Civil Engineering Reference
In-Depth Information
Table 8.1
Average values of thermal conductivity of different materials at room temperature
Thermal conductivity
k
in the temperature range 273.15-373.15 K
0-100
C 32-212
F
Material
Pure aluminum
W/m
K
(
a
)
228
F
Btu/h
ft
(
b
)
¼
(
a
)/1.731
132
in/ft
2
F
Btu
h
(
c
)
¼
(
b
)
12
1,581
Cast iron
W/m
K
48
Btu/h
ft
F
28
Btu
in/ft
2
F
h
333
Fiber glass
W/m
K
0.058
Btu/h
ft
F
0.034
in/ft
2
Btu
F
h
0.402
Ceramic fiber
W/m
K
0.055
F
Btu/h
ft
0.032
in/ft
2
F
Btu
h
0.381
Cellular polyurethane
W/m
K
0.025
F
Btu/h
ft
0.014
in/ft
2
F
Btu
h
0.173
Thermal conductivities of various materials at room temperature or higher are
reported in Table
8.1
. If the temperature rises, thermal conductivity decreases with
homogeneous metals and it increases in composite materials almost linearly in an
interval range of 323-373 K (50-100
C; 122-212
F). With insulating materials,
because of the composite or multilayer structure where also air can be present, the
k
coefficient must take into account the presence of all the heat transfer basic modes
and its value is assessed on empirical bases. For detailed evaluations, manufacturer
specifications should be consulted.
Radiation
: heat is transferred as radiant energy and does not require any medium of
diffusion. Radiation can take place even in a vacuum. Solid surfaces, gases, and
liquids all emit, absorb, and transmit thermal radiation in varying degrees.
The rate at which energy is emitted from a system with a surface
A
situated
inside a large space is quantified by the basic relationship:
T
s
T
o
Q
¼ ε σ
A
ðÞ
W
where
T
s
and
T
o
are the surface and output surrounding absolute temperatures (K),
ε
(0
1) is an adimensional quantity that indicates how effectively the surface
radiates (
< ε <
ε ¼
1 for the black body or the perfect emitter), and
σ
is the Stefan-
K
4
). As thermal radiation is
associated with the fourth power of surface absolute temperature, the importance of
this mode of heat transfer increases rapidly with the temperature.
Values of
10
8
W/(m
2
Boltzmann constant equal to 5.67
for different operating conditions (materials, different states of
surfaces) are shown in Table
8.2
. As with the conduction energy transfer, different
units are commonly used alternatively to the SI units.
ε
