[JMM_UC3M]

Hi all, I am trying to set up a model in Fluent, an easy one, and there is an "edge effect". Let me explain the problem (or discrepancy):
1)Model configuration --> The model is just a tube (with some fluid inside, so forced convection), around the tube there is air (natural convection). and the tube is receiving the solar radiation. There is also an adiabatic wall in the back side of the tube, 0,05 m away from the tube.
2) Boundary conditions --> The setup is that I define a semi-transparent wall in front of the tube (0,5 meters away) in order to introduce within the domain the solar radiation (concentrated solar radiation) (the semi transparent wall do not absorbe any radiation, and I have tried specular and diffusive, just to clarify hehe, also I have tried to do the same with just a wall and this wall at a specifc temperature, and results are exactly the same, as I expected)
3) Problem --> There is what I call "edfe effect", because the radiation close to the edges is lower than in the center of the pipe (attached picture) (the radiation is not the same in the radial direction, this is correct), and I have tried many tests (like defining a black body temperature in inlet and outlet of same of the irradation wall), and the effect is more or less the same (edge effect).

Well, I think it is because Fluent needs to do that in order to calculate the total energy, but, it does not make sense 100%, because if I calculate a control domain, the tube should receive same radtiation flux in the middle of the tube than in the edges... I am trying to figure out how it works, but not a clear overview. To be honest, I have tried many different configurations, and my conclusion is that this is how Fluent calculates the radiation, so, I cannot use the edges, and I need to make sure any parameter I sue is within the real control volum, so smaller than the domain I define in Fluent.

Does anybody know why is that effect? and in positive case, how to parameterize it? Many thanks in advance and best regards.

PS: In addition to this, I do not understand the difference between radiation-->incident radiation vs wall flux-->surface incident radiation, the second one is what I expect, b ut the first one is higher and does not make sense.... If anybody knows about it I would really appreciate!