non-premixed combustion in the open space
 

hello, everyone
i am trying to simulate a poolfire in the open space these days. but i can't get the right result. the boundary conditions are labeled on the following picture.
chosen models: K-ε model; non-premixed combustuion model;prePDF
material: ch4-air


[IMG]file:///D:/geo.jpg[/IMG]
and the temperture field is as the following picture
[IMG]file:///D:/result.jpg[/IMG]


it seems that the outside air can't disperse into the computation area smoothly.
Please tell me the reason of the bad result, thank you !