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perdita December 6, 2010 06:00

fluent definitions for mass fractions

im working towards making a model to look at NOx emissions, and im new to fluent, so i've been going through the tutorials and i have a question.

in tutorial 12 (modelling species transport & gaseous combustion), they have a cylindrical combustor burning methane in air. the fuel comes in through a separate fuel nozzle.

when defining the mass fractions at the boundary conditions - im kind of confused as to how they do it. at the main inlet with only air coming in, the mass fraction of O2 is defined as 0.23 (with everything else zero) - which makes sense according to the chemical equation for combustion which they use (CH4 + O2 -> CO2 + H2O). at the fuel injector however, they use a mass fraction of 1 for CH4.

why 1? is it because its only fuel coming in? and if that is the case, then why dont they use 1 for the O2 mass fraction at the main inlet as well? otherwise shouldnt the mass fraction of CH4 be 0.116? or is the 0.23 supposed to be the fraction of O2 in ambient air (in which case isnt the accepted value more like 20-21%)?

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