Table of contents

In this paper, we give a detailed review of recent work carried out on the numerical characterization of non-thermal gas discharge plasmas in air at atmospheric pressure. First, we briefly describe the theory of discharge development for dielectric barrier discharges, which is central to the production of non-equilibrium plasma, and we present a hydrodynamic model to approximate the evolution of charge densities. The model consists of the continuity equations for electrons, positive and negative ions coupled to Poisson's equation for the electric field. We then describe features of the finite element flux corrected transport algorithm, which has been developed to specifically aim for accuracy (no spurious diffusion or oscillations), efficiency (through the use of unstructured grids) and ease of extension to complex 3D geometries in the framework of the hydrodynamic model in gas discharges. We summarize the numerical work done by other authors who have applied different methods to various models and then we present highlights of our own work, which includes code validation, comparisons with existing results and modelling of radio frequency systems, dc discharges, secondary effects such as photoionization and plasma production in the presence of dielectrics. The extension of the code to 3D for more realistic simulations is demonstrated together with the adaptive meshing technique, which serves to achieve higher efficiency. Finally, we illustrate the versatility of our scheme by using it to simulate the transition from non-thermal to thermal discharges.

We conclude that numerical modelling and, in particular, the extension to 3D can be used to shed new light on the processes involved with the production and control of atmospheric plasma, which plays an important role in a host of emerging technologies.

Electrical transport (90–300 K) and magnetic (5–375 K) properties of the La_0.67Ca_0.33MnO₃ + x% V₂O₅ (x = 0, 2, 5, 15 and 30) type magnetic–nonmagnetic composite system show appreciable grain boundary contribution. These composites, except for the one with x = 30%, show metal–insulator transitions at T = T_p which decrease with increasing V₂O₅ concentration. The high temperature (T > T_p) semiconducting part of the resistivity (ρ) data follow a small polaron hopping conduction mechanism. The magnetoresistance (MR) in this system increases with increasing V₂O₅ content, showing a maximum value (25% at 93 K and field H = 1.15 T) for the sample with x = 15%. Spin polarization is considered to be responsible for such an increase of MR. Neither thermoelectric power (between 90 and 300 K) nor magnetization data indicate any noticeable change of the Curie temperature (T_C) with change of x, suggesting that V₂O₅ remains at the grain boundary. The complex impedance (Z^*) of the composite system, measured over a frequency range of 0.01 kHz–13 MHz, at 300 K shows strong frequency dependence and the impedance plane plots (Z'–Z'') are semicircular for different V₂O₅ content. An equivalent resistor–capacitor circuit (R_g (R_gbC_gb)) has been proposed to explain the impedance results in this composite system.

Formation of Cd_1−xZn_xTe (CdZnTe) ternary compounds as a result of Zn diffusion in CdTe thin films, the structural and optical properties of CdZnTe compounds and I–V characteristics of CdZnTe/CdTe heterojunctions are presented. X-ray diffraction study of Zn/CdTe structures exposed to thermal annealing indicates the formation of CdZnTe compounds. Analysis of the absorption spectra of Zn/CdTe structures, annealed at 500 °C, and CdTe thin films show that the energy band gap of the former (1.65 eV) is larger than that of the latter (1.50 eV). Such an increase of the band gap of annealed two-layer Zn/CdTe is attributed to the reactive diffusion of Zn in CdTe films accompanied by the formation of Cd_1−x Zn_xTe compounds with an average value of x = 0.22. The temperature dependence of the effective diffusion coefficient of Zn in CdTe at 430–520 °C is described as D = 2.5 × 10⁻³ exp(−1.30 kT⁻¹)~cm² s⁻¹.

In this paper, the radial distributions of emissivity of the species present in the helium–nitrogen, helium–oxygen and helium–argon plasmas sustained by the axial injection torch at atmospheric pressure are studied by optical emission spectroscopy using the Abel inversion procedure. The spectrum was scanned for atomic and ionic lines together with rotational bands of neutral and ionic molecules of such species. The variation of the emissivity with different mixture composition was also studied. No emission from oxygen molecular species (O₂, ) was found in the discharge or from the atomic ions N⁺ or O⁺. The interaction processes between helium and the introduced species are discussed. A decrease in helium emissivity when compared with the pure helium plasma was found for every plasma gas mixture composition.

This work is a contribution to the understanding of the electrical arc behaviour in low voltage breaking devices—circuit breakers, contactors, switches and switch-disconnectors. A structured and detailed study based on a bibliographical synthesis of research work on experimental devices, the switching arcs and their mathematical models are presented. According to this study and under the assumption that the flow is stationary, a 3D modelling in the air for a 50 A current intensity is achieved. The external magnetic field is taken into account, which makes it possible to quantify the distortion of the confined electrical arc for each of the breaking devices. Two particular shapes are observed: a 'U' shape in the case of the circuit breaker and an 'S' shape in the case of the contactor, the switch and the switch-disconnector.

