A computational modeling study of streamer propagation in a cold, atmospheric-pressure, helium jet in ambient air is presented. A self-consistent, multi-species, multi-temperature plasma model with detailed finite-rate chemistry and photoionization effects is used to provide fundamental insights into the structure and dynamics of the streamers. A parametric study of the streamer properties as a function of important discharge geometric and operating conditions is performed. The fluid mechanical mixing layer between the helium jet core and the ambient air is instrumental in guiding the propagation direction of the streamer and gives the plasma jet a visibly collimated appearance. The key chemical reactions which drive the streamer propagation are electron-impact ionization of helium neutral and nitrogen molecules. Photoionization plays a role in enhancing the propagation speed of the streamer, but is not necessary to sustain the streamer. The streamer yields a large radical concentration through chemical reactions in the streamer head and the body. The streamer propagation speed increases with reduced helium jet radius and increased helium-air mixing layer width. Impurities in the helium jet result in a significant increase in the discharge propagation speed within the tube through photoionization, but not after the streamer propagates into the open ambient region. It is also observed that thinner electrodes produce stronger electric-field concentrations that increase discharge propagation speeds within the tube but have a smaller influence on the discharge after it emerges out of the tube as a streamer.

We describe a computational modeling study of a cold atmospheric pressure plasma jet interacting with a dielectric surface placed normal to the jet axis. The plasma jet is generated by the application of a nanosecond pulse voltage applied to a dielectric tube through which the jet issues into ambient air. A base fluid flow field is pre-computed using a Navier-Stokes model for the helium jet impinging on the dielectric target surface with a two-species description for laminar diffusional mixing of the helium and ambient air streams. A self-consistent, multiple species, two-temperature model is used to describe the non-equilibrium plasma discharge dynamics in the presence of the base jet flow field. A single nanosecond pulse discharge event starting from initial breakdown in the dielectric tube, to propagation into the open gap, and finally the interaction with the dielectric surface is simulated. Initially, the plasma forms within the dielectric tube and propagates along the tube surface as a surface discharge driven by large induced electric fields produced by trapped charge on the dielectric surface. When the discharge reaches the end of the dielectric tube, the discharge transitions to a constricted fast ionization wave that propagates along the helium-air interface. The fast ionization wave eventually reaches the dielectric target surface where charged species are deposited as the discharge propagates parallel to the wall as a surface driven discharge. The surface driven discharge ceases to propagate once the quantity of air to helium is sufficient enough to quench the hot electrons and prevent further ionization. Due to the low speed of the flow discharge and the short life times of the radical species such as O, most of the radical species delivered to the surface are a result of the surface discharge that forms after the plasma bullet impinges against the surface. It is found that factors such as the thickness of the target dielectric and the profile of the stagnation helium-air jet significantly impact the net quantity of reactive particles delivered to the surface.

A luminous plasma jet is produced when helium gas issuing into atmospheric pressure ambient air is excited by high voltage nanosecond pulsing of a dielectric covered electrode. A detailed computational modeling study of such a discharge is presented. The dynamics of streamer propagation, its dependence on the diffusional mixing layer between helium and air species, and the role of photoionization are discussed.

We present a computational simulation study of non-equilibrium streamer discharges in a coaxial electrode and a corona geometry for automotive combustion ignition applications. The streamers propagate in combustible fuel-air mixtures at high pressures representative of internal combustion engine conditions. The study was performed using a self-consistent, two-temperature plasma model with finite-rate plasma chemical kinetics. Positive high voltage pulses of order tens of kV and duration of tens of nanoseconds were applied to the powered inner cylindrical electrode which resulted in the formation and propagation of a cathode-directed streamer. The resulting spatial and temporal production of active radical species such as O, H, and singlet delta oxygen is quantified and compared for lean and stoichiometric fuel-air mixtures. For the coaxial electrode geometry, the discharge is characterized by a primary streamer that bridges the inter-electrode gap and a secondary streamer that develops in the wake of the primary streamer. Most of the radicals are produced in the secondary streamer. For the corona geometry, only the primary streamer is observed and the radicals are produced throughout the length of the primary streamer column. The stoichiometry of the mixture was observed to have a relatively small effect on both the plasma discharge structure and the resulting yield of radical species.

Non-equilibrium plasma generated from positive-pulsed nanosecond electrical discharges into desiccated air is simulated in this paper using a multi-dimensional, multi-physics plasma solver. A pin-to-pin electrode configuration is used with a fixed 5.2 mm gap spacing. Peak pulse voltages range between 10.2 and 22.5 kV. Care is taken to match the exact electrode profile from the experiments, and adjust the electron collision frequency so that breakdown limits closely match those from corresponding experimental results. The optimized numerical simulations predict qualitative streamer structure that is in close agreement with experimental observations. Quantitative measurements of atomic oxygen at the anode tip and qualitative estimates of streamer gas heating are closely matched by simulations. The model results are used to provide insight into the spatial and temporal development of the transient plasma. The work performed in this paper delivers a numerical tool that can be extremely useful to link the post-discharge plasma properties to low-temperature plasma ignition mechanisms that are of great interest for the automotive industry.