Weizong Wang

Thermophysical properties of carbon–argon and carbon–helium plasmas

The calculated values of thermodynamic and transport properties of mixtures of carbon and argon, and carbon and helium, at high temperatures are presented in this paper. The thermodynamic properties are determined by the method of Gibbs free energy minimization, using standard thermodynamic tables. The transport properties including electron diffusion coefficients, viscosity, thermal conductivity and electrical conductivity are evaluated using the Chapman–Enskog method expanded up to the third-order approximation (second order for viscosity). Collision integrals are obtained using the most accurate cross-section data that could be located. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30u2009000u2009K for different pressures between 0.1 and 10u2009atm. The results are compared with those of previously published studies. Good agreement is found for pure argon and helium. Larger discrepancies occur for carbon and mixtures of carbon and argon, and carbon and helium; these are explained in terms of the different values of the collision integrals that were used. The results presented here are expected to be more accurate because of the improved collision integrals employed.

Simulation of Arc Characteristics in Miniature Circuit Breaker

This paper focuses on the numerical investigation of arc plasma behavior in low-voltage circuit breaker with arc-splitting process included. A 3-D simulation model of a certain type miniature circuit breaker product is built and calculated, which is based on magnetohydrodynamics theory. Aside from coupled electromagnetic and gas dynamic interactions being considered as usual, a thin layer of nonlinear electrical resistance elements is used to represent the voltage drop of plasma sheath and the formation of new arc roots. Thus, the arc-splitting process with ferromagnetic plates can be included. Arc motion is described in detail by the temperature distribution. Some interesting phenomena are observed in the simulation, such as the arc root jumping from contact to arc runner, the arc back commutation before splitting into series arcs by metallic plates, new arc root formation in splitting process, and, finally, the arc go across the splitter plates in the chamber. Moreover, the simulation result is compared with experimental result, which shows good agreement.

Theoretical investigation of the decay of an SF 6 gas-blast arc using a two-temperature hydrodynamic model

The behaviour of a decaying SF6 arc, which is representative of the approach to the final current-zero state of switching arcs in a high-voltage circuit breaker, is theoretically investigated by a two-temperature hydrodynamic model, taking into account the possible departure of the plasma state from local thermodynamic equilibrium (LTE). The model couples the plasma flow with electromagnetic fields in a self-consistent manner. The electrons and heavy species are assumed to have different temperatures. The species composition, thermodynamic properties and transport coefficients of the plasma under non-LTE conditions are calculated from fundamental theory. The model is then applied to a two-dimensional axisymmetric SF6 arc burning in a supersonic nozzle under well-controlled conditions; for this configuration, experimental results are available for comparison. The effect of turbulence is considered using the Prandtl mixing-length model. The edge absorption of the radiation emitted by the arc core is taken into account by a modified net emission coefficient approach. The complete set of conservation equations is discretized and solved using the finite volume method. The evolution of electron and heavy-particle temperatures and the total arc resistance, along with other physical quantities, is carefully analysed and compared with those of the LTE case. It is demonstrated that the electron and heavy-particle temperature diverge at all times in the plasma–cold-flow interaction region, in which strong gas flow exists, and further in the transient current-zero period, in which case the collision energy exchange is ineffective. This study quantitatively analyses the energy exchange mechanisms between electrons and heavy particles in the high-pressure supersonic SF6 arcs and provides the foundation for further theoretical investigation of transient SF6 arc behaviour as the current ramps down to zero in gas-blast circuit breakers. (Some figures may appear in colour only in the online journal)

Thermodynamic and transport properties of two-temperature SF6 plasmas

This paper deals with thermodynamic and transport properties of SF6 plasmas in a two-temperature model for both thermal equilibrium and non-equilibrium conditions. The species composition and thermodynamic properties are numerically determined using the two-temperature Saha equation and Guldberg-Waage equation according to deviation of van de Sanden et al. Transport properties including diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are calculated with most recent collision interaction potentials by adopting Devoto’s electron and heavy particle decoupling approach but expanded to the third-order approximation (second-order for viscosity) in the frame of Chapman–Enskog method. The results are computed for various values of pressures from 0.1u2009atm to 10u2009atm and ratios of the electron temperature to the heavy particle temperature from 1 to 20 with electron temperature range from 300 to 40 000u2009K. In the local thermodynamic equilibrium regime, results are compared with avail...

Investigation on critical breakdown electric field of hot sulfur hexafluoride/carbon tetrafluoride mixtures for high voltage circuit breaker applications

Sulfur hexafluoride (SF6) gas, widely used in high-voltage circuit breakers, has a high global warming potential and hence substitutes are being sought. The use of a mixture of carbon tetrafluoride (CF4) and SF6 is examined here. It is known that this reduces the breakdown voltage at room temperature. However, the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown depends on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/CF4 mixtures corresponding to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300u2009K to 3500u2009K. The equilibrium compositions of hot SF6/CF4 mixtures under different mixing fractions were determined based on Gibbs free energy mi...

Dielectric breakdown properties of hot SF6/He mixtures predicted from basic data

Sulfur hexafluoride (SF6) gas has a quite high global warming potential and hence it is required that applying any substitute for SF6 gas. Much interest in the use of a mixture of helium and SF6 as arc quenching medium was investigated indicating a higher recovery performance of arc interruption than that of pure SF6. It is known that the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown relies on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/He mixtures related to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300u2009K to 3500u2009K. The critically reduced electric field strength of these mixtures was obtained by balancing electron generation and...

