ENGR 260 Section 6.5 – 6.8

1. ENGR 260Section 6.5 – 6.8

2. Heat Pump Refrigerator Heat Engine

3. Kelvin-Planck Statement • The Second Law of Thermodynamics • It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

4. Kelvin Planck Statement • Heat Engine must have a low temperature sink!

5. Clausius Statement of Second Law • It is impossible to construct a device that operates in a cycle and produces no effect other than to transfer heat from a low temperature body to a higher temperature body.

6. Clausius Statement • Heat Pumps and Refrigeration must have work input!

7. Perpetual Motion Machines Two Types: • PMM1 ~ violates the first law of thermo • PMM2 ~ violates the second law of thermo • Some PMMs actually violate both laws

8. Perpetual Motion Machines • PMM1 ~ violates the first law of thermo

9. Perpetual Motion Machines • PMM2 ~ violates the second law of thermo

10. Perpetual Motion Machines

11. Perpetual Motion Machines

12. Perpetual Motion Machines Quiz for Tuesday: Find an example of a perpetual motion machine Show it in class Tuesday Explain if it is a PMM1 or PMM2

13. Reversible and Irreversible Processes • Reversible process ~ a process that can be reversed without leaving any trace on the surroundings • the system and the surrounding are returned to their initial state • net work and heat exchange between process and surrounding is zero for the combined process

14. Reversible Processes

15. Reversible Processes

16. Reversible Processes Can a reversible process really occur? No! So why study them? 1) They are easy to analyze. 2) They are idealized models to predict theoretical limits of corresponding actual processes.

17. Reversible Processes • Deliver the most • Consume the least work

18. Reversible and Irreversible Processes • Reversible process ~ a process that can be reversed without leaving any trace on the surroundings • the system and the surrounding are returned to their initial state • net work and heat exchange between process and surrounding is zero for the combined process

19. Irreversibilities • Friction • Unrestrained expansion of a gas • Mixing of two fluids • Heat transfer through a finite temp differential • Electric Resistance • Inelastic deformation of solids • Chemical Reactions

20. Friction Energy supplied as work is converted to heat. Heat is transferred to bodies in contact. This is seen as a temperature rise. When reversed heat is not converted back to work.

21. Unrestrained expansion of a gas Only way to restore system: Compress to initial volume Transfer heat from gas to return to original temperature Involves transferring heat to work which violates 2nd law

22. Heat Transfer Violates 2nd Law Clausius Statement – cannot transfer heat from low temp body to high temp body without work

23. Clausius Statement • Heat Pumps and Refrigeration must have work input!

24. Internally/Externally Reversible • Internally Reversible: • No irreversibilities occur within system boundaries (quasi-equilibrium) • Externally Reversible: • No irreversibilities occur outside system boundaries • Totally Reversible: • No irreversibilites occur within system or its surroundings

25. Chapter 5 Example An adiabatic air compressor is to be powered by a direct-coupled adiabatic steam turbine that is also driving a generator. Steam enters the turbine at 12.5 MPa and 500oC at a rate of 25 kg/s and exits at 10 kPA and a quality of 0.92. Air enters the compressor at 98 kPa and 295 K at a rate of 10 kg/s and exits at 1 Mpa and 620 K. Determine the net power delivered to the generator by the turbine. Mass flow = 25 kg/sec Mass flow = 10 kg/sec hairout = 628.07 kJ/kg (Table A-17) hsteam in= 3343.6 kJ/kg (Table A-6) Quality = 0.92 hairin = 295.17 kJ/kg (Table A-17) hwaterout = 2392.5 kJ/kg (Table A-5)

26. Heat Engine Review • Heat engines are cyclic devices in that the working fluid returns to it original state at the end of each cycle. • Work is done by the fluid in part of the cycle and on the fluid during another part of the cycle. • Efficiency of a cycle is dependent on the processes that make up a cycle. • Efficiency can be maximized by using reversible processes.

27. Carnot Cycle • Proposed by a French engineer Sadi Carnot in 1824 • Theoretical heat engine • Comprised of four reversible processes. 2 isothermal and 2 adiabatic

28. Carnot Cycle • Consider a closed system containing gas in an adiabatic piston-cylinder assembly.

29. Reversible Isothermal Expansion • Process 1-2 • TH is constant. • Cylinder head in close contact with source at TH • Gas expands slowly doing work on surroundings • Reversible heat transfer process • Amount of heat transferred is QH

30. Reversible Adiabatic Expansion • Process 2-3 • Reservoir is removed, replaced with insulation • Gas expands doing work on surroundings • Temp drops from TH to TL • Frictionless piston and quasi-equilibrium • Reversible and adiabatic

31. Reversible Isothermal Compression • Process 3-4 • TL is constant. • Cylinder head in close contact with sink at TL • Piston is pushed with external force doing work • Reversible heat transfer process • Amount of heat rejected is QL

32. Reversible Adiabatic Compression • Process 4-1 • Reservoir is removed, replaced with insulation • Gas is compressed to original state • Temp rises from TL to TH • Frictionless piston and quasi-equilibrium • Reversible and adiabatic

33. Carnot P-V Diagram (Heat Engine)

34. Reverse Carnot Cycle (Refrigeration)

35. Carnot Principles • The efficiency of a irreversible heat engine is always less than a reversible one operating between the same two reservoirs. • The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.