Brief communication The chemical effect of CO 2 replacement of N 2 in air on the burning velocity of CH 4 and H 2 premixed flames Fengsham Liu*, Hongsheng Guo, Gregory J. Smallwood Combustion Technology Group, Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario, K1A 0R6 Canada Received 30 May 2002; received in revised form 16 January 2003; accepted 17 January 2003 1. Introduction When CO 2 is added to air or is used to replace N 2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the fol- lowing three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO 2 , and iii) the enhanced radiation transfer by CO 2 . Experimental measurements of the burning velocity of CH 4 /O 2 / CO 2 mixtures at various equivalence ratios and pres- sures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO 2 on the burning velocity of CH 4 /O 2 /N 2 /CO 2 mixtures has been recently studied by Ju et al. [2] and Ruan et al. [3]. Moreover, it has also been pointed out in several studies that CO 2 is not inert but directly participates in chemical reactions primarily through CO + OH 7 CO 2 + H [4-6]. The objective of this study is to numerically investigate the chemical effects of CO 2 replacement of N 2 in air on the burning velocity of lean to stoichiometric CH 4 /O 2 /N 2 / CO 2 and H 2 /O 2 /N 2 /CO 2 mixtures at 1 atm. 2. Numerical model The conservation equations for steady planar freely propagating premixed flames were solved us- ing a CHEMKIN-based code [7]. The thermochemi- cal and transport properties of species were obtained using the codes cited in [6] along with the GRI-Mech 3.0 databases [8]. At a spatial location of x = 0.05 cm, the mixture temperature is fixed at 400 K. In all the calculations, the upstream location (fresh mix- ture) is always kept at x =-2.5 cm. The computa- tional domain was sufficiently long to achieve adia- batic equilibrium in the downstream. The gas mixture temperature at the upstream boundary was kept at 298 K and zero-gradient conditions were specified at the downstream boundary. Chemical reactions are modeled using the GRI Mech 3.0 mechanism [8]. A sample calculation for a stoichiometric mixture of CH 4 /O 2 /N 2 /CO 2 (30% N 2 in air is replaced by CO 2 ) indicated that the NO x chemistry lowers its burning velocity by only about 0.01%. Therefore, reactions and species related to NO x formation were removed in the present calculations. Radiation heat loss was not taken into account. Because the burning velocity, especially for hydrogen, was found to be sensitive to how the multicomponent diffusion velocity was cal- culated [9], the present calculations were conducted with thermal diffusion (Soret effect) included and using the multicomponent expression for diffusion velocity calculation. The strategy proposed in our previous study [6] to numerically isolate the chemical effects of CO 2 was employed. In this strategy, two numerical solutions are obtained: one for CO 2 replacement of N 2 in air, the other for FCO 2 replacement of N 2 . Here FCO 2 is a fictitious species that has identical thermal and transport properties as CO 2 but is artificially ex- cluded from chemical reactions. In addition, FCO 2 was assigned the same third-body collision efficiency as CO 2 in all relevant reactions. * Corresponding author. Tel.: 613-993-9470; fax: 613- 957-7869. E-mail address: fengshan.liu@nrc.ca (F. Liu). Combustion and Flame 133 (2003) 495– 497 0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved. doi:10.1016/S0010-2180(03)00019-1