Advantage of using oxygen as plasma jet feedstock for methane/air ignition

Methane/air ignition capability was investigated through ignition tests with nitrogen, air, and oxygen plasma jets under Mach 2 methane/air streams in combination with spectroscopic measurements and numerical simulations. Experimental results show that oxygen plasma jets can ignite methane/air streams even at an input power of 600 W, whereas air and nitrogen plasma jets need 3200 and 4900 W, respectively, all at the same feedstock flow rate of 30 L/min. Spectroscopic measurements clarify that the process of methane/air ignition is affected differently by the additive radicals N or O produced in each feedstock. N radicals, induced strong emission of CN and NH at the initial stage. The temperature obtained by spectroscopy suggests that the superiority of oxygen to nitrogen is not based on differences in their molar dissociation energies, but on differences in their ignition processes. Numerical simulations show that a combination of additive O and O-2 is superior for shortening the ignition delay time to a combination of additive N and N-2. The overall results obtained by this study reveal that using oxygen as plasma jet feedstock for methane/air ignition is advantageous not only because of the active radicals produced from feedstock, but more importantly because of the nonradicalized feedstock, which contributes to reaction enhancement through the change of local mixture concentration.