creating a round burner

[Dukeswp]

hello guys, as a novice using FDS, I want to know how to create a round burner in Pyrosim. I have tried to create the round burner by rotating obstacles, but it does not look good. And second, If I choose to create a circular vent from the vent menu, the burner does not burn. Meanwhile, I tried to create one by CAD. However, I found that I was unable to select one side (the top interface) of the model as a burning interface. I do really want to know how you there build a satisfying round burner?
Thanks!

[rmcdermo]

Have a look at the FDS user guide sections on MULT and Complex Geometry (GEOM).