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This work presents a numerical model to evaluate the energy deposition pathways activated during plasma-assisted ignition of a stoichiometric ammonia/air mixture at atmospheric conditions. To that end, zero-dimensional simulations are conducted of ignition by nanosecond repetitively pulsed discharges (NRPD), for which a detailed energy tracking is performed from the electrical input to the mechanisms that directly influence ignition. The analysis of the plasma energy deposition pathways is relevant to gain insight into complex plasma kinetic mechanisms and help optimize actuation strategies. Results show that the main pathways activated are: (i) slow-gas heating, (ii) fast-gas heating, (iii) dissociation of NH3 into NH2, (iv) dissociation of O2, and (v) dissociation of NH3 into NH. The exact proportion of energy deposited in each pathway is highly dependent on the reduced electric field profile, in particular the "down-slope" dominates the energy breakdown. The influence of the reduced electric field magnitude is studied for square pulses. The correlation between the various plasma energy deposition pathways and NOx emissions is quantified. 

Nanosecond pulsed plasmas have been demonstrated, both experimentally and numerically, to be beneficial for ignition, mainly through gas heating (at different timescales) and radical seeding. However, most studies focus on specific gas conditions, and little work has been done to understand how plasma performance is affected by fuel and oxygen content, at different gas temperatures and deposited energies. This is relevant to map the performance envelope of plasma-assisted combustion across different regimes, spanning from fuel-lean to fuel-rich operation, as well as oxygen-rich to oxygen-vitiated conditions, of interest to different industries. This work presents a computational effort to address a large parametric exploration of combustion environments and map out the actuation authority of plasmas under different conditions. The work uses a zero-dimensional plasma-combustion kinetics solver developed in-house to study the ignition of CH4/O2/N2 mixtures with plasma assistance. A main contribution of the study is the detailed tracking of the energy, from the electrical input all the way to the thermal and kinetic effects that result in combustion enhancement. This extends prior works that focus on the first step of the energy transfer: from the electrical input to the electron-impact processes. Independent of the composition, four pathways stand out: (i) vibrational-translational relaxation, (ii) fast gas heating, (iii) O2 dissociation, and (iv) CH4 dissociation. Results show that the activated energy pathways are highly dependent on gas state, composition, and pulse shape, and can explain the observed range in performance regarding ignition enhancement. The approach can be used to calculate the fractional energy deposition into the main pathways for any mixture or composition, including new fuels, and can be a valuable tool to construct phenomenological models of the plasma across combustion environments. 

In this paper we explore the effects of nanosecond repetitively pulsed discharges (NRPD) on combustion instabilities and NOx emissions of CH4/NH3 flames, in a swirl-stabilized, 6 kW combustor. The combustor has a large dump plane ratio resulting in a turbulent flame with a single compact macrostructure across all operating conditions. Discharges are setup with a central pin-to-cylindrical-wall type electrode configuration with a discharge gap of 4.55 mm at the dump plane. For pure methane, instabilities occur at various frequencies and amplitudes, up to 1000 Pa. NPRD actuation was successful in suppressing instabilities across all conditions, with best reduction being as much as 23 dB. The discharge voltages ranged from 6-9 kV at a pulse rate of 9 kHz, equivalent to 5-12 mJ/pulse energies and plasma powers less than 99W. For pure CH4 flames, the NRPD actuation increased NOx emissions, but for high NH3/CH4 blends, the NRPD actuation reduced NOx emissions. 

We propose that the upcoming Decadal Survey on Solar and Space Physics describe prominent contributions of lightning and its impacts beyond the troposphere, particularly within the NASA Heliophysics portfolio. We present a brief review of several topics highly relevant to NSF and NASA. We opt to unify these topics into one white paper, with longer reviews/references included.