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Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3 | Next
Emissivity |
RADIATION HEAT TRANSFER
Heat Transfer
RADIANT HEAT TRANSFER
Radiant heat transfer is thermal energy transferred by means of electromagnetic
waves or particles.
EO 1.10
DESCRIBE how the following terms relate to radiant
heat transfer:
a.
Black body radiation
b.
Emissivity
c.
Radiation configuration factor
Thermal Radiation
Radiant heat transfer involves the transfer of heat by electromagnetic radiation that arises due to
the temperature of a body. Most energy of this type is in the infra-red region of the
electromagnetic spectrum although some of it is in the visible region. The term thermal radiation
is frequently used to distinguish this form of electromagnetic radiation from other forms, such
as radio waves, x-rays, or gamma rays. The transfer of heat from a fireplace across a room in
the line of sight is an example of radiant heat transfer.
Radiant heat transfer does not need a medium, such as air or metal, to take place. Any material
that has a temperature above absolute zero gives off some radiant energy. When a cloud covers
the sun, both its heat and light diminish. This is one of the most familiar examples of heat
transfer by thermal radiation.
Black Body Radiation
A body that emits the maximum amount of heat for its absolute temperature is called a black
body. Radiant heat transfer rate from a black body to its surroundings can be expressed by the
following equation.
(2-12)
Q
sAT
4
where:
=
heat transfer rate (Btu/hr)
Q
s
=
Stefan-Boltzman constant (0.174 Btu/hr-ft2-°R4)
A =
surface area (ft2)
T
=
temperature (°R)
HT-02
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