Study of a transferred-arc plasma reactor with a converging wall and flow through a hollow cathode

In this paper the behavior of an arc in a transferred-arc plasma reactor with a converging wall geornetrr and flow through a hollow cathode is investigated numerically with emphasis on the fluid dynamics. The general conservation equations and auxiliary relations for the calculation domain are established based on reasonable assumptions Then, the coupled nonlinear differential equations are solved with suitable boundary conditions and temperature-dependent argon plasma properties at atmospheric pressure, by employing an efficient finite-difference method. The results, for a hollow cathode geornetrr with low injection flow rates, clear/y demonstrate the existence of the Maecker elect which is responsible Joy the formation of two recirculation zones. As the plasma gas flow rate is increased, the downstream recirculation zoner is swept away leaving only an upstream recirculation zone.     

Hydrodynamics and heat transfer in a plasma spouted bed reactor

The literature reveals very little intformation about plasma spouted bed hydrodynamics. Spouting of corindon particles with diameters ranging from 0.4 to 3.36 mm with argon plasma was conducted in a 90-mm-diameter column in the temperature range 300–1300°C. It was found that the maximum spoutable height (H_m) decreases with increasing particle diameter and decreasing mean bed temperature. A relation between the inlet plasma velocity and H_m is proposed. Concerning heat transport phenomena in the annulus, measurements and calculations indicate a large axial diffusivity but a poor radial mixing. Typical values of D_z and D_r are proposed on the basis of an identification procedure.Notation Ar Archimede number - Ar d ³ p (p — f) _f g ² - C_p specific heat - d_p particle diameter - d_e core diameter (or spout diameter) - D_i fluid inlet orifice diameter - D_e column diameter - D_r and D_z radial and axial diffusivity, respectively - g acceleration due to gravity - H packed static bed height - H_b bed height - H_m maximum spoutable bed height - P power     

Heat transfer to nonspherical particles in a rarefied plasma flow

The interaction of a nonspherical metallic or nonmetallic particle with a rarefied thermal plasma flow is considered. Heat transfer to a particle of arbitrary shape with an extremely thin plasma sheath due to, respectively, gas molecules, electrons, and ions is described. Analytical expressions are derived for charge and heat fluxes in the particular case of a spheroidal metallic or nonmetallic particle in a subsonic plasma flow. It has been shown that the intensity of heat exchange is greatly influenced by gas ionization, charge transfer processes, and particle shape, velocity, and orientation in the plasma flow.     

Heat transfer from a rarefied plasma flow to a metallic or nonmetallic particle

Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio T_e/T_h.     

Heat transfer to a single particle exposed to a thermal plasma

This paper is concerned with an analytical study of the heat and mass transfer process of a single particle exposed to a thermal plasma, with emphasis on the effects which evaporation imposes on heat transfer from the plasma to the particle. The results refer mainly to an atmospheric-pressure argon plasma and, for comparison purposes, an argon-hydrogen mixture and a nitrogen plasma are also considered in a temperature range from 3000 to 16,000 K. Interactions with water droplets, alumina, tungsten, and graphite particles are considered in a range of small Reynolds numbers typical for plasma processing of fine powders. Comparisons between exact solutions of the governing equations and approximate solutions indicate the parameter range for which approximate solutions are valid. The time required for complete evaporation of a given particle can be determined from calculated values of the vaporization constant. This constant is mainly determined by the boiling (or sublimation) temperature of the particles and the density of the condensed phase. Evaporation severely reduces heat transfer to a particle and, in general, this effect is more pronounced for materials with low latent heat of evaporation.     

One-dimensional analysis of the wall region for a multiple-temperature argon plasma

The present analysis is restricted to the wall region for a confined gas plasma and applied specifically to an argon plasma. The wall may be either positive or negative in potential with respect to the plasma, and the electric current may flow either parallel or normal to the wall. Estimates of the Debye shielding distance and the mean free path of various components are made to obtain the range of validity of the analysis, in addition to the situation where the wall acts like a cathode, an anode, or an electrical insulation. Analysis is for a one-dimensional case with an outer boundary, where the plasma temperature is specified. The computational domain is split into a continuum region, where both equilibrium compositions for a two-temperature plasma and a chemically reacting plasma are studied, and a free-fall region. The results allow a quantitative assessment of temperature nonequilibrium and electrical potential distribution in the free-fall region.     

Heat transfer to a particle under plasma conditions with vapor contamination from the particle

