Characteristics of Propagating H2-Enriched CH4-Air Flames

Abstract

The effects of H2 enrichment on the propagation of laminar CH4-air triple 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. Flames are ignited in a jet-mixing layer far downstream of the burner. Following ignition, a well-defined triple flame is formed that propagates upstream with nearly constant flame displacement speed towards the burner along the stoichiometric mixture fraction line. As the flame approaches the burner, it transitions to a double flame, and subsequently to a burnerstabilized nonpremixed flame. Predictions are validated using measurements of the flame displacement speed. Detailed simulations are used to examine the effects of H2 enrichment on the propagation characteristics of CH4-air triple flames. As H2 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 H2 addition. In addition, the flammability limits associated with the triple flames are progressively extended with the increase in H2 concentration. The flame structure and flame dynamics are also markedly modified by H2 enrichment, which substantially increases the flame curvature and mixture fraction gradient, as well as the hydrodynamic and curvatureinduced stretch near the triple point. For all the H2-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 H2 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.