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05 Basic Heat transfer

Infrared Thermography Basic heat transfer

Daniel514
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05 Basic Heat transfer

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  1. Basic heat transfer How heat can be exchanged between objects and move from place to place ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  2. Lesson objectives • Understand heat transfer • Conduction • Understand what four factors affect conduction, and how they do that • Convection • Natural versus forced convection • Radiation, understand the concepts of • Emission • Absorption ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  3. Lesson objectives – cont. • Know the difference between steady state and transient heat transfer • Know how thermal capacity affects transient heat flow • Understand how evaporation and condensation can affect the surface temperature of a target object ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  4. Convection Evaporation/condensation Conduction Radiation Heat transfer modes ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  5. Conduction – definition Heat conduction is the direct transfer of thermal energy from molecule to molecule, caused by collisions between the molecules. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  6. Conduction Direct transfer from molecule to molecule Transfer of energy of movement (kinetic energy) between molecules Can occur in solids, liquids and gases It is the only one that occurs in solids! Very important for a thermographer to understand ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  7. Demo - conduction ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  8. What happened exactly? Electrical energy was converted to heat Heat was conducted from the iron to the aluminium block Heat was conducted from molecule to molecule inside the aluminium block A distinctive conduction pattern was created ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  9. Conductive heat transfer rate The rate of heat flow under steady state conditions is directly proportional to the thermal conductivity of the object, the cross-sectional area of the object through which the heat flows, and the temperature difference between the two ends of the object. It is inversely proportional to the length, or thickness, of the object. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  10. Conductive heat transfer formula • The result Q/t, is a measure of heating power, i.e. the rate of heat flow • It is valid in a steady state condition Formula number 1 ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  11. Conductive heat transfer formula • Four factors: • Conductivity of the material, k (W/m*K) • Cross sectional area, A (m2) • Temperature difference, T1 – T2 (K) • Conductive path length, L (m) Formula number 1 ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  12. Power and energy Conduction formula gives us the heating power, or the rate of heat flow Multiply power with time and you get energy (or work), 1 joule = 1 watt-second For heat flow to continue at the same rate, energy must be continuously added, if not, the temperature difference will start to decrease ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  13. Thermal conductivity values Material Conductivity (W / m* K) Copper 401 Aluminium 237 Steel 52 Pure ice 2.04 Brick 1 Glass (window) 0.9 Water 0.6 Wood 0.14 Fibreglass (Mineral wool, Rockwool) 0.04 Air (still) 0.025 Argon 0.018 Xenon 0.0051 ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  14. Insulation material has k=0.04 Wooden stud has k=0.14, same thickness (L in the formula) Wooden stud conducts energy at a higher rate Corners are always cooler, but for other reasons… Example 1: Conductivity ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  15. Example 2: Wall area • Heated houses in winter, -10°C • Same temperature inside, 20 °C • Same wall thickness (0.2 m) and material (k=0.05 W/m*K) • Small house: 120 m2 • Big house: 480 m2 ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  16. Example 2: Wall area • Small house: • Delta T = 30K • L = 0.2 m • k = 0.05 W / m* K • A = 120 m2 • Result: The heat loss is 900 W, or 0.9 kW • Big house: • Delta T = 30K • L = 0.2 m • k = 0.05 W / m* K • A = 480 m2 • Result: The heat loss is 3 600 W, or 3.6 kW To this number, we must add losses through the roof, by ventilation, and to the ground. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  17. Example 3: Temperature difference Coffee inside the cup is hotter than the air Heat is conducted through at a higher rate where there is hot coffee We can see the level of coffee in the cup – also from the side ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  18. Example 4: Conductive path length • Refractory insulated torpedo ladle • Used to transport molten steel within the steel plant • Loaded from furnace at the top arrow (a) • Heavy wear on refractory at (b) a b ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  19. Molten steel – HOT! L1 L2 Hot spot Cool outside air Example 4: Conductive path length • Worn refractory has shorter path length (L2 < L1) for heat conduction • Smaller L means higher T • Hotter area shows up where refractory is worn ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  20. Steady state and transient heat transfer Two types of heat transfer situations Steady state is a stable condition without changes Transient condition is one where changes occur In reality, steady state is a rare condition Our conduction formula is valid for steady state ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  21. Steady state • Heating power and cooling power are the same; A = B • Temperature difference (T) remains stable over time • Note! Heat transfer does take place! ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  22. Temperature Heating Cooling Time Transient condition Temperature changes over time ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  23. Steady state and transient condition Steady state – input equals output Transient condition Transient condition Temp. 50 45 40 35 Graph 1 30 25 Time 30 00 30 00 14 15 16 Trending period: 2002-01-29 14:03:53 to 16:03:34 ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  24. Thermal capacity Ability to store thermal energy Thermal inertia High thermal capacity objects react slower to temperature changes Hence, high thermal capacity materials require more energy to change their temperature by one degree ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  25. Thermal capacity values Material Conductivity (k) Capacity (W / m* K) (kJ / kg* K) Copper 401 0.39 Aluminium 237 0.90 Steel 52 0.46 Ice 2.04 2 Brick 1 0.75 Glass (window) 0.9 0.84 Water 0.6 4.18 Wood 0.14 1.80 - 2.80 Fibreglass (Mineral wool, Rockwool) 0.04 - Styrofoam 0.03 - Air (still) 0.025 1.0 Xenon 0.0051 - ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  26. Thermal capacity and transients • Tank is not actively heated or cooled • Variations in heat flow during daily cycle • Different capacity of water and air makes the tank level visible Morning Afternoon ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  27. Convection - definition Convection is a heat transfer mode where a fluid is brought into motion, either by gravity or another force, thereby transferring heat from one place to another. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  28. Convection Heat transfer by moving fluids (liquid, gas) Can occur in liquids and gases, not solids Can create difficulties for the thermographer! ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  29. Demo - Convection Let’s look into a bucket of warm water… ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  30. What happened? In hotter parts of the fluid, molecules are further apart ”Further apart” means lower density Lower density means less gravitational force Less gravitational force means ”lighter” fluid Lighter fluid will rise, heavier fluid will sink ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  31. Convection ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  32. Natural and forced convection • Natural: • The fluid is affected purely by gravitational forces around the object of study • Forced: • The fluid is also affected by an outside force • Wind, fan, pump ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  33. Example 1: Wind cooling With wind load Without wind load Oil-filled circuit breaker ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  34. Example 1: Wind cooling • Conclusion: Do not do surveys under high wind conditions!!! • You will miss problems, and/or misinterpret • DeltaT will also decrease when you have wind!!! Detected problems! Undetected problems? Obvious problem in both cases ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  35. Example 2: Air infiltration • This heated building has air leaks • Low pressure inside draws cool air into the room • Cool airflow creates a pattern on the surfaces ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  36. Evaporation and condensation Liquid turns into gas – evaporation Gas turns into liquid – condensation Energy is exchanged, by evaporation and condensation alone Evaporation cools the surface Condensation heats the surface ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  37. Demo: Evaporation Hot! Cold! Let’s wet a sheet of paper with warm water… ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  38. Consequences of evaporation The cooling effect of evaporation (and the heating effect of condensation) can often be misleading. An example is in building applications. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  39. Radiation heat transfer - definition Heat transfer by emission and absorption of thermal radiation is called radiation heat transfer. ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  40. Thermal radiation Thermal radiation is a kind of electromagnetic radiation All objects emit thermal radiation No medium (matter) is required Passes easily through most gases Will pass with difficulty, or be blocked, by liquids and solids ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  41. Radiation heat transfer Hotter Cooler Heat is transferred by emission and absorption Both objects emit and absorb radiation The net heat transfer is the difference ITC Level 1 Training © Publ. no. 1560097_G-en_GB

  42. Questions Which modes of heat transfer have we discussed so far in this course? In what kind of medium can we expect convection to take place? What is the formula for steady state conductive heat transfer? If you double the thickness of a wall, what will happen to the heat transfer through the wall? If you reduce the temperature difference to half, what happens then? What is the principal mechanism behind conduction? How does conduction work? What is characteristic for a ”steady state” condition? What does ”transient condition” mean? What is convection? What is the difference between ”natural” and ”forced” convection? How does condensation affect the surface it condenses on? Heat up, cool down or remain the same? (yes, it does get wet, but…) What sort of medium does radiation heat transfer require? What is the physical mechanism behind radiation heat transfer? Which is the only heat transfer mode that (normally) works in a solid? If two objects radiate heat towards each other, what would determine the heat transfer direction? ITC Level 1 Training © Publ. no. 1560097_G-en_GB

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