Judy Cooper
Ansys Employee

Hi Johann:

The results of the partially premixed model are sensitive to turbulence, so an area that is borderline for ignition may be quenched/reignited if turbulence is low/high.  When there is no boundary layer mesh at walls, articifial turbulence may be generated by the solution of the turbulence equations, because bad values are produced. 

For the issue at the outlet, pay attention to the turbulence and mixture fraction backflow values.  I believe the turbulence intensity defaults to unrealistically high values on backflow.  If the outlet is experiencing any backflow or recirculation, this can also generate artificial turbulence. It may be better to extend it so that backflow is avoided entirely, however.