Validity of the commonly used simplified models for predicting local and overall radiant heat interchange among real surfaces is critically examined. The spectral local irradiation was measured at wavelengths of 3.08 and 4.51 μ in a configuration consisting of three plane parallel surfaces of finite extent. The test surfaces were gold with mechanical roughness and temperature varying from 0.02 to 7.1 μ and 77 to 760 deg K respectively. Comparisons between experimental data and predictions for six different models ranging from the simple diffuse to the most detailed diffuse-plus-specular-directional-property model with specular component calculated according to Beckmann are presented. It is shown that the calculation of the overall irradiation using appropriate constant-property models agrees well with the experimental data to within combined experimental and analytical uncertainty. In general it is concluded that for accurate prediction of local irradiation the directional characteristics of the surfaces must be considered.

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