Semi-transparent boundary conditions
 

Dear community,

I am simulating a hot gas flow, low Re number flow through a square tube with quartz semi-transparent walls.

The gas flow inlet is 1600 K.
The average velocity is 1.9 m/s
The chimney is 55x55 mm cross section

I'm using the k-omega SST model with cell Re numbers close to the walls around 1. (I have a small but fast jet in the center of the chimney, making it non-laminar, but not affecting the flow to much at the wall)

I'm having trouble with the boundary conditions. The heat transport across the walls are essential to model correct, therefore my questions are fundamental:
- How do I estimate the radiation contribution in the best way through the semi-transparent wall (a few mm in thichness), I do not have any material specific information on this particular material.
- If I enable the mixed thermal boundary condition, the solution generates reversed flows in the outlet that will not dissappear?
- is it correctly understood, that if I enable the mixed contribution, define the external gas temperature, then it will autmatically simulate free convection and conduction on the external side without resolving it?
- I'm having doubts about how to interpret the radiation boundary condition panel?

Thanks in advance