Heat Transfer
RADIATION HEAT TRANSFER
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.
Q
sA(T
4
1
T42 )
where:
A
=
surface area of the first body (ft2)
T1
=
temperature of the first body (°R)
T2
=
temperature of the second body (°R)
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.
Emissivity
Real objects do not radiate as much heat as a perfect black body. They radiate less heat than a
black body and are called gray bodies. To take into account the fact that real objects are gray
bodies, Equation 2-12 is modified to be of the following form.
Q
esAT
4
where:
e = emissivity of the gray body (dimensionless)
Emissivity is simply a factor by which we multiply the black body heat transfer to take into
account that the black body is the ideal case. Emissivity is a dimensionless number and has a
maximum value of 1.0.
Radiation Configuration Factor
Radiative heat transfer rate between two gray bodies can be calculated by the equation stated
below.
Q
fa fe sA(T41
T42 )
Rev. 0
Page 27
HT-02