Gliding Arc Plasmatron: providing an alternative method for carbon dioxide conversion

Low-temperature plasmas are gaining a lot of interest for environmental and energy applications. A large research field in these applications is the conversion of CO2 into chemicals and fuels. Since CO2 is a very stable molecule, a key performance indicator for the research on plasma-based CO2 conversion is the energy efficiency. Until now, the energy efficiency in atmospheric plasma reactors is quite low, and therefore we employ here a novel type of plasma reactor, the gliding arc plasmatron (GAP). This paper provides a detailed experimental and computational study of the CO2 conversion, as well as the energy cost and efficiency in a GAP. A comparison with thermal conversion, other plasma types and other novel CO2 conversion technologies is made to find out whether this novel plasma reactor can provide a significant contribution to the much-needed efficient conversion of CO2 . From these comparisons it becomes evident that our results are less than a factor of two away from being cost competitive and already outperform several other new technologies. Furthermore, we indicate how the performance of the GAP can still be improved by further exploiting its non-equilibrium character. Hence, it is clear that the GAP is very promising for CO2 conversion.

The Reactive Thermal Conductivity of Thermal Equilibrium and Nonequilibrium Plasmas: Application to Nitrogen

The accuracy of numerical simulation on plasma behavior depends strongly on the reliability of thermophysical property data. Large number of studies for thermal plasma properties in the local thermodynamic equilibrium (LTE) exist; however, the database for thermal nonequilibrium plasmas is still far from completeness. This paper derives a general expression of total reactive thermal conductivity (TRTC) with great applicability to monatomic, diatomic, and polyatomic gases in terms of a two-temperature model. The derived formula is applied to nitrogen plasmas under thermal equilibrium and nonequilibrium conditions, considering its wide use in plasma systems and switching devices. Typical calculated results of TRTC with two different Saha equations and Guldberg-Waage equations in the temperature range of 300 K-40 000 K under different degrees of nonequilibrium are given and compared with those computed according to Brokaw and Butlers derivation for the special case of LTE plasmas, which shows excellent agreement. The influence of different expressions for Saha equations and Guldberg-Waage equations, together with different pressures of 0.1, 1, 3, 5, and 10 atm, on the TRTC evaluated by this newly developed expression is presented as well. These provide reliable reference data for use in the simulation of plasmas.

Modeling plasma-based CO2 conversion: crucial role of the dissociation cross section

Plasma-based CO2 conversion is gaining increasing interest worldwide. A large research effort is devoted to improving the energy efficiency. For this purpose, it is very important to understand the underlying mechanisms of the CO2 conversion. The latter can be obtained by computer modeling, describing in detail the behavior of the various plasma species and all relevant chemical processes. However, the accuracy of the modeling results critically depends on the accuracy of the assumed input data, like cross sections. This is especially true for the cross section of electron impact dissociation, as the latter process is believed to proceed through electron impact excitation, but it is not clear from the literature which excitation channels effectively lead to dissociation. Therefore, the present paper discusses the effect of different electron impact dissociation cross sections reported in the literature on the calculated CO2 conversion, for a dielectric barrier discharge (DBD) and a microwave (MW) plasma. Comparison is made to experimental data for the DBD case, to elucidate which cross section might be the most realistic. This comparison reveals that the cross sections proposed by Itikawa and by Polak and Slovetsky both seem to underestimate the CO2 conversion. The cross sections recommended by Phelps with thresholds of 7 eV and 10.5 eV yield a CO2 conversion only slightly lower than the experimental data, but the sum of both cross sections overestimates the values, indicating that these cross sections represent dissociation, but most probably also include other (pure excitation) channels. Our calculations indicate that the choice of the electron impact dissociation cross section is crucial for the DBD, where this process is the dominant mechanism for CO2 conversion. In the MW plasma, it is only significant at pressures up to 100 mbar, while it is of minor importance for higher pressures, when dissociation proceeds mainly through collisions of CO2 with heavy particles.

CO_{2} conversion in a gliding arc plasma : 1D cylindrical discharge model

CO2 conversion by a gliding arc plasma is gaining increasing interest, but the underlying mechanisms for an energy-efficient process are still far from understood. Indeed, the chemical complexity of the non-equilibrium plasma poses a challenge for plasma modeling due to the huge computational load. In this paper, a one-dimensional (1D) gliding arc model is developed in a cylindrical frame, with a detailed non-equilibrium CO2 plasma chemistry set, including the CO2 vibrational kinetics up to the dissociation limit. The model solves a set of timedependent continuity equations based on the chemical reactions, as well as the electron energy balance equation, and it assumes quasi-neutrality in the plasma. The loss of plasma species and heat due to convection by the transverse gas flow is accounted for by using a characteristic frequency of convective cooling, which depends on the gliding arc radius, the relative velocity of the gas flow with respect to the arc and on the arc elongation rate. The calculated values for plasma density and plasma temperature within this work are comparable with experimental data on gliding arc plasma reactors in the literature. Our calculation results indicate that excitation to the vibrational levels promotes efficient dissociation in the gliding arc, and this is consistent with experimental investigations of the gliding arc based CO2 conversion in the literature. Additionally, the dissociation of CO2 through collisions with O atoms has the largest contribution to CO2 splitting under the conditions studied. In addition to the above results, we also demonstrate that lumping the CO2 vibrational states can bring a significant reduction of the computational load. The latter opens up the way for 2D or 3D models with an accurate description of the CO2 vibrational kinetics.