Non-gray radiation exchange: the internal fractional function reconsidered
Author(s)Lienhard, John H.
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The radiation fractional function is the fraction of black body radiation below a given value of λT. Edwards and others have distinguished between the traditional, or “external”, radiation fractional function and an “internal” radiation fractional function. The latter is used for simplified calculation of net radiation from a non-gray surface when the temperature of an effectively black source is not far from the surface’s temperature, without calculating a separate total absorptivity. This paper examines the analytical approximation involved in the internal fractional function, with results given in terms of the incomplete zeta function. A rigorous upper bound on the difference between the external and internal emissivity is obtained. Calculations using the internal emissivity are compared to exact calculations for several models and materials. A new approach to calculating the internal emissivity is developed, yielding vastly improved accuracy over a wide range of temperature differences. The internal fractional function can be useful for certain simplified calculations. ©2018
DepartmentMassachusetts Institute of Technology. Department of Mechanical Engineering; Rohsenow Kendall Heat Transfer Laboratory (Massachusetts Institute of Technology)
ASME International Mechanical Engineering Congress and Exposition
Lienhard, John H., V, "Non-gray radiation exchange: the internal fractional function reconsidered." Proceedings of the ASME 2018 International Mechanical Engineering Congress & Exposition (IMECE 2018), November 9-15, 2018, Pittsburgh, USA. Volume 8B: Heat transfer and thermal engineering (New York, N.Y.: ASME, 2018): no. IMECE2018-86386 doi 10.1115/IMECE2018-86386 ©2018 Author(s)
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