Plasma-to-flame transition at the AC driven filamentary discharge ignition

Non-equilibrium plasmas make a new effective way for plasma-assisted ignition. The mechanisms of transition toward a flame kernel are investigated for the case of ignition initiated by the ac-driven discharge thanks to fast optical techniques. The study is done under 1-10 bars pressure range for air-propane mixtures. At atmospheric pressure, filaments of the discharge present two regions with the hotter one near the electrode. Time-resolved imaging coupled with emission spectroscopy reveals that the inflammation takes place all along the hottest part of the filamentary structure. At higher pressure, the region of inflammation extends over the whole discharge because the more important heating of the gas inside. Thus, the plasma-to-flame transition indicates the energy and so the gas temperature distribution along the filaments. By scanning the spatial expansion of the kernel flame, it can be correlated with the chemical and thermal properties of the discharge. It results that thermal effect presents the governing mechanism of the ignition enhanced by the presences of reactive species created by the discharge. These species could play an important role during the spatial expansion of the kernel flame.