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.

[zhang.limin]

diffuse reflectivity = diffuse fraction(1-internal emissivity)
specular reflectivity = 1- diffuse fraction
absorptivity= diffuse fraction* internal emissivity

[HoussameYi]

Dear Zhang ?
Where did u come with those relations ? are you sure they 're right ?

[Nimmy Kovoor]

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.

[amir98]

Hi. I'm working with DO model. I'm confused with absorptivity and absorption coefficient of material.
we have an aluminum wall and we set the absorption coefficient at material part.
we have a formula that absorptivity= diffuse fraction * internal emissivity. what is the difference?
when we have radiation which one determine the absorbed energy?

[waynetsu]

Originally Posted by amir98

Hi. I'm working with DO model. I'm confused with absorptivity and absorption coefficient of material.
we have an aluminum wall and we set the absorption coefficient at material part.
we have a formula that absorptivity= diffuse fraction * internal emissivity. what is the difference?
when we have radiation which one determine the absorbed energy?

Hi, there.

I am also confused about the absorption coefficient and absorptivity.
I read some theory about radiative transfer and it seems that the integration of absorption coefficient over a geometric path can be connected to absorptivity. You can check the book named Radiative Heat Transfer by Michael F. Modest (equation 9.5~9.7).

Wayne