Adiabatic burning velocity and cellular flame characteristics of H2–CO–CO2–air mixtures

The objective of this work was to study the effect of dilution with carbon dioxide on the adiabatic burning velocity of syngas fuel (with various H2/CO ratios)-air(21% O2–79% N2 by volume) mixtures along with detailed understanding of cellular flame structures. Heat flux method with a setup similar to that of de Goey and co-workers [1] was used for measurement of burning velocities. Validation experiments were done for H2 (5%)–CO (95%)–air and H2 (5%)–CO (45%)–CO2 (50%)–air mixtures at various equivalence ratios and the results were in good agreement with published data in the literature. The mixtures considered in this work had 1:4, 1:1 and 4:1 H2/CO ratio in the fuel and 40%, 50% and 60% CO2 dilution. The burning velocity increased significantly with the increase in H2 content in mixture of H2–CO with fixed CO2 dilution. The burning velocity reduced remarkably with carbon dioxide dilution in H2–CO mixture due to reduction in heat release, flame temperature and thermal diffusivity of the mixture. The location of peak adiabatic burning velocity shifted from ϕ = 1.6 for 40% CO2 to ϕ = 1.2 for 60% CO2, whereas it remained unchanged with variation of H2:CO ratio (4:1, 1:1 and 1:4) at a given CO2 dilution. A comparison of experiments and simulations indicated that the Davis et al. [2] mechanism predicted burning velocities well for the most of experimental operating conditions except for rich conditions. For some lean mixtures, flames exhibited cellular structures. In order to explain the structures and generate profiles of various field variables of interest, computations of three dimensional porous burner stabilized cellular flames were performed using commercial CFD software FLUENT. Simulations for lean H2 (25%)–CO (25%)–CO2 (50%)–air mixtures (ϕ = 0.6 and 0.8) produced cellular flame structures very similar to those observed in the experiments. It was found that the in the core region of a typical cell, stretch rate was positive, the volumetric heat release rate was high and the net reaction rate for the reaction O + H2 ⇄ H + OH and the net consumption rate of H2 were both high.