Thermodynamics Lecture Series

1. Thermodynamics Lecture Series Assoc. Prof. Dr. J.J. Entropy – Quantifying Energy Degradation Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA email: drjjlanita@hotmail.comhttp://www5.uitm.edu.my/faculties/fsg/drjj1.html

2. Quotes • “The principal goal of education is to create men and women who are capable of doing new things, not simply repeating what other generations have done” Jean Piaget “What we have to learn to do, we learn by doing” Einstein

3. Win Wout Mass in Mass out Qin Qout System E1, P1, T1, V1 To E2, P2, T2, V2 Review - First Law Properties will change indicating change of state How to relate changes to the cause Dynamic Energies as causes (agents) of change

4. Review - First Law Energy Balance Ein – Eout = Esys, kJ or ein – eout = esys, kJ/kg or

5. Review - First Law Mass Balance min – mout = msys, kg or

6. Review - First Law Energy Balance – Control Volume Steady-Flow Steady-flow is a flow where all properties within boundary of the system remains constant with time DEsys= 0, kJ; Desys= 0 , kJ/kg, DVsys= 0, m3; Dmsys= 0 or min = mout , kg

7. Review - First Law Mass & Energy Balance–Steady-Flow: Single Stream Mass balance Energy balance qin – qout+ win – wout = qout – qin, kJ/kg

8. Second Law Working fluid: Water High T Res., TH Furnace Purpose: Produce work, Wout, out qin = qH qin - qout = out - in qin = net,out + qout Steam Power Plant net,out net,out = qin - qout qout = qL Low T Res., TL Water from river An Energy-Flow diagram for a SPP

9. Second Law Thermal Efficiency for steam power plants

10. Second Law Working fluid: Ref-134a High T Res., TH, Kitchen room / Outside house qout– qin = in - out qout = qH Refrigerator/ Air Cond net,in qin = qL net,in = qout - qin Purpose: Maintain space at low T by Removing qL Low Temperature Res., TL, Inside fridge or house net,in = qH - qL An Energy-Flow diagram for a Refrigerator/Air Cond.

11. Second Law Coefficient of Performance for a Refrigerator

12. Second Law Working fluid: Ref-134a High Temperature Res., TH, Inside house Purpose: Maintain space at high T by supplying qH qout = net,in + qin qout = qH Heat Pump net,in net,in = qout - qin qin = qL net,in = qH - qL Low Temperature Res., TL, Outside house An Energy-Flow diagram for a Heat Pump

13. Second Law Coefficient of Performance for a Heat Pump

14. Second Law – Energy Degrade What is the maximum performance of real engines if it can never achieve 100%?? Factors of irreversibilities • less heat can be converted to work • Friction between 2 moving surfaces • Processes happen too fast • Non-isothermal heat transfer

15. Second Law – Dream Engine Carnot Cycle • Isothermal expansion • Slow adding of Q resulting in work done by system (system expand) • Qin – Wout = U = 0. So, Qin = Wout . Pressure drops. • Adiabatic expansion • 0 – Wout = U. Final U smaller than initial U. • T & P drops.

16. Second Law – Dream Engine Carnot Cycle • Isothermal compression • Work done on the system • Slow rejection of Q • - Qout + Win = U = 0. So, Qout = Win . • Pressure increases. • Adiabatic compression • 0 + Win = U. Final U higher than initial U. • T & P increases.

17. P, kPa qin 1 2 4 3 qout , m3/kg Second Law – Dream Engine Carnot Cycle P -  diagram for a Carnot (ideal) power plant

18. P, kPa qout 1 4 2 3 qin , m3/kg Second Law – Dream Engine Reverse Carnot Cycle P -  diagram for a Carnot (ideal) refrigerator

19. Second Law – Dream Engine Carnot Principles • For heat engines in contact with the same hot and cold reservoir • All reversible engines have the same performance. • Real engines will have lower performance than the ideal engines.

