8_INTRO TO THERMO MECH 101 mechanical eng.pptx

THERMODYNAMICS

1. INTRODUCTION TO THERMODYNAMICS FOR ENGINEERING MECHANICS PART ONE ENGINEERING MECHANICS (MECH 101) By Eng. S. Chinguwa Office number ED 23 IME

2. Thermo Dynamics The word thermodynamics is made up from two words: Thermo - from a Greek word meaning hot, or heat, and, Dynamics - the study of matter in motion - again from a Greek word meaning power, or Powerful.

3. Components thermodynamic studies Thus, the word thermodynamics means the study of heat related to matter in motion. Much of the study of engineering (or applied) thermodynamics is concerned with work producing or utilizing machines such as; engines , turbines and compressors together with the working substances used in such machines.

4. All thermodynamic systems require some working substance The working substances are, in general, fluids which are capable of deformation that they can readily be expanded and compressed. The working substance also takes part in energy transfer. For example it can receive -or reject heat energy or it can be the means by which work is done. Common examples of working substances used in thermodynamic systems are air and steam .

5. Laws of Thermodynamics W= Q where, W= Work transfer Q = Heat transfer If work is transferred during the cycle then, since there is no final change in the properties of the working substance, the energy to provide the work must have been transferred as heat and must exactly equal the work. Now, during some processes in a cycle, work will be done by the substance, while, during others, work is done on the substance. Similarly, during some processes heat is transferred out of the substance, while, during others, it is transferred into the working substance. Thus, for a cycle, since there is no net property change,

6. First law cont

7. Conservation of energy An engine which could provide work transfer without heat transfer would violate the first law because it would create energy.

8. Another statement of 1 st law Q = Heat transfer (stored inside the substance)  

9. 2 nd law of thermodynamics Natural heat transfer down a temperature gradient degrades energy to a less valuable level. A limit of value occurs when temperatures become equal and are thus in thermal equilibrium. (it directional flow). To violate 2 nd law you have to do external work.

10. Some implications of the 2 nd law of thermodynamics Heat transfer will only occur, and will always naturally occur, when a temperature difference exists, and always naturally down the temperature gradient. If, due to temperature difference, there is heat transfer availability, then work transfer is always possible. However, there is always some heat transfer loss. Temperature can be elevated but not without the expenditure of external energy. Elevation of temperature cannot occur unaided. There is no possibility of work transfer if only a single heat energy source or reservoir at a fixed temperature is available

11. By virtue of the second law of thermodynamics it is essential that all fuels should be used as efficiently as possible in order that fuel stocks may be preserved for as long as possible.

12. The 3 rd law of thermodynamics This law is concerned with the level of availability of energy. It is the concept that at the absolute zero of temperature the entropy of a perfect crystal of a substance is zero . Random translational , rotational and vibrational types of motion of the atoms and molecules making up the substance are reduced to zero , then the substance is considered to become of a perfect crystal form and the energies associated with these forms of motion will be reduced to zero . The substance is at ground state .

13. Rudolf Clausius ( 1822-1888) Statement It is impossible for a self-acting machine, unaided by any external agency, to convey heat from a body at a low temperature to one at a higher temperature. Note that the implication here is that, unless external energy is inputted.

14. Max Planck (1858-1947) : It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir, and do an equivalent amount of work on the surroundings . This statement implies the inability to completely convert heat transfer into work transfer . Kelvin-Planck: It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature. This combination of individual statements implies that it is not possible to produce work transfer if a heat engine system is connected only to a single heat energy source or reservoir which is at a single fixed temperature.

15. Sadi Carnot (17 96-1832) : Whenever a temperature difference exists, motive power can be produced . Lord Kelvin (1824-1907): We cannot transfer heat into work merely by cooling a body already below the temperature of the coldest surrounding objects . This implies that when a body reaches the temperature of the coldest surrounding objects no further heat-transfer is possible and hence no further work transfer is possible.

16. State point If two independent properties of a pure substance are defined, then all other properties, or the state of the substance, are also defined. If the state of the substance is known then the phase or mixture of phases of the substance are also known. E.g.

17. Phases of matter When a substance is of the same nature throughout its mass it is said to be in a phase. Matter can exist in three phases , solid , liquid and vapour or gas . If the matter exists in only one of these forms then it is in a single phase . If two phases exist together then the substance is in the form of a two-phase mixture .

18. Process When the state of a substance is changed by means of an operation or operations having been carried out on the substance, then the substance is said to have undergone a process. Typical processes are the expansion and compression of a gas or the conversion of water into steam. A process can be analyzed by an investigation into the changes which occur in the properties of a substance, and the energy transfers which may have taken place.

19. Cycle If processes are carried out on a substance such that, at the end, the substance is returned to its original state, then the substance is said to have been taken through a cycle. This is commonly required in many. engines. A sequence of events takes place which must be repeated and repeated. In this way the engine continues to operate. Each repeated sequence of events is called a cycle.

20. Diesel cycle illustrated

21. The constant temperature process This is a process carried out such that the temperature remains constant throughout the process. It is often referred to as an isothermal process. Particular cases of the constant temperature process will be dealt with in the text.

22. The constant volume & pressure processes The constant pressure process. This is a process carried out such that the pressure remains constant throughout the process. It is often referred to as an isobaric or isopiestic process. The constant volume process. This is a process carried out such that the volume remains constant throughout the process. It is often referred to as an isometric or isochoric process

23. Energy and Work Energy is defined as that capacity a body or substance possesses which can result in the performance of work. Here, work is defined, as in mechanics, as the result of moving a force through a distance. Work . If a system exists in which a force at the boundary of the system is moved through a distance, then work is done by or on the system. work is therefore a transient quantity being descriptive of that process by which a force is moved though a distance.

24. Work and Energy Cont Force on the piston remaining constant )  

25. Boyle's law During a change of state of any gas in which the mass and the temperature remain constant, the volume varies inversely as the pressure, or PV = C, a constant This is known as Boyle's Law, named after its discoverer, Robert Boyle

26. Boyle’s law example

27. Charles law and absolute temperature Consider now an experiment in which the pressure of a fixed mass of gas is kept constant while the volume and temperature are varied.

28. Charles’ law and absolute temp cont Further experiments at different pressures, with different masses and with different gases give a similar result. An interesting point however is that if all the straight lines obtained are produced back to cut the temperature axis, they all cut this axis at the same point.

29. Using the new origin The law of the graph becomes V = TC graph cuts the temperature axis at approximately -273 o C This is called absolute temperature scale. The temp is absolute zero of temperature . This is Charles’ law by Jacques A. Charles

30. Charles’ law example

31. Combining Boyle and Charles’ laws PV = C  

32. Characteristic equation of ideal gas

33. The specific heat capacities of a gas If a mass of gas m has its temperature changed from T 1 to T 2 then the change of internal energy can be determined by the expressions. If the temperature change is the same.

34. Polytropic process

35. Polytropic process

36. Polytropic equations Polytropic law Characteristic equation

37. Polytropic example 1

38. Polytropic example 2

39. Polytropic example 3

40. Polytropic example 4

41. Tutorial

42. Other processes Work and the hyperbolic process (isothermal)

43. Isothermal process

44. Constant pressure process

45. Adiabatic process  

46. Adiabatic process example