Heat Transfer

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Heat Transfer

  1. 1. © Group 6 Heat Transfer 2013 Conduction, Convection and Radiation
  2. 2. © Group 6 Heat Transfer 2013 Heat Transfer A temperature difference must exist for heat transfer to occur. Heat is always transferred in the direction of decreasing temperature. • Temperature is a scalar • Heat flux is a vector quantity.
  3. 3. © Group 6 Heat Transfer 2013 ● Conduction- transfer of energy between objects that are in physical contact. ● Convection- transfer of energy between an object and its environment due to fluid motion. ● Radiation- transfer of energy to or from a body by means of the absorption of electromagnetic radiation. Heat energy only flows when there is a temperature difference from a warmer area to a cooler area. During heat transfer, thermal energy always moves in the same direction: HOT COLD
  4. 4. © Group 6 Heat Transfer 2013 Metals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms. When a metal is heated, the free electrons gain kinetic energy. How do metals conduct heat? heat This means that the free electrons move faster and transfer the energy through the metal. This makes heat transfer in metals very efficient. Insulators do not have free electrons and so they do not conduct heat as well as metals.
  5. 5. © Group 6 Heat Transfer 2013 How do non-metals conduct heat?
  6. 6. © Group 6 Heat Transfer 2013
  7. 7. © Group 6 Heat Transfer 2013 The rate of heat flux is linearly proportional to the temperature gradient. Fourier’s Law Equation :
  8. 8. © Group 6 Heat Transfer 2013 L Q k t1 t2 A For the steady state, unidirectional flow, Fourier’s Equation: Units metric English Q (Heat) A (Area) t (Temp) L (Thickness) (heat flux) (Temp.Gradient) F/in . F/ft2 k (thermal conductivity) Btu*in/f t2-F-hr W m In, ft
  9. 9. © Group 6 Heat Transfer 2013 Conversion of Units English x Metric
  10. 10. © Group 6 Heat Transfer 2013 Variation of Thermal Conductivity Thermal conductivity varies depending on temperature. Depending on the material of the conductor, as the temperature rises the thermal conductivity of the material often raises as well, increasing the flow of energy. Material Temperatur e T Volume V Density Pressure P Thermal Conductivity K Solids increase ------ ------ ------ increase Metals increase ------ ------ ------ decrease Liquids increase -------- decrease -------- decrease Gases increase decrease increase increase increase
  11. 11. © Group 6 Heat Transfer 2013 1. The conductivities of all homogeneous solid materials are relatively high; practically all good (heat) insulators are porous, cellular, fibrous, or laminated materials. 2. In general, the conductivity increases with the density. 3. With rare exceptions, the conductivity of insulating materials increases very materially with the temperature. 4. The absorption of moisture greatly impairs the insulating value of porous materials. Generalizations concerning conductivities of solids
  12. 12. © Group 6 Heat Transfer 2013 Sample Problem # 1 K= 80 W/m.k A= 5000 cm2 = 0.5m2 T1 = 150 C + 273 = 423 K T2 = 140 C + 273 = 413 K L = 2 cm = .02m Compute the amount of heat per second through an iron plate 2 cm thick and area of 5000 cm2 if one face has a temperature of 150 C and the other face is 140 C. The thermal conductivity for iron is 80W/m.k. Given Solution Q = (80 W/m—K * 0.50m2 )(10K/.02m) Q = 20000 J/s or 20 kW Q = kA(t1-t2)/L
  13. 13. © Group 6 Heat Transfer 2013 DAISY
  14. 14. © Group 6 Heat Transfer 2013 Conduction through plane wall
  15. 15. © Group 6 Heat Transfer 2013 Steady-state flow same area (A) different area (A)
  16. 16. © Group 6 Heat Transfer 2013 Any one section of the wall Thermal Conductance (c) Unit Conductance Thermal Resistance (R) Unit Resistance Composite plane wall Thermal Conductance (c) Unit Conductance Thermal Resistance (R) Unit Resistance
  17. 17. © Group 6 Heat Transfer 2013
