Effects of Pressure and Preheat on Super-Adiabatic Flame Temperatures in Rich Premixed Methane/Air Flames

Abstract

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.

Keywords:

• Preheat effect
• Pressure effect
• Rich premixed freely propagating flames
• Super-adiabatic flame temperature