Conduction

1
Conduction
2
Transfer Mechanisms

• Heat can be transferred in three ways.
• Transfer can include more than one way.

• Conduction
• Energy flow from direct thermal contact

• Radiation
• Energy radiating from an object into surroundings

• Convection
• Fluid flow carrying energy

3
Direct Contact

• Items in direct contact transfer heat.
• Molecules in hot regions have greater kinetic
  energy.
• Elastic collisions with cool molecules
• Kinetic energy transfer at boundary

4
Thermal Conductivity

• Heat flow within an object is due to transfer by
  conduction.
• Thermal conductivity (k) measures the ability for
  heat to move in a material.
• Measured in W / m-K
• High number means high rate of transfer

• Material Thermal Cond.
• Air 0.026 W/m-K
• Stryrofoam 0.029 W/m-K
• Wood 0.11 W/m-K
• Water 0.61 W/m-K
• Glass 0.8 W/m-K
• Concrete 1.0 W/m-K
• Steel 46 W/m-K
• Aluminum 240 W/m-K
• Copper 400 W/m-K

5
Heat Flow Rate

• The rate of heat flow depends on the temperature
  gradient.
• Change in temperature with distance
• Depends on surface area A for contact.

A
H
T
T DT
Dx
6
Conductors and Insulators

• Thermal conductors have high values of k.
• Metals with conducting electrons
• Greater than 10 W/m-K
• Still air is an excellent thermal insulator.
• Materials that trap air are good wood, styrofoam
• Vacuum would be the best.

7
Swimming Hole

• A lake with a flat bottom and steep sides has a
  surface area 1.5 km2 and is 8.0 m deep. The
  surface is at 30 ?C and the bottom is at 4 ?C.
• What is the rate of heat conduction through the
  lake?

• Convert area to m2.
• 1.5 km2 1.5 x 106 m2
• Use the equation for heat flow.
• H -kA(DT/Dx)
• -(0.61 W/m-K)(1.5 x 106 m2) (26 K) / (8.0 m)
• H -3.0 x 106 W.

8
Two Layers

• If there are two layers in thermal contact, the
  rate of heat flow must be the same for both.
• Energy doesnt accumulate in the layer.

T1
T2
T3
H
H
Dx2
Dx1
9
Thermal Resistance

• For an arbitrary set of layers the intermediate
  temperature is unknown.
• Define thermal resistance
• For multiple layers R adds

T1
T2
T3
H
H
R2
R1
10
R-Factor

• In the US, thermal resistance is measured per
  unit area.
• R Dx / k
• Units are ft2 ?F hr / BTU
• 1 BTU 1055 J

• Material R-factor
• Glass (1/8) 1
• Brick (3½) 0.6 1
• Plywood (1/2) 0.6
• Fiberglass
• insulation (1) 4

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