I have a case in which the global equivalence ratio is about 0.5. The fuel is methane. Based on the Kinetic Modelling, the flame temperature of that equivalence ratio should be around 1800 K. However, my CFD simulation showed the flame (max) temperature to be about 2400 K. I used the Flamelet Generated Manifold for the combustion model and GRI Mech 3.0 for the Chemical Mechanism. Both the Kinetic Modelling and the CFD were on the same operating conditions, i.e. pressure ~ 16 bar and initial temperature ~ 673 K.
If it is a non-uniform mixture (as in a partially premixed flame), even though the global equivalence ratio is 0.5, there may be regions where the mixture is locally stoichiometric or slightly rich. The flame temperatures can be high in such regions. In that case, it will be better to compare the average temperature of the combustion products, rather than the maximum temperature.
I am puzzled why you are asking this question. You know much more about the case than I do, yet the answer seems apparent to me, an outsider. As you say, equivalence applies to the bulk properties, not to the conditions at the flame front. Isn't the combustion condition at the flame front more "rich" than stoichiometric? The flame mixture passes through stoichiometric at some point, and I would expect a temperature close to the thermodynamic value there.
In many cases, it is simply as a consequence of the bad mesh quality. I have an experience in simulating reactive distillation of methanol, where the maximum temperature is ideally around 400K. However, I observed some locations with the temperature point of higher than 700K. Second possibility is that you did not correctly define Cp in the material properties as a function of temperature. Also one need to ensure that the residuals drop to certain levels of tolerance.
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