Spectral emissivities of electrode materials

Abstract

Many of the high temperature metals and carbides being considered as electrodes in thermionic convertors possess spectral emissivities that are sensitive to both wavelength and temperature. For these materials the gray-body procedure for computing the net radiant heat exchange can yield results considerably different from those obtained by the more exact method of summing the monochromatic heat flux over several hundred increments of wavelength. For tungsten, the flux obtained by summation has been shown to be as much as 25 per cent greater than the gray-body flux when both computational procedures were based on the same emissivity data. The present paper will discuss current progress towards obtaining spectral emissivity data on other materials (such as tantalum); the amount of emissivity data required for any given material in order to calculate summed net heat flows having good accuracy; the extent to which simple equations can be made to fit these heat flow data. The analytical procedure used by the NASA data-reduction center to compute net heat flux for particular geometries will be mentioned. Emphasis will be placed on how the procedure can be applied to a variety of geometries and materials.