A Comparison Between MHD Modeling and Experimental Results in a 3-Phase AC Arc Plasma Torch: Influence of the Electrode Tip Geometry

Arc behavior in 3-Phase AC plasma technology remains poorly explored. This system noticeably differs from the classical DC plasma torches and aims to overcome certain limitations, such as efficiency, equipment cost and reliability. A MHD model of a 3-Phase AC plasma torch was recently developed at Mines-ParisTech. The model does not include the electrodes in the computational domain. In parallel, experiments were conducted using a high-speed video camera shooting 100,000 frames per second. In this paper, the comparison between MHD modeling and experimental results shows that the arc behavior is in line with the results from the MHD model. Particularly, the strong influences of both the electrode jets and Lorentz forces on the arc motion are confirmed. However, some differences between experimental and numerical electrical waveforms are observed and particularly in the current–voltage phase shift. A new model was then developed by integrating the electrodes into the computational domain and adjusting the electrode tip geometry. With this simulation, we were able to reproduce the phase shift, power and voltage values with a good accuracy showing the strong influence of electrode tip geometry on the 3-Phase arc plasma discharge.     

Large Eddy Simulation of a Free-Burning Arc Discharge in Argon with a Turbulent Cross Flow and External Field

In most of the numerical approaches proposed for modeling high-intensity plasma-arcs, the effects of turbulence on the arc structure are often excluded because of the intricate physics originating from the interaction of turbulent scales, high-temperature gas dynamics, magnetohydrodynamics (MHD) and chemical kinetics. The goal of this study is threefold: to develop a generic turbulent MHD model to simulate free-burning arc discharges, to validate the code with available experimental data, and to investigate the effect of an external field and turbulent cross flow on the free-burning arc configuration. The governing equations are solved in conservative form using a hybrid scheme that combines a high-order monotonic upwind scheme with a second-order central scheme. The fluid and MHD turbulence are resolved using a large eddy simulation (LES) approach with a recently developed sub-grid closure model. An implicit scheme is used to compute the magnetic diffusion term appearing in the magnetic induction equation to alleviate the severe time-step constraint. The comparison of the model prediction with experimental data for Argon arcs at different current intensities shows generally good agreement. When an external field is applied, the overall shape of the free-burning arc drastically changes. The straightening of the arc indicates the potential for stabilization of a free-burning arc by magnetic forces. Even though the turbulence is significantly attenuated as a result of the thermal expansion near the cathode, it adds an unsteady characteristic to the arc and, in general, has a negative impact on the stabilization of the electrical discharge.     

Steam Torch Plasma Modelling

Numerical modelling of physical properties and processes in an electric arc stabilized by a water vortex (steam torch) has been summarized in this review paper. One-fluid MHD equations are numerically solved for an axisymmetric thermal plasma flow inside a discharge chamber of the steam plasma torch. The steady state solution results are discussed for the range of currents 300–600 A with relatively low steam flow rate of about 0.3 g s^?1. The maximum obtained velocities and temperatures—8500 m s^?1, 26,300 K, are reported at the centre of the nozzle exit for 600 A. The evaporation of water, i.e. mass flow rate of steam, was predicted from a comparison between the present simulation and experiments. The generated plasma is mildly compressible (M < 0.7) with the inertial forces overwhelming the magnetic, viscous, centrifugal and Coriolis forces with the factor of 10³. Our calculations showed that the most significant processes determining properties of the arc are the balance of the Joule heat with radiation and radial conduction losses from the arc. Rotation of plasma column due to the tangential velocity component has a negligible effect on the overall arc performance, however, the rotation of water induces fluctuations in the arc and in the plasma jet with characteristic frequency which is related to the frequency of rotation of water. Reabsorption of radiation occurs at the radial position higher than 2.5 mm from the arc axis. The amount of reabsorbed radiation is between 17 and 28%. LTE conditions are satisfied in the arc column with the 2 mm radius. Comparison between the present simulations and experiments shows good agreement with the current–voltage characteristics, radial velocity and temperature profiles, as well as with the other related numerical simulation.     

The Dependence of Gliding Arc Gas Discharge Characteristics on Reactor Geometrical Configuration

The dependence of gliding arc gas discharge characteristics, including gas flow field, arc column motion and volatile organic compounds (VOCs) decomposition performance, on reactor configuration parameters was investigated based on numerical simulation and laboratory experimental findings. For a given supply voltage (10 kV) and a certain nozzle outlet diameter (1.5 mm), increasing the electrodes gap (1–4 mm) or decreasing vertical distance between electrode throat and nozzle outlet (25–10 mm) will increase the gas flow rate through the electrode throat, the gas velocity in the plasma region, the arc column velocity, the maximum attainable position of the arc column and the electrical power consumption, also, higher VOCs decomposition rate and lower specific energy requirement are observed according to the n-butane and toluene decomposition experiments.     

