simulation a free turbulent jet flames to air atmosphere with Fluent6.3

Hi, everyone:

I am trying to use CFD to simulate a lifted free jet with/without combustion to air atmosphere.I am using Fluent6.3 to find its velocity,pressure and temperature distribution in the air.

My settings of model are:

1.Time : Steady      2.Viscous:Realizable k-epsilon turbulence model      3. Combustion: non-premixed model     4. gravity is included                                    5. material: methane  6. density: ideal gas 

The settings of boundary condition are:

1.inlet: mass flow inlet      2. outlet:pressure-outlet

Problem:

when i simulated a free turbulent jet without combustion & my answer was correct and flow enter to my domain but when i simulated a free turbulent  jet flames & i enabled non-premixed model for combustion my result became incorrect because part of flow is going out of lateral boundary while i expect to enter more flow to my domain because suction at combustion state is more than turbulent state without combustion?

My questions are:

1. what boundary conditions is suitable for outlet boundary, pressure outlet or pressure far field?

2. how to define the size of computational domain? It seems to me that Fluent is very sensitive to the size of domain.

3. could anybody give me some reference about the simulation of free jet.

Thanks in advance!