Yann Cressault

Net emission coefficient of air thermal plasmas

We have calculated the net emission coefficient of air plasmas at atmospheric pressure in the temperature range between 300 and 40?000?K, in the assumption of local thermodynamic equilibrium and isothermal plasmas. This calculation takes into account the radiation due to the atomic continuum, the molecular continuum, the molecular bands (several systems for O2, N2, NO and N2+) and the atomic lines. Special attention has been devoted in this paper to the description of the molecular bands radiation. The results show that in the high temperature range where molecules are dissociated, the radiation properties of air plasmas are mainly due to those of nitrogen plasmas, the resonance atomic lines playing an important role in spite of their strong self-absorption. At low temperature (T<6000?K) the role of the molecular bands of oxygen (O2) and NO is predominant.

Influence of metallic vapours on the properties of air thermal plasmas

This paper deals with properties of air thermal plasmas containing vapours of iron, silver or copper. The plasma is supposed to be in local thermodynamic equilibrium, for temperatures ranging from 2000 to 30 000 K. First, the equilibrium composition and thermodynamic properties are presented. Then, the radiative properties are calculated using the method of the net emission coefficient. Finally, the viscosity, electrical and thermal conductivities are calculated using the method of Chapman–Enskog. For all mixtures, mole fractions have been used. The results are computed for various values of pressure, plasma size and proportions of vapours. The influence of metallic vapour is important on the electrical conductivity and on the radiation, even at low concentration. All the metallic vapours present a similar behaviour except iron, which has a stronger radiation emission than the other components.

Mixing rules for thermal plasma properties in mixtures of argon, air and metallic vapours

Modelling of electric arcs and thermal plasmas in mixtures of gases and vapours needs prior knowledge of rather large data banks corresponding to thermodynamic functions, transport coefficients and radiation properties. For a given pressure these data are functions of temperature and gas proportions in the mixture. In order to reduce the memory or because some properties of the mixtures are not known, some mixing laws can be useful. These mixing rules allow estimation of the properties of the mixtures when only the corresponding properties of the pure gases or vapours are known. In this paper we study several mixing rules for mixtures of argon or air with metallic vapours. Simple laws such as linear interpolations are compared with relations deduced from physical consideration and some general behaviour is given in conclusion: mixing rules for electrical conductivity, viscosity and net emission coefficient are good. For other properties the use of a mixing law may produce rather large errors.

Thermal plasma properties for Ar–Al, Ar–Fe and Ar–Cu mixtures used in welding plasmas processes: I. Net emission coefficients at atmospheric pressure

This article is devoted to the calculation of the net emission coefficient (NEC) of Ar?Al, Ar?Fe and Ar?Cu mixtures at atmospheric pressure for arc welding processes. The results are given in data tables for temperatures between 3?kK and 30?kK, for five plasma thicknesses (0, 0.5, 1, 2, 5?mm) and ten concentrations of metallic vapours (pure gas, 0.01%, 0.1%, 1%, 5%, 10%, 25%, 50%, 75% and pure metal vapours in mass proportions). The results are in good agreement with most of the works published on the subject for such mixtures. They highlight the influence of three parameters on the radiation of the plasma: the NEC is directly related to temperature and inversely related to plasma radius and is highly sensitive to the presence of metal vapours. Finally, numerical data are supplied in tables in order to develop accurate computational modelling of welding arc and to estimate both qualitatively and quantitatively the influence of each metallic vapour on the size and on the shape of the weld pool.

Improvements of radiative transfer calculation for SF6 thermal plasmas

We present a comparison between an exact calculation of radiative transfer in SF6 thermal plasma based on a fine description of the spectrum with 300 000 spectral points for each temperature value, for simplified conditions (1D and 2D geometries with imposed symmetrical temperature profiles and local thermodynamic equilibrium) and two kinds of approximated calculations. The first is the classical net emission coefficient largely used in arc modelling. The second one is based on a very simplified spectral description with only seven intervals assuming a grey body condition within each interval and using the Planck and the Rosseland averaging for deducing the mean absorption coefficient (MAC). At high temperature the use of the Rosseland averaging is not satisfactory. The other two approximated methods (net emission and the Planck averaging) are acceptable but the radiative flux is in general not very accurate. The radiative transfer calculation can be improved following three ways: a better knowledge of the basic processes and in particular of the absorption coefficients for diatomic and polyatomic molecules, the use of different definitions of MACs (Planck averaging at high temperature and mean natural value at low temperature) and a careful choice of the spectral intervals.

