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

Abstract

This work numerically studies the ignition enhancement of NH3/air mixtures by non-equilibrium excitation in a nanosecond pulsed plasma discharge. A NH3/O2/N2 plasma-combustion 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 N2 and the dissociation of NH3 and O2 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 NH2 + NO = NNH + OH and NO + HO2 = NO2 + 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.