For using radiation only in solids, you can use DO model in Fluent. In your model you will need to define cell zone as solid. Then you will need to enable Energy equation under Setting Up Physics > Models. Then in Radiation, please select DO model. Under boundary condition of solid walls, please select semi-transparent under BC Type in Radiation tab. Give other inputs. In Solving tab, go to Controls > Equations and deselect Flow so that flow equations are not solved. Keywords: DO, Radiation, Solids, Fluent