Pascal Jeanney

OH kinetic in high-pressure plasmas of atmospheric gases containing C2H6 studied by absolute measurement of the radical density in a pulsed homogeneous discharge

The absolute value of the hydroxyl radical was measured in the afterglow of an homogeneous photo-triggered discharge generated in N2/O2/H2O/C2H6 mixtures, using a UV absorption diagnostic synchronized with the discharge current pulse. Measurements show that OH is efficiently produced even in the absence of water vapour in the mixture, and that the radical production is closely linked to the degradation kinetic of the hydrocarbon. Experimental results for dry mixtures, both for OH and for the removal of ethane in the discharge volume, are compared with predictions of a self-consistent 0D discharge and the kinetic model. It appears that the oxidation reaction of the ethane molecule by O(3P) atoms plays a minor role. Dissociation of the hydrocarbon through quenching collisions of the nitrogen metastable states are of great importance for a low oxygen concentration value. Also, the oxidation of ethane by O(1D) cannot be neglected at high oxygen concentration. The most probable exit channel for N2 states quenching collisions by ethane is the production of ethene and hydrogen molecules. Afterwards C2H4 should be dissociated to produce H and H2. As previously suggested from the study of the OH density time evolution in relative value, the recombination of H and O atoms appears as a main process for the production of OH in transient low temperature plasmas generated in atmospheric gases at high pressure. Another important reaction is the reduction of the HO2 radical by O, this radical coming from the addition of H on the oxygen molecule. H atoms come from numerous kinetic processes, amongst which is the dissociation of ethene.

Sub-nanosecond time resolved light emission study for diffuse discharges in air under steep high voltage pulses

Pin-to-plane discharges in centimetre air gaps and standard conditions of pressure and temperature are generated under very high positive nanosecond scale voltage pulses. The experimental study is based on recordings of sub-nanosecond time resolved and Abel-processed light emission profiles and their complete correlation to electrical current waveforms. The effects of the voltage pulse features (amplitude between 20 and 90 kV, rise time between 2 and 5.2 ns, and time rate between 4 and 40 kV ns−1) and the electrode configuration (gap distance between 10 and 30 mm, pin radius between 10 and 200 µm, copper, molybdenum or tungsten pin material) are described. A three time period development can be found: a glow-like structure with monotonic light profiles during the first 1.5 ns whose size depends on time voltage rate, a shell-like structure with bimodal profiles whose duration and extension in space depends on rise time, and either diffuse or multi-channel regime for the connection to the cathode plane according to gap distance. The transition of the light from monotonic to bimodal patterns reveals the relative effects and dynamics of streamer space charge and external laplacian field. A classical 2D-fluid model for streamer propagation has been used and adapted for very high and steep voltage pulses. It shows the formation of a strong space charge (streamer) very close to the pin, but also a continuity of emission between the pin and the streamer, and electric fields higher than the critical ionization field (28 kV cm−1 in air) almost in the whole gap and very early in the discharge propagation.

Towards a kinetic understanding of the ignition of air-propane mixture by a non-equilibrium discharge: the decomposition mechanisms of propane

The decomposition of propane in non-thermal plasmas of N2/C3H8 and N2/O2/C3H8 mixtures (oxygen percentage up to 20%) at low temperature is studied in a photo-triggered discharge. Quenching of nitrogen metastable states dissociate C3H8 to produce propene and hydrogen. Oxidation reactions are growing in importance when the O2 concentration increases, but the dissociation quenching reactions still occurs for the air-based mixture. Even for a low concentration of oxygen, OH is an important specie involved in the conversion of the hydrocarbon. A kinetic analysis emphasises that OH comes in great part from the production of H, in which the methyl radical plays a role, strengthening the role of the dissociation processes of propane and propene in the medium reactivity. Results of PLIF measurements performed on OH during the diffuse afterglow of a nanosecond corona discharge correlate with results obtained on the photo-triggered discharge.

Filamentation of a Nanosecond Pulse Corona Discharge in Air–Propane Mixtures at Atmospheric Pressure

The space and time development of a single nanosecond pulsed corona discharge is studied in atmospheric air-propane mixtures with different concentrations of propane up to 8%. Time-resolved imaging of the discharge shows a complete diffuse pattern in pure dry air, which becomes more and more filamentary when propane is added. Filaments are thinner with higher concentrations of propane, and the whole discharge energy increases and gets saturated.

Ignition of Lean Air / Hydrocarbon Mixtures at Low Temperature by a Single Corona Discharge Nanosecond Pulse

A great number of experimental studies have demonstrated that non-thermal plasmas produced by high voltage pulse discharges, running at a given pulse repetition frequency, are able to ignite air / hydrocarbon mixtures at a low initial temperature and atmospheric pressure. In this paper, we show that ignition can also be achieved using a single nanosecond pulse corona discharge generated under a very strong overvoltage. Experiments were conducted in air / propane and air / n-heptane mixtures. For such a discharge, ignition of n-heptane requires less released electrical energy than propane and lean mixtures can be completely burnt with reasonable energy values of several tens of mJ. Extended flame kernels or several points of ignition can be induced. For n-heptane, we have developed a simplified combustion model using a reduced mechanism, enabling us to suggest a kinetic explanation for ignition at a low temperature. In particular, it appears that oxygen atoms produced in the plasma induce a significant reduction of the ignition time. This model lays the foundation for a more complete study, including the production of other active species by the discharge, such as the first excited state of the oxygen atom, or even dissociation products of the hydrocarbon molecule following electron collisions or quenching of the nitrogen excited states.

On the Influence of Anode Composition on the Pattern of a Nanosecond Diffuse Discharge at Atmospheric Pressure

A diffuse discharge is generated in atmospheric air under a nanosecond range overvoltage. The influence of the anode composition on the diffuse pattern is studied and images of the discharges developed under these conditions are presented. The focus is made on two different anode compositions, tungsten or copper, and results show a much more diffuse discharge created for a tungsten-anode in a shorter delay.

Point-to-Plane Discharge on an Anisotropic Ceramic-Matrix Composite Layer at Atmospheric Pressure

In this paper is presented the recording of a point-to-plane dielectric barrier discharge in dry air, in which the dielectric layer has been replaced by a layer of anisotropic ceramic-matrix composite material (Streamer). An original high-intensity discharge is observed above a voltage threshold. The electrical parameters and the spatial structure of the discharge are presented. The energy deposition in the gas is compared to the one obtained for a purely dielectric layer (Al2O3).

Nanosecond Pulsed Discharge Phenomenology in Micrometer-Sized Radially Confined Air Gap

Geometry effects on nanosecond-scale pulsed corona discharges propagating in rectangular glass tubes are reported in this paper. Both the section and thickness of the tubes are the parameters which are used to investigate the discharge phenomenology. Three picosecond high-speed ICCD cameras are used to derive the propagation velocities from a single discharge event. Propagation is significantly speeded up in radially highly confined gaps and promoted close to edges. Inception of weak discharges along the tubes can be observed.