Simulations of Nanosecond Pulse Plasmas in Supersonic Flows for Combustion Applications

摘要

Nanosecond pulse plasma discharges have demonstrated the ability to significantly reduce ignition delay time in the combustion of supersonic fuel-oxidizer mixtures. Although it is generally believed that such plasmas enhance combustion through fast production of radicals, there is still uncertainty over the detailed kinetics that take place during the pulse, as well as the importance of other mechanisms such as gas heating. In this work, we develop a computational model for study of nanosecond pulse discharges interacting with supersonic oxygen/hydrogen (O_2/H_2) flows and pure-argon flows. Both positive (anodic) and negative (cathodic) pulse polarities are investigated at voltages ranging from 4 to 8 kV. Results indicate that the plasma develops as filamentary streamers, with O radical densities of ~10~(21) m~(-3) (~0.5% by volume of the mixture) in addition to other important radicals such as H, OH, O(~1D), O_2(a_1A_g), and O_2(b_1∑ _g~+). Gas heating is most intense near the electrode edges, due to ion Joule heating densities of ~10~13 W/m~3, with a temperature increase of approximately hundreds of kelvins. Gas heating due to quenching of ion and metastable species also occurs within the bulk of the plasma, but is about an order of magnitude less than ion Joule heating. Although gas heating is observed within the bulk plasma for all O_2/H_2 pulses, heating only takes place at the electrode edges for the argon anodic pulse and no heating is observed in the argon cathodic pulse. Increasing voltages increases plasma volume, peak species densities, and peak gas temperatures. The anodic pulse streamers produce radicals over a greater volume and deeper into the flow than cathodic pulses, which suggests that anodic pulses are better suited to combustion applications.