Étude d'un modèle périodique de diffusion non linéaire en régime stationnaire pour une réaction réversible

For a partially inhibited solid electrode a periodic model of non-linear diffusion is defined, permitting the calculation of the attenuation factor, ?, of the diffusion current density as a function of the size and the closeness of active or inactive sites and of the diffusion layer thickness. Values of ? are compared with those obtained from the Landsberg's model [3]. In the case of a disk electrode rotating at N r.p.s., the influence of the characteristic parameters of the inhibiting coverage on the relationship between the diffusion current and N^1/2 is analysed in the experimentally accessible laminar flow domain.     

Generation of Water-In-Oil-In-Water (W/O/W) Double Emulsions by Microfluidics

Novel compartment microparticles prepared with double emulsion droplets as templates provide a protected internal space for material encapsulation. The effect of three-phase flow rate on the micro-droplet generation of double emulsion mechanism is available for reference to produce precise size and highly monodisperse particles. The influence of three-phase flow rate on the formation mode and size of the emulsion droplets is investigated by combination of experiment and numerical simulation. The size of compound droplets decreases and frequency increases with the increasing outer fluid flow rate. The monodispersity of the double emulsion reduces due to transition from dripping to narrowing jetting regime. Outer droplet size increases with the increasing flow rate of the middle fluid, whereas inner droplet size is the opposite. The frequency increases and then stabilizes, which leads to a widening regime. When Q₂/Q₁ > 6, the multi-core type double emulsion droplets are produced. Droplet coalescence occurs when surfactants is not involved. As Q₁ increases, there is an increasing tendency for inner drop size. The outer drop size is proportional to the sum of the inner and middle flow rate, and that is irrelevant to Q₁/Q₂. For drop size, the ratio of core-shell and internal structure is precisely controlled by adjusting three-phase flow rate respectively.     

Comparative Study of Flow Characteristics Inside Plasma Torch with Different Nozzle Configurations

A numerical analysis of the influence of different nozzle configurations on the plasma flow characteristics inside D.C plasma torches is presented to provide an advanced nozzle design basis for plasma spraying torches. The assumption of steady-state, axis-symmetric, local thermodynamic equilibrium, and optically thin plasma is adopted in a two-dimensional modeling of plasma flow inside the plasma torch. The PHOENICS software is used for solving the governing equations, i.e. the conservation equations of mass, momentum, and energy along with the equations describing the K-epsilon model of turbulence. The calculated arc voltages are consistent with the experimental results when arc current, gas inflow rate, and working gas are the same as the experimental parameters. Temperature, axial velocity contours inside plasma torches, profiles along the torch axis and profiles at the outlet section are presented to show the plasma flow characteristics. Comparisons are made among those torches. The results show that torches with different anode nozzle configurations produce different characteristics of plasma flows, which suggest some important ideas for the advanced nozzle design for plasma spraying. In order to validate the model and to show its level of predictivity, a comparison of the model with experimental results encountered in the literature is presented in the last part.     

Numerical Simulation of Metal Vapour Behavior in Double Electrodes TIG Welding

Metal vapour from the weld pool in double electrodes tungsten inert gas welding is taken into account by a unified numerical model including the arc plasma and the weld pool. The thermodynamic properties and transport coefficients of the arc plasma are dependent on both the local temperature and the mass fraction of the metal vapour. A second viscosity approximation is used to describe the diffusion coefficient of the metal vapour in the arc plasma. The temperature and the flow fields of both the arc plasma and the weld pool are calculated together with the metal vapour concentration. The simulated results are presented for the cases of 3 and 9 mm electrode separation, respectively. It is shown that the metal vapour behavior is much different in these two cases. In the case of 3 mm electrode separation, the metal vapour above the mass fraction of 0.2% is concentrated just above the weld pool surface, while in the case of 9 mm electrode separation, the metal vapour is diffused to the most region of the arc plasma for the same range of mass fraction. In addition, the arc plasma temperature as well as the heat flux at the weld pool is constricted by the presence of the metal vapour. The constricted heat flux at the weld pool results in an increase in the temperature of the weld pool about 100 K or less but a slight shrinkage of the weld pool shape.     

