[Volker P.]

Since some versions Fluent offers the possibility to display the Heat of Reaction via the Flux-report for the Total Sensible Heat Transfer Rate. Unfortunately, there are unexplainable discrepancies:

I am currently modeling a combustion (with Fluent 2023R2) using the simple eddy dissipation model. The simplified reaction equation looks like this:
A) 1 Fuel + 7.025 O2 -> 6.3 CO2 + 3.55 H2O .
Typically we model the combustion by a source/sink specification: Air disappears and exhaust gas is released. All our custom field functions (cff) are based on the mass fraction of the “species exhaust gas”. In order to be able to use our cff with the combustion calculation, my idea was to include N2 in the reaction equation. Since the molar proportion of N2 is in a fixed ratio to O2 (air without trace gases was used: 21.1 vol% O2 and 78.9 vol% N2), the following new reaction equation becomes:
B) 26.269 N2 + 1 Fuel + 7.025 O2 -> 6.3 CO2 + 3.55 H2O + 26.269N2'
where N2' has identical properties to N2 and was introduced do be able to differ between N2 belonging to "air" or belonging to flue gas.

If I do my simulation with equation A), the heat of reaction (Energy Sources Due to Reaction under Fluxes and Sensible Heat Release Rate) of Fluent is reported in the same way as the analytical calculation using the standard enthalpies of formation (deviation < 0.1 %). With equation B, on the other hand, deviations of up to +/-20% are observed compared to ΔH_reac=∑_nu_i ΔHi_0; with ΔH_0=standard enthalpie of formation ; nu_i = stoichiometric coefficient

This behavior is incomprehensible to me, as N2 has a standard enthalpy of formation of 0, so it should have no influence on the reaction, and certainly none that leads to an increase in the reaction value.





Does anyone have any idea why this all might be?