Chapter 3
Steady Heat Conduction
Chapter 3
STEADY HEAT CONDUCTION
Steady Heat Conduction In Plane Walls
31C
(a) If the lateral surfaces of the rod are insulated, the heat transfer surface area of the cylindrical rod
is the bottom or the top surface area of the rod,
4
/
2
D
A
s
. (b) If the top and the bottom surfaces of
the rod are insulated, the heat transfer area of the rod is the lateral surface area of the rod,
A
DL
.
32C
In steady heat conduction, the rate of heat transfer into the wall is equal to the rate of heat transfer
out of it. Also, the temperature at any point in the wall remains constant. Therefore, the energy content of
the wall does not change during steady heat conduction. However, the temperature along the wall and thus
the energy content of the wall will change during transient conduction.
33C
The temperature distribution in a plane wall will be a straight line during steady and one
dimensional heat transfer with constant wall thermal conductivity.
34C
The thermal resistance of a medium represents the resistance of that medium against heat transfer.
35C
The combined heat transfer coefficient represents the combined effects of radiation and convection
heat transfers on a surface, and is defined as
h
combined
=
h
convection
+
h
radiation
. It offers the convenience of
incorporating the effects of radiation in the convection heat transfer coefficient, and to ignore radiation in
heat transfer calculations.
36C
Yes. The convection resistance can be defined as the inverse of the convection heat transfer
coefficient per unit surface area
since it is defined as
R
hA
conv
1/ (
) .
37C
The convection and the radiation resistances at a surface are parallel since both the convection and
radiation heat transfers occur simultaneously.
38C
For a surface of
A
at which the convection and radiation heat transfer coefficients are
h
h
conv
rad
and
,
the single equivalent heat transfer coefficient is
h
h
h
eqv
conv
rad
when the medium and the surrounding
surfaces are at the same temperature. Then the equivalent thermal resistance will be
R
h
A
eqv
eqv
1/ (
)
.
39C
The thermal resistance network associated with a fivelayer composite wall involves five singlelayer
resistances
connected in series.
310C
Once the rate of heat transfer
Q
is known, the temperature drop across any layer can be
determined by multiplying heat transfer rate by the thermal resistance across that layer,
T
QR
layer
layer
31
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Chapter 3
Steady Heat Conduction
311C
The temperature of each surface in this case can be determined from
(
) /
(
)
(
) /
(
)
Q
T
T
R
T
T
QR
Q
T
T
R
T
T
QR
s
s
s
s
s
s
s
s
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
where
R
i
is the thermal resistance between the environment
and surface i.
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 Spring '08
 BENARD
 Heat Transfer

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