[jlo1124]

Currently doing a thermal analysis to predict the internal temperature of a nuclear fuel rod assembly. I'm struggling with setting up the radiation boundary conditions so I was hoping to get some input here.

Geometry:
I am using a very simplified model for this analysis. It is a solid 2D circle with several smaller holes within to represent nuclear fuel rods. The solid is simply a vacuum.

Boundary Conditions:
These nuclear fuel rods have radiation and internal heat generation. I am applying these conditions to the edges of the smaller holes.
The outside will be submersed in water so I have a temperature boundary condition on the outer edge of the solid 2D circle.

Solution:
Laminar viscous model and DO radiation model. Flow eq is off.

Result:
The entire fluid domain is heating up to a constant temperature that I specify for the radiation temperature. Only near the outer wall does the temperature drop to the wall temp condition. There should be a much more prominent temperature gradient to the outer wall as the fuel rods should be getting hotter towards the center.


QUESTIONS:
For radiation it requests an internal and external emissivity. It seems that the internal emissivity has little to no effect on the resulting temperature. BC type is opaque with diffuse fraction set to 1 (this also doesn't seem to have much of an effect at all). Only when I set the BC to semi-transparent do I get a gradient more similar to what I'm looking for but I don't believe this to be correct.
Are there any recommendations as to why I am getting a constant temperature through the entire fluid domain? The results are much different than the ones from ANSYS static - thermal which is what I'm wanting to achieve.

Eventually I will have solid surfaces where the fuel rods would be but I am getting confused with all the contact surfaces, which will be a problem for another day.

Any help is appreciated!