Superiority of oxygen as feedstock for a plasma jet igniter in supersonic methane/air streams

Ignition tests by plasma jets were conducted with N-2, O-2, and air feedstocks to clarify what radicals play a more important role in methane/air reaction. Under test conditions of flow Mach 2 and total temperature of 400 K, ignition limits were measured by changing the electrical input power and feedstock flow rates. The experimental results indicated that oxygen plasma jets were superior to nitrogen or air plasma jets, and when the oxygen feedstock flow rate was 60 L/min, the input power at ignition limits was about 0.7 kW, only 13% of the value using air as the feedstock. Spectroscopy measurements revealed the presence of radicals such as nitrogen and oxygen atoms in the torch exhausts and identified the chemical species produced by the reactions of methane/air and radicals by plasma jets. The chemical intermediate products were greatly changed depending on whether the added radical was an oxygen atom or a nitrogen atom; oxygen atoms generated H and OH, important species for augmentation of the reaction rates. These results show that the superiority of oxygen plasma jets for ignition is due to the chemical effect of radicals produced in the jet, and accordingly that oxygen atoms are particularly effective in methane/air from reaction standpoints.