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Heat Transfer

Heat Transfer. Conduction. Direction. From hot to cold. Heat Transfer by Conduction. Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Heat Transfer by Convection.

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Heat Transfer

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  1. Heat Transfer

  2. Conduction Direction From hot to cold. Heat Transfer by Conduction Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move.

  3. Heat Transfer by Convection Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Heated fluid rises and is then replaced by cooler fluid, producing convection currents. Convection Convection is significantly affected by geometry of heated surfaces. (wall, ceiling, floor)

  4. Radiation Atomic Sun Heat Transfer by Radiation Radiation is the process by which heat energy is transferred by electromagnetic waves. No medium is required !

  5. Kinds of Heat Transfer Consider the operation of a typical coffee maker: Think about how heat is transferred by: Conduction Convection Radiation

  6. Steam Ice Heat Current The heat currentH is defined as the quantity of heat Q transferred per unit of time t in the direction from high temperature to low temperature. Typical units are: J/s, cal/s, and Btu/h

  7. t1 t2 H = Heat current (J/s) A = Surface area (m2) Dt = t2 - t1 Dt = Temperature difference L = Thickness of material Thermal Conductivity The thermal conductivity k of a material is a measure of its ability to conduct heat.

  8. Hot Cold For Copper: k = 385 J/s m C0 The SI Units for Conductivity Taken literally, this means that for a 1-m length of copper whose cross section is 1 m2 and whose end points differ in temperature by 1 C0, heat will be conducted at the rate of 1 J/s. In SI units, typically small measures for length L and area A must be converted to meters and square meters, respectively, before substitution into formulas.

  9. Dt = 1 F0 t = 1 h A=1 ft2 Q=1 Btu L = 1 in. Older Units for Conductivity Older units, still active, use common measurements for area in ft2 time in hours, length in seconds, and quantity of heat in Btu’s. Glass k = 5.6 Btu in./ft2h F0 Taken literally, this means that for a 1-in. thick plate of glass whose area is 1 ft2 and whose sides differ in temperature by 1 F0, heat will be conducted at the rate of 5.6 Btu/h.

  10. Material 385 2660 Copper: Concrete or Glass: 0.800 5.6 0.040 0.30 Corkboard: Thermal Conductivities Examples of the two systems of units used for thermal conductivities of materials are given below:

  11. 2050 kJ/s 4980 Btu/h Aluminum: 3850 kJ/s 9360 Btu/h Copper: Concrete or Glass: 8.00 kJ/s 19.4 Btu/h 0.400 kJ/s 9.72 Btu/h Corkboard: Examples of Thermal Conductivity Comparison of Heat Currents for Similar Conditions: L = 1 cm (0.39 in.); A = 1 m2 (10.8 ft2); Dt = 100 C0

  12. 200C 120C t = 1 h A Q = ? Dt = t2 - t1 = 8 C0 0.015 m Example 1: A large glass window measures 2 m wide and 6 m high. The inside surface is at 200C and the outside surface is at 120C. How many joules of heat pass through this window in one hour? Assume L = 1.5 cm and that k = 0.8 J/s m C0. A = (2 m)(6 m) = 12 m2 Q = 18.4 MJ

  13. ti -200C 250C HA Steady Flow 8 cm 12 cm Example 2: The wall of a freezing plant is composed of 8 cm of corkboard and 12 cm of solid concrete. The inside surface is at -200C and the outside surface is +250C. What is the interface temperature ti? Note:

  14. ti -200C 250C HA Steady Flow 8 cm 12 cm Example 2 (Cont.):Finding the interface temperature for a composite wall. Rearranging factors gives:

  15. ti -200C 250C HA Steady Flow 8 cm 12 cm Example 2 (Cont.):Simplifying, we obtain: 0.075ti + 1.50C = 250C - ti ti = 21.90C From which: Knowing the interface temperature tiallows us to determine the rate of heat flow per unit of area, H/A. The quantity H/A is same for cork or concrete:

  16. ti -200C 250C HA Steady Flow 8 cm 12 cm Example 2 (Cont.):Constant steady state flow. Over time H/A is constant so different k’s cause different Dt’s Cork:Dt = 21.90C - (-200C) = 41.9 C0 Concrete:Dt = 250C - 21.90C = 3.1 C0 Since H/A is the same, let’s just choose concrete alone:

  17. ti -200C 250C HA Cork:Dt = 21.90C - (-200C) = 41.9 C0 Concrete:Dt = 250C - 21.90C = 3.1 C0 Steady Flow 8 cm 12 cm Example 2 (Cont.):Constant steady state flow. Note that 20.7 Joules of heat per second pass through the composite wall. However, the temperature interval between the faces of the cork is 13.5 times as large as for the concrete faces. If A = 10 m2, the heat flow in 1 h would be ______? 745 kW

  18. Summary: Heat Transfer Conduction: Heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Radiation is the process by which heat energy is transferred by electromagnetic waves.

  19. The thermal conductivity k of a material is a measure of its ability to conduct heat. t1 t2 H = Heat current (J/s) A = Surface area (m2) Dt = t2 - t1 Dt = Temperature difference L = Thickness of material Summary of Thermal Conductivity

  20. Summary of Formulas

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