Apparent Thermal Emissivity From Surfaces With Multiple v-Shaped Grooves

A requirement of the advancement in space and high-temperature technology is the transfer of heat by thermal radiation. The efficient transfer of heat by radiation can be accomplished by surfaces of high thermal emissivity., Such surfaces are difficult to obtain, particularly at very high temperatures. This paper presents certain experimental and analytical results on metallic surfaces with multiple V-shaped grooves. The radiant interchange within an infinitely long V-shaped groove is studied, and its apparent thermal emissivity is found as a function of the included angle and the emissivity of the surfaces including the angle. The view factor and the apparent emissivity are plotted for various angles vs the surface emissivity. The experimental results for a 30° included angle are in agreement with the analysis. It is found that, with an included angle of 30° and surface emissivity of 0.3, an apparent emissivity of 0.64 can be obtained. Experimental measurements of the total thermal emissivity of a ground molybdenum surface at various temperatures also are presented. A discussion on actual application of grooves on radiating surfaces and the effects of imperfections present in the grooves due to cutting processes is given.