  18. 18. © Group 6 Heat Transfer 2013 FILM COEFFICIENT(h) -film conductance, surface conductance Some examples of the surface coefficients:  Inside building walls, still air, design value h = 1.65 Btu/hr-ft2-°F  Outside walls, 15-mph, design value h = 6.00 Btu/hr-ft2-°F  Evaporating refrigerants in tube, typical value h = 200 Btu/hr-ft2-°F  Condensing steam in tube, typical value h = 2000 Btu/hr-ft2-°F
  19. 19. © Group 6 Heat Transfer 2013 maj
  20. 20. © Group 6 Heat Transfer 2013 Conduction through a hollow sphere Consider a hollow sphere having inner radius ri and outer radius ro. The inner and outer surface temperature of the sphere is maintained at ti and to (ti>to) respectively. From the FOURIER’S Law we can write
  21. 21. © Group 6 Heat Transfer 2013 To calculate the total rate of heat transfer, we have to integrate the previous equation. or 2 1 2 1 or General Formula Conduction through a hollow sphere
  22. 22. © Group 6 Heat Transfer 2013
  23. 23. © Group 6 Heat Transfer 2013 Sample Problem # 3 ; and t1 = 426.7 C. If the heat from the sphere is 439.6 W, what is to ? Solution Let 0.0763 m 0.127m 699.7 K Find: to
  24. 24. © Group 6 Heat Transfer 2013 Solution
  25. 25. © Group 6 Heat Transfer 2013 CONDUCTION THROUGH CURVED WALL ( CYLINDER)
  26. 26. © Group 6 Heat Transfer 2013 CONDUCTION THROUGH CURVED WALL i w
  27. 27. © Group 6 Heat Transfer 2013 CONDUCTION THROUGH CURVED WALL
  28. 28. © Group 6 Heat Transfer 2013 Multilayered Cylinder
  29. 29. © Group 6 Heat Transfer 2013 Sample Problem # 5 Calculate: The rate of heat loss per unit length of pipe. Given R1= 5.25/2 = 2.625 cm R2= 6.03/2 = 3.015 cm R3= 3.015 + 2 = 5.015cm L = 1 m
  30. 30. © Group 6 Heat Transfer 2013
  31. 31. © Group 6 Heat Transfer 2013 Jass
  32. 32. © Group 6 Heat Transfer 2013 How does convection in a liquid occur?
  33. 33. © Group 6 Heat Transfer 2013 How does convection in a gas occur?
  34. 34. © Group 6 Heat Transfer 2013 Flow of Fluids
  35. 35. © Group 6 Heat Transfer 2013
  36. 36. © Group 6 Heat Transfer 2013 Combination of Conduction , Radiation and Transport of fluid Free Convection - Forced Convection - occurs when the fluid circulates, the heavier parts of the fluid moving downward under the force of gravity When work is done to blow or pump the fluid What is Convection Heat?
  37. 37. © Group 6 Heat Transfer 2013 Why is the freezer compartment at the top of a fridge? The freezer compartment is at the top of a fridge because cool air sinks. This warmer air rises and so a convection current is set up inside the fridge, which helps to keep the fridge cool. Why is convection important in fridges? The freezer cools the air at the top and this cold air cools the food on the way down. It is warmer at the bottom of the fridge.
  38. 38. © Group 6 Heat Transfer 2013 What is Film Coefficient (h) ? Three (3) Conditions 1. Forced Convection with Turbulent flow 2. Forced Convection with Laminar flow 3. Free Convection General Equation for (h) Where : L = length of the pipe/duct D = diameter of pipe/duct h = film coefficient
  39. 39. © Group 6 Heat Transfer 2013 Film Coefficient , Turbulent Flow
  40. 40. © Group 6 Heat Transfer 2013 Sample problem #
  41. 41. © Group 6 Heat Transfer 2013 arnel
  42. 42. © Group 6 Heat Transfer 2013 Laminar Flow For laminar flow inside pipe Re<2100 Equation A where ṁ = mass flow rate L = heated length of the straight pipe c = specific heat of the liquid 0.14 b b 0.14
  43. 43. © Group 6 Heat Transfer 2013 Equation B (Sieder – Tate Equation) 0.14 0.14 Laminar Flow
  44. 44. © Group 6 Heat Transfer 2013 Sample Problem # 2
  45. 45. © Group 6 Heat Transfer 2013 0.14 0.14
  46. 46. © Group 6 Heat Transfer 2013 Film Coefficient for annular space Double pipe Heat Exchanger that consists of two concentric tubes Drawing TBC For turbulent flow through concentric annular area 0.8 n b b where :
  47. 47. © Group 6 Heat Transfer 2013 For laminar tube through concentric annular area 0.14 where :
  48. 48. © Group 6 Heat Transfer 2013 Sample Problem #
  49. 49. © Group 6 Heat Transfer 2013
  50. 50. © Group 6 Heat Transfer 2013 Rachel
  51. 51. © Group 6 Heat Transfer 2013 Logarithmic Mean of Temperature Difference The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.