This paper is concerned with the numerical simulation of high-intensity discharge lamps, focusing on the influence of pressure, current and electrode gap on plasma, anode and cathode properties. The interaction between electrodes and plasma is taken into account. Comparisons between experimental findings and numerical results show that the observed increase in the total lamp voltage caused by an increasing lamp current is a superposition of an increasing lamp voltage due to an increasing pressure (caused by the increasing lamp current) and a decreasing lamp voltage due to the increasing lamp current (if the pressure would be constant). Furthermore, the numerical results reveal that the plasma potential in the whole plasma strongly depends on the electrode gap and that even the electrode temperature and the arc attachment to the electrodes are influenced by the size of the electrode gap.

The efficient production of syngas from a CH₄+CO₂ mixture in an atmospheric pulsed glow discharge, sustained by corona pre-ionization, has been investigated. The products were mainly syngas (CO, H₂) and hydrocarbons up to C₄, with acetylene having the highest selectivity. The energy efficiency was within 15–40% for different experimental conditions, which demonstrates a comprehensive improvement relative to the achievements of other types of non-equilibrium plasma. These values are, however, comparable with the efficiencies obtained by gliding arc plasmas but this plasma operates at near room temperature. Furthermore, it has been shown that the energy efficiency is increased by decreasing the effective residence time. The effects of molar ratio CH₄ : CO₂, voltage, repetition rate and gas flow rate on conversion, energy efficiencies and the selectivities have also been investigated. The higher efficiency obtained in this kind of plasma is discussed and attributed to the short pulse regime and electric field uniformity.

This paper presents results of singlet oxygen generation experiments in a high-pressure, non-self-sustained crossed discharge. The discharge consists of a high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage dc discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen (SDO). The results demonstrate operation of a stable and diffuse crossed discharge in O₂–He mixtures at static pressures of at least up to P₀ = 380 Torr and sustainer discharge powers of at least up to 1200 W, achieved at P₀ = 120 Torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N = 0.3 × 10⁻¹⁶ to 0.65 × 10⁻¹⁶ V cm², which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O₂(a ¹Δ) excitation. Measurements of visible emission spectra O₂(b ¹Σ → X ³Σ) in the discharge afterglow show the O₂(b ¹Σ) concentration to increase with the sustainer discharge power and to decrease as the O₂ fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O₂–90% He flows at P₀ = 120 Torr and mass flow rates of are 365–465 K. SDO yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0, 0) band of the O₂(a ¹Δ → X ³Σ) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modelling calculations using a quasi-one-dimensional nonequilibrium pulser–sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O₂(a ¹Δ) yield by a factor of 2–2.5, which suggests that the model's description of nonequilibrium O₂–He plasma kinetics at high pressures is not quite adequate.

Dissociation reactive thermal conductivity (DRTC) is the transfer of dissociation energy in plasma with temperature and, therefore, composition gradient. In the existing theories, calculation of the DRTC coefficient consists of the calculation of diffusion coefficients and plasma composition. The heat flux is then calculated by assigning to every molecule the dissociation energy and by the multiplication of the molecule flux density by this energy.

This approach, correct for the LTE plasma, is not adequate for the non-equilibrium two-temperature plasma: it does not allow one to separate the total DRTC coefficient into two components responsible for the heat transfer by electrons and heavy particles (atoms, molecules, ions). Only at LTE, during atom reassociation, do the electrons recuperate the energy they spent during the molecule dissociation. Therefore, in order to separate these two components of DRTC, the kinetics of the dissociation–reassociation processes should be considered. This is done in this paper for nitrogen plasma at atmospheric pressure. F_e, the electron fraction of the total DRTC coefficient, was calculated for T_e (electron temperature) in the range 0.4–1.0 eV and T_h (heavy particles temperature) from 0.2 eV to T_e. It is shown that F_e depends mostly on the electron temperature and increases with increasing electron temperature.

The purpose of this study was to show that the thermal properties of foam neoprene under hydrostatic pressure cannot be predicted by theoretical means, and that uni-axial pressure cannot simulate hydrostatic compression. The thermal conductivity and compressive strain of foam neoprene were measured under hydrostatic pressure. In parallel, uni-axial compressive strain data were collected. The experimental set-up and data were put into perspective with past published studies. It was shown that uni-axial compression yielded strains 20–25% greater than did hydrostatic compression. This suggests the need for direct hydrostatic pressure measurement. For comparison to hydrostatic experimental data, a series of thermal conductivity theories of two phase composites based on particulate phase geometry were utilized. Due to their dependence on the porosity and constituent thermal conductivities, a model to predict porosity under hydrostatic pressure was used and an empirical correlation was derived to calculate the thermal conductivity of pure neoprene rubber from experimental data. It was shown that, although some agreement between experimental data and thermal conductivity theories was present, no particular theory can be used because they all fail to model the complex structure of the pores. It was therefore concluded that an experimental programme, such as reported here, is necessary for direct measurement.