Quantification of transient stretch effects on kernel-vortex interactions in premixed methane-air flames

Relative flame speeds of time-dependent highly curved premixed methane-air flames (spark-ignited flame kernels) interacting with a laminar vortex have been quantified using high-speed chemiluminescence imaging, particle image velocimetry, and piezoelectric pressure measurements. The goals of this study are to improve fundamental understanding of transient stretch effects on highly curved premixed flames, to provide practical insight into the turbulent growth of spark-ignited flame kernels in internal combustion (IC) engines burning light hydrocarbon fuels, and to provide data for IC engine ignition and combustion model development. Lean and rich CH₄-O₂-N₂ flames were tested (φ{symbol} = 0.64, 0.90, and 1.13, with nitrogen dilution to equalize the flame speeds (S_b) in the absence of vortex interaction). Transient stretch rates were varied using three different vortex strengths, and the size of the flame kernel at the start of the vortex interaction controlled by time delay between ignition and vortex generation. Vortex interactions with small (∼5 mm radius) flame kernels were found to increase burning rates for lean (φ{symbol} = 0.64) flame kernels substantially. Burning rates for rich (φ{symbol} = 1.13) flames were decreased, with total flame kernel extinction occurring in extreme cases. These small flame kernel-vortex interactions are dominated by transient stretch effects and thermodiffusive stability, in agreement with premixed flame theory. However, vortex interactions with larger methane-air flame kernels (∼30 mm radius) led to slight flame speed enhancements for both lean and rich flame kernels, with the flame-vortex process dominated by increased flamefront area generated by vortex-induced flame wrinkling.