Brief communicationThe chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames
Introduction
When CO2 is added to air or is used to replace N2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the following three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO2, and iii) the enhanced radiation transfer by CO2. Experimental measurements of the burning velocity of CH4/O2/CO2 mixtures at various equivalence ratios and pressures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO2 on the burning velocity of CH4/O2/N2/CO2 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 CO2 is not inert but directly participates in chemical reactions primarily through CO + OH → CO2 + H [4], [5], [6]. The objective of this study is to numerically investigate the chemical effects of CO2 replacement of N2 in air on the burning velocity of lean to stoichiometric CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures at 1 atm.
Section snippets
Numerical model
The conservation equations for steady planar freely propagating premixed flames were solved using a CHEMKIN-based code [7]. The thermochemical 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 mixture) is always kept at x = −2.5 cm. The computational domain was sufficiently long to achieve
Results and discussion
In all the results presented below, the mole fraction of O2 in the oxidizer is kept at 0.21. Fig. 1 shows the burning velocities of methane and hydrogen mixtures. In the CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures, 30% (volume basis) N2 in air is replaced by CO2. Also shown in Fig. 1 are the experimental data of Zhu et al. [1] for CH4/air and CH4/O2/CO2 mixtures and the data of Law [11] for H2/air mixtures.
The effect of the very distinct thermal and transport properties of CO2 from N2 on the
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