Mapping the performance envelope and energy pathways of plasma-assisted ignition across combustion environments

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 ${\mathrm{CH}}_4/O_2/N_2$ 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) $O_2$ dissociation, and (iv) ${\mathrm{CH}}_4$ 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.

Novelty and significance statement

This work maps the performance of plasma-assisted ignition over a broader range of combustion environments than prior works. Whereas most works focus on fuel/air mixtures, this work quantifies the impact of fuel content and oxygen dilution on plasma actuation. This is relevant to determine the possibilities of using plasma ignition across industries. The novelty of the model presented is the accurate tracking of the energy deposited by the plasma and the identification of the chemical pathways activated by the plasma. Although it is recognized as critical in the description of plasma-gas interactions, detailed energy tracking is often omitted in plasma chemical simulations. In this work, we present a methodology applicable to any plasma chemical kinetic model, which allows for the analysis of the energy share at multiple timescales.