Two-temperature Kinetic Model Research Articles

Dominant particles and reactions in a two-temperature chemical kinetic model of a decaying SF6 arc

This paper is devoted to the computation of the non-equilibrium composition of an SF6 plasma, and determination of the dominant particles and reactions, at conditions relevant to high-voltage circuit breakers after current zero (temperatures from 12 000 K to 1000 K and a pressure of 4 atm). The non-equilibrium composition is characterized by departures from both thermal and chemical equilibrium. In thermal non-equilibrium process, the electron temperature (Te) is not equal to the heavy-particle temperature (Th), while for chemical non-equilibrium, a chemical kinetic model is adopted. In order to evaluate the reasonableness and reliability of the non-equilibrium composition, calculation methods for equilibrium composition based on Gibbs free energy minimization and kinetic composition in a one-temperature kinetic model are first considered. Based on the one-temperature kinetic model, a two-temperature kinetic model with the ratio Te/Th varying as a function of the logarithm of electron density ratio (ne/) was established. In this model, T* is introduced to allow a smooth transition between Th and Te and to determine the temperatures for the rate constants. The initial composition in the kinetic models is obtained from the asymptotic composition as infinite time is approached at 12 000 K. The molar fractions of neutral particles and ions in the two-temperature kinetic model are consistent with the equilibrium composition and the composition obtained from the one-temperature kinetic model above 10 000 K, while significant differences appear below 10 000 K. Based on the dependence of the particle distributions on temperature in the two-temperature kinetic model, three temperature ranges, and the dominant particles and reactions in the respective ranges, are determined. The full model is then simplified into three models and the accuracy of the simplified models is assessed. The simplified models reduce the number of species and reactions by a factor of about 2, while providing results that agree closely with the full model. Thus, the physicochemical processes of SF6 arc can be characterized by relatively few species and reactions in each temperature range. It is noted that the simplified models can also be applied to a wide range of pressures, 1–16 atm, conditions which cover most circuit breaker applications. The simplified species and reactions will allow the computing time of multi-dimensional models, taking into account departures from both thermal and chemical equilibrium, to be decreased dramatically while capturing the main physicochemical processes in SF6 arcs.

Substantiation of the two-temperature kinetic model by comparing calculations within the kinetic and fluid models of the positive column plasma of a dc oxygen discharge

Results from kinetic and fluid simulations of the positive column plasma of a dc oxygen discharge are compared using commercial CFDRC software ( http://www.cfdrc.com/˜cfdplasma ), which enables one to perform numerical simulations in an arbitrary 3D geometry with the use of both the fluid equations for all the components (fluid model) and the kinetic equation for the electron energy distribution function (kinetic model). It is shown that, for both the local and nonlocal regimes of the formation of the electron energy distribution function (EEDF), the non-Maxwellian EEDF can satisfactorily be approximated by two groups of electrons. This allows one to take into account kinetic effects within the conventional fluid model in the simplest way by using the proposed two-temperature approximation of the nonequilibrium and nonlocal EEDF (2T fluid model).

Nonequilibrium discharges in air and nitrogen plasmas at atmospheric pressure

Abstract Diffuse glow discharges were produced in low temperature (<2000 K) atmospheric pressure air and nitrogen plasmas with electron number densities in excess of 1012 cm­3, more than six orders of magnitude higher than in thermally heated air at 2000 K. The measured discharge characteristics compare well with the predictions of a two-temperature kinetic model. Experimental and modeling results show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a behavior resulting from competition between ionization and charge-transfer reactions. Non-Maxwellian effects are shown to be unimportant for the prediction of steady-state electron number densities. The power requirements of DC discharges at atmospheric pressure can be reduced by several orders of magnitude using short repetitive high-voltage pulses. Between consecutive pulses, the plasma is sustained by the finite rate of electron recombination. Repetitive discharges with a 100-kHz, 12-kV, 10-ns pulse generator were demonstrated to produce over 1012 electrons/cm3 with an average power of 12 W/cm3, 250 times smaller than a DC discharge at 1012 cm­3.

A two-temperature kinetic model of SF6 plasma

Studying the influence of thermal departures from equilibrium in SF6 circuit-breakers, we develop a two-temperature kinetic model to calculate the composition. Such a kinetic approach has not been adopted until now for SF6 plasma because of the complexity of chemical processes. Our model takes into account the collisional mechanisms responsible for the creation and disappearance of atoms and molecules through 19 species linked by 66 chemical reactions. To solve the conservation equations, the model uses the direct rates of reactions, that mainly proceed from the literature, and reverse rates, that are computed by two-temperature micro-reversibility laws. Thus, we point out the importance of the choice of the expression of Saha law, comparing Potapov and van de Sanden formulations of this law. We then discuss the impact of thermal departures from equilibrium on plasma composition, on `mean path' of molecules before dissociation in the plasma, and on the reactions that govern the disappearance of electrons.

Assessment of two-temperature kinetic model for ionizing air

A two-temperatur e chemical-kinet ic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock tubes, ballistic ranges, and flight experiments. In the model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium flow in shock tubes are obtained using the computer code STRAP (shock-tube radiation program) and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code SPRAP (stagnation-point radiation program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures, the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibriu m airflow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of 6 m radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of freestream conditions. Assumptions, approximations, and limitations of the model are discussed. Nomenclature = average molecular speed ^/$kT/nm, cm s ~ ! = pre-exponential factor in reaction rate coefficient, cm3mole~ 1 s~ * - average vibrational energy per particle, erg = average vibrational energy per particle under equilibrium, erg = reaction energy, erg

Assessment of a two-temperature kinetic model for dissociating and weakly ionizing nitrogen

The validity of the author's two-temperature, chemical/kinetic model which the author has recently improved is assessed by comparing the calculated results with the existing experimental data for nitrogen in the dissociating and weakly ionizing regime produced behind a normal shock wave. The computer program Shock Tube Radiation Program (STRAP) based on the two-temperature model is used in calculating the flow properties behind the shock wave and the Nonequilibrium Air Radiation (NEQAIR) program, in determining the radiative characteristics of the flow. Both programs were developed earlier. Comparison is made between the calculated and the existing shock tube data on (1) spectra in the equilibrium region, (2) rotational temperature of the N2(+) B state, (3) vibrational temperature of the N2(+) B state, (4) electronic excitation temperature of the N2 B state, (5) the shape of time-variation of radiation intensities, (6) the times to reach the peak in radiation intensity and equilibrium, and (7) the ratio of nonequilibrium to equilibrium radiative heat fluxes. Good agreement is seen between the experimental data and the present calculation except for the vibrational temperature. A possible reason for the discrepancy is given.