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1. Streamer models for development of predictive tools for plasma-assisted combustion

   Reyes, Stuart; Dhali, Shirshak K (October 2024, APS)

   Many positive laboratory results that have been reported in which non-thermal plasmas, particularly repetitive nano second pulses, showed a reduction of ignition time delay and extension of flammability limits. However, there is a need for predictive models for designing practical systems. We present the results of a self-consistent model and simulation results of plasma assisted combustion of hydrogen air fuel mixture. The electrical discharge phase is modeled as a streamer discharge which is followed by the combustion kinetics phase. Nonequilibrium population of excited states leads to an increase in the reactivity and facilitates ignition and flame propagation. We have quantified some macroscopic properties of streamers such as radical production efficiency which will lead to the development of predictive tools. The concentration of radicals depends on the electrical energy density which is critical in determining ignition. We find that short duration streamers do not deposit enough energy to ignite hydrogen air mixtures. Also, the spatial and temporal electric energy density will influence the ignition delay and flame propagation velocity etc. 

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2. Plasma Chemistry and Applications of Atmospheric Pressure Transient Electrical Discharges

   Dhali, Shirshak K (July 2025, Springer Series on Atomic, Optical, and Plasma Physics; Springer: Berlin/Heidelberg, Germany.)

   Guharay, S; Wada, M (Ed.)

   At or near atmospheric pressure, overvolted gas breakdown results in a streamer formation. In many applications of non-thermal plasma where efficient excited species generation is critical, the streamers are quenched to prevent it from reaching the arc phase. This can be achieved by repetitive nano second pulsing or dielectric barrier discharges were the dielectric charging quenches the arc formation. In such discharges, the plasma characteristics such as electron and ion densities and the production of excited species is determined by the streamer properties. Over the past five decades, a vast amount of experimental and computational work has been accumulated to establish a well-accepted theory of streamer formation and propagation. In this article we discuss the fluid models for streamers and quantify some macroscopic properties which can inform specific applications. We discuss in detail the fluid equations needed to model streamers and several schemes of parametrization of the transport and electron collisional processes. From an application point of view, the steamer simulations are used to quantify the excited species production by electron impact. This information is used to predict the specific outcomes via the plasma chemical conversion pathways. We present results of streamer discharges for three applications which are of technological importance to illustrate this approach: Plasma-assisted combustion, remediation of toxic gases, and plasma medicine. For plasma-assisted combustion the results of hydrogen ignition are discussed since non-hydrocarbon-based fuels such as hydrogen and ammonia are potential fuel candidates to reduce greenhouse gases. For the remediation of toxic gases, we discuss the removal of SOX/NOx from flue gas. Plasma medicine is a relatively new field and repetitive nano-second pulsed discharges in a helium gas carrier shows promise as a reactive plasma source for treating biological material. We discuss the helium metastable production in a streamer discharge since this species leads to the production of OH radicals which plays an important role. 

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3. Effects of spatiotemporal plasma power distribution on the modeling of ignition kernel evolution in quiescent and turbulent methane/air mixtures

   https://doi.org/10.1088/1361-6463/ad6881  

   Johnson, Praise_Noah; Taneja, Taaresh_Sanjeev; Yang, Suo (August 2024, Journal of Physics D: Applied Physics)

   Abstract The present work improves a phenomenological plasma-assisted combustion model by integrating the spatiotemporal distribution of plasma power density, thereby considering the evolution of plasma streamers in the modeling, and subsequently, better predicting the ignition kernel evolution. The improved phenomenological model is validated against experiments representing the plasma discharge and post-discharge ignition kernel evolution. Specifically, the new model demonstrates a more accurate prediction of ultrafast gas heating and O₂dissociation during the plasma discharge, compared to the original model. In addition, the new model is found to closely match the experimental pressure wave and heated channel profiles post-discharge without the need for tuning the energy deposition (unlike the original model), highlighting its accuracy of post-discharge ignition kernel dynamics. The improved phenomenological model is then employed to investigate ignition kernel evolution for a stoichiometric methane-air discharge across various discharge gap configurations. Simulations reveal a non-uniform temperature and streamer distribution progressing from the electrode tips toward the center, contrasting uniform cylindrical discharges previously described in the original model. Streamer propagation is observed to be faster for larger gaps when maintained at the same average electric field for different discharge gaps. The tendency of smaller gaps to produce detached toroidal ignition kernels is observed, while larger gaps promote cylindrical and attached ignition kernels. Interactions between successive ignition kernels from consecutive discharges varied significantly, with the smallest gap (1 mm) promoting the quenching of the preceding ignition kernel due to the initial kernel–kernel separation. The intermediate gap (2 mm) promotes detached kernel growth. In contrast, in the largest gap (4 mm), kernels consistently combine and expand attached to electrodes. The impact of homogeneous isotropic turbulence is also explored, showing the persistence of ignition kernels early on but eventually quenching due to enhanced radical and heat losses with pronounced turbulence intensity. 

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4. 1D Simulation of Avalanche to Streamer to Spark Transition of Plasma Discharge in Ammonia-Air Combustion

   https://doi.org/10.2514/6.2024-2607  

   Taneja, Taaresh Sanjeev; Yang, Suo; Sitaraman, Hariswaran (January 2024, American Institute of Aeronautics and Astronautics)

   The backward problem of plasma assisted combustion emphasizes evaluating the effect of the evolving thermochemical state on the plasma discharge. This paper investigates the dependence of avalanche to streamer to spark formation dynamics and kinetics on the gas composition and temperature at different points in an ammonia-air premixed laminar flame using a self-consistent multigrid-based 1D plasma solver. Different values of alpha, the coefficient for effective ionization events per unit length, have been reported for electron avalanches in air and stoichiometric NH3-air mixtures. The streamer inception has been shown to obey the Meek’s criterion. An exponential reduction in streamer and spark formation time has been observed from plasma simulations at different points in the unburnt, pre-heat zone, reaction zone and the fully burnt regions of the premixed flame. While the enhancement of the reduced electric field with increasing temperature affects effective ionization, there exists a minimum breakdown field for streamer formation, which does not vary proportionally with the changing number density of the gas. The change in the mixture from reactants (NH3, O2, N2) to products of complete combustion of ammonia in air (N2, H2O) has also been shown to affect the streamer and spark formation. Finally, the major pathways during the streamer and spark phases which are responsible for producing important radicals used in combustion of NH3 are also discussed. 

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5. Energy Pathways in Plasma-Assisted Ignition of Ammonia

   Dijoud, Raphael J; Guerra-Garcia, Carmen (January 2025, AIAA SCITECH 2025 Forum, American Institute of Aeronautics and Astronautics)

   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. 

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