I know radiation can be used as a heating source in thermal simulation but I can't find any resources describing how to apply it in a flow simulation. I am modeling a furnace where the pipes are used to cool the furnace so they have water flowing through them and hot air moves outside the pipes. The melted steel is several feet below the pipes and I want to see if the glow from the steel has a significant effect on the pipes' temperature. The steel is 3000F.