RADIATION HEAT TRANSFER
Heat Transfer
where:
fa =
is the shape factor, which depends on the spatial arrangement of the two objects
(dimensionless)
fe =
is the emissivity factor, which depends on the emissivities of both objects
(dimensionless)
The two separate terms fa and fe can be combined and given the symbol f. The heat flow
between two gray bodies can now be determined by the following equation:
(2-13)
Q
fsA(T41
T42 )
The symbol (f) is a dimensionless factor sometimes called the radiation configuration factor,
which takes into account the emissivity of both bodies and their relative geometry. The radiation
configuration factor is usually found in a text book for the given situation.
Once the
configuration factor is obtained, the overall net heat flux can be determined. Radiant heat flux
should only be included in a problem when it is greater than 20% of the problem.
Example:
Calculate the radiant heat between the floor (15 ft x 15 ft) of a furnace and the roof, if
the two are located 10 ft apart. The floor and roof temperatures are 2000°F and 600°F,
respectively. Assume that the floor and the roof have black surfaces.
Solution:
A1 = A2 = (15 ft) (15 ft) = 225 ft2
T1 = 2000oF + 460 = 2460°R
T2 = 600oF + 460 = 1060°R
Tables from a reference book, or supplied by the instructor, give:
f1-2
= f2-1 = 0.31
Q1-2
= sAf(T14- T24)
= (0.174
Btu
hr ft2 oR4
) (225 ft2) (0.31) [(2460oR)4
(1060oR)4]
= 4.29 x 1014 Btu/hr
HT-02
Page 28
Rev. 0
