Any differential equation needs boundary condition to solve! Why we don't need to set outlet B.C. for species transport combustion modeling?

that is how the pressure outlet bc has been defined by fluent..

the mass fractions that you specify will be used in case of reverse flows..

the fact that you dont specify a bc does not mean that it is not using a bc for solving..ofcourse to solve an elliptic equation, u need 2 bcs for every dimension in space..

in fluent, when u initialize the solution, the values at the outlet boundary will be set..

palani.