hay there,,,,,,,

if we havnot any knowen information about the outlet condition what i can do in the boundry condition for outlet region ?

say i make mixture between air and natural gas in aburner

i dont know what the result of mixture?

and

if i want study length of flame the study finish in the end of geometry and dosnot complet copetional domain ?

i want  advice ? what i can do to simulat this burner?