Plasma enhanced combustion in methane-, ethane-, propane-, and butane-air mixtures below self-ignition threshold

The paper presents results of temporal dynamics of hydroxyl radicals in premixed hydrocarbon-air flows excited by a nanosecond pulsed streamer discharge plasma below a self-ignition threshold. The experiments have been conducted for four different hydrocarbons - methane, ethane, propane and butane with equivalence ratio 0.1 at six different temperature points varying from 300 K to 800 K. Using laser induced fluorescence (LIF) technique the measurements of absolute OH radicals concentration dynamics is achieved by adjusting triggering synchronization between pulsed high voltage generator and Nd-YAG laser. The plasma was generated by sequential pulses of high-voltage (~20 kV), short pulse duration (~10 ns) and extremely short rise time (<; 1 ns) at repetition rate of 10 Hz. The high reduced electric field guarantees efficient electronic excitation and molecular dissociation, while the picosecond scale rising time greatly improves the discharge stability and helps sustaining uniform nonequilibrium plasma. The streamer discharge in premixed hydrocarbon-air flow results in extra large concentration of OH radicals and only about 10 percent increase of gas temperatures, inferred from nitrogen second positive band system spectra. The experiments gave quantitative information on the hydrocarbon reaction dynamics under self-ignition threshold with high initial concentration of radicals.