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Black
Body Radiation - Heat Transfer
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[ Heat
Transfer Table of Contents]
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.
Stefan-Boltzman Constant = 5.67 x 10-8 Watts-m-2-°K-4
= 0.174 x 10-8 BTU/Hr-ft2-°R4
Two black bodies that radiate toward each
other have a net heat flux between them. The net flow
rate of heat between them is given by an adaptation of
Equation 2-12.
All bodies above absolute zero temperature
radiate some heat. The sun and earth both radiate heat toward
each other. This seems to violate the Second Law of
Thermodynamics, which states that
heat cannot flow from a cold body to a hot body. The paradox
is resolved by the fact that each
body must be in direct line of sight of the other to receive
radiation from it. Therefore, whenever
the cool body is radiating heat to the hot body, the hot body
must also be radiating heat
to the cool body. Since the hot body radiates more heat (due
to its higher temperature) than the
cold body, the net flow of heat is from hot to cold, and the
second law is still satisfied.
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