Effects of CO₂ Addition on the Turbulent Flame Front Dynamics and Propagation Speeds of Methane/Air Mixtures

Exhaust gas recirculation (EGR) is one of the most promising methods of improving the performance of power-generating gas turbines. CO₂ is known to have the largest impact on flame behavior of any major exhaust species, but few studies have specified its thermal, kinetic, and transport effects on turbulent flames. Therefore, in this study, methane/air mixtures diluted with CO₂ are experimentally investigated in a reactor-assisted turbulent slot (RATS) burner using OH planar laser-induced fluorescence (PLIF) measurements. CO₂ addition is tested under both constant adiabatic flame temperature and variable adiabatic flame temperature conditions in order to elucidate its thermal, kinetic, and transport effects. Particular attention is paid to CO₂'s effects on the flame surface density, progress variable, turbulent burning velocity, and flame wrinkling. The experimental measurements reveal that CO₂'s thermal effects are the dominant factor in elongating the turbulent flame brush and decreasing the turbulent burning velocity. When thermal effects are removed by holding the adiabatic flame temperature constant, CO₂'s kinetic effects are the next most important factor, producing an approximately 5% decrease in the global consumption speed for each 5% of CO₂ addition. The transport effects of CO₂, however, tend to increase the global consumption speed, counteracting 30-50% of the kinetic effects when the adiabatic flame temperature is fixed. It is also seen that CO₂ addition increases the normalized global consumption speed primarily through an enhancement of the stretch factor.