With the Non-Premixed or Partially Premixed Combustion Models you can specify a so-called “Secondary Stream”. With the Secondary Stream you can model at least three different inlet compositions. In certain situations, you can model even more inlet compositions by combining streams. If you want to specify a mixture containing the Secondary Stream it is important to understand that the normalized mixture fraction (p_sec) is used when the user is prompted for Secondary Mixture Fraction in the Boundary Condition Panel. For example: If you want to specify at an inlet a mixture of 50 % of Fuel Stream composition and 50 % of Secondary Stream Composition, you can write: f_fuel + f_sec + f_ox = 1 0.5 + 0.5 = 1 and f_sec = p_sec * ( 1- f_fuel ) p_sec = f_sec / ( 1- f_fuel) p_sec = 0.5 / (1-0.5) p_sec = 1 –> At the inlet boundary condition you have to specify: Mean Mixture Fraction = 0.5 Secondary Mixture Fraction = 1