An integrated theoretical and numerical framework is developed to study the dynamics of energy coupling, gas heating and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBDs) in air. The work represents one of the first attempts to simulate, in a self-consistent manner, multiple (more than 100) nanosecond pulses. Detailed information is obtained about the electric-field transients during each voltage pulse, and accumulation of plasma generated species and gas heating over ms timescales. The plasma is modelled using a two-temperature, detailed chemistry scheme, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. The analysis is conducted with pressures and pulsing frequency in the range 40-100 Torr and 1-10 5 Hz, respectively. The input electrical energy is directly proportional to the number density, and remains fairly constant on a per molecule basis from pulse to pulse. Repetitive pulsing results in uniform production of atomic oxygen in the discharge volume via electron-impact dissociation during voltage pulses, and through quenching of excited nitrogen molecules in the afterglow. The ion Joule effect causes rapid gas heating of ∼40 K/pulse in the cathode sheath and generates weak acoustic waves. Conductive heat loss to the walls during the time interval between voltage pulses prevents overheating of the cathode layer and development of ionization instabilities. A uniform 'hat-shaped' temperature profile develops in the discharge volume after multiple pulses, due to chemical heat release from quenching of excited species. This finding may explain experimentally observed volumetric ignition (as opposed to hot-spot ignition) in fuel-air mixtures subject to NS DBD.

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The present work combines numerical and experimental efforts to investigate the effect of nanosecond pulsed plasma discharges on the low-temperature oxidation of C 2 H 4 ∕O 2 ∕Ar mixtures under reduced pressure conditions. The nonequilibrium plasma discharge is modeled using a one-dimensional framework, employing separate electron and neutral gas temperatures, and using a detailed plasma and combustion chemical kinetic mechanism. Good agreement is seen between the numerical and experimental results, and both results show that plasma enables lowtemperature C 2 H 4 oxidation. Compared to zero-dimensional modeling, the one-dimensional modeling significantly improves predictions, probably because it produces a more complete physical description (including sheath formation and accurate reduced electric field). Furthermore, the one-and zero-dimensional models show very different reaction pathways, using the same chemical kinetic mechanism and thus suggest different interpretations of the experimental results. Two kinetic mechanisms (HP-Mech and USC Mech-II) are examined in this study. The modeling results from HP-Mech agree better with the experimental results than those of USC Mech-II because USC Mech-II does not include the OH C 2 H 4 CH 2 CH 2 OH reaction pathway. The model shows that 75-77% of the input pulse energy is consumed during the breakdown process in electron impact dissociation, excitation, and ionization reactions, which efficiently produce reactive radical species, fuel fragments, and excited species. The modeling results using HP-Mech reveal that reactions between O 1 D and C 2 H 4 generate 24% of OH, 19% of HCO, 60% of CH 3 , 63% of CH 2 , and 17% of CH 2 O. These in turn significantly enhance hydrocarbon oxidation, since 83% of CO comes from HCO and 53% of CO 2 comes from CH 2 under the present low-temperature environment and short time scale. Nomenclature E = electric field, V · cm −1 F EHD i = electrohydrodynamic force per unit volume, kg · cm −2 · s −1 Gt = nondimensional heat transfer parameter J e;s = wall boundary flux of electrons, cm −2 · s −1 J k = flux of kth species, cm −2 · s −1 J ϵ = flux of electron energy, eV · cm −2 · s −1 J ϵ;s = wall boundary flux of electron energy, eV · cm −2 · s −1 J ;s = wall boundary flux of positive ions, cm −2 · s −1 J −;s = wall boundary flux of negative ions, cm −2 · s −1 J ;− = net positive and negative charge fluxes, respectively, cm −2 · s −1

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A self-consistent 1D theoretical framework for plasma assisted ignition and combustion is reviewed. In this framework, a "frozen electric field" modeling approach is applied to take advantage of the quasi-periodic behaviors of the electrical characteristics to avoid the recalculation of electric field for each pulse. The correlated dynamic adaptive chemistry (CO-DAC) method is employed to accelerate the calculation of large and stiff chemical mechanisms.The time-step is updated dynamically during the simulation through a three-stage multitimescale modeling strategy, which takes advantage of the large separation of timescales in nanosecond pulsed plasma discharges. A general theory of plasma assisted ignition and combustion is then proposed. Nanosecond pulsed plasma discharges for ignition and combustion can be divided into four stages. Stage I is the discharge pulse, with timescales of O(1-10 ns). In this stage, most input energy is coupled into electron impact excitation and dissociation reactions to generate charged/excited species and radicals. Stage II is the afterglow during the gap between two adjacent pulses, with timescales of O(100 ns). In this stage, quenching of excited species not only further dissociates O2 and fuel molecules, but also provides fast gas heating. the remaining gap between pulses, with timescales of O(1-100 μs). The radicals generated during Stages I and II significantly enhance the exothermic reactions in this stage. Stage IV is the accumulative effects of multiple pulses, with timescales of O(1 ms -1 sec), which include preheated gas temperatures and a large pool of radicals and fuel fragments to trigger ignition. For plasma assisted flames, plasma significantly enhances the radical generation and gas heating in the pre-heat zone, which could trigger upstream auto-ignition.Keywords: plasma assisted combustion, plasma fluid modeling, nanosecond plasma discharge, low temperature chemistry, ignition. Nomenclature