Fully coupled modeling of nanosecond pulsed plasma assisted combustion ignition

A coupled 2D computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H-2 mixture (dry air) in a high pressure environment is discussed. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effects of three important plasma properties that influence combustion ignition and flame propagation, namely (a) plasma gas temperature, (b) plasma produced primary combustion radicals O, OH and H densities, and (c) plasma generated charged and electronically excited radical densities. Preliminary OD studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduce the ignition delay of the H-2-air mixture and become especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H-2-air mixture (phi = 0.3) at 3.3 atm and an initial temperature of 1000K. The plasma is accompanied by fast gas heating due to N-2 metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (similar to 0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H-2-air mixture. The generation of other plasma specific species results only in a slight similar to 10% improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H-2(+), O-2(-), O-) and oxygen metastable species (O-2(a1), O-2(b1), O-2*) primarily contributing to combustion enhancement.These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.