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

Abstract

This paper presents measurements of the time evolution of hydroxyl (OH) radicals in premixed hydrocarbon-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 a repetition rate of 10 Hz. The tip electrode shape ensured a uniform streamer discharge in a gap of 8 mm. The reactant flow rate was set at about 20 cm/s to ensure 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 ensure 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. The [OH] profiles show that after generation by the plasma the [OH] persists at significant level for a long time that lengthens with increasing temperature (starting at 500 K), which is not predicted in current kinetic models. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.