Ansys Employee
The flame also looks very short, and that may be a lack of mesh resolution. Use domain decomposition and/or bodies of influence to refine the mesh local to the inlets. Use a poly mesh too, tets can be very diffusive in combustion cases.  What did you set the inlet/outlet radiation temperature as? I'm wondering if you're losing too much heat in that way. Try with the radiation model off and compare the results.