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Meaning of diffuse fraction,emissivity,absorption.

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Old   February 19, 2007, 12:24
Default Meaning of diffuse fraction,emissivity,absorption.
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Ferdinando
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Hi all, i'm working with DO radiation in Fluent. I want to know better the meaning of "diffuse fraction","internal emissivity","absorption". If i have a UV lamp defined as wall that emitted radiation 500 W/m^2, how can i define a near wall as a mirror? which parameters i must set? i want a specular reflection, not absorption,not trasmissivity. Please help me. Thanks in advance.
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Old   August 16, 2009, 13:00
Default diffuse reflectivity specular reflectivity absorptivity
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limin
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diffuse reflectivity = diffuse fraction(1-internal emissivity)
specular reflectivity = 1- diffuse fraction
absorptivity= diffuse fraction* internal emissivity
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Old   March 31, 2017, 08:58
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Houssame
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Dear Zhang ?
Where did u come with those relations ? are you sure they 're right ?
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Old   November 7, 2017, 08:51
Default DO modelling of a mirror like surface
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Diffuse fraction: decides the portion of the reflected radiation that is diffusely reflected. The remaining fraction would be specularly reflected. Diffusely reflected energy= (diffuse fraction)*(1-internal emissivity)*incident radiation. Specularly reflected energy = (1-diffuse fraction)*(incident radiation).

Internal emissivity: it is the emissivity of the wall that faces the computational domain (the cell zone). Absorption at the wall surface assumes that the absorptivity is equal to the emissivity. Internal emissivity value is therefore quintessential to model the absorption and diffuse reflection according to the equations: Absorbed energy = (diffuse frcation)*(internal emissivity)*(incident radiation). And diffusely reflected energy follows the equation above.

Absorption: Explained above

Basically the conservation equation of reflectance + transmittance + absorptance =1 should be respected. In case of an opaque wall, there is no transmittance and hence only absorption (which is modelled via internal emissivity) and reflection exists. Now, if the value of internal emissivity at an opaque wall is set to 0 (that would make the absorption to be 0 also), then all the incident energy at that wall is reflected. Now, again if the diffuse fraction value is put in as 0, this would then mean that all the incident radiation is reflected specularly like a mirror.
So, in the radiation tab of the wall select: opaque wall + internal emissivity=0 + diffuse fraction = 0 this combination of parameters should basically mean a mirror like wall.
Follow the link :
http://www.afs.enea.it/project/neptu...th/node115.htm
for all relating equations.

Last edited by Nimmy Kovoor; November 7, 2017 at 08:54. Reason: Typo errors
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