20. Second Law Working fluid: Not a factor High T Res., TH Furnace qin = qH P1: 1 = 2 = 3 Steam Power Plants net,out P2: real < rev qout = qL Low T Res., TL Water from river An Energy-Flow diagram for a Carnot SPPs

21. Second Law Working fluid: Not a factor High T Res., TH, Kitchen room / Outside house qout = qH Rev. Fridge/ Heat Pump net,in qin = qL Low Temperature Res., TL, Inside fridge or house An Energy-Flow diagram for Carnot Fridge/Heat Pump

22. Second Law – Will a Process Happen Carnot Principles • For heat engines in contact with the same hot and cold reservoir • P1: 1 = 2 = 3 (Equality) • P2: real < rev (Inequality) Processes satisfying Carnot Principles obeys the Second Law of Thermodynamics

23. Second Law – Will a Process Happen Clausius Inequality : • Sum of Q/T in a cyclic process must be zero for reversible processes and negative for real processes reversible real impossible

24. Second Law – Will a Process Happen Source Carnot SPP Processes satisfying Clausius Inequality obeys the Second Law of Thermodynamics qin = qH Steam Power Plant net,out qout = qL Sink

25. Entropy – Quantifying Disorder Source Entropy qin = qH Entropy Change in a process Steam Power Plant net,out qout = qL Sink

26. Entropy – Quantifying Disorder • Entropy • Quantitative measure of disorder or chaos • Is a system’s property, just like the others • Does not depend on process path • Has values at every state

27. Entropy – Quantifying Disorder Entropy • Quantifies lost of energy quality • Can be transferred by heat and mass or • generated due to irreversibilty factors: • Frictional forces between moving surfaces. • Fast expansion & compression. • Heat transfer at finite temperature difference.

28. Surrounding System Entropy – Quantifying Disorder Increase of Entropy Principle The entropy of an isolated (closed and adiabatic) system undergoing any process, will always increase. For pure substance:

29. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Clausius Inequality Entropy Change So: Then:

30. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Then entropy change for the closed system:

31. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Then entropy change for the closed system:

32. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Then entropy change for the closed system: For adiabatic process:

33. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Then entropy change for the closed system: For adiabatic reversible system: Isentropic or constant entropy process

34. irreversible 2 1 reversible Entropy – Quantifying Disorder Increase of Entropy Principle Proven • Consider the following cyclic process containing an irreversible forward path and a reversible return path Then entropy change for the closed system: For isolated (adiabatic & closed) system:

35. Entropy – Quantifying Disorder T – S diagram Area of curve under P – V diagram represents total work done Area of curve under T – S diagram represents total heat transfer Hence total heat transfer is Recall Then Area under T- S diagram is amount of heat in a process

36. T, C The infinite area dA = area of strip = Tds 1 TH qin=qH T TL 2 s, kJ/kgK ds Entropy – Quantifying Disorder T – s diagram Adding all the area of the strips from state 1 to state 2 will give the total area under process curve. It represents specific heat received for this process

37. Entropy – Quantifying Disorder Factors affecting Entropy (disorder) • Entropy will change when there is • Heat transfer (receiving heat increases entropy) • Mass transfer (moving mass changes entropy) • Irreversibilities (entropy will always be generated)

38. Entropy – Quantifying Disorder Entropy Balance • For any system undergoing any process, • Energy must be conserved (Ein – Eout = Esys) • Mass must be conserved (min – mout = msys) • Entropy will always be generated except for reversible processes • Entropy balance is (Sin – Sout + Sgen = Ssys)

39. Entropy – Quantifying Disorder Entropy Balance –Closed system Energy Balance: Entropy Balance:

40. Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Then:

41. In State 1 Out State 2 A2 << A1 Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Nozzle: Assume adiabatic, no work done, pemass = 0 where Entropy Balance

42. In Out Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Turbine: Assume adiabatic, kemass = 0, pemass = 0 where Entropy Balance

43. Qin Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Heat exchanger: energy balance; 2 cases where Assume kemass = 0,pemass = 0 Case 1 Case 2

44. Qin Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Heat exchanger: Entropy Balance where Case 1 Case 2

45. 3 1 2 Entropy – Quantifying Disorder Entropy Balance –Steady-flow device Mixing Chamber: where