On the competing roles of turbulence and differential diffusion in facilitated ignition

Recent experiments (Wu etal., 2014) demonstrated that ignition of a combustible mixture by a high-energy kernel can be facilitated by turbulence for certain conditions. This is contrary to the common notion that ignition in turbulence is more difficult than in quiescence because of the increased dissipation of the deposited energy. In this study, we extend our investigation to a larger range of mixtures and turbulence intensities to explore possible limitations of such facilitated behavior. First, we calibrate our ignition system to quantify the deposited ignition energy. Next, using a lean hydrocarbon/air mixture with Le?>?1, we show that: (1) it is possible to ignite a mixture in a turbulent environment with an ignition energy that is not sufficient to ignite the same mixture in a quiescent environment, and (2) there exists an upper limit of turbulence intensity beyond which such facilitation is not feasible. Through detailed experiments an ignition map for such mixtures is constructed, which shows a window of ignition energy within which this non-monotonic transition is observed. The study also identifies a minimum ignition energy required to achieve either quiescent or turbulence-facilitated ignition for the Le?>?1 mixtures, and demonstrates that Le?<?1 mixtures do not show such facilitation. These results are further validated with diluted hydrogen/oxygen mixtures.