Thermodynamic properties and transport coefficients in Ar–H2–Cu plasmas

This paper deals with the calculation of thermodynamic properties and transport coefficients in Ar?H2?Cu mixtures at atmospheric pressure in the temperature range between 300 and 25?000?K. From the equilibrium composition and the species thermochemical properties, the thermodynamic properties are obtained directly. The calculation of the transport coefficients?viscosity, electrical conductivity and thermal conductivity?is based on the Chapman?Enskog approximation. The necessary data are collision integrals, particle densities and thermodynamic properties. Special attention has been devoted in this paper to the study and the choice of the collision integrals for each interaction. The first results concern the influence of this choice on the transport coefficients. The general results show that the influence of the copper vapour presence on the thermodynamic properties and on the transport coefficients can be important at intermediate temperatures.

Calculation of radiative properties of SF6–C2F4 thermal plasmas—application to radiative transfer in high-voltage circuit breakers modelling

Radiative transfer is a key point for accurate simulations of arcs in high voltage circuit breakers where the plasma is mainly composed, at high current, of a mixture of SF6 and PTFE vapours (C2F4 and decomposition products). Assuming local thermodynamic equilibrium, we have built a database of absorption coefficients over almost 300 000 spectral points, for a pressure range between 1 and 100 bar, temperatures from 300 to 50 000 K, and proportions from pure SF6 to pure C2F4. From these data, we have calculated the mean absorption coefficients (MAC) by considering several definitions of the mean coefficient and several spectral ranges or intervals. The choice between the various definitions was operated using a one dimensional radiative transfer model with imposed temperature profiles. The results showed that a combination of a normal average over the molecular continuum at low temperature, with a mixed definition of Planck average at high temperature gives the most accurate results. The optimization of the number of intervals for the definition of the MAC database was performed and showed that the accuracy on the radiative flux and on the divergence of the flux depends on the temperature profile. A good compromise is obtained with five or seven intervals.

Thermal plasma properties for Ar–Cu, Ar–Fe and Ar–Al mixtures used in welding plasmas processes: II. Transport coefficients at atmospheric pressure

This article is devoted to the calculation of thermodynamic properties (mass density, enthalpy and specific heat at constant pressure) and transport coefficients (viscosity, electrical conductivity, thermal conductivity and combined diffusion coefficients) of mixtures of argon and aluminum, iron and copper vapour at atmospheric pressure. Data are presented for the temperature range 300 to 30 000 K, for different concentrations of the metal vapours. The dependence of the properties on metal vapour type and concentration are discussed. Mixtures of argon and metal vapour occur in the arc welding and in other plasma applications. Tabulations of the data are presented, and will be of use in computational modelling of such applications.

Experimental studies on power frequency breakdown voltage of CF3I/N2 mixed gas under different electric fields

To verify the feasibility of replacing SF6 by CF3I/N2, we compared their power frequency breakdown performance with the influence of gas pressure, mixing ratio, and electric field utilization coefficient. Under different electric fields and mixing ratios, the power frequency breakdown voltage of CF3I/N2 increases linearly along with gas pressure. Besides, with the rise of the electric field utilization coefficient, the linear growth rate of breakdown voltage along with gas pressure gradually rises. The sensitivity of pure CF3I to electric field is particularly high and can be improved by the addition of N2. The mixture 30% CF3I/70% N2 at 0.3 MPa could replace pure SF6 in equipment requiring a low insulation, but the gas pressure or the content of CF3I need to be increased for higher insulation requirements.

Net emission of H2O–air–MgCl2/CaCl2/NaCl thermal plasmas

This paper is devoted to the calculation of the net radiation emitted by water–air–MgCl2/CaCl2/NaCl mixtures. Assuming a spherical, homogeneous and isothermal plasma, the net emission coefficient (NEC) is calculated for several pressures between 1 and 10 bar in the temperature range 300–30 000 K under the assumption of local thermodynamic equilibrium. The various contributions to the NEC (atomic continuum, molecular continuum, molecular band and spectral lines) are presented and described. The influence of the pressure and of the proportion of air, MgCl2, CaCl2 or NaCl is analysed. It is also demonstrated that the alkaline salt resonance lines (Ca+, Ca2+, Na+, Mg+, Mg2+, Cl+ and Cl2+) have a significant effect on the NEC value in spite of their strong self-absorption.