Page 162 - Industrial Ventilation Design Guidebook

I 24 CHAPTER 4 PHYSICAL FUNDAMENTALS

For direct net radiation between two blackbodies (from Eq. (4.220)),

Radiation heat transfer in a hollow can be represented by electrical
analogy as
/• x

current = conductance x potential difference
where (e/a)M m = 17 = radiation potential, which is dependent on the
temperature; e is dependent on the radiation properties of the surface and
the temperature; and a is dependent on the spectrum of the incoming ra-
diation. (a/p)A = G = radiation conductance between the potentials U
and M.
For a gray body

When the surface is black, G = °° or R = 0, while a = 1. and (7=0;
points U and M unite and the potential is M m.
When the surface is thermally insulated, $ net = 0. Points U and M unite
and the potential is M = (e/a)M m = UE = M..
For two surfaces, $ z/ net = E ijA i - E^Aj = A ;/(M ; - M,), so by analogy A i;
is the radiation conductance between potentials M r and M^ (see Fig. 4.32).
When there are only two surfaces in the hollow, the net thermal radiation is

If surface 1 is convex or planar, all the incoming radiation is from surface 2,
and F 12 = 1, while the visibility factor expresses that part of radiation coming
from this surface. If surface 2 is concave, a part of the radiation is also from
this surface.

FIGURE 4.32 Radiation net for a hollow with four surface