Learning Forum

[gopalakrishna.gangisetty]

I have a skylight for roof top modeled with two different materials

1.        regular window glass which is opaque for longwave radiation.

2.       ZnS glass is 75% (0.75) transparent to the longwave radiation.

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

I made four bands, 0 – 4, 4 – 8, 8-14, 14-50 µm in Model - Radiation - Number of Bands,

Lets assume that :

Topside window glass and top side ZnS glass are exposed to ambient surroundings at 296 K

bottom side window glass and bottom side ZnS glass are located inside building space at 298 K

Now, I turned on solar calculator,

Input the data such as Latitude, longitude and time and data, (this is fine).

Direct solar  irradiation as 660 W/m²

Diffuse solar irradiation as 125 W/m²

Then, I turned on solar BC options for both top side window glass and top side ZnS glass.

Direct and diffuse irradiation values are directly calculated by Ansys Fluent,

I think, Diffuse fraction = Diffuse irradiation/(direct irradiation + diffuse irradiation).

Now, the question is:

For bottom side window glass, for bottom side ZnS glass, I want to set

Direct irradiation = ? Perhaps it is zero in all bands´(1 to 4)

Diffuse irradiations = band 1 to band 4 = (need help here).

Diffuse fraction= band 1 to band 4 = (need help here).  

Please find the attached screen shots for better idea.

 

[AG]

Switch following option ON and fluent will automatically calculate Diffuse and Direct irradiation depending on the solar beam angle. For setting of diffuse fraction user same analogy used for top window glass. Refer 15.3.8.3. Wall Boundary Conditions for the DO Model for details. Diffuse Fraction is the fraction of the reflected radiation from the wall that is treated as diffuse between 0 and 1. 

[gopalakrishna.gangisetty]

 

Hello Sir,

Thank you for your response. In this situation, the top side window glass and top side ZnS glass are exposed to solar radiation as they are located in an open environment. I enabled solar BC options (see screen shot please). 

However, the bottom side window glass and the ZnS glass on the bottom side are inside the building space and not exposed to direct solar radiation. Given this, should I enable the solar calculator? I would appreciate your guidance on this matter.

 

[AG]

Yes, as in this case the bottom side window glass and the ZnS glass on the bottom side are inside the building space and not exposed to direct solar radiation  you can directly put these values of direct and diffuse irradiation to 0 and no need to activate Solar BC option.  

[gopalakrishna.gangisetty]

 

 

Dear Sir,

Thank you once again for your response. I still have a few uncertainties and would appreciate your guidance. (see the picture Sir). 

 

We have assumed a scenario where both the bottom window glass and the bottom ZnS glass are at 298 K. In this setup, diffuse irradiation will be emitted from the bottom window and ZnS glass into the volume inside the skylight, with heat being removed from the top ZnS glass. Given this,  direct irradiation can be set to 0 but can the diffuse irradiations are 0 across all bands? If so, should the diffuse fraction be set to 1 or 0 for all bands?

Our main objective is to eliminate longwave heat radiation from the bottom ZnS glass to the top ZnS glass, considering that the ZnS is 75% transparent to longwave radiation. Could you also provide guidance on the appropriate values of Theta and Phi for the bottom glass surfaces?

Thank you for your assistance.

 

 

[AG]

These are totaly different inputs and concepts. Hope following representative schematic will make it clear:

1. Diffuse and Direct irradiation represent outer solar iradiations on outer wall of the domain. As these irradiation will enter simulation domain only through transparent walls, you will see these settings available for transparent walls only. In your case only top surfaces are exposed to sun and accordingly irradiation values are specified as you did in snapshot above. 
2. Bottom surfaces are not exposed to the sun and should have 0 value for direct and diffuse irrradiations. Read Semi-Transparent Walls for the DO Model from user manual for details. 
3. Diffuse fraction is totally different concept. Whatever radiation get reflected from surface, is it a specular or diffuse in nature that is getting defined by this. Read Semi-Transparent Walls for the DO Model completely to get full understanding of this. It is explined in detail. 
4. As you expect complete beam to get absorrbed in glass and 25% should get absorbed in ZnS glass. This gets defined by propery of material called as Absorption coefficient. You can define it differently for different bands. Read  Gray-Band Absorption Coefficient in user manual for details. 

 

[gopalakrishna.gangisetty]

Dear AG Sir,

Thank you for the help and guiding me, I managed to calculate Absorption Coefficients based on Beer- lambert law.

a_λ = [-ln(tau)/d]; where  tau = transmittance and d = thickness

a_λ for bottom and top window glasses are :

band 1 = 21.07 m^-1;

band 2 = band 3 = band 4 = 1842 m^-1

since I assume tau = 0.1 in band 1, tau = 0.0001 for band 2 to band 4.

Refractive Index I choose 1.5 for all the bands in a regular window glass.

----------------------------------

a_λ for bottom and top ZnS glasses are :

band 1 = band 2 = band 3 = 57.5 m^-1

 band 4 = 1842 m^-1

since I assume tau = 0.75 in band 1, band 2 and band 3; and in band 4 = 0.0001

Refractive Index I choose. Band 1 = 2.2, band 2 = 2.2, band 3 = 2.2 and band 4 = 1.5 [this is from manufacturer data sheet).

 

I would still request your help in understanding in assuming the diffuse fractions for

·         top side window glass, Top side ZnS glass from band 1 to band 4 -----> exposed to solar

·         bottom side window glass , bottom side ZnS glass from band 1 to band 4. -----> not exposed to solar. 

Can you please enlighten me in setting diffuse fraction values in band 1 to band 4?

Thank you Sir.

[gopalakrishna.gangisetty]

Hello Sir,

can you please help me? thank you.