What is Enthalpy?
The energy carried by a flowing gas — both the part due to temperature and the part stored in chemical bonds.
Heating duty, mixing temperature and flame temperature all come from an energy balance written in terms of enthalpy.
A methane–air mixture at room temperature holds a lot of energy in its chemical bonds; burning releases it as a temperature rise.
Used by the mixer (enthalpy is conserved), the heater (heat duty) and equilibrium in HP mode. Values come from the mechanism's thermodynamic data.
h = u + P v
with internal energy u and specific volume v. Because it includes enthalpies of formation, total enthalpy is conserved through reactions in an adiabatic, constant-pressure process.