When solving a case in CFX-Solver using Discrete Transfer radiation model with the Surface to Surface (S2S) option, the solver may hang at the first iteration for a long time. Since the discrete transfer model traces the case’s domains with 8 (default number) rays leaving each cell face, solving a very fine mesh with this model is computationally expensive. This, in conjunction with the S2S model’s incapability to coarsen the mesh, causes hangups on iteration 1. A workaround to this problem is to run the case using the Participating Media option instead of S2S. This is because Participating Media coarsens the mesh, causing CFX to solve it faster. When implementing Participating Media, ensure that the coarsening rate produces a few hundred radiation elements so that it will result in a similar solution to one solved under S2S. For more information, please refer to section 10.7.1.2. in the CFX-Solver Modeling Guide, entitled “Transfer Mode”.