About radiation model
 

I used DOM, WSGGM, equilibrium combustion model(non premixed) for pure oxygen combustion.

The temperature profile continuously decrease toward downstream, but on the other hand the absorption coefficient increase to flat region.

A steeply flat region exist from middle to outlet. what is the problem?

The flat region of absorption coefficient which is max. 1.4 is also the same as that of temperature profile which is max. 2900K.