Ansys Employee

You'll need to carefully review the paper to determine this: as I've mentioned before we're here to offer guidance on the modelling, not to try and second guess an author of a paper.  Radiation settings on the inlet & outlet depend entirely on what's up & downstream. 

The two models are covered in the Theory Manual. Very simply DO can deal with absorbing materials, S2S can be computationally cheaper as it uses a one-off ray tracing method.