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  • 1
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 10 (1990), S. 167-188 
    ISSN: 1572-8986
    Schlagwort(e): Nonequilibrium plasma ; modeling ; RF plasma sintering
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract Plasma sintering experiments in this laboratory at reduced pressures revealed efficient heating of the ceramic sample due to recombination of dissociated and/or ionized species on the surface. For establishing a model for this plasma sintering process, it is necessary to first consider the plasma itself. Therefore, a suitable model for an RF inductively coupled plasma has been developed considering reduced pressures. As the pressure decreases, the electron density also decreases at a fixed electron temperature, causing substantial deviations from chemical equilibrium. Due to the poor collisional coupling between electrons and heavy particles at reduced pressures, large deviations from kinetic equilibrium have also to be expected. The model is based on a rotationally symmetric plasma contained in a quartz tube. The power level ranges from 1.5 to 3 kW and the operating pressure is varied from 1 to 0.01 atm. Both deviations from chemical and kinetic equilibrium are included in this model. Thermodynamic and transport properties for two-temperature plasmas are used for this modeling work. The results indicate that for pressures below 0.1 atm, there is a strong ambipolar flux of charge carriers to the confining walls, leading to significant variations of the temperature across the tube. The electron temperature increases rapidly as the pressure decreases, whereas the heavy-particle temperature decreases.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 10 (1990), S. 305-319 
    ISSN: 1572-8986
    Schlagwort(e): Thermal plasma sintering ; reduced pressures ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract Numerical simulation of the thermal behavior of sintering specimen under RF plasma conditions at reduced pressure is considered. A two-dimensional approach is adopted for describing flow, temperature, and electromagnetic fields in the reactor with appropriate boundary conditions. Slip boundary conditions are imposed for the velocity field at the sample surface and at the wall of the reactor, and corresponding jump boundary conditions are specified for the temperature field. Simple kinetic theory is employed for the calculation of the heat flux from the plasma to the specimen. The so-called capture-radiative-cascade model is adopted for ionization and recombination processes. The results indicate that ion-electron surface recombination is the dominant heat transfer mechanism to the sintering specimen under reduced pressure conditions.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 11 (1991), S. 103-128 
    ISSN: 1572-8986
    Schlagwort(e): RF plasma torch ; injection probe ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract Flow, temperature, and electromagnetic (EM) fields in a radio-frequency thermal plasma torch designed for the preparation of superconducting powders or films have been analysed by using a new two-dimensional modeling approach with the electric field intensity as the fundamental EM field variable. The insertion of a stainless steel injection tube into the torch leads to large induction currents in this tube. Although such large induction currents cause pronounced changes of the EM fields near the injection tube, flow and temperature fields are little affected. There exists only one large toroidal vortex in the upper part of the present torch, while the maximum temperature appears at an off-axis location within the coil region.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 11 (1991), S. 129-150 
    ISSN: 1572-8986
    Schlagwort(e): Thermal plasma reactor ; transferred arc ; converging wall ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract 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.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 11 (1991), S. 229-249 
    ISSN: 1572-8986
    Schlagwort(e): Thermal plasma reactor ; counterflow ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract Modeling of a counterflow plasma reactor is presented, using liquid injection for the synthesis of fine particles. An experimental reactor has been developed in this laboratory, and feasibility has been demonstrated for synthesizing advanced ceramic powders. The flow field calculations show two major recirculating regions which are of importance for increasing the particles' residence time inside of the reactor. In addition, the temperature within these recirculation zones remains relatively uniform. For simulation, water droplet trajectories have been calculated for droplets produced by an injection probe. It is shown that the droplets in a size range below 50 μm in diameter will follow the streamlines and evaporate completely within a short traveling distance. This finding suggests that this reactor configuration provides a favorable environment for the synthesis of fine particles using liquid precursors.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 5 (1985), S. 119-141 
    ISSN: 1572-8986
    Schlagwort(e): Heat and mass transfer ; thermal plasma ; vapor contamination effect ; analytical studies
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract 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.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 3 (1983), S. 97-113 
    ISSN: 1572-8986
    Schlagwort(e): Knudsen effect ; heat transfer ; small particles ; thermal plasmas ; analytical studies
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract The Knudsen effect on heat transfer to a particle exposed to a thermal plasma is important for many practical situations experienced in plasma chemistry and plasma processing. This paper provides theoretical results of this effect based on the “heat conduction potential jump” approach. It is shown that a correction factor which depends on the Knudsen number must be introduced into the expressions for heat fluxes obtained previously based on the continuum approach. The Knudsen effect is stronger for smaller particles and it is also more pronounced for an Ar-H2 plasma (compared to Ar and nitrogen plasmas at the same temperature). Since the Knudsen effect depends on the surface temperature of a particle, calculation of particle heating becomes more complicated.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 2 (1982), S. 185-212 
    ISSN: 1572-8986
    Schlagwort(e): Heat and mass transfer ; small particles ; thermal plasmas ; exact and approximate solutions ; analytical studies
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract 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.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 45-71 
    ISSN: 1572-8986
    Schlagwort(e): Thermal plasma flows ; particulates ; three-dimensional effects ; swirl component ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract Injection of particulate matter into a thermal plasma represents one of the approaches used in thermal plasma processing. The injected particles are usually treated as a dispersed phase, governed by the equation of motion and the rate equations for heat and mass transfer in Lagrangian coordinates. A stochastic approach is introduced to take particle dispersion into account due to turbulent fluctuations by randomly sampling instantaneous flow fields. Three-dimensional effects are also considered which are mainly due to particle injection and the presence of a swirl component. A modified approach for investigating noncontinuum effects on plasma-particle heat transfer is proposed, incorporating both electric and aerodynamic effects on the boundary layer around a particle immersed into a thermal plasma. Comparisons of theoretical predictions based on the present model with available experimental data are, in general, in reasonable agreement.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 10
    Digitale Medien
    Digitale Medien
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 291-328 
    ISSN: 1572-8986
    Schlagwort(e): Thermal plasma jets ; turbulence ; vortex flow ; modeling
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Technik allgemein
    Notizen: Abstract This work is concerned with analytical studies of thermal plasma jets, which are finding increasing interest for thermal plasma processing. A two-dimensional model for turbulent plasma jets with superimposed vortex flow has been developed, incorporating multiple time scales for velocity and temperature fluctuations and a density-weighted averaging for the density fluctuation effect. Results show that adding swirl to the flow field for confined and free jets induces strong axial and radial pressure gradients near the nozzle exit, causing a rapid decay of the axial velocity with increasing distance from the nozzle. Comparisons with cold flow show similar trends close to the nozzle exit, but further downstream, the axial velocities increase again, especially for larges swirl numbers. Comparisons of theoretical predictions based on the present model with available experimental data are, in general, in reasonable agreement.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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