# DO Model radiation fluent (Semi-transparent) problem

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 January 29, 2018, 11:21 DO Model radiation fluent (Semi-transparent) problem #1 New Member   Mostafa Join Date: Sep 2014 Posts: 9 Rep Power: 11 Hi, I am using DO model in fluent to simulate 3D double glass layers and the water flows between them. The solar irradiance applied at the first glass layer at 800 w/m2 and this value was divided between three wavelength bands, (1) from 280 nm to 400 nm with (40 w/m2), (2) from 400 nm to 700 nm with (344 w/m2) and (3) from 700 nm to 2000 nm with (416 w/m2). Both the absorption coefficient and refractive index of water and glass are varying with each band. the top surface of first glass pattern is defined as semi-transparent surface and the direct irradiance was applied as mentioned before (boundary condition). During the calculation of fluent, the residual of DO intensity equation does not converge more than 2e-4. After 500 iterations, the do intensity equation still constant. After the calculation process finished (3000 iterations), I checked the radiation heat flux (from wall fluxes). I found that there is no any irradiance transmit to the top surface of the water which interfaces directly with the bottom surface of the first glass layer, Secondly, the backside of first glass layer has negative values of radiation heat flux. Although, the energy equation converged until 1e-13, as well as the temperatures of all walls, are very reasonable and logical. Actually, I tried with all ways to tackle this problem but I didn't find any solution. But when I increased the direct irradiance value to 20000 (w/m2) (not logic), the solar radiation transmits to the water. if anyone has a solution or suggestion to overcome this problem, please don't hesitate to help me. Thanks in advance regards, mostafa

February 20, 2018, 04:35
#2
New Member

Petar Filipovic
Join Date: Nov 2015
Location: Zagreb, Croatia
Posts: 9
Rep Power: 10
Hi Mostafa,

Did you manage to solve your problem?

I am using DO model with semi-transparent BC to simulate heat transfer inside solar collector. So far I discovered that defining material properties of solid (glass in my case) has the biggest impact on the result. I have observed some strange radiation fluxes (surface incident radiation, transmitted radiation etc.) which strongly depend on the refractive index of a material.

In the enclosure, I send you my excel file with wall flux reports in my case.

Best regards, Petar
Attached Files
 air_glass_air.xlsx (18.3 KB, 54 views)

February 20, 2018, 09:20
#3
New Member

Mostafa
Join Date: Sep 2014
Posts: 9
Rep Power: 11
Quote:
 Originally Posted by Petar Filipovic Hi Mostafa, Did you manage to solve your problem? I am using DO model with semi-transparent BC to simulate heat transfer inside solar collector. So far I discovered that defining material properties of solid (glass in my case) has the biggest impact on the result. I have observed some strange radiation fluxes (surface incident radiation, transmitted radiation etc.) which strongly depend on the refractive index of a material. In the enclosure, I send you my excel file with wall flux reports in my case. Best regards, Petar
Dear Petar,

really, i have solved my problem through different procedures. Firstly, for the mesh interfaces, you should convert all mesh interfaces setting into the coupled wall (thermally coupled) and avoid the overlapping in contacts regions.

Secondly, if your incident solar beam is normal to the glass surface, you can set the glass refractive index @ 1. (for the non-gray model).

Thirdly, in order to check the incident radiation heat flux on surfaces, Surface integrals >>>> Area-Weighted Average >>>> Heat Fluxes >>>> Radiation Heat Flux. (if you find its value in negative sign that means the radiation heat flux leaves the surface).

Best regards,
Mostafa

 February 20, 2018, 09:31 #4 New Member   Petar Filipovic Join Date: Nov 2015 Location: Zagreb, Croatia Posts: 9 Rep Power: 10 Dear Mostafa, Thank you for your answer! Can you specify why refractive index can be set at 1 if my beam is normal to the glass surface? Best regards, Petar

February 20, 2018, 10:31
#5
New Member

Mostafa
Join Date: Sep 2014
Posts: 9
Rep Power: 11
Quote:
 Originally Posted by Petar Filipovic Dear Mostafa, Thank you for your answer! Can you specify why refractive index can be set at 1 if my beam is normal to the glass surface? Best regards, Petar
Dear Petar,

When I reviewed the relations to other quantities part regarding refractive index in the following link, I have concluded that.

[URL="http://en.wikipedia.org/wiki/Refractive_index"]https://en.wikipedia.org/wiki/Refractive_index

Best regards,
Mostafa

August 2, 2019, 07:01
#6
New Member

Join Date: Aug 2019
Posts: 1
Rep Power: 0
Quote:
 Originally Posted by sasa mohamed Hi, I am using DO model in fluent to simulate 3D double glass layers and the water flows between them. The solar irradiance applied at the first glass layer at 800 w/m2 and this value was divided between three wavelength bands, (1) from 280 nm to 400 nm with (40 w/m2), (2) from 400 nm to 700 nm with (344 w/m2) and (3) from 700 nm to 2000 nm with (416 w/m2). Both the absorption coefficient and refractive index of water and glass are varying with each band. the top surface of first glass pattern is defined as semi-transparent surface and the direct irradiance was applied as mentioned before (boundary condition). During the calculation of fluent, the residual of DO intensity equation does not converge more than 2e-4. After 500 iterations, the do intensity equation still constant. After the calculation process finished (3000 iterations), I checked the radiation heat flux (from wall fluxes). I found that there is no any irradiance transmit to the top surface of the water which interfaces directly with the bottom surface of the first glass layer, Secondly, the backside of first glass layer has negative values of radiation heat flux. Although, the energy equation converged until 1e-13, as well as the temperatures of all walls, are very reasonable and logical. Actually, I tried with all ways to tackle this problem but I didn't find any solution. But when I increased the direct irradiance value to 20000 (w/m2) (not logic), the solar radiation transmits to the water. if anyone has a solution or suggestion to overcome this problem, please don't hesitate to help me. Thanks in advance regards, mostafa
Dear Mostafa,
I'm using DO model in fluent, the solar radiation is about 740 w/m^2 and I don't know how to divide this value between 5 wavelength bands (like from 200 to 400 nm - 400 to 600 nm and ...) .
Did you use any formula or plot?
I would really appreciate it if you could help me with that.
Sincerely

 October 2, 2023, 08:47 #7 New Member   Bk Join Date: Sep 2016 Posts: 10 Rep Power: 9 I want to do CFD analysis of solar flate plate collector how to set my problem in 2D analysis in ansys fluent....top glass middle air bottom absorber plate... should I go with DO model or Heat flux apply on plate