A Numerical Study on the Effects of CO2/N2/Ar Addition to Air on Liftoff of a Laminar CH4/Air Diffusion Flame

The addition of exhaust gas to a combustor may cause liftoff of a diffusion flame due to several possible mechanisms. Understanding the relative influence of these mechanisms is of importance for the further development of exhaust gas recirculation combustion technology. The authors present a numerical study on the effects of CO2, N2 (two of primary exhaust gas components) and Ar addition on the liftoff of a laminar CH4/air diffusion flame. A gradual switch-off approach was used to identify the relative importance of the different mechanisms. A detailed reaction scheme and complex thermal and transport properties were employed. The simulation results were validated by comparing the calculated and previously measured critical ratios of the 3 additives for liftoff. The results show that the numerical simulation successfully reproduced the previously measured critical ratios of liftoff for all 3 studied additives. Detailed analysis of the numerical results suggests that the addition of N2 affects flame liftoff due to the sole effect of dilution. On the other hand, the addition of CO2 causes flame liftoff due to the dilution, thermal and chemical effects, with the dilution effect being the most significant one, followed by the thermal and chemical effects. All 3 effects tend to reduce combustion intensity and cause flame liftoff, leading to the smaller critical ratio of CO2 than that of N2. The radiation and transport property effects are negligible for CO2 addition. Ar addition affects flame liftoff due to dilution, thermal, and transport property effects. However, whereas the dilution effect tends to reduce combustion intensity and cause flame liftoff, the thermal and transport property effects tend to increase combustion intensity and resist flame liftoff for Ar addition, which results in the greater critical ratio of Ar than that of N2. Therefore, for the 3 studied additives in this paper, CO2 has the minimum critical ratio, whereas Ar has the maximum for liftoff.