The Simulation of Apparent Directional Emissivity in a Three-Dimensional Non-Isothermal Medium by the DRESOR Method

The emissivity as a thermal property plays an important role required for heat transfer calculations and temperature measurement. In an isothermal purely absorption medium, the emissivity can be calculated by the formula, but no general formula for the emissivity will suit the system with scattering of medium and reflection of walls in a coal-fired boiler or an industrial heating furnace. In this study, a new approach was proposed to scale the apparent field directional emissivity by DRESOR method combined with two-color method in a three-dimensional non-isothermal participating medium with reflection of walls. The results obtained by the new method were compared with those calculated by the formula to verify the validity and accuracy of new method in an isothermal purely absorption medium. Then the new method was extended to examine the effect of absorption coefficient, scattering coefficient and reflection of walls on the apparent directional emissivity in the isothermal and non-isothermal cases. It is found that when there is scattering in the medium, the emissivity cannot be equal to the entity, even if the medium is optically thick. In the condition of walls with cold or low temperature, such as in the case of a coal-fired boiler, the apparent emissivity increases with the increase of absorption coefficient and reflectivity of walls, because radiation from hot media plays a dominated role in emissivity in this situation; Meanwhile, in the case of walls with high temperature, such as in the case of an industrial heating furnace in metallurgy or glass melting industry, the apparent emissivity decreases with the increase of absorption coefficient, because the emissivity is mainly determined by the wall radiation in this situation. And when scattering coefficient increases, the apparent emissivity decreases for all isothermal and non-isothermal cases.