  52. 52. © Group 6 Heat Transfer 2013 Two primary classifications of flow arrangement Parallel-flow arrangement Fluids flow in the same direction through the heat exchanger
  53. 53. © Group 6 Heat Transfer 2013 Counter flow arrangement The fluids enter the exchanger from opposite ends
  54. 54. © Group 6 Heat Transfer 2013 Sample Problem # When a heat exchanger was designed its overall heat transfer co- efficient was 600 kcal/hr mt²°C. The heat transfer area provided = 10mt². Over a period of time, its overall heat transfer co-efficient has fallen to 450 kcal/hr mt²°C due to fouling. Data: Specific heat of hot fluid = 1 kcal/kg °C Hot fluid entering temperature = 80°C Hot fluid leaving temperature = 60°C Cold fluid entering temperature = 25 °C Cold fluid leaving temperature = 40°C Calculate: How much additional area is to be added to maintain the same rate of heat transfer? Assumption: Flow is assumed in counter current direction
  55. 55. © Group 6 Heat Transfer 2013
  56. 56. © Group 6 Heat Transfer 2013 infrared waves The Earth is warmed by heat energy from the Sun. There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. ? The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space. How does heat travel through space? How does this heat energy travel from the Sun to the Earth?
  57. 57. © Group 6 Heat Transfer 2013 Heat can move by travelling as infrared waves. This means that infrared waves act like light waves:  They can travel through a vacuum.  They travel at the same speed as light – 300,000,000 m/s.  They can be reflected and absorbed. What are infrared waves? Infrared waves heat objects that absorb them and are also known as thermal radiation. These are electromagnetic waves, like light waves, but with a longer wavelength.
  58. 58. © Group 6 Heat Transfer 2013 Investigating thermal emission
  59. 59. © Group 6 Heat Transfer 2013 Emitting thermal radiation All objects emit (give out) some thermal radiation. Matt black surfaces are the best emitters of radiation. white silver matt black best emitter worst emitter Certain surfaces are better at emitting thermal radiation than others. Shiny surfaces are the worst emitters of radiation. Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle?
  60. 60. © Group 6 Heat Transfer 2013 Investigating thermal absorption
  61. 61. © Group 6 Heat Transfer 2013 Absorbing thermal radiation Infrared waves heat objects that absorb (take in) them. best emitter worst emitter best absorber worst absorber white silver matt black Matt black surfaces are the best absorbers of radiation. Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers. Shiny surfaces are the worst emitters because they reflect most of the radiation away. Why are solar panels that are used for heating water covered in a black outer layer?
  62. 62. © Group 6 Heat Transfer 2013 When thermal radiation strikes a body, it can be absorbed by the body, reflected from the body or transmitted through the body. The fraction of the incident radiation which is absorbed by the body is called absorptance (symbol α). Other fractions of incident radiation which are reflected and transmitted are called reflectance(symbol ρ) and transmittance(symbol τ), respectively. The sum of these fractions should be unity α + ρ + τ = 1.
  63. 63. © Group 6 Heat Transfer 2013 States that the amount of radiation from a black body is proportional to the fourth power of the absolute temperature, where σ = ( 0.1713) (10-8) is the Stefan-Boltzmann constant when Q is in Btu/hr, and A is the radiating area in square feet of the black body whose surface temperature is T°R. STEFAN-BOLTZMANN LAW Q= σAT4 The amount of radiation with emissivity Ɛ is Q= ƐσAT4
  64. 64. © Group 6 Heat Transfer 2013 Sample Problem # A solid metal sphere (emissivity = 0.65) has a radius of 5 cm is heated to 600ºC and suspended from a thin wire in the center of a room. At what rate is heat released to the room?
