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

This paper presents measurements of time evolution of hydroxyl (OH) radicals in premixed HC-air flows in the afterglow of a nanosecond pulsed discharge at atmospheric pressure. The temperature ranged from 300 to 800 K (below the self-ignition point). The fuels were methane, ethane, propane and butane, at an equivalence ratio of 0.1 from 400 to 800 K. The plasma was generated by 20 kV pulses of 10 ns duration and a <1 ns rise time at repetition rate of 10 Hz. The tip electrode shape ensured a uniform streamer discharge in the gap of 8 mm. The reactant flow rate was set at ~20 cm/s so that each discharge pulse occurred in a fresh gas mixture. Laser induced fluorescence was used to measure the concentration of OH radicals after the discharge. The energy of the excitation laser was adjusted to insure that the measurements were made under saturation conditions for all experiments. The time evolution of OH radicals was tracked by adjusting the delay time between the high-voltage pulse and the concentration measurement.