Page 161 - Industrial Ventilation Design Guidebook
P. 161
4.3 HEAT AND MASS TRANSFER 1 23
The radiation intensity of a surface element is the sum of emission and reflection:
where p = reflectance and
giving an integral formula for the radiation intensity function,
Equation (4.221) is difficult to solve, and for practical cases approximate
methods are used.
Usually the surface is divided into zones, and with sufficient accuracy M is
considered constant over this area, giving
where
Also, by integration,
where
F ki is the visibility factor between two finite surfaces, and A tF ki = A ki is the
geometrical radiation surface. Equation (4.225) shows that the geometrical ra-
diation surface is symmetric and therefore
Integrated over the semispace, the integrals (4.223) are 1. Thus in the hollow,
This sum includes all the hollow surfaces and also the surface k, if it is
concave, in which case the portion F kk of the received radiation is re-
flected back.
The net radiation power falling the surface is the difference between the incom-
ing and outgoing radiation and the difference between absorption and emission:
Eliminating £ when e = a,
