Chapter 4 The First Law of T hermodynamics

1. Chapter 4 The First Law of Thermodynamics Steam power still reigns on the Cumbres and Toltec Scenic Railway in Colorado and New Mexico.

2. 4. The First Law of Thermodynamics • Conservation of Energy • Energy Balance • = Energy transferred across system boundary • ECV = Energy contained within system boundary Chapter 4 The First Law of Thermodynamics

3. system boundary QOUT QIN WIN or WOUT 4.1 Closed Systems • Mass Balance • dmCV/dt = 0 • mCV = constant • Energy Balance • ECM = U + KE + PE • KE = mCMv2/2gC • PE = mCMzg/gC • Mass does not cross system boundary • Energy crosses system boundary. Chapter 4 The First Law of Thermodynamics

4. 4.2 Open (Control Volume) Systems • Denote with CV subscript (e.g., mCV) • Mass and energy cross system boundary • On the following slides, • Compare combustion in open and closed systems • See a gas turbine that is analyzed as an open system Chapter 4 The First Law of Thermodynamics

5. 4.2 Open (Control Volume) Systems PowerPoint frozen? Click here and try again Chapter 4 The First Law of Thermodynamics

6. 4.2 Open (Control Volume) Systems PowerPoint frozen? Click here and try again Chapter 4 The First Law of Thermodynamics

7. 4.2.1 Conservation of Mass • Rate Basis • Time Interval • Useful Relations • = Volumetric flow rate [m3/s or ft3/s] • AX = cross-sectional flow area [m2 or ft2] Chapter 4 The First Law of Thermodynamics

8. 4.2.2 Flow Work and Enthalpy • Mass crossing system boundary • Carries energy u + ke + pe per unit mass flow • Does flow work Pv per unit mass flow • Recall enthalpy, h = u + Pv • Total energy entering/leaving system due to mass transfer is u + ke + pe + Pv = h + ke + pe per unit mass flow. Chapter 4 The First Law of Thermodynamics

9. 4.2.3 The First Law • Change in energy for open system is sum of • Shaft work: Present if rotating shaft crosses boundary • Boundary (PdV) work: Present if dVCV/dt  0 • Heat Transfer • Energy transfer by mass transfer (u + ke + pe) Chapter 4 The First Law of Thermodynamics

10. 4.3 Steady-State Steady-Flow Processes • Steady-State (SS): where ( )CV is any property of the system (e.g., m or E) • Steady-Flow (SF): where ( )CV is any transfer across the system boundary (e.g., Q, W or m) . . . . . Chapter 4 The First Law of Thermodynamics

11. 4.3 Steady-State Steady-Flow Processes • Steady-State Steady-Flow (SSSF) = No changes with time • Mass Balance • If 1 stream (i.e., 1-inlet and 1-outlet) Chapter 4 The First Law of Thermodynamics

12. 4.3 Steady-State Steady-Flow Processes • SSSF Energy Balance • If 1 stream (i.e., 1-inlet and 1-outlet) and dividing by mass flow rate Chapter 4 The First Law of Thermodynamics

13. A nozzle converts high pressure, low speed flow to low pressure, high speed flow A diffuser converts high speed, low pressure flow to low speed, high pressure flow 4.3.1 Nozzles and Diffusers • On next page, see a nozzle in a turbojet engine Chapter 4 The First Law of Thermodynamics

14. 4.3.1 Nozzles and Diffusers PowerPoint frozen? Click here and try again Chapter 4 The First Law of Thermodynamics

15. 4.3.1 Nozzles and Diffusers • Common Assumptions • SSSF • No work or heat transfer • Neglect changes in pe • Energy Balance: Crossing out terms assumed 0 Chapter 4 The First Law of Thermodynamics

16. 4.3.2 Throttling • Throttling: Reduces Pressure • Common Assumptions: • SSSF • No work or heat transfer • Neglect changes in pe and ke • Energy Balance: Throttling Valve • Isenthalpic (h = constant) Process Chapter 4 The First Law of Thermodynamics

17. 4.3.3 Pumps, Fans, and Blowers • Pumps: Pressurize or move liquids • Fans & Blowers: Move air • Common Assumptions: • SSSF • No heat transfer • Neglect changes in pe and ke • EnergyBalance for fan & blower • EnergyBalance for pump (assuming ICL) Pump Schematic Chapter 4 The First Law of Thermodynamics

18. 4.3.4 Turbines • Turbine: Enthalpy  Shaft work • Used in • Almost all power plants • Some propulsion systems (e.g., turbofan and turbojet engines) • Working Fluid: • Liquids (e.g., hydro power plants) • Vapors (e.g., steam power plants) • Gases (e.g., gas power plants) Chapter 4 The First Law of Thermodynamics

19. 4.3.4 Turbines • Common assumptions for turbine: • SSSF • Adiabatic (q = 0) • Neglect kinetic and potential energies • Turbine energy balance (Single Stream) Chapter 4 The First Law of Thermodynamics

20. 4.3.4 Compressors • Compressor: Shaft work  Increase pressure & enthalpy of vapor or gas • Often like turbine run in reverse • Used in • Gas power plants (e.g., gas turbine engine) • Turbo propulsion systems (e.g., turbofan and turbojet engines). • Industry (e.g., supply high pressure gas) • Working Fluids • Gas • Vapor • Not Liquid (pump used) Chapter 4 The First Law of Thermodynamics

21. 4.3.4 Compressors • Common assumptions for compressor: • SSSF • Adiabatic (q = 0) • Neglect kinetic and potential energies • Compressor energy balance Chapter 4 The First Law of Thermodynamics

22. 4.3.5 Heat Exchangers • Allows heat transfer from one fluid to another without mixing • Example: Car Radiator Chapter 4 The First Law of Thermodynamics

23. 4.3.5 Heat Exchangers in Steam Power Plant PowerPoint frozen? Click here and try again Chapter 4 The First Law of Thermodynamics

24. 4.3.5 Heat Exchangers • Common Assumptions • SSSF • Externally adiabatic • Neglect kinetic and potential energies • Energy Balance Chapter 4 The First Law of Thermodynamics

25. 4.3.6 Mixing Devices • Combine 2 or more streams • Common in industrial processes • Common assumptions • SSSF • Adiabatic • Neglect kinetic and potential energies • Energy Balance (Streams 1 & 2 mixing to form 3) Chapter 4 The First Law of Thermodynamics

26. 4.3.6 Mixing Devices PowerPoint frozen? Click here and try again Chapter 4 The First Law of Thermodynamics

27. 4.4 Transient (Unsteady) Analysis • Typically open system not at steady state • Tank Filling • Tank Emptying • Mass Balance: • Energy Balance: Chapter 4 The First Law of Thermodynamics

28. 4.4.1 Uniform State Uniform Flow (USUF) • Uniform State: All properties uniform across system at any instant in time • Uniform Flow: All mass flow properties at each inlet and outlet are uniform across the stream • Neglect kinetic and potential energies • Mass Balance: • Energy Balance: Chapter 4 The First Law of Thermodynamics

29. 4.4.2 Tank Filling • Simplest USUF analysis: • No outlet flow • Assume adiabatic • Mass Balance: • Energy Balance: Chapter 4 The First Law of Thermodynamics