High-temperature redox behavior of doped SrTiO3 and LaCrO3

The stability of high-temperature fuel cell electrodes to their ambient environment is important for the long-term reliability of fuel cells. In this report the behavior of oxide electrode materials as a function of oxygen activity and temperature is considered. Models for the oxidation-reduction behavior of both p- and n-type oxides are presented. These models take into account the absorption and evolution of oxygen which take place as oxygen activity is varied. The resulting instability in electrical conductivity is explained as a consequence of changes in carrier concentration due to variability in ionic defect concentration. The proposed models are applied to acceptor-doped LaCrO₃ and donor-doped SrTiO₃. It is shown that the models explain the experimental data well and as a consequence diagrams can be made which show the regions of oxygen activity and temperature for which stability of electrical conductivity and defect structure might be expected.     

High Speed Video Camera and Electrical Signal Analyses of Arcs Behavior in a 3-Phase AC Arc Plasma Torch

A 3-phase AC plasma torch has been developed and aims at overcoming some limits of the classical DC torches in terms of efficiency, cost and reliability. However, the arc behavior in 3-phase plasma torch remains poorly explored. This paper is dedicated to the high speed video camera at 100,000 frames per second and electrical signal analyses of arcs behavior in a 3-phase AC arc plasma torch. First, a reference case at 150 A, in nitrogen as working gas, has been deeply analyzed. Afterwards, a parametric study based on current and inter-electrode gap has been carried out. Results show that only one arc can exist at a given time and arcs rotate by switching from a pair of electrodes to another one, following the maximal electrical gap potential. However, a particular “abnormal” arc behavior was sometimes observed. Indeed, the arc motion within the inter-electrode gap increases the heat exchange and stabilizes the 3-phase discharge whereas the system is unbalanced when the arc is in the periphery. The analysis highlights that the arc motion is strongly influenced by the electrode jet velocity and repulsive Lorentz forces. The parametric study shows that the current increases both jet velocity and arc discharge stability. Elsewhere, the increase of the inter-electrode gap can also stabilizes the electrical 3-phase arc discharge. Furthermore, the correlation between arc motion and current waveform is highlighted. This work is likely to open the way toward a better understanding of 3-phase discharges in the perspective of their further optimization.     

Improvement of the proton exchange membrane fuel cell performances by optimization of the hot pressing process for membrane electrode assembly

The present study deals with PEM fuel cells, namely with the optimization of the hot pressing process for membrane electrode assembly (MEA) fabrication. Designs of experiments (DoE) have been used for evaluating the effect of hot pressing parameters (pressure, temperature, and time) on the MEA electrical performances. Full factorial 2³ DoE showed that the most important parameter is the pressing temperature. Surface response methodology indicated a non-monotonous behavior of the MEA electrical performances with respect to the pressing temperature. The MEA electrical performances increased with the pressing temperature in the temperature range from 100 to 115 °C, and decreased significantly in the temperature range from 115 to 130 °C. This behavior was attributed to drastic changes of the Nafion® 112 membrane properties and membrane/electrode interface over this temperature range. Observations of the MEA cross-section structure by scanning electron microscopy confirmed such hypotheses. Thermo-mechanical properties of Nafion® as determined by dynamic scanning calorimetry allowed estimating the glass transition temperature at ca. T _g?≈?117 °C in the conditions of the present study. The higher H₂/air fuel cell performance of ca. 0.8 W cm^?2 was obtained with the optimized pressing temperature for MEA fabrication of ca. 115 °C close to the T _g temperature of Nafion® 112, whereas for higher temperature the structure of the Nafion® membrane and of the membrane–electrode interface is damaged.     

Effect of the electrical double layer on the behavior of molecules: Regio-and stereoselective electrochemical reduction of 2,4-pentadienol

An approach to studying the effect of the electrical double layer (EDL) on the behavior of molecules in the electrode vicinity is proposed. The approach consists of comparing the results of a “direct” electrolysis of a model substance (process P ¹) with the results of its electrolysis performed in the mode of a homogeneous catalytic endoergic electron transfer (process P ₂). Two EDL effects are discovered in the electrochemical reduction of deuterated 2,4-pentadienol (I): the predominant formation of positional isomer IVa ? (IVa ? IVb) (regioselective synthesis) and a drastic increase in the concentration of cis-isomers of both IVa and IVb compounds (stereoselective synthesis). Possible reasons for these effects are discussed. In particular, it is shown that the regioselectivity of electrosynthesis is due to the “effect of heredity” that initial molecule I and its ether and ester produce on the behavior of an intermediate pentadienyl anion (PDA). A hypothesis is considered for the nonequilibrium orientation of PDA near the electrode as a result of the superviscous state of the medium (heredity of orientation of the initial unsymmetrical molecule). A principle of EDL tomography based on electrolysis with gradual movement of the reaction layer from the electrode into the bulk solution under conditions of mixed P ₁₂ process is proposed.     

