metaenthalpy, which means beyond enthalpy)

  7. 4-2 The Steady-Flow Process 4-2-1 Definition of Steady-Flow Process A process during which a fluid flows through a control volume steadily 4-2-2 Characteristics of Steady-Flow Process • No properties in thecontrol volume change with time • 2. No properties changes at the boundaries of the control volume with time

  8. 3. The heat and work interactions between a steady flow system and its surroundings do not change with time 4-2-3 Energy Equation of Steady-Flow Process From the above discussion we can conclude that: 2.The net change in energy ofCV isZero

  9. Total energy of mass leaving CV per unit time Total energy crossing boundary per unit time Total energy of mass entering CV per unit time = - Energy Equation of Steady-Flow

  10. Dividing the equation by yields Using subscript 1and subscript 2 for denoting inlet and exit states

  11. Work

  12. 3-3 Some Steady-Flow Engineering devices 3-3-1 Nozzles and Diffusers q = Δh + ΔV2 / 2 + gΔz + w q = 0 Δz = 0 w = 0

  13. 3-3-2 Steam Turbine from q = Δh + ΔV2 / 2 + gΔz + ws if q = 0; Δc = 0; Δz = 0 then ws = -Δh = h1 - h2

  14. 3-3-3 Throttling Valves From q = Δh + ΔV2 / 2 + gΔz + ws as q = 0; ws = 0; z = 0 ; ΔV2 / 2 =0 then Δh =0 That is h1 = h2

  15. 3-3-4 Mixing Chambers From q = Δh + ΔV2 / 2 + gΔz + ws as q = 0; ws = 0; z = 0 ; ΔV2 / 2 =0 then Δh =0 That is ∑hin = ∑ hexit

  16. 3-3-5 Heat exchangers Fromq = Δh + ΔV2 / 2 + gΔz + ws as q = 0; ws = 0; Δ z = 0;ΔV2 / 2 =0 Δh =0 That is ∑hin = ∑ hexit

  17. 3-4 Unsteady-Flow Processes 3-4-1 Model 1. The net energy transferred to the system: dQ ， e1dm1， p1v1dm1 2. The net energy transferred out of the system: dWs， e2dm2，p2v2dm2

  18. 3-4-2 Conversation of Energy

  19. Divide dτ on each side

  20. 3-5 Uniform-Flow Processes 1. At any instant during the process, the state of control volume is uniform 2. The fluid properties may differ from one inlet or exit to another, but the fluid flow at an inlet or exit is uniform and steady

  21. The end of this chapter Thanks for your attention!

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