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Ideal gas: Ilexan, heat transfer medium, Illingworth, A, Imbedded fins, Immersed bodies: Immersed tubes, in fluidized beds, heat transfer to, Immiscible liquids, condensation of vapors producing Impairment of heat transfer in combined free and forced convection in a vertical pipe, Imperfectly diffuse surfaces: Impingement damage in heat exchangers, Impingement plate: Impingement protection, in shell-and-tube heat exchangers, Impinging jets: Implicit equations, solution of Inclined enclosures, free convective heat transfer in, Inclined flow, effect of on heat transfer to cylinders, Inclined pipes: Inclined surfaces, free convective heat transfer from, Inconel, spectral characteristics of reflectance from oxidized surface of, Induced flow instabilities, in augmentation of heat transfer, Injection: Inlet effects in shell-and-tube heat exchangers, In-line tube banks: Inorganic compounds, solutions of, as heat transfer media, Inorganic substances: Instability, parallel channel, in condensers, Insulators, thermal conductivity of, Integral condensation: Integral finned tubes: Interaction coefficients in heat exchangers, Interaction parameters for binary systems, tables, Interfacial friction, in three-phase (liquid-liquid-gas) stratified flows, Interfacial resistance, in condensation, Interfacial roughness, relationships for, in annular gas-liquid flow, Interfacial shear stress, effect on filmwise condensation, on vertical surface, Intergrannular corrosion, of Intermating troughs, as corrugation design in plate heat exchangers, Intermittent flows: Internal heat sources, temperature distribution in bodies with, Internal heat transfer coefficient, use in transient conduction calculations, Internal reboilers (in distillation columns), characteristics advantages and disadvantages of, Internally finned tubes: International codes for pressure vessels, Interpenetrating continua (as representation of heat exchangers): Intertube velocity, in tube banks, Inviscid flow, compressible, with heat addition, Iodine: Iodobenzene: Iodoethane: Iodomethane: ISO codes for mechanical design of heat exchangers, Isobutane: Isobutanol: Isobutylamine: Isobutylformate: Isobutyric acid: Isoparaffins: Isopentane: Isopentanol: Isopropanol: Isopropylacetate: Isopropylamine: Isopropylbenzene: Isopropylcyclohexane: Isothermal flow, compressible, in ducts, Isothermal gas, radiation heat transfer to walls from, Isotropic materials, elastic properties, Isotropic scattering, Italy, guide to national practice for heat exchanger mechanical design,
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Radiation Transfer Between Specular and Imperfectly Diffuse Surfaces

DOI 10.1615/hedhme.a.000207

2.9 HEAT TRANSFER BY RADIATION
2.9.4 Radiation transfer between specular and imperfectly diffuse surfaces

A. Specular and imperfectly diffuse surfaces

The concept of a perfectly diffuse surface was an artifice introduced to simplify formal mathematical analysis of radiant transfer. The concept is widely used in engineering design and analysis for its convenience, and, surprisingly, the answers so obtained are found in many instances to be remarkably close to the answers found with more realistic analytical models. There are situations — for example, in transmission through long passages with specular side walls — where the assumption of perfectly diffuse reflection will lead to serious error, however. Thus the designer or analyst needs to be able to carry out calculations when one or more surfaces are not perfectly diffuse.

Perfectly diffuse reflection is where the bidirectional reflectance is a constant independent of all four angles, the two angles of incidence and the two of emergence. The antithesis of perfectly diffuse reflection is specular reflection, where the bidirectional reflectance is identically zero for all directions of emergence except the specular angle where it has an integrable singularity. Imperfectly diffuse is the term usually used to denote that the bidirectional reflectance is nonzero but not constant with angles of emergence. Mixed specular diffuse reflection occurs when there is a specular component, for example, from the smooth surface of the binder of a glossy enamel paint, and a (perfectly or imperfectly) diffuse component, for example, from the underlying particles of pigment of such a glossy enamel.

B. The mirror-image concept

The mirror-image concept was introduced formally into thermal radiation transfer analysis by Eckert, Sparrow, and co-workers (Eckert and Sparrow,1961; Sparrow et al., 1964). The concept is useful primarily when the enclosure in question contains only a few plane specular surfaces arranged so that the number of multiple specular reflections is either limited or forms an easily summed chain. The concept is based on the fact that a ray coming from an element of diffuse surface i and reflected by mirror m to an element of diffuse surface j can be regarded as an uninterrupted straight line from i to the mirror image of j. Thus the shape factor of Equation 206.5 can be applied between surface i and the mirror image of j in calculating the transfer between i and j via m. The image of j as seen in m is denoted j(m). The mirror-image shape factor is then written Fi–j(m).

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