Mixture fraction & equivalence ratio invariant to the reaction progress

[CanteraUser]

Hi,

sorry for such a basic question, I am a newbie. How can it be:

"Mixture fraction and equivalence ratio are invariant to the reaction progress. For example, they stay constant if the mixture composition changes to the burnt state"

according to tutorial: https://cantera.org/examples/python/thermo/equivalenceRatio.py.html

From my understanding the equivalence ratio and mixture fraction  values are based on local fuel/air ratio (wiki). In equilibrium state (assuming I understand equilibrium correctly) there is no more reactants (fuel). So how it can be invariant to the reaction progress?

I would be grateful for some clarification.

CanteraUser

[Ssue]

I guess the initial value of equivalence ratio of mixture is chosen as parameter in simulation or experiment. Mixture fraction seems does not change along reaction process since its value is influenced by dilution but not reaction. 

[Ray Speth]

Hi,

You can define equivalence ratio and fuel air ratio in a way that includes the intermediate and product species, and is subject only to knowing what the fuel and oxidizer originally were. This is done by computing the mixture fraction, which in turn is a function of the elemental mass fractions for the fuel and oxidizer. The formulas are given in the C++ documentation for ThermoPhase::equivalenceRatio and ThermoPhase::mixtureFraction.

Regards,

Ray