Transcript Conduction - NIU - Northern Illinois University

Conduction
Transfer Mechanisms

Heat can be transferred in three ways.
Transfer can include more than one way.

Conduction

• Energy flow
from direct
thermal contact

Convection
• Fluid flow
carrying energy

Radiation
• Energy radiating
from an object
into
surroundings
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
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
Air
Stryrofoam
Wood
Water
Glass
Concrete
Steel
Aluminum
Copper
Thermal Cond.
0.026 W/m-K
0.029 W/m-K
0.11 W/m-K
0.61 W/m-K
0.8 W/m-K
1.0 W/m-K
46 W/m-K
240 W/m-K
400 W/m-K
Heat Flow Rate

• Change in temperature with
distance
A
H
T + DT
The rate of heat flow
depends on the temperature
gradient.
T

Depends on surface area A
for contact.
Dx
H
DQ
DT
 kA
Dt
Dx
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.
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.
Two Layers

T3
T2
H
T1
H
If there are two layers in
thermal contact, the rate of
heat flow must be the same
for both.
• Energy doesn’t accumulate
in the layer.
Dx2 Dx1
H  k 2 A
T3  T2
T T
 k1 A 2 1
Dx2
Dx1
Thermal Resistance

T3
T2
T1
H
H
For an arbitrary set of layers
the intermediate temperature
is unknown.
• Define thermal resistance
R
R2
R1
DT
H
R
Dx
kA
• For multiple layers R adds
R  R1  R2
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
Glass (1/8”)
Brick (3½”)
Plywood (1/2”)
Fiberglass
insulation (1”)
R-factor
1
0.6 – 1
0.6
4
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