Learning Forum

[Jiezougt]

Hello All,

In fluent, I am trying to model asphalt on top of concrete under radiation, conduction, and convection. Below are the questions:

1. There is a Absorption Coefficient [m-1] in the material definition.
2. There is a Absorptivity under Boundary Conditions/Wall/Any wall window. There is a section call "Absorptivity" for both Direct Visible and Direct IR.

Which one is the place to set absorptivity of concrete (absorption coefficient of concrete = 0.65)? Why there are two different locations for the potentially same parameter?

Emissivity of concrete (0.88) should be set under Boundary Conditions/Wall/Any wall window, under "Internal Emissivity". Is this correct?

Thanks!

Brandon

[DrAmine]

Absorption Coefficient is related to the medium participating in radiation and refers to the amount of radiation being absorbed over certain length (second term of the RTE equation on the LHS). Absorptivity is surface related phenomenon. If radiation hits an opaque wall a portion is is absorbed and part is reflected (1-emissivity). When a ray travel through a medium it gets absorbed according to the absorption coefficient so the intensity is reduced.

I think what you are referring to it to be provided under Radiation Tab for the walls.
The internal emissivity is used to compute the radiative exchange in the domain whereas external emissivity refers to radiation exchange with the exterior and it is there even if you do not activate any radiation model. -> Internal property.

To answer to the whole questions we need more information about model and if you have transparent walls etc but perhaps the explanation provided is already enough.

[Jiezougt]

Thanks for the answer DrAmine!
I would love to explain the whole model information.
A concrete box girder bridge with concrete deck.
Top surface of the concrete deck to be modeled as asphalt (through changing the parameters under Radiation Tab for the Walls)
All walls are opaque. No transparent walls.
Please see attachments: Overall model, Absorption Coefficient, Absorptivity, and Internal Emissivity.
After reading your answer, I come to an understanding that:
Absorption Coefficient (alpha in RTE) is the absorption for fluid medium (in this case,is the air), or glass, which is not opaque. So concrete surfaces should not need this parameter. Related to this fluid thickness. Unit:1/length.
Absorptivity (surface related) is for the opaque body, "flux absorbed/flux received" which is unitless.
There is no external emissivity option in the wall boundary under radiation tab. Only internal emissivity.
In this case, the internal emissivity will be for the domain of concrete body, and domain of the air surrounding the concrete, which from research is around 0.65.

[Jiezougt]

[Rob]

We are not permitted to open the attachments.
Re the absorption coefficient for solids, if the solid is opaque then the material doesn't need an absorption coefficient, however the solver doesn't know if a solid is used in a semi-transparent region or not at the point the material properties are assigned so a value is requested. The value is related to the material property, and not it's dimension.