Emissivity of materials from E+ Eng Ref
Posted by Richard Raustad on 05 October 2010 02:33 PM
In the Engineering Reference, there is a section named "Outside Surface Heat Balance"... which contains a subsection named "External Longwave Radiation":

External Longwave Radiation
q"LWR is a standard radiation exchange formulation between the surface, the sky, and the
ground. The radiation heat flux is calculated from the surface absorptivity, surface
temperature, sky and ground temperatures, and sky and ground view factors.
The longwave radiation heat exchange between surfaces is dependent on surface
temperatures, spatial relationships between surfaces and surroundings, and material
properties of the surfaces. The relevant material properties of the surface, emissivity  and
absorptivity , are complex functions of temperature, angle, and wavelength for each
participating surface. However, it is generally agreed that reasonable assumptions for
building loads calculations are (Chapman 1984; Lienhard 1981):

* each surface emits or reflects diffusely and is gray and opaque (absorptivity = emissivity, transmittance = 0, reflectivity = 1- emissivity)
* each surface is at a uniform temperature
* energy flux leaving a surface is evenly distributed across the surface,
* the medium within the enclosure is non-participating.

These assumptions are frequently used in all but the most critical engineering applications.
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