Chemical Effects of Plasma Gases on Flame Kernel Development

A study of the plasma jet ignition of lean methane-air mixtures was conducted to determine the effect of different plasma gases on flame kernel development. A plasma igniter, incorporating a shutter that separated the gases in the igniter from the reactant gases in a combustion chamber before discharge, allowed any combination of gases to be used without mixing. Measurements of the two-dimensional distribution of OH concentration by laser-induced fluorescence in a diametral plane above the igniter yielded information on the ignition process and the subsequent flame kernel development. The methane-air mixtures chosen for study had equivalence ratios, φ, near the lean flammability limit of φ = 0.53. To differentiate OH formation in the initial plasma from that generated during mixing and reaction with gas in the combustion chamber, experiments were conducted using the following combinations of plasma media and reactant gases: H_2 and 2H_2 + O_2 plasmas into air; Ar, N_2, H_2 and 2H_2 + O_2 plasmas into φ = 0.50 CH_4-air; and 2H_2 + O_2 plasma into φ = 0.65 CH_4-air. In addition to OH measurements, the pressure in the combustion chamber was measured, and Schlieren photographs were taken. Results indicated relatively small, chemically active regions, generally off-axis and often associated with vortices. Measurements in lean mixtures that are known to discriminate strongly between plasma of different effectiveness confirm the higher incendivity, for the same total energy, of chemically active plasmas and demonstrate the higher concentration and longer persistence of OH during the approach to steady state flame conditions. Such chemically active plasmas promote combustion for hundreds of milliseconds in normally non-flammable sub-limit mixtures.