Fundamentals of Heat Transfer (Basic Concepts)

Transcription

1 Fundamentals of Heat Transfer (Basic Concepts) 1

2 Topics to be covered History Thermodynamics Heat transfer Thermodynamics versus Heat Transfer Areas and Applications of Heat Transfer Heat Transfer problems 2

3 Historical Background Heat: Perceived to be something that produces in us a sensation of warmth Heat is fluid like substance called caloric that is mass-less, colorless, odorless, tasteless that can be poured from one body into another : Caloric Theory James P. Joule: Published in 1843 that heat was not a substance

4 Historical Background 4

5 Thermodynamics and Heat Transfer 5

6 Thermodynamics Thermodynamics deals with the science of motion (dynamics) and/or the transformation of heat (thermo) and energy into various other energy containing forms. System: It is the subject of the investigation. open system, closed system, isolated system Surroundings: Everything external to the system is the surroundings. Boundary: It is a closed surface surrounding a system through which energy and mass may enter or leave the system. 6

7 Thermodynamics Science of thermodynamics deals with the amount of heat transfer as system undergoes a process from one equilibrium state to another without any information concerning the nature of interaction or the time rate at which it occurs. Study of the relationship between heat, work and energy. Deals with the equilibrium and feasibility of process and properties of system, direction of change etc. 7

8 Energy The capacity to do work or transfer heat. Work The energy required to move an object against a force. Work = force distance 8

9 Energy can exist in various forms such as thermal mechanical kinetic potential electrical magnetic chemical nuclear Their sum constitutes the total energy E of a system. 9

10 Internal energy is the energy stored in a body. Internal energy is the sum of kinetic and potential energy of all particles in the body. Unit of internal energy: joule (J) It increases when the temperature of the body rises or when the body changes from solid to liquid or from liquid to gas. 10

11 Temperature is a measure of velocity and hence kinetic energy of the molecule of the system. When all the molecular motion ceases and there is no motion energy within the molecules, the temperature of the molecules is absolute zero. 11

12 Heat According to the modern or dynamic theory of heat: Heat is a form of energy. The energy in transit is termed as heat. Energy flow due to temperature difference is called heat. The mean kinetic energy per molecule of the substance is proportional to its absolute temperature. Unit of heat: joule (J) 12

13 Facts Whenever there is an exchange of heat, heat is consumed (heat lost by the hot body is always equal to heat gained by the cold body). The heat flow takes place from high to lower temperature. The substances expand on heating. In order to change the state of a body from solid to liquid to gas, certain amount of heat is required. When a body is heated or cooled, its weight does not change. 13

14 Heat Transfer Heat transfer is transmission of energy from one region to another region as a temperature difference between them. Whenever there exists a temperature difference in mediums or within medium, heat transfer must occur (always from the high temperature medium to the lower temperature one) Heat transfer stops when the two mediums reach the same temperature (thermal equilibrium). 14

15 When two bodies of different temperatures touch each other, energy is transferred from the hot body to the cold body until they reach the same temperature. (The bodies are then said to be in thermal equilibrium.) 15

16 Heat Transfer: Driving potential: Temperature difference Mass transfer: Driving potential: Concentration difference (concentrate on mass motion which result in changes in composition and are caused by variations in concentration of the various constituent species. This transfer is also known as diffusion ) 16

17 Heat Transfer Science of thermodynamics and fluid mechanics Heat transfer is a branch of thermal science which deals with analysis of rate of heat transfer and temperature distribution taking place in a system as well as the nature of heat transfer. Heat transfer cannot be measure directly but the effects produced by it can be observed and measured. 17

18 Note: Heat cannot be measured directly by an instrument as temperature is by a thermometer. 18

19 Heat Temperature It is a form of energy called thermal energy. Measured in kcal, BTU etc. Colorimeter is used for measurement. Extensive property It is a thermal state of a body which distinguishes a hot body from a cold body It is measure of degree of hotness or coldness of system Measured in 0 C, 0 F, K etc. Thermometer is used for measurement. Intensive property Extensive properties (i.e. mass) are dependent upon the amount of a substance, while intensive properties (i.e. density) are independent of quantity. 19

20 Difference between Thermodynamics and Heat Transfer 20

21 Interested in how long it takes for the hot coffee in a thermos bottle to cool to a certain temperature which can not be determined from a thermodynamic analysis alone 21

22 Consider the cooling of a hot steel bar placed in water bath. Thermodynamic analysis will predict the final equilibrium temperature of the composite system comprised by steel bar-water combination but it will not predict how long it takes to reach this equilibrium condition or what the temperature of bar will be after a certain length of time before the equilibrium condition is attained. 22

23 But heat transfer study will predict the temperature of both the bar and water as a function of time. That is the temperature at all points of interest within the bar or temperature at any specific point (such as at the center of the bar where it is the highest) at any time can be predicated. Also the instantaneous heat transfer rate can be predicated from all or from any part of the surface of the bar at any time. Heat transfer studies mainly require the knowledge of thermodynamics, fluid mechanics, physics and mathematics. 23

