Two-dimensional simulation of cool and double flame formation induced by the laser ignition under shock-tube conditions

The laser ignition-induced spherical cool and double flame's initiation, propagation, and transition are studied using a two-dimensional simulation with a detailed mechanism using n-heptane/O₂/Ar/He mixture. The current study's primary goal is to reveal how the radicals and flow field produced by the laser ignition lead to cool, hot, or double flame formation and to determine the flame propagation modes. It is found that after the laser spark, the over-driven shock wave results in vortex pairs, which significantly distorts the temperature field and the spatial distribution of the radicals. A torus-like hot flame is ignited at the ignition kernel center instantaneously. The results show that the laser-ignited hot flame can transit to a cool or double flame depending on the laser pulse energy. On the one hand, the hot flame extinguishes under a small laser ignition energy condition, at which the radicals at the hot flame front are transported by the vortex pair and lead to a cool flame formation. On the other hand, when the laser spark energy is large, a transient premixed double flame structure is observed. The double flame can be localized, and the appearance location is primarily affected by the spatial distribution of laser-induced radicals. The cool flame can coexist with a trailing hot flame for milliseconds, dramatically accelerating the hot flame propagation and fundamentally impacting the flame speed interpolation. Computational singularity perturbation (CSP) analysis identifies key reactions that belong to typical low-temperature chemistry in the double flame zone. A comparative simulation case with an initial temperature above the low-temperature chemistry regime confirms that no double flame structure can be formed under high-temperature conditions. The present study provides essential insight and guidance for the flame speed measurement using laser ignition under engine-relevant conditions.