Ignition and propagation characteristics of hydrogen-enriched methane flames

The effects of H-2 enrichment on the ignition and propagation of laminar CH4-air flames in axisymmetric coflowing jets are numerically investigated. A comprehensive, time-dependent computational model, which employs a detailed description of chemistry and transport, is used to simulate the transient ignition and flame propagation phenomena. Because fuel-air mixtures can potentially be ignited due to the presence of either a high-temperature zone or a radical pool, we explore temperature-induced ignition and radical -poo l-i nduced ignition. It is observed overall that radical-pool-induced ignition is more effective than temperature- induced ignition for igniting any H-2-CH4-air mixture. With increasing the radical-pool concentration ignition is facilitated and the total ignition time for the mixture to reach the ignition conditions is significantly reduced. In addition, as H2 mole fraction in the fuel jet increases, the mixture is more easily ignited. To study the propagation characteristics of these flames only radical-induced ignition is used since it is more effective. Following ignition, a well-defined triple flame, containing a rich premixed, a nonpremixed, and a lean premixed reaction zone, is formed that propagates upstream with nearly constant flame displacement speed towards the burrier. As the flame approaches the burner, it transitions to a double flame, and subsequently to a burner-stabilized nonpremixed flame. Predictions are validated using measurements of the flame displacement speed. Detailed simulations are used to examine the effects of H-2 enrichment on the propagation characteristics of CH4-air triple flames. As H-2 concentration in the fuel blend is increased, the flame displacement and propagation speeds increase progressively due to the enhanced chemical reactivity, diffusivity, and preferential diffusion caused by H-2 addition. In addition, the flammability limits associated with the triple flames are progressively extended with the increase in H-2 concentration. The flame structure and flame dynamics are also markedly modified by H-2 enrichment, which substantially increases the flame curvature and mixture fraction gradient. For all the H-2-enriched methane-air flames investigated in this study, there is a negative correlation between flame speed and stretch, with the flame speed decreasing almost linearly with stretch, consistent with previous studies. The effect of H-2 addition is to modify the flame sensitivity to stretch, as it decreases the Markstein number (Ma) and increases the flame tendency towards diffusive-thermal instability (i.e. Ma -> 0). These results are consistent with the previously reported experimental results for outwardly propagating spherical flames burning a mixture of natural gas and hydrogen.