Microdischarge Channels Research Articles

A computational model of a barrier discharge in air at atmospheric pressure: the role of residual surface charges in microdischarge formation

A two-dimensional (axially symmetric) computational model of the microdischarge formation in a barrier discharge for short (1–2 mm) gaps in air at atmospheric pressure is proposed. A non-homogeneous electric field, caused by a residual non-uniform charging of the dielectric barriers, is considered as an important reason for the filament formation by a Townsend mechanism. The ion–electron emission from the dielectric covering the cathode is treated as the principal secondary process; the secondary emission coefficient in the model is selected to be consistent with the Paschen voltage. For an applied sinusoidal voltage, this follows the microdischarge development on a microsecond scale. It is shown that even a slight inhomogeneity of the initial electric field leads to the formation of a narrow microdischarge channel.The two-dimensional dynamics of the radiation from a microdischarge for the case of the second positive and the first negative systems of nitrogen is simulated and compared with recent experimental data. The effects of the secondary emission coefficient and of a distribution of residual surface charges are investigated. It is shown that the level of inhomogeneity of the residual surface charge distribution does not affect the radius of the microdischarge channel, but affects its two-dimensional structure.The proposed model explains satisfactorily the experimental results for the velocity of the cathode-directed ionizing wave and the emission of the N2 second positive system from a microdischarge.

Analysis of two-dimensional microdischarge distribution in dielectric-barrier discharges

The two-dimensional spatial distribution of microdischarges in atmospheric pressure dielectric-barrier discharges (DBDs) in air was studied. Experimental images of DBDs (Lichtenberg figures) were obtained using photostimulable phosphors. The storage phosphor imaging method takes advantage of the linear response of the phosphor for characterization of microdischarge intensity and position. A microdischarge interaction model in DBDs is proposed and a Monte Carlo simulation of microdischarge interactions in the discharge is presented. Comparison of modelled and experimental images indicates interactions and short-range structuring of microdischarge channels.

A Kinetic Study of Ozone and Nitric Oxides in Dielectric Barrier Discharges for O2/NOx Mixtures

A simple model is described to simulate kinetic processes in dielectric barrier discharges for O2/NOx mixtures. A threshold of ozone production found experimentally is confirmed by the calculations of this modeling, and the underlying chemical reaction mechanisms are discussed. It is also found that the effects of diffusion processes in the period of the lifetime of O atoms are not important to microdischarge channels with a large radius, i.e. larger than 150 μm.

Kinetics of ozone and nitric oxides in dielectric barrier dischargesin O2/NOx and N2/O2/NOx mixtures

Concentrations of NO, NO2, NO3, N2O5, and O3 weremeasured by classical absorption spectroscopy in dielectric barrierdischarges in flowing O2/NOx and N2/O2/NOx mixtures. Theresults of measurements in different parts of the discharge chamber and inits exhaust are compared to a numerical zero-dimensional kinetic model and goodagreement is found. The experimentally found upper limit of the NOxconcentration allowing ozone production is confirmed by the kineticcalculations for both gas mixtures. The rotational temperature of differentnitrogen bands was measured by high-resolution emission spectroscopy. Theresults are explained on the basis of a simplified model and related to thegas temperature in the microdischarge channel and the surrounding gas.

Comparative modelling of NOx and SO2 removal from pollutantgases using pulsed-corona and silent discharges

A comparative modelling of pulsed-corona and silent dischargesfor removal of NOx, NyOx, SO2, CO andCH2O using operating equipment is presented. The mainpurpose is to compare, by modelling, the energy efficiency on removal oftoxic impurities between two types of discharges at similar gas compositionand temperature. Three different gas compositions: diesel engine exhaust,methane combustion products and pollutant air are considered. Thesimulation is based on an approximate mathematical model for plasmacleaning of a waste gas. The influence of non-uniform species distributionsarising from a great number of streamer or microdischarge channels in adischarge chamber is taken into account. The modelling is carried out for apulse series, considering after each discharge pulse the chemical anddiffusion processes inside and outside the streamer trace. The distinctionsof the cleaning processes in the pulsed-corona and barrier discharges arepresented. It is also recommended when each of the examined dischargesshould be used.

Two-dimensional modelling of the dielectric barrier discharge in air

A self-consistent two-dimensional modelling of microdischarges in devices in which one of the electrodes is covered with a dielectric is presented. The discharge development can be divided into four phases, a Townsend, an ionization wave or streamer, a cathode layer formation, and a decay phase. While during the Townsend phase the initial field strength distribution is hardly distorted, an ionization wave propagates towards the cathode during the following phase. On the wave reaching the cathode, a cathode layer develops. Its radical extension is determined by the increase of the current. During the decay phase the distributions of, for example, field strength and charge carriers are nearly frozen. The discharge fades because of the slow decrease of the field strength within its column. Energy and temperature distributions of microdischarge channels in air at atmospheric pressure are given.