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Chapter 2 The First Law Of Thermodynamics

Chapter 2 The First Law Of Thermodynamics. The First Law Of Thermodynamics. §2-1.The central point of first law §2-2. Internal energy and total energy §2-3.The equation of the first law §2-4.The first law for closed system §2-5.The first law for open system

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Chapter 2 The First Law Of Thermodynamics

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  1. Chapter 2 The First Law Of Thermodynamics

  2. The First Law Of Thermodynamics §2-1.The central point of first law §2-2. Internal energy and total energy §2-3.The equation of the first law §2-4.The first law for closed system §2-5.The first law for open system §2-6.Application of the energy equation

  3. §2-1.The central point of first law 1.Expression In a cyclic process, the algebraic sum of the work transfers is proportional to the algebraic sum of the heat transfers. Energy can be neither created nor destroyed; it can only change forms. The first law of thermodynamics is simply a statement of energy principle.

  4. §2-1.The central point of first law 2.Central point The energy conservation law is used to conservation between work and heat. Perpetual motion machines of the first kind.(PMM1) Heat: see chapter 1; Work: see chapter 1;

  5. §2-2.Internal Energy 1.Definition: Internal energy is all kinds of micro-energy in system. 2. Internal energy is property It include: • Kinetic energy of molecule (translational kinetic, vibration, rotational energy) • Potential energy • Chemical energy • Nuclear energy

  6. §2-2.Internal Energy 3.The symbol u: specific internal energy, unit –J/kg, kJ/kg ; U: total internal energy, unit – J, kJ; 4.Total energy of system E=Ek+Ep+U Ek=mcf2/2 Ep=mgz ΔE=ΔEk+ΔEp+ΔU per unit mass: e=ek+ep+u Δe=Δek+Δep+Δu

  7. §2-3. The equation of the first law 1. The equation ( inlet energy of system) – (outlet energy of system) = (the change of the total energy of the system) Ein-Eout=ΔEsystem

  8. Q W §2-4.The first law in closed system 1. The equation Ein-Eout=ΔEsystem

  9. §2-4.The first law in closed system Q-W=ΔEsystem=ΔU Q=ΔU+W Per unit mass: q= Δu+w dq=du+dw If the process is reversible, then: dq=du+pdv This is the first equation of the first law. Here q, w, Δu is algebraic.

  10. §2-4.The first law in closed system The only way of the heat change to mechanical energy is expansion of working fluid.

  11. §2-5. The first law in open system 1. Stead flow For stead flow, the following conditions are fulfilled: • The matter of system is flowing steadily, so that the flow rate across any section of the flow has the same value; • The state of the matter at any point remains constant; • Q, W flow remains constant;

  12. §2-5. The first law in open system 2. Flow work Wflow=pfΔs=pV wflow=pv p V

  13. §2-5. The first law in open system 3. 技术功 “ Wt” are expansion work and the change of flow work for open system. 4. 轴功 “ Ws” is “ Wt” and the change of kinetic and potential energy of fluid for open system.

  14. §2-5. The first law in open system 5. Enthalpy for flow fluid energy: U+pV +mcf2/2+mgz H =U+pV unit: J, kJ For Per unit mass: h=u+pv unit: J/kg, kJ/kg

  15. Q W §2-5. The first law in open system 6. Energy equation for steady flow open system , mcf12/2, mgz1 U1+p1V1 H1 U2+p2V2 H2 , mcf22/2, mgz2

  16. §2-5. The first law in open system

  17. §2-5. The first law in open system Per unit mass:

  18. §2-5. The first law in open system If neglect kinetic energy and potential energy , then: If the process is reversible, then: This is the second equation of the first law.

  19. Q W §2-5. The first law in open system 7. Energy equation for the open system Inlet flows Out flows 1 1 Open system 2 2 …… … … i j

  20. §2-5. The first law in open system Energy equation for the open system

  21. Q Wi §2-6. Application of The Energy Equation 1. Engine a). Turbines energy equation: Ein-Eout=Esystem=0 Wi=H2-H1 wi=h2-h1 , mcf12/2, mgz1 =0 U1+p1V1 H1 Q≈0 U2+p2V2 H2 mcf22/2, mgz2 =0

  22. H2 H1 Wt §2-6. Application of The Energy Equation 1. Engine b). Cylinder engine energy equation: Wt=H2-H1+Q=(U+pV) 2-(U+pV) 1 +Q Ek1, Ep1≈0 Q Ek1, Ep1≈0

  23. H2 H1 Wc Q≈0 §2-6. Application of The Energy Equation 2. Compressors Energy equation: Wc=- Wt =H2-H1 Ek1, Ep1≈0 Ek1, Ep1≈0

  24. §2-6. Application of The Energy Equation 3. Mixing chambers Energy equation: m1h1 + m2h2 -m3h3=0 Mixing water: m3h3 hot water: m2h2 Cold water: m1h1

  25. m3h3 m5h5 m２h２ m1h1 m4h4 m6h6 §2-6. Application of The Energy Equation 4. Heat exchangers Energy equation: (m1h1 + m2h2 + m3h3)-(m4h4 + m5h5 + m6h6)= 0

  26. h2 §2-6. Application of The Energy Equation 5. Throttling valves Energy equation: h1 -h2 =0 h1

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