Heat transfer to a copper particle immersed into an argon plasma is considered in this paper, including the effects of contamination of the plasma (transport coefficients) by copper vapor from the particle. Except for cases of high plasma temperatures, the vapor content in the plasma is shown to have a considerable influence on heat transfer to a nonevaporating particle, and, to a lesser extent, on heat transfer to an evaporating particle. Evaporation itself reduces heat transfer to a particle substantially as shown in a previous paper [Xi Chen and E. Pfender, Plasma Chem. Plasma Process.,2, 185 (1982)]. Comparisons of the calculated results with those based on a method suggested in the above reference show that the simplified assumptions employed, i.e., that the surface temperature is equal to the boiling point and that plasma properties based on a fixed composition are applicable, can be employed to simplify calculations for many cases. This study reveals that a considerable portion of a particle must be vaporized before a steady concentration distribution is established around the particle.Nomenclature C _p specific heat at constant pressure - D diffusion coefficient of copper in the mixture - D _a diffusion coefficient of copper atoms in the mixture - D _i ambipolar diffusion coefficient of copper ions in the mixture - f mass fraction of copper in the mixture - f _a mass fraction of copper atoms in the mixture - f _i mass fraction of copper ions in the mixture - f mass fraction of copper in the plasma far away from the particle - f _s mass fraction of copper at the particle surface - G total mass flow rate due to evaporation - G _a mass flow rate of copper atoms - G _i mass flow rate of copper ions - H function defined in Eq. (19) - h specific enthalpy - h _s specify enthalpy at the particle surface - h specific enthalpy corresponding toT andf - k thermal conductivity - L latent heat of evaporation - M ₁ molecular weight of argon (M ₁=39.99) - M ₂ molecular weight of copper (M ₂=63.55) - p ₀ pressure of the gas mixture - p _s partial pressure of copper vapor at the particle surface - Q ₀ heat flux to a particle without evaporation - Q ₁ heat flux to a particle with evaporation - R gas constant - r radical coordinate - r _s particle radius - S heat conduction potential defined in Eq. (4) - S _s surface value ofS, corresponding toT _s andf _s - S free-stream value ofS, corresponding toT andf - T temperature - T _b boiling temperature of particle material - T _s particle surface temperature - T plasma temperature - density - T temperature step for numerical integration     

Heat transfer in RF plasma sintering: Modeling and experiments

The mechanisms of heat transfer from an argon RF plasma, generated in a water-cooled quartz tube, to a sintering sample immersed into the plasma and to the walls of the plasma torch have been studied both analytically and experimentally for pressures from 1 to 50 torr. The model, based on the assumption of chemical equilibrium in a two-temperature plasma with rotational symmetry, includes the influence of the magnetic field and of the Knudsen number on the thermal conductivity of the plasma. At pressures below 20 torr heat transfer to the sintering sample is enhanced compared to heat transfer to the wall of the plasma torch. This nonsymmetry is attributed to the Hall parameter and Knudsen number effect. The relative importance of the two effects is a function of the pressure. A comparison with experiments, based on calorimetric and indirect heat transfer measurements for a range of pressures and power levels, indicates satisfactory agreement with analytical predictions, with the exception of larger discrepancies at higher power levels and relatively low pressures. For pressures below 5 torr, the chemical equilibrium assumption becomes questionable, i.e., the sintering model underestimates the heat transfer to the sintering sample.     

Nusselt number correlations for heat transfer to small spheres in thermal plasma flows

Seven different equations predicting heat transfer rates to small spheres in plasma flows are examined considering argon and nitrogan as plasma gases from 300 to 21,000 K at 1 atm. For argon there is a general consensus up to 9000 K, beyond which wide deviations in behavior occur. For nitrogen, the seven correlations are in good agreement up to 4000 K, but show substantial deviations beyond this value. Comparisons with the sparsely available experimental data are made for argon from 300 to 17,000 K and for nitrogen up to 5500 K. Disagreement between the various correlations and experiment can exceed one order of magnitude.     

Effect of a substrate on the temperature distribution in an argon-hydrogen thermal plasma jet

Gas temperature profiles in the plume of an argon-hydrogen thermal plasma jet were determined /torn Rayleigh scattered laser light. Measured profiles were found to be well fitted by a Gaussian curve. Temperature data were compared with values obtained from thermocouples and showed an increasing discrepancy for temperatures higher than 800 K. The presence Q1 a cooled substrate in the flow was observed to increase the center-line temperature by about 22 at the substrate. By, combining the temperature results with calorimetric measurements of heal fox, a heat transfer coefficient to a copper substrate held at 300 K Iras determined to be in the range 400–1000 W/m². K under typical plasma spraying conditions.     

Effect of particle charging on momentum and heat transfer from rarefied plasma flow

The features of interaction of a spherical metallic particle with a rarefied thermal plasma flow due to the presence o charges-electrons and ions in the gaseous phase-are considered. Analytical expressions describing charge, momentum, and energy exchange between the plasma and the particle für the cases of strong and weak Debye screening are obtained. It is illustrated that the efficiency of particle heating in the plasma considerably grows as compared with a hot molecular gas due to participation of electrons and ions in file transfer processes.     

Application of a chemical equilibrium model to thermodynamic functions of transfer of alcohols from water to aqueous surfactants

In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models.     

Environmental application of elemental speciation analysis based on liquid or gas chromatography hyphenated to inductively coupled plasma mass spectrometry—A review

In recent years the number of environmental applications of elemental speciation analysis using inductively coupled plasma mass spectrometry (ICP-MS) as detector has increased significantly. The analytical characteristics, such as extremely low detection limits (LOD) for almost all elements, the wide linear range, the possibility for multi-elemental analysis and the possibility to apply isotope dilution mass spectrometry (IDMS) make ICP-MS an attractive tool for elemental speciation analysis. Two methodological approaches, i.e. the combination of ICP-MS with high performance liquid chromatography (HPLC) and gas chromatography (GC), dominate the field. Besides the investigation of metals and metalloids and their species (e.g. Sn, Hg, As), representing “classic” elements in environmental science, more recently other elements (e.g. P, S, Br, I) amenable to ICP-MS determination were addressed. In addition, the introduction of isotope dilution analysis and the development of isotopically labeled species-specific standards have contributed to the success of ICP-MS in the field. The aim of this review is to summarize these developments and to highlight recent trends in the environmental application of ICP-MS coupled to GC and HPLC.