  65. 65. © Group 6 Heat Transfer 2013 Iana
  66. 66. © Group 6 Heat Transfer 2013
  67. 67. © Group 6 Heat Transfer 2013 Radiation between Gray Bodies Let the amount of radiant heat q, leaving surface A, be
  68. 68. © Group 6 Heat Transfer 2013 From the equation
  69. 69. © Group 6 Heat Transfer 2013
  70. 70. © Group 6 Heat Transfer 2013 In an analogous circuit we have the form shown in this figure W1 J1 q1 Resistance due to the space configuration
  71. 71. © Group 6 Heat Transfer 2013 Example – Two Gray Surfaces in Shape W1 J1 Total Resistance
  72. 72. © Group 6 Heat Transfer 2013 4 4 44 44
  73. 73. © Group 6 Heat Transfer 2013 4 4
  74. 74. © Group 6 Heat Transfer 2013 Configuration Space Factors for Radiation Equation
  75. 75. © Group 6 Heat Transfer 2013 RADIATION BETWEEN ELEMENT AND RECTANGULAR PARALLEL SURFACE ABOVE ELEMENT
  76. 76. © Group 6 Heat Transfer 2013 RADIATION SHAPE FACTOR FOR PARALLEL, CONCENTRIC, DISCS
  77. 77. © Group 6 Heat Transfer 2013 RADIATION SHAPE FACTOR FOR PARALLEL, DIRECTLY OPPOSED, RECTANGLES
  78. 78. © Group 6 Heat Transfer 2013 RADIATION SHAPE FACTOR FOR PERPENDICULAR RECTANGLES WITH S COMMON EDGE.
  79. 79. © Group 6 Heat Transfer 2013 RADIATION SHAPE FACTOR FOR CONCENTRIC CYLINDERS OF FINITE LENGTH.
  80. 80. © Group 6 Heat Transfer 2013 RADIATION SHAPE FACTOR FOR INFINITESIMAL PLANES AND SPHERES
  81. 81. © Group 6 Heat Transfer 2013
  82. 82. © Group 6 Heat Transfer 2013 Conduction Soldering iron • Iron rod is a good conductor of heat with copper tip. • The handle is made of plastic which is a good insulator. Home electrical appliances • The handles of kettles, hot iron, cooking utensils are made of wood and plastics which are the good insulators of heat.
  83. 83. © Group 6 Heat Transfer 2013 Convection Electric kettle • The heating element is always placed at the bottom of the kettle. • So that hot water at the bottom which is less dense will rise up. • Cooler water at the top which is denser will sink to the bottom. • Convection current is set up to heat up the water. Refrigerator • The freezer is always placed at the top of the refrigerator. • So that cold air at the top will sinks to the bottom. • Warmer air at the bottom will rise to the top. • Convection current is set up to cool down the refrigerator.
  84. 84. © Group 6 Heat Transfer 2013 Radiation Cooling fins at the back of a refrigerator • A black and rough surface is a good radiator of heat. White paint for houses • In hot countries, houses are painted in white to reduce absorption of heat energy from the Sun Teapot • Has smooth, shiny and silvery surface.
  85. 85. © Group 6 Heat Transfer 2013 Vacuum Flask •The shiny bright silvering surface on glass wall reduces heat loss by radiation. •cork stopper which is made of poor conductors reduces heat loss by conduction and convection
  86. 86. © Group 6 Heat Transfer 2013 Glossary  absorber – A material that takes in thermal radiation.  conduction – The method of heat transfer in solids.  conductor – A material that lets heat flow through it.  convection – The method of heat transfer in fluids, which occurs because hot fluids are less dense than cold fluids.  emitter – A material that gives out thermal radiation.  free electrons – Electrons in a metal that are free to move through the metal.  heat transfer – The flow of heat energy from a hotter area to a colder area.  radiation – Heat energy transferred by infrared waves. This method of heat transfer does not need particles.

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