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In radiative heat transfer
Radiative heat transfer

In radiative heat transfer, heat is transferred between bodies by electromagnetic radiation. In natural radiative heat transfer , the spectrum of this radiation is that of a black body, and its power depends on the fourth power of the absolute temperature of the body....
, a view factor is the proportion of all that radiation which leaves surface and strikes surface .

In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surfaces and surface segments.

View factors are also sometimes known as configuration factors, form factors or shape factors.

example, consider a case where two blobs, 'A' and 'B' are floating around in a cavity 'C'.






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In radiative heat transfer
Radiative heat transfer

In radiative heat transfer, heat is transferred between bodies by electromagnetic radiation. In natural radiative heat transfer , the spectrum of this radiation is that of a black body, and its power depends on the fourth power of the absolute temperature of the body....
, a view factor is the proportion of all that radiation which leaves surface and strikes surface .

In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surfaces and surface segments.

View factors are also sometimes known as configuration factors, form factors or shape factors.

Summation of view factors


Because all of the radiation leaving a surface is a fixed amount, one can add up all of the view factors from a given surface , and they will always add up to one:

For example, consider a case where two blobs, 'A' and 'B' are floating around in a cavity 'C'. All the radiation that leaves surface A must either hit surface B or the cavity surface C, or if the surface A is concave, might again hit A. In terms of fractions, 100% of the radiation leaving surface A is divided up between surfaces A, B, and C, which is equivalent to the expression above.

Confusion often arises when considering the radiation that arrives at a target surface. In that case there, summation of view factors does not apply, because each 'incoming' view factor is a fraction of the radiation leaving some other surface: there is no reason for those fractions to add up to anything at all; in fact they can add up to less than one, or many times one, but this value has no significance.

Self-viewing surfaces


For a convex
Convex

The word convex means curving out or bulging outward.Convex or convexity may refer to:Mathematics:* Convex set, a set of points containing all line segments between each pair of its points...
 surface, no radiation can leave the surface and then hit it later, because radiation travels in straight lines. Hence, for convex surfaces,

For concave
Concave function

In mathematics, a concave function is the additive inverse of a convex function. A concave function is also synonymously called concave downwards, concave down, convex cap or upper convex....
 surfaces, this doesn't apply, and so for concave surfaces

Reciprocity


The reciprocity theorem for view factors allows one to calculate if one already knows . Using the areas of the two surfaces and ,

View factors in a triangle


For a triangle with sides of length , and , the view factor is given by

Hottel's crossed string rule


The crossed string rule allows calculation of radiation transfer between opposite sides of a quadrilateral, and furthermore applies in some cases where there is partial obstruction between the objects. For a derivation and further details, see .

See also

  • Radiosity
    Radiosity

    Radiosity is a global illumination algorithm used in 3D computer graphics rendering . Radiosity is an application of the finite element method to solving the rendering equation for scenes with purely diffuse surfaces....
    , a matrix calculation method for solving radiation transfer between a number of bodies.


External links

A large number of 'standard' view factors can be calculated with the use of tables that are commonly provided in heat transfer
Heat transfer

Heat transfer is the transition of thermal energy or simply heat from a hotter object to a cooler object . When an object or fluid is at a different temperature than its thermodynamic system or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surround...
 textbooks.

  • , a computer program (FOSS
    Foss

    Foss may refer topeople*Foss , people with the last name Foss*Foss Shanahan , New Zealand diplomat*Foss Westcott , English bishop*Lasse Bergan-Foss , Norwegian footballer...
    ) for calculating view factors in 2D and 3D.