Effective emissivity of a cylindrical cavity source has already been known by approximation methods or highly accurate numerical solutions. For a cavity source with an arbitrary shape, the calculation by De Vos' method is more useful than ordinary one. In this paper the effective emis-sivity of a conical cavity source is evaluated by De Vos' method.
It is shown that the average effective emissivity of the bottom of a conical cavity source with its opening closed by a lid with a circular hole in its center is greater than that of a cavity source of cylindrical shape with equal max. sizes (i.e., max. diameter and length). Average effective emissivity of this conical cavity is given by
1-(_??_+_??_),
where _??_=2π(α')
2·_??_
rω00y(
l-
y)
dy/{(
l-
y)
2+(
yα')
2}
3/2, _??_<<_??_
α'=tan α, α=half apex angle,
l=length of cavity,
p=ratio of max. radius of the cavity to radius of the circular hole,
rω00=reflectivity of cavity surface for the radiation from hole lid back to hole lid.
When the surface has perfect diffusivity, _??_
1, and _??_
2 are replaced by _??_
1 and _??_
2 respectively, and we have
_??_=2ρ(α')
2/√1+(α')
2_??_
y(
l-
y)
dy/{(
l-
y)
2+(
yα')
2}
2 where ρ is the reflectivity for perpendicular incidence.
_??_=4ρ
2(α')
6l/1+(α')
2_??_
dy_??_
dx×
xy2(
x_??_
y)(
l-
x)/{(
l-
x)
2+(
xα')
2}
2{(
x-
y)
2+(
xα')
2}
2 Numerical values of _??_
1, _??_
1' and _??_
2' for ρ=1.00, 0.75, 0.50 and 0.25 and
l=2, 4, 6, 8, and 10 are given in tables and shown by graphs. It seems that the effective emissivity of conical cavity increases when the surface is not perfectly diffusive.
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