Effects of pressure and preheat on super-adiabatic flame temperatures in rich premixed methane/air flames

The structure of freely propagating rich CH4/air flames was studied numerically using detailed thermal and transport properties and the GRI-Mech 3.0 mechanism. Different fresh mixture temperatures and ambient pressures were considered in the simulation to investigate the effects of preheat and pressure on the super-adiabatic flame temperature (SAFT) phenomenon. The occurrence of SAFT in rich CH4/air flames is chemical kinetics in nature and is associated with the overproduction of CH2CO and H2O in the reaction zone followed by their endothermic dissociations in the post-flame region. Preheat lowers the degree of H2O concentration overshoot and results in faster depletion of CH2CO concentration in the post-flame region. Preheat accelerates a rich CH4/air flame to approach equilibrium and suppresses the occurrence of SAFT. Increased pressure reduces the H radical concentration in the reaction zone and increases the overshoot of H2O concentration and the peak concentration of CH2CO. Although pressure also accelerates the approach of a rich CH4/air flame to equilibrium, it actually enhances the degree of SAFT.