Air fuel ratio incongruity
 

Hi everybody,
I have a problem with the calculation of air fuel ratio. I have air and methane entering in a domain with a ratio between their mass flow rates of 17.2. Air is only O2+N2, the flow is non reacting (I use reactingFoam deactivating the reactions). In steady state condition, the air fuel ratio, computed as (N2+O2)/CH4, that is ratio between mass fractions, is lower than 17.2 everywhere in the unique outlet patch. There is no back flow at the inlet.
How is it possible?
How would you compute the air fuel ratio?