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Fundamental Radiation
Published in Je-Chin Han, Lesley M. Wright, Analytical Heat Transfer, 2022
In addition to emission, a surface can reflect, absorb, or transmit any oncoming radiation energy (irradiation). Figure 11.5 sketches an energy balance between irradiation (radiation coming to the surface) and reflection, absorption, and transmission. Absorptivity, reflectivity, and transmissivity are defined as the fractions of irradiation that are absorbed, reflected, and transmitted, respectively. For many engineering gray and diffuse surfaces, we can assume that surface absorptivity is the same as surface emissivity (ελ, θ = αλ, θ, ελ = αλ, then ε = α, but in general ε ≠ α), and the transmissivity approaches zero (except window glass); therefore, the reflectivity can also be determined from the emissivity.
Heat Transfer
Published in Robert K. McMordie, Mitchel C. Brown, Robert S. Stoughton, Solar Energy Fundamentals, 2021
Robert K. McMordie, Mitchel C. Brown, Robert S. Stoughton
Surfaces not only emit photons, they also absorb photons. Consider an enclosure that is maintained at a uniform temperature, and inside the enclosure a perfect vacuum exists. Due to the vacuum, there will be no conduction or convection in the enclosure, and radiation will be considered the only mode of heat transfer present. All the interior surfaces of the enclosure and all objects within the enclosure will emit radiation according to Equation (5-11). For the objects within the enclosure to remain at a uniform temperature, which we know will indeed happen, there must be an absorption of radiant energy (photons) to balance the emission. This is the case and it can be proven that at a given wavelength (radiation has a wave characteristic as well as a particle characteristic), the emissivity is identically equal to the absorptivity (Kirchoff’s Law). Absorptivity is defined as the percentage of incident radiant energy that is absorbed by a surface.
Radiative Properties at Interfaces
Published in John R. Howell, M. Pinar Mengüç, Kyle Daun, Robert Siegel, Thermal Radiation Heat Transfer, 2020
John R. Howell, M. Pinar Mengüç, Kyle Daun, Robert Siegel
Absorptivity is defined as the fraction of energy incident on a body that is absorbed by the body. The incident radiation depends on the radiative conditions (spectral intensity) at the source of the incident energy. The spectral distribution of incident radiation is independent of the temperature or physical nature of the absorbing surface unless radiation emitted from the surface is partially reflected from the source or surroundings back to the surface. Compared with emissivity, the absorptivity has additional complexities, because directional and spectral characteristics of the incident radiation must be included along with the absorbing surface temperature. It is desirable to have relations between emissivity and absorptivity so that measured values of one will allow the other to be calculated. These relations are developed in this section, along with the absorptivity definitions.
Zonal network solution of temperature profiles in a ventilated wall module
Published in Journal of Building Performance Simulation, 2018
T. M. J. Rakotomahefa, Feng Wang, Tengfei (Tim) Zhang, Shugang Wang
By definition, the absorptivity α and the transmitivity τ of a material are its effectiveness in absorbing and transmitting radiant energy, respectively. They are dimensionless numbers that indicate the fraction of incident electromagnetic power that is absorbed or transmitted in a material. For a homogeneous material, α and τ are intimately linked by the equation α + τ + r = 1, where r denotes the reflectivity.