Electrochemical Properties of Electrode Materials of the Composition (Yb,Y)0.5Ca0.5MnO3

Electrochemical characteristics, such as the parameter of resistance (/d) of an intermediate layer at the electrode material/electrolyte interface and the polarization resistance (R ) of the three-phase boundary gas/solid-electrolyte/electrode under study, are studied as a function of the electrode layer thickness, temperature, and oxygen content in the gas phase for electrode materials of the composition (Yb, Y)_0.5Ca_0.5MnO₃. Established is the similarity in the behavior of /d and R from the above parameters. The determined effect on these characteristics is exerted by the composition of the electrode material. In the temperature range 300–1100 K, at = 10²–10⁵ Pa, the least values of the parameter of resistance and polarization resistance are intrinsic to samples with the thickness of the electrode layer 100 mg cm^–2 of the composition Y_0.5Ca_0.5MnO₃. For systems studied, the least temperature of establishment of electrochemical equilibrium at the three-phase boundary is 875 (±5) 5 K.     

Degradation of Phenol in Aqueous Solutions by Gas–Liquid Gliding Arc Discharges

The efficiency of a gliding arc reactor designed with the aim to degrade aqueous phenol solutions is studied as a function of supply voltage, electrode gap distance, and gas–liquid flow properties. This efficiency, which steeply increases when increasing the supply voltage, can reach 96% when the minimum electrode distance is fixed at 3 mm. Experiments show that phenol degradation efficiency also depends on solution pH, Fe²⁺ addition, gas nature and gas flow rate. Furthermore, degradation pathways of phenol in aqueous solutions are proposed.     

Remarks on the determination of low-frequency measurements of the dielectric response of colloidal suspensions

Electrode impedance is a significant artifact in low frequency dielectric measurements involving conducting media. In their recent review article regarding the dielectric dispersion of aqueous colloidal systems, Grosse and Delgado [1] presented an electrode polarization model that provides a physical explanation of the effect of electrolyte concentration and mobility, electrode spacing, and frequency. Although the model properly predicts the undesired phenomenon, the low frequency scaling, often used to identify electrode polarization effects, is incorrect. The apparent dielectric constant actually follows an ω^? 2 frequency dependence for ω/κ²D ? 1, where κ^? 1 is the Debye length and D is an average ion diffusion coefficient. Strictly speaking, the predicted scaling with exponent ? 1.5 is applicable only for sufficiently high frequencies, where electrode polarization is insignificant. This letter is intended to help clarify matters: the asymptotic behavior of the polarization model is examined, and the approximate expressions representing the real part of the complex dielectric constant of a parallel plate cell containing electrolyte solutions or colloidal suspensions are discussed.     

Graphene/poly(ortho-phenylenediamine) nanocomposite material for electrochemical supercapacitor

Reduced graphene oxide was synthesized by simple chemical processing of graphite. Electron microscopy investigations of synthesized graphene showed slightly folded transparent sheets with a few square micrometers dimension. Poly(ortho-phenylenediamine)/graphene/Pt electrode was electrochemically fabricated in a 2.0-M H₂SO₄ solution by means of multiple potential cycling. Due to the catalytic effect of graphene on the oxidative electropolymerization of ortho-phenylenediamine, the ortho-phenylenediamine/graphene (PoPD/GR) nanocomposite showed greatly enhanced electrical properties and excellent capacitive behavior. Electrochemical impedance spectroscopy, galvanostatic charge/discharge curves, and voltammetric investigations revealed that PoPD/GR nanocomposite represented good capacitive behavior with a specific capacitance as high as 308.3 F g^?1 at 0.1 A g^?1. It is almost three times higher than that of pure graphene (111.7 F g^?1). In addition, the nanocomposite electrode retained more than 99 % of the initial capacity after 1,500 cycles at a current density of 1 A g^?1.     

