Fluids

Topics relate to Fluent, CFX, Turbogrid and more

Semi transparent materials – Emissivity in different wavelengths

• Gopalakrishna Gangisetty
Subscriber

Q. How can I give different transmissivity with more than 3 wavelength region?

I used Discrete Ordinates (DO) Model since it is the only model with 'semi-transparent' and 'non-gray' mode.

I want to give material A (semi-transparent) transmissivity or emissivity like this:

ex) t=0.75 in 0 to 4 µm, or e = 0.25

t= 0.75 in 4 ~8 µm, or e = 0.25

t=0.75 in 8~14 µm, or e = 0.25

t = 0.0001 in 14 – 1000 µm, or e = 0.9999
(t is transmissivity), e = emissivity.

But when I change BC type of  a material A from opaque to semi-transparent in boundary condition - radiation tab,
I made four bands, 0 – 4, 4 – 8, 8-14, 14-1000 µm in Model - Radiation - Number of Bands, but I could only change Direct / Diffuse Irradiation and Diffuse Fraction of each band, not transmissivity or emissivity of each band.

Please, someone can guide me, it would be a great help,

Is it possible to give each band different emissivity?

• C N
Ansys Employee

Hello Gopalakrishna,

I recommend you to set the absorption coeffciient for different bands in materials and also set up the wall thickness with some temperature in shell conduction or create a wall solid zone . This will give the absorptivity for each band .

If the semi-transparent wall has thickness, then the thickness and the absorption coefficient determine the absorptivity of the ‘thin’ wall.  Although incident radiation can be absorbed in a semi-transparent wall that has thickness, the default is that the absorbed radiation flux does not affect the energy equation, which can result in an energy imbalance and possibly an unexpected temperature field. The exception is when shell conduction is used (available in 3D only). With shell conduction there is full correspondence between energy and radiation. If the wall is expected to have significant absorption/emission, then it may be better to model the thickness explicitly with solid cells, where practical.

Ansys Fluent does not include emission from the surface of semi-transparent walls (due to defined internal emissivity), except when a temperature boundary condition is defined.

(absorptivity= diffuse fraction*internal emissivity )from this relation also you can try to set the diffuse fraction for each band seperately and also absorptivity through the thickness and absorption coeffcient of the different band in the materials,

By this way, you can apply the emissivity for different bands for non gray semi transparent material in DO model.

I am attaching the user guide link for your reference.

5.3.6.7.1. Semi-Transparent Interior Walls

I hope this helps your simulation.

Thanks,

Chaitanya Natraj