24 Thermodynamics It is concerned with equilibrium states and precludes existence of temperature gradient It helps to determine quantity of heat and work interactions when a system changes from one equilibrium state to another but do not provide information on nature of interaction and time rate at which interaction occurs u = Q ± W We can not obtain temperature profile Heat Transfer It is concern with nonequilibrium states. Temperature gradient must exist for exchange of heat It helps to predict rate at which energy is exchanged and also predicts temperature distribution as a function of coordinates and time within regions of matter We can obtain temperature profile 24

25 Summary: 1. In thermodynamics, no consideration is given to time or temperature difference required to bring about the transfer of heat energy and whether or not there is uniform temperature within the thermodynamic system. 2. The subject of heat transfer seeks to provide answer to the question such as Possibility of removal or addition of heat at a desire rate Temperature distribution existing within the system Amount of heat to be transferred Time taken (duration of heating and cooling) for a certain duty and surface area required to accomplish that duty. 25

26 Law of Thermodynamics Thermodynamic entails four laws 1. Zeroth Law of thermodynamics 2. First Law of thermodynamics 3. Second Law of thermodynamics 4. Third Law of thermodynamics 26

27 Zeroth Law of Thermodynamics The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. 27

28 First Law of Thermodynamics The first law, also known as Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system. 28

29 Second Law of Thermodynamics The second law of thermodynamics states that the entropy of any isolated system always increases. 29

30 Second law of Thermodynamics indicates the limit of converting heat into work and introduces the principle of increase of entropy. OR It states that "Heat will flow naturally from one reservoir to another at a lower temperature but not in opposite direction without assistance. (This law established the direction of energy transport). 30

31 Third Law of Thermodynamics The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature: The entropy of a perfect crystal at absolute zero is exactly equal to zero 31

32 Zeroth law of Thermodynamics deals with thermal equilibrium and establishes a concept of Temperature. First law of Thermodynamics throws light on concept of internal energy. Second law of Thermodynamics Thermal energy is transferred whenever a temperature gradient exists and the free flow of heat energy is always from a higher temperature to a lower temperature in accordance with the second law of thermodynamics.

33 Third law of Thermodynamics defines absolute zero of entropy. 33

34 Importance of Heat Transfer Heat transfer study is carried out for the follows purpose: 1. To estimate the rate of flow of energy as heat through the boundary of a system under study (both under steady and transient conditions). 2. To determine the temperature field under steady and transient conditions 34

35 Application Areas of Heat Transfer Energy production and Conversion: For thermal design of boilers, steam turbines, condensers, gas turbines, cooling towers, feed heaters, internal combustion engine Have to make a detailed heat transfer analysis (amount of heat to be transmitted as well as the rate at which heat is to be transferred) Refrigeration and Air Conditioning: The thermal design of compressors, evaporators, incinerators involves an intensive study of heat transfer. 35

36 Application Areas of Heat Transfer Electric Machines: Insulations provide on electric wires need a heat transfer analysis to avoid conditions which will cause overheating. heat transfer analysis must also be accounted in the design of electronic components, electric machines, transformers and bearings to avoid overheating and damage of equipment 36

37 Application Areas of Heat Transfer Civil Engineering: Design of buildings for the minimization of heat losses need heat transfer study. Construction of dams and other heavy structures, calculation of thermal expansion of suspension bridge and railway tracks. Manufacturing process: The casting of metals, extrusion, metal cutting and heat treatment of metals involve heat transfer study. 37

38 Application Areas of Heat Transfer Chemical and Petrochemical operations Environmental Engineering Earth Sciences Astronomy Agriculture and Food processing 38

39 Areas covered under discipline of Heat Transfer 1. Design of thermal and nuclear power plants including heat engines, steam generators, condensers and other heat exchange equipment, catalytic converters, heat shields for space vehicles, furnaces, electronic equipment etc. 2. Internal combustion engine 3. Refrigeration and air conditioning units 4. Design of cooling systems for electric motors, generators and transformers 5. Heating and cooling of fluids etc. in chemical operations 39

40 Areas covered under discipline of Heat Transfer 6. Construction of dams and structures; minimization of building-heat losses using improved insulation techniques 7. Thermal control of space vehicles 8. Heat treatment of metals 9. Dispersion of atmospheric pollutants 40

41 Application Areas of Heat Transfer 41

42 Importance of Study of Heat Transfer NPTEL Video Lecture - 1 Introduction on Heat and Mass Transfer (Time: 6.57 to 30.36) 42

43 Engineering Heat Transfer Heat transfer equipment such as heat exchangers, boilers, condensers, radiators, heaters, furnaces, refrigerators, and solar collectors are designed primarily on the basis of heat transfer analysis. The heat transfer problems encountered in practice can be considered in two groups: (1) rating and (2) sizing problems. 43

44 Engineering Heat Transfer An engineering device or process can be studied either experimentally (testing and taking measurements) or analytically (by analysis or calculations). The experimental approach has the advantage that we deal with the actual physical system, and the desired quantity is determined by measurement, within the limits of experimental error. However, this approach is expensive, time consuming, and often impractical. 44

45 Questions 1. The capacity to do work is. 2. Heat is measured in. 3. The form of energy that produced feeling of hotness is called. 4. is neither created nor destroyed it can only change one form to another. 5. What is the driving force for a heat transfer? 6. Heat transfer takes place according to law of thermodynamics. 7. How does the science of heat transfer differ from the science of thermodynamics? 8. How do rating problems in heat transfer differ from the sizing problems? 45