Y Cressault

Mean absorption coefficient of H2O?air?MgCl2/CaCl2/NaCl thermal plasmas

Under the local thermodynamic equilibrium hypothesis, the mean absorption coefficients (MACs) were calculated for H2O?air?MgCl2/CaCl2/NaCl thermal plasmas in a temperature range from 300 to 30?000?K and at atmospheric pressure. The MACs were computed under the hypothesis of isothermal plasmas which allows a good description of the radiation absorbed in cold regions. In this study, we took into account the absorption radiation resulting from the atomic continuum, molecular continuum, atomic lines and some molecular bands. Free?free transitions (bremsstrahlung) and free?bound (electron?ion recombination and electron attachment) or bound?free transitions in terms of absorption were considered for the calculation of atomic continuum. For bound?bound transitions, natural, resonance, van der Waals, Stark and Doppler effects were taken into account for the line broadenings while the escape factors were used to treat the self-absorption of the resonance lines. Molecular continuum was considered for the main molecules (H2, O2, N2, OH, NO, H2O, N2O, NO2, O3, NO3 and N2O5) whereas we studied only diatomic systems O2, N2, NO and for the absorption of molecular bands. The influence of the proportion of MgCl2, CaCl2 or NaCl in a water?air mixture was analysed as the effect of the strong self-absorbed resonance lines of the alkaline salts (Ca, Ca+, Na, Na+, Mg, Mg+, Cl and Cl+). Our results show that a low concentration of alkaline salts (less than 1% in molar proportions) in the plasma increased the MACs at low temperatures (T?<?10?000?K) due to the resonance lines mainly localized in the near-UV and visible spectral regions in opposition to hydrogen, oxygen or nitrogen species for which 90% of them exist in ultraviolet. In addition to the atomic and molecular continuum, the absorption radiation of molecular bands is important at low temperatures.

Radiative properties of argon–helium–nitrogen–carbon–cobalt–nickel plasmas used in CNT synthesis

This work presents the radiative properties of argon–helium–nitrogen–carbon–nickel–cobalt thermal plasmas by the computation of net emission coefficients (NECs) under the assumption of a local thermodynamic equilibrium and at temperature range 1000–20 000 K. These mixtures were often used in the study of carbon nanotubes (CNTs) synthesis with arc plasma which becomes one of the most useful techniques in terms of flexibility of carbon nanostructures produced with fewer defects. The values of NEC allow estimation of total radiation losses in plasmas, by taking into account the emission radiation resulting from the atomic continuum, the molecular continuum, the atomic lines and some molecular bands. Free–free transitions (Bremsstrahlung) and free–bound (electron–ion recombination), have been considered for the calculation of atomic continuum. For bound–bound transitions, natural, resonance, Van der Waals, Stark and Doppler effects have been taken into account in the calculation of the lines broadenings while the self-absorption of the resonance lines has been treated using their escape factors. Molecular continuum has been only considered for N2, C2 and CN molecules whereas we have only taken into account diatomic systems N2, , CN and C2 for the emission of the molecular bands. The results obtained show that even for low concentrations of Ni and Co in the plasma, the NECs are modified and considerably increase only at a low temperature (T < 8000 K) and the major contribution in the total radiation arises from the lines emission. However, the effect of the thickness of the plasma on plasma radiation has been analysed based on the self absorption phenomenon of resonance lines.

Calculation of the net emission coefficient of an air thermal plasma at very high pressure

The aim of this paper is to present an accurate evaluation of the phenomena appearing for high pressure air plasmas supposed to be in local thermodynamic equilibrium (LTE). In the past, we already calculated the net emission coefficient for air mixtures at atmospheric pressure and for temperatures up to 30kK (molecular contribution being restricted to 10kK). Unfortunately, the existence of high pressures does not allow us to use this database due to the non-ideality of the plasma (Viriel and Debye corrections, energy cut-off ...), and due to the significant shifts of molecular reactions towards upper temperatures. Consequently, this paper proposes an improvement of our previous works with a consideration of high pressure corrections in the composition algorithm in order to take into account the pressure effects, and with a new calculation of all the contributions of the plasma radiation (atomic lines and continuum, molecular continuum, and molecular bands) using an updated database. A particular attention is paid to calculate the contribution of all the major molecular band systems to the radiation: O2 (Schumann–Runge), N2 (VUV, 1st and 2nd positive), NO (IR, β, γ, δ, ) and N2+ (1st negative and Meinel). The discrete atomic lines and molecular bands radiation including the overlapping are calculated by a line-by-line method up to 30kK and 100 bar. This updated database is validated in the case of optically thin plasmas and pressure of 1bar by the comparison of our integrated emission strength with the published results. Finally, this work shows the necessity to extend the molecular radiation database up to 15kK at high pressure (bands and continuum) since their corresponding contributions could not be neglected at high temperature.

Pressure Rise in Electrical Installations due to Internal Arcing in CO2 as Insulating Gas

Internal arcs cause a rapid increase in pressure in electrical installations. The type of insulation gas has influence on pressure development. Typically SF6 is used incompact metal-clad switchgear, however, it has a high global warming potential. Because of this, the replacement of SF6 by alternative gases such as CO2 is under discussion. The pressure developments in a closed vessel filled with air, SF6 and CO2 are measured and compared. During internal arcing in gas-insulated switchgear, overpressure causes a rupture of a burst plate and hot gas escapes into the surrounding room mixing with air. In order to predict the pressure development in electrical installations reliably, the portion of energy causing pressure rise, arc voltage as well as reliable gas data i.e., thermodynamic and transport properties, must be known in a wide range of pressure and temperature. These data are up to now not available for CO2/air mixtures. The thermodynamic properties are directly calculated from the number densities, internal partition functions and enthalpies of formation. The transport coefficients are deduced using the Chapman-Enskog method. Comparing measured and calculated pressure developments in a test arrangement demonstrates the quality of the calculation approach.

Calculation of the resonance escape factor of magnesium spectral line shapes in the case of a MgCl2—water plasma

The resonance escape factors for the lines emitted by a neutral magnesium atom MgI at 285.2127 nm (3 1S–3 1P) and of ionic magnesium MgII at 279.5528 nm (3 2S–3 2P) are calculated assuming a Voigt profile and in the case of MgCl2‐water plasma. The dependence of the escape factor on the optical thickness τ0 from the line center which itself depends on the two main spectral line shape broadening mechanisms (pressure and Doppler effects) are considered. The variation of the resonance escape factors with the temperature and the MgCl2 molar proportion are also investigated. This calculation is useful for the application of laser induced breakdown spectroscopy in the quantitative analysis of elemental composition.