Atmospheric Pressure Ion Source Development: Experimental Validation of Simulated Ion Trajectories within Complex Flow and Electrical Fields

Three-dimensionally (3D) resolved ion trajectory calculations within the complex viscous flow field of an atmospheric pressure ion source are presented. The model calculations are validated with spatially resolved measurements of the relative sensitivity distribution within the source enclosure, referred to as the distribution of ion acceptance (DIA) of the mass analyzer. In previous work, we have shown that the DIA shapes as well as the maximum signal strengths strongly depend on ion source operational parameters such as gas flows and temperatures, as well as electrical field gradients established by various source electrode potentials (e.g., capillary inlet port potential and spray shield potential). In all cases studied, distinct, reproducible, and, to some extent, surprising DIA patterns were observed. We have thus attempted to model selected experimental operational source modes (called operational points) using a validated computational flow dynamics derived 3D-velocity field as an input parameter set for SIMION/SDS, along with a suite of custom software for data analysis and parameter set processing. Despite the complexity of the system, the modeling results reproduce the experimentally derived DIA unexpectedly well. It is concluded that SIMION/SDS in combination with accurate computational fluid dynamics (CFD) input data and adequate analysis software is capable of successfully modeling operational points of an atmospheric pressure ion (API) source. This approach should be very useful in the computer-aided design of future API sources.

Cyclic voltammetric investigation of Eu<Superscript>3+</Superscript> on a MWCNTs/SDS-modified glassy carbon (GC) electrode

Electrochemical reduction behavior of Eu³⁺ on a multi-walled carbon nanotubes (MWCNTs)/sodium lauryl sulfate (SDS) (MWCNTs/SDS)-modified glassy carbon (GC) electrode was investigated by cyclic voltammetry (CV). Results indicated that the electrochemical reduction process of Eu³⁺ at the MWCNTs/SDS-modified GC electrode is a quasi-reversible and diffusion-controlled process. The value of standard rate constant (k _s) at the MWCNTs/SDS-modified GC electrode was estimated to 1.96 × 10⁻² cm s⁻¹. CV studies showed that the electrochemical response of Eu³⁺ was directly related to the ratio of MWCNTs to SDS, and the tube diameter of MWCNTs had a slight influence on the electrochemical behavior of Eu³⁺, whereas the tube length. of MWCNTs had a strong influence. CVs results also proved that s-MWCNTs (with shorter tube length)-modified GC electrode showed better response to the electrochemical reaction of Eu³⁺.     

Modelling of solid state voltammetry of immobilized microcrystals assuming an initiation of the electrochemical reaction at a three-phase junction

The electrochemical reduction of a solid compound characterized by mixed ionic/electronic conductivity, immobilized on an electrode surface and in contact with an electrolyte solution, has been studied theoretically. The uptake or expulsion of electrons and electrolyte cation is coupled to maintain electroneutrality and is assumed to obey Fick's law of diffusion. Starting with the fully oxidized species, the simultaneous uptake of cations and electrons will be possible at the three-phase junction only, where electrode, solid and electrolyte solution meet. From this point, electrons and cations diffuse perpendicularly into the crystal lattice. The reaction zone grows owing to the formation of the electronically and ionically conducting reduced product. Two- and three-dimensional models have been utilized to simulate the diffusion and the current flow in response to an applied potential step. The resulting chronoamperometric curves have been analyzed with the help of fitting procedures. Under certain conditions, a transition of the three-phase reaction to a pure two-phase reaction occurs. This transition to a two-phase condition is the reason that a number of equations for the exhaustive conversion are similar to those known for planar diffusion, for example. To illustrate this, and for a better understanding of the phenomena, concentration profiles are presented for different degrees of the reaction and for varied simulation conditions. It is demonstrated how geometrical properties like crystal shape (cuboid with x ≠ y ≠ z) and crystal size as well as physical properties, e.g. the diffusion coefficients, govern the electrochemical behavior of mixed ionic/electronic conductors and form the basis of the current-time functions. The numerical simulation of a two-dimensional semi-infinite model of the reaction at the three-phase junction gives results comparable to an algebraical approach. The finite-difference method turned out to be suitable to solve the problems arising from the three-dimensional and finite diffusion conditions and from different crystal shapes. Received: 24 November 1999 / Accepted: 22 February 2000     

Lipophilic salts as membrane additives and their influence on the properties of macro- and micro-electrodes based on neutral carriers

The influence of salts containing lipophilic cations and anions on the electrical resistance of the membranes of calcium ion-selective macro- and micro-electrodes based on a neutral carrier is described. The resistance of macroelectrodes was decreased by a factor of about 50 or of about 3 compared to membranes without and with potassium tetrakis- (p-chlorophenyl)borate, respectively. No significant reduction of the membrane resistance was achieved for microelectrodes. The lower detection limit and the Ca²⁺/K⁺ selectivity factor were improved for both types of electrode.