Radiation heat exchange between non-diffuse gray surfaces separated by isothermal absorbing-emitting gas

Abstract

In many industrial thermal systems and plants thermal radiation is one of the major means of energy transfer. Analysis of radiation heat exchange in enclosures is often simplified introducing the following assumptions: emissivity and reflectivity are independent of direction and wavelength; the reflected energy is diffuse and uniform. These approximations greatly simplify the enclosure theory but as it can be seen (Rohsenow and Hartnett, Handbook of Heat Transfer. McGraw-Hill, New York (1973)) many engineering materials show direction-dependent properties. The non-diffuse nature of thermal radiation is often approximated by a model based on the assumption that whole emission is diffuse while the reflection is partly specular and partly diffuse (Sarofim and Hottel, J. Heat Transfer 88(1), 37-44 (1966)). However, many materials have also non-diffuse emission (Rohsenow and Hartnett, Handbook of Heat Transfer. McGraw-Hill, New York (1973)). In this work a model based on non-diffuse emission and reflection of surfaces separated by isothermal absorbing-emitting gas is presented. The radiation from surfaces and gas volume is simulated by an artificial, random process, while the determination of beam extinguishing place enables the calculation of geometrical system characteristics (like absorption factor) as well as the formulation of heat balance equations. The model presented is tested on the simple geometry of a cube and the results show the influence of non-diffuse emission and reflection as well as the presence of an intervening medium in such systems. © 1991.