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

• Published in: Combustion science and technology Vol. 182; no. 11-12; pp. 1549 - 1563
• Main Authors: Guo, Hongsheng, Min, Jiesheng, Galizzi, Cedric, Escudié, Dany, Baillot, Françoise
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01-11-2010; Taylor & Francis
• Subjects: Carbon dioxide; Chemical effects; Combustion; Computer simulation; Diffusion flame; Diffusion flames; Engineering Sciences; Exhaust; Exhaust gas recirculation combustion; Flameless combustion; Laminar; Liftoff; Mechanics; Physics; Thermics; Transport properties; Diffusion flame Exhaust gas recirculation combustion Flameless combustion Liftoff
• ISSN: 0010-2202; 1563-521X
• DOI: 10.1080/00102202.2010.497074

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 CO
2
, N
2
(two of primary exhaust gas components) and Ar addition on the liftoff of a laminar CH
4
/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 N
2
affects flame liftoff due to the sole effect of dilution. On the other hand, the addition of CO
2
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 CO
2
than that of N
2
. The radiation and transport property effects are negligible for CO
2
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 N
2
. Therefore, for the 3 studied additives in this paper, CO
2
has the minimum critical ratio, whereas Ar has the maximum for liftoff.