Joseph Yan

Plasma-assisted conversion of CO2 in a dielectric barrier discharge reactor: understanding the effect of packing materials

A cylindrical dielectric barrier discharge (DBD) reactor has been developed for the conversion of undiluted CO2 into CO and O2 at atmospheric pressure and low temperatures. Both the physical and chemical effects on reaction performance have been investigated for the addition of BaTiO3 and glass beads into the discharge gap. The presence of these packing materials in the DBD reactor changes the physical characteristics of the discharge and leads to a shift of the discharge mode from a typical filamentary discharge with no packing to a combination of filamentary discharge and surface discharge with packing. Highest CO2 conversion and energy efficiency are achieved when the BaTiO3 beads are fully packed into the discharge gap. It is found that adding the BaTiO3 beads into the plasma system enhances the average electric field and mean electron energy of the CO2 discharge by a factor of two, which significantly contributes to the enhancement of CO2 conversion, CO yield, and energy efficiency of the plasma process. In addition, the highly energetic electrons (>3.0 eV) generated by the discharge could activate the BaTiO3 photocatalyst to form electron–hole pairs on its surface, which contributes to the enhanced conversion of CO2.

A comparative study of turbulence models for SF6 arcs in a supersonic nozzle

A comparative study of the two most popular turbulence models, the Prandtl mixing length model and the K-epsilon model, has been carried out for turbulent SF6 arcs burning in a supersonic nozzle under direct current conditions. The computed radial temperature profiles are compared with those measured previously by Leseberg and Pietsch at currents of 100 A, 600 A and 1800 A, respectively. It has been found that the turbulence parameters of the K-epsilon model are dependent upon the arcing current while those of the Prandtl mixing length model are applicable for all the currents investigated. For turbulent SF6 arcs in a supersonic nozzle, consideration of the range of applicability of the turbulence parameters and the cost of computation have made the Prandtl mixing length model preferable to the K-epsilon model.

Thermophysical properties of nitrogen plasmas under thermal equilibrium and non-equilibrium conditions

Calculated thermophysical properties of nitrogen plasmas in and out of thermal equilibrium are presented. The cut-off of the partition functions due to the lowering of the ionization potential has been taken into account, together with the contributions from different core excited electronic states. The species composition and thermodynamic properties are determined numerically using the Newton–Raphson iterative method, taking into account the corrections due to Coulomb interactions. The transport properties including diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are calculated using the most recent collision interaction potentials by adopting Devoto’s electron and heavy particle decoupling approach, expanded to the third-order approximation (second-order for viscosity) in the framework of Chapman–Enskog method. Results are presented in the pressure range of 0.1 atm–10 atm and in electron temperature range from 300 to 40 000 K, with the ratio of electron temperature t...

The development of PC based CAD tools for auto-expansion circuit breaker design

It is shown that, with the current understanding of basic arc physics, CAD tools based on a mathematical model encompassing all important physical processes can be developed to aid the design of auto-expansion circuit breakers. The model takes account of radiation transport, arc radiation induced inner wall ablation, turbulence enhanced momentum and energy transport and the moving parts of the breaker. With the aid of the PC based PHOENICS, a commercial general purpose computational fluid dynamics (CFD) package, the conservation equations for the gas and arc together with the relevant Maxwells equations can be solved. The temperature, velocity and electric fields within the breaker can easily be visualised and the thermal interruption capability predicted. The effects of changing design parameters can be simulated in detail, thus reducing the number of short-circuit tests. Results are verified by available tests.

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 30 000 K for different pressures between 0.1 and 10 atm. 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.

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 0.1 atm to 10 atm and ratios of the electron temperature to the heavy particle temperature from 1 to 20 with electron temperature range from 300 to 40 000 K. In the local thermodynamic equilibrium regime, results are compared with avail...

Dielectric breakdown of a residual SF/sub 6/ plasma at 3000 K under diatomic equilibrium

A residual plasma at 3000 K and under diatomic equilibrium is left inside an axially blown gas blast SF/sub 6/ circuit breaker after the thermal extinction of an arc. The breakdown of such a residual plasma depends on the electron generation and loss mechanisms. Altogether six ionization and chemical reactions relevant to the time scale of breakdown have been taken into account. It has been found that the critical field strength for breakdown is proportional to the pressure and is equal to 2.0 V/(m.Pa). This field strength is in agreement with the experimental results.

Thermophysical Properties of High-Temperature Reacting Mixtures of Carbon and Water in the Range 400–30,000 K and 0.1–10 atm. Part 1: Equilibrium Composition and Thermodynamic Properties

This paper is devoted to the calculation of the chemical equilibrium composition and thermodynamic properties of reacting mixtures of carbon and water at high temperature. Equilibrium particle concentrations and thermodynamic properties including mass density, molar weight, entropy, enthalpy and specific heat at constant pressure, sonic velocity, and heat capacity ratio are determined by the method of Gibbs free energy minimization, using species data from standard thermodynamic tables. The calculations, which assume local thermodynamic equilibrium, are performed in the temperature range from 400 to 30,000 K for pressures of 0.10, 1.0, 3.0, 5.0 and 10.0 atm. The properties of the reacting mixture are affected by the possible occurrence of solid carbon formation at low temperature, and therefore attention is paid to the influence of the carbon phase transition by comparing the results obtained with and without considering solid carbon formation. The results presented here clarify some basic chemical process and are reliable reference data for use in the simulation of plasmas in reacting carbon and water mixtures together with the need of transport coefficients computation.

Three-dimensional modelling of a dc arc plasma in a twin-torch system

A three-dimensional steady-state arc model has been developed for simulating the behaviour of a twin-torch plasma system operating in an atmospheric argon environment. The governing equations for plasma flow, electric and magnetic fields are solved in a Cartesian coordinate system with suitable domain size and appropriate boundary conditions. Radiation from the arc is accounted for using the concept of net emission coefficient. Results over the current range 300–900 A suggest that the two arc columns attached, respectively, to anode and cathode are coupled by a thin tissue-like conducting layer with a thickness in the range 3–6 mm, serving as a critical part of the current path. Lorentz force resulting from the arcs current interacting with its own magnetic field plays a decisive role in bending the two arc columns and shaping the coupling zone. Turbulence is introduced to account for the effect resulting from instability of the coupling zone as observed in experiment and its effectiveness is analysed in detail using different length scale parameters. Suggestions to further improve the model are proposed.