Ignition enhancement of NH₃/air mixtures by non-equilibrium excitation in a nanosecond pulsed plasma discharge

This work numerically studies the ignition enhancement of NH₃/air mixtures by nonequilibrium excitation in a nanosecond pulsed plasma discharge. A NH₃/O₃/N₃ plasmacombustion kinetic model validated by in-situ laser diagnostics is employed. The results of nanosecond discharge assisted ignition show that there exists a non-monotonic dependence of ignition delay time on reduced electric field strength. The optimum ignition enhancement is achieved at the electric fields at which the production of electronically excited N₃ and the dissociation of NH₃ and O₃ are most effective. The ignition enhancement by vibrationally excited species at low reduced electric fields is less efficient because the gas heating by vibrational-translational relaxation only provides thermal effects. At fuel-lean conditions, reactions NH₃+ NO = NNH+ OH and NO + HO₃ = NO₃ + OH have larger sensitivities on ignition enhancement due to NO production. The results show that reaction NH₂ + O = NH + OH has a promotive effect on ignition enhancement at fuel-lean conditions whereas shows an inhibitive effect at fuel-rich conditions. The results indicate that ignition enhancement by plasma is more effective at fuel-lean conditions due to the production of O and O(¹D). This work provides insights to understand the kinetic enhancement on NH₃ ignition and the control of NOx emission by non-equilibrium plasma.