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Radiation Exchange between Surfaces: Participating Media. Chapter 13 Section 13.5. General Considerations. General Considerations. The medium separating surfaces of an enclosure may affect radiation at each
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Radiation Exchange between Surfaces:Participating Media Chapter 13 Section 13.5
General Considerations General Considerations • The medium separating surfaces of an enclosure may affect radiation at each • surface through its ability to absorb, emit and/or scatter (redirect) radiation. • Participating media may involve semitransparent solids and liquids, as well • as polar gases such as CO2, H2O(v), CH4, and O3. • Radiation transport in participating media is a volumetric phenomenon, and for • polar gases is confined to discrete wavelength bands. • Beer’s law: A simple relation for predicting the exponential decay of radiation • propagating through an absorbing medium. • Transmissivity and absorptivity of medium of thickness L: • Emissivity of medium. Assuming applicability of Kirchhoff’s law:
An approximate procedure involves use of a mean beam length, • to apply emissivity data obtained for a hemispherical gas mass to other gas • geometries. radius of a hemispherical gas mass whose emissivity, is equivalent to that for the geometry of interest. • Emissivity data have been obtained for a hemispherical gas mass with radiating • species of H2O (v) and/or CO2 in a mixture with other nonradiating • gases. Gaseous Emission and Absorption Radiant Heat Flux from an Emitting/Absorbing Gas to an Adjoining Surface Results depend on Tg – the gas temperature pw, pc – the partial pressures of H2O (v) and CO2 p – the total pressure of the mixture L – the radius of the hemisphere
Fig. 13.20 Figs. 13.18, 13.19 Figs. 13.16, 13.17 correction factor for mutual absorption of radiation for H2O (v) and CO2 Eq. (13.42) Eq. (13.41) Gaseous Emission and Absorption (cont) • For application to other gas geometries, replace L by Le. • Rate of heat transfer to a surface of area As due to emission from an adjoining • gas is • Net rate of radiation exchange between a black surface and an adjoining • gas is
Problem 13.130 Problem 13.130: Heat recovery from flue gases to convert saturated water to saturated vapor.
