Kinetic roles of energy transformation during ignition enhancement of NH3/air mixture by non-equilibrium plasma discharge via nanosecond repetitive pulsed discharge

Abstract

In this work, a numerical study and multi-scale process analysis of plasma assisted ammonia ignition with nanosecond repetitive pulse discharge at room temperature and pressure are carried out under varied applied voltages. Analysis of theoretical plasma thermal-chemical instability and Gibbs free energy confidence to ignition spontaneity through plasma intervention are performed. In addition, the present study extends the ability of modelling deposition energy transformation accounted for plasma kinetics interact with vibrational-translational relaxation, electron attachment/detachment. To identify the significant reaction paths on plasma systems reactivity, a plasma-based global pathway analysis (PGPA) was derived from element-flux transfer including repeated nodes within cyclic reaction step (de-excitation to ground state). It is concluded that although a large amount of plasma generated in the pulse discharge causes a rapid but short-lived temperature rise in the system, heat release from chemical reactions during the pulse interval is the primary cause of combustion system heating. Kinetics analysis discloses that oxygen decomposition (O₂=>O) and ammonia dehydrogenation (NH₃=>NH₂) are two key processes stimulating ignition. Furthermore, O₂ is decomposed into O and O(1D) through collisions with electrons and excited state N₂ (N₂(B) and N₂(C)) in the pulse discharge, and O(1D) subsequently relaxes and quenches into O. Ammonia dehydrogenation occurs at the same time as a result of collision dissociation. Provisions on activated radicals and energy transfer at low temperature are made due to plasma participant, thus facilitating to trigger subsequent NH₃ oxidation chain reactions. The present study provides insights and guidance to discover the underlying plasma kinetic roles when performing ignition enhancement of NH₃/air mixture.