Effect of direction and wavelength dependent surface properties on radiant heat transfer.

In this paper, the validity and accuracy of the commonly used simple methods of radiant heat-transfer predictions are examined by more realistic and refined analyses. The radiant interchange problem between surfaces having nonelementary emission and reflection characteristics is formulated in terms of an integral equation for the spectral intensity of radiation leaving a differential area on an opaque surface and is then specialized to the parallel plate configuration. Solutions for the local radiant heat flux and the incident flux were obtained for seven different models approximating the radiation characteristics of surfaces. The models ranged from the simple diffuse, specular, and diffuse-specular constant property models to a very detailed directional emission and bidirectional reflection model. The general conclusions are that directional dependent properties should be employed to calculate radiant heat flux or irradiation for specularly or nearly specularly reflecting surfaces, and that the simple diffuse model predicts values that agree remarkably well with more detailed analyses and available experimental data.