I'm studying a chimney, gases come from a natural gas furnace. I am not interested in the combustion, so I will measure the composition of the gases an input these results to the inlet of my sistem.

My question is, how can I do this? I'm reading fluent`s manual in the mixture fraction chapter but I am not sure how to start.

Dear Rodrigo Madariaga

It seems that your question is not related to the CFD or Fluent, but to the couple of the theory of chemistry or thermodynamics.

Are you interested in the gas composition, only ? Then you can obtain it by combustion calculation by the chemical relation with operating condition of the furnace. You can calculate the composition by spread sheet calculation if you have operating condition of the furnace, fuel flow rate, fuel composition and air flow rate.

You can not obtain the information from Fluent manual. You can obtain it at the chemistry or thermodynamics textbook. If you can't, send e-mail to me.

When you obtain species composition, then refer 'multiple species' calculation with Fluent.

One additional comment : I think that you do not need multiple species calculation, because your system is already sufficiently mixed. So, single species calculation is enough. You need only material properties for the mixture. If you can calculate gas composition, you can calculate the properties of mixture by couple of simple calculation.

Sincerely, Jinwook