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Thermal and radiation boundary conditions settings for atmospheric pressure inlet/out

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Old   November 19, 2022, 19:25
Default Thermal and radiation boundary conditions settings for atmospheric pressure inlet/out
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Jane
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Hello,

I am calculating the thermal radiation emitted from a turbulent diffusion flame generated by a burner placed on the ground. I always find the distribution of the incident thermal radiation on the bottom wall or say ground, unreasonable. The radiation model used here is P-1. I suspect it is caused by wrong boundary conditions settings. Please see the attached picture for my case description and the boundary conditions applied. (My model is 3D but I showed it as 2D here for simplicity)

What should be the internal emissivity for the pressure inlet/outlet when they stand for the atmospheric boundary? And what about the thermal setting for my bottom wall which represents the ground?

Thanks!

Case description and BC.JPG

Pressure inlet and outlet thermal BC.JPG

Plenum wall BC.JPG
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Old   November 28, 2022, 02:57
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Quote:
Originally Posted by DomBadger View Post
Hello,

I am calculating the thermal radiation emitted from a turbulent diffusion flame generated by a burner placed on the ground. I always find the distribution of the incident thermal radiation on the bottom wall or say ground, unreasonable. The radiation model used here is P-1. I suspect it is caused by wrong boundary conditions settings. Please see the attached picture for my case description and the boundary conditions applied. (My model is 3D but I showed it as 2D here for simplicity)

What should be the internal emissivity for the pressure inlet/outlet when they stand for the atmospheric boundary? And what about the thermal setting for my bottom wall which represents the ground?

Thanks!

Attachment 92384

Attachment 92385

Attachment 92386
Hello DomBadger:
I think each of the boundary conditions were set such that not heat flux was generated, and the internal emissivity values were set to 0. This ensures that radiation generated within the domain comes solely from the flame. I hope the advice should help you.
In addition, I have a question to ask you. I'm calculating a flare radiation and I think Fluent gives me wrong results. As the thermal radiation decays from the center of the flame, it should be 0 kW/m2 at a distance, but fluent's incident radiation is about 1.83 kW/m2. I want some advices!!!
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Old   November 28, 2022, 10:53
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Hello He,

I noticed a non-zero incident thermal radiation value at a distance as well. The value is also ~1.83 kW/m2. I am thinking of just having air filled in my domain and the same atmospheric boundary conditions set to see what the incident thermal radiation is on the ground. Probably that is ~1.83kW/m2 everywhere. But still, even if this is true, what is wrong in our Fluent setting? I don't expect such a high value emitted from merely the air at ~300K. Have you found a solution yet?

Additionally, I don't quite agree with your opinion of setting internal emissivity to be 0 for pressure inlet/outlet. In this way, the thermal radiation emitted from the flare will all be, reflected back, which is not true for the infinite atmosphere around.

Update: when I have air filled in the domain and the same atmospheric boundary conditions set to see the incident thermal radiation on the ground, it turned out the value indeed is ~1.83kW/m2 everywhere

Last edited by DomBadger; November 29, 2022 at 22:13.
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Old   November 29, 2022, 22:10
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So I found out that the right one to be used in this scenario is Wall fluxes--Surface incident radiation. With the DO model, this option is available.

However, my question is, when using the P1 model and this surface incident radiation is not available, what should I do?
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boundary condition wrong, emissivity, pressure boundary, radiation model, wall boundary condition

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