Page 160 - Industrial Ventilation Design Guidebook

1 22 CHAPTER 4 PHYSICAL FUNDAMENTALS

FIGURE 4.31 Direction and nature of incident radiation.

where dA cos $ is the projection surface of the opening in the direction concerned.
Thus the radiation power is distributed to different directions at a ratio of cos #.
This is the cosine law of Lambert. It is valid for a blackbody and approximately
valid for insulating materials, but it cannot be used for bright metal surfaces.
Substituting in Eq. (4.211) gives

and integrating for the radiation intensity to the semispace by a blackbody gives

where M m — TrL m = crT .

4.3.5.4 Thermal Radiation inside a Vacuum (without Gas)
Surface element dAj is located on the spatial surface A ; sending radiation
to the surface A z. The radiation power from the surface element dAj to the ele-
ment dAi according to Eq. (4.211) is

The solid angle da)j = dA { cos fy/r . Equation (4.211) gives

where £,y is the proportion of the radiation from dAj relative to the radiation in-
tensity of dAj. If Lambert's cosine law is valid, L is not dependent on direction and

Thus

The visibility factor of the surface element <p /; depends on the geometry and
gives that part of the radiation intensity of dAj that falls directly on the surface
dAj or vice versa.