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  • 1
    Electronic Resource
    Electronic Resource
    An experimental investigation of dispersion in layered porous media (1994)
    Springer
    Transport in porous media 15 (1994), S. 15-30 
    Details
    ISSN: 1573-1634
    Keywords: Experiment ; dispersion ; layered heterogeneity ; permeability ; averaging ; permutation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Experiments were run in three linear, homogeneous, nonuniform porous media constructed in lucite columns using spherical glass beads. The columns were also joined end to end to create an in series layered heterogeneous porous media. Each column, all combinations of columns and several permutations were studied with a factorial experimental design to determine the effects of porosity, permeability, velocity, length, and column order upon dispersion. Attempts to predict the heterogeneous results from the homogeneous results were made, and a statistical regression based on the factorial design was calculated. Results showed that no simple averaging procedure accurately predicted the heterogeneous results. The statistical regression showed permeability, velocity, viscosity, length and column order to be significant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01046156
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  • 2
    Electronic Resource
    Electronic Resource
    A LGA model for dispersion in heterogeneous porous media (1994)
    Springer
    Transport in porous media 17 (1994), S. 19-32 
    Details
    ISSN: 1573-1634
    Keywords: LGA model ; heterogeneous porous media ; miscible displacement ; dispersion ; tracer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The lattice gas automaton (LGA) model proposed in the previous paper is applied to the problem of simulating dispersion and mixing in heterogeneous porous media. We demonstrate here that tracer breakthrough profiles and longitudinal dispersion coefficients can be computed for heterogeneous porous media.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00624048
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  • 3
    Electronic Resource
    Electronic Resource
    Fluid boundary layer effects in atmospheric-pressure plasma diamond film deposition (1993)
    Springer
    Plasma chemistry and plasma processing 13 (1993), S. 169-187 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; chemical vapor deposition ; diamond film ; impinging jet ; atomic carbon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract Diamond films were deposited in an atmospheric-pressure radio frequency plasma reactor. Hydrogen and methane were injected coaxially into the plasma as a high-velocity jet which impinged on the molybdenum substrate. In some cases argon was added to the reactant jet to increase its momentum, thereby reducing the boundary layer thickness. In most cases argon addition substantially, improved diamond growth. A numerical model was developed, which calculated two-dimensional reactor temperature and velocity, distributions, and the chemical kinetics in the boundary layer. The calculations indicate that under the experimental conditions argon addition reduced the thickness of the hydrogen nonequilibrium boundary layer from 3.5 to 1.0 mm. In addition, the calculations suggest that monatomic carbon may be a key diamond growth species under thermal plasma conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01466040
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  • 4
    Electronic Resource
    Electronic Resource
    Modelling fluid flow in cross-bedded sections (1993)
    Springer
    Transport in porous media 12 (1993), S. 143-159 
    Details
    ISSN: 1573-1634
    Keywords: Heterogeneity ; layers ; displacement ; numerical simulation ; flow in porous media ; dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Beadpack experiments and numerical simulations have been carried out to study flow displacements, effluent profiles and streamline patterns for layered systems with flow not parallel to the layers. The effects of layer thickness, permeability contrast, angle of layer to flow direction, mobility ratio and flood rate have been examined. Each of these parameters influence the displacement profiles, and disperse the flood front. Such real effects must be considered when subsuming reservoir heterogeneities in average reservoir parameters in simulation studies, or interpreting core tests.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00616977
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  • 5
    Electronic Resource
    Electronic Resource
    Fractal and superdiffusive transport and hydrodynamic dispersion in heterogeneous porous media (1993)
    Springer
    Transport in porous media 13 (1993), S. 3-40 
    Details
    ISSN: 1573-1634
    Keywords: Diffusion ; dispersion ; percolation ; fractals ; scaling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract We review and discuss diffusion and hydrodynamic dispersion in a heterogeneous porous medium. Two types of heterogeneities are considered. One is percolation disorder in which a fraction of the pores do not allow transport to take place at all. In the other type, the permeabilities of various regions of the pore space are fractally distributed with long-range correlations. Both systems give rise to unusual transport in which the mean square displacement 〈r 2(t)〉 of a particle grows nonlinearly with time. Depending on the heterogeneities and the mechanism of diffusion and disperison, we may havefractal transport in which 〈r 2〉 growsslower than linearly with time, orsuperdiffusive transport in which 〈r 2〉 growsfaster than linearly with time. We show that percolation models can give rise to both types of transport with scale-dependent transport coefficients such as diffusivity and dispersion coefficients, which are consistent with many experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00613269
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  • 6
    Electronic Resource
    Electronic Resource
    A theory of macrodispersion for the scale-up problem (1993)
    Springer
    Transport in porous media 13 (1993), S. 97-122 
    Details
    ISSN: 1573-1634
    Keywords: Scale up ; dispersion ; porous media ; random field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Dispersion is the result, observable on large length scales, of events which are random on small length scales. When the length scale on which the randomness operates is not small, relative to the observations, then classical dispersion theory fails. The scale up problem refers to situations in which randomness occurs on all length scales, and for which classical dispersion theory necessarily fails. The purpose of this article is to present non-Fickian, theories of dispersion, which do not assume a scale separation between the randomness and the observed consequences, and which do not assume a single length scale. Porous media flow properties are heterogeneous on all length scales. The geological variation on length scales below the observational length scale can be regarded as unknown and unknowable, and thus as a random variable. We develop a systematic theory relating scaling behavior of the geological heterogeneity to the scaling behavior of the fluid dispersivity. Three qualitatively distinct regimes (Fickian, non-Fickian and nonrenormalizable) are found. The theory gives consistent answers within several distinct analytic approximations, and with numerical simulation of the equations of porous media flow. Comparison to field data is made. The use of Kriging to generate constrained ensembles for conditional simulation is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00613272
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  • 7
    Electronic Resource
    Electronic Resource
    Spectroscopic study of a magnetically rotating arc between opper electrodes in contaminated argon (1992)
    Springer
    Plasma chemistry and plasma processing 12 (1992), S. 1-16 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; electrode erosion ; plasma spectroscopy ; metal vapor plasma ; electrode contamination
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract The effects of N2 and CO contaminants in atmospheric-pressure argon on an arc rotating between two concentric copper electrodes has been studied using optical spectroscopy of copper lines. The axial temperature of the magnetically driven arc in Ar + %N2 was determined to be around 10,000 K for arc currents of SO to 200 A. The diffusion process of the copper vapor from the cathode was also studied. A copper density maximum 1 mm from the cathode along the arc column was found in Ar + %N2. Removal of the contaminated cathode surface layers by the arc when contaminant injection in the plasma gas was stopped was found to be a slow process with a time scale depending on the type of the gas contaminant. The presence of gas contaminant in the electrode material controls the cathode erosion mechanism and the overall arc behavior in the transition between a contaminated to a pure argon arc.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01447941
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  • 8
    Electronic Resource
    Electronic Resource
    Computational fluid dynamics modeling of multicomponent thermal plasmas (1992)
    Springer
    Plasma chemistry and plasma processing 12 (1992), S. 299-325 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; plasma processing ; rnulticomponent ; chemistry ; diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract A comprehensive computational model has been developed Jbr flowing thermal plasmas in the absence of electromagnetic fields, with particular emphasis on plasma jets. The plasma is represented as a rnulticomponent chemicalh, reacting ideal gas with temperature-dependent thermodynamic and transport properties. The plasma flow is governed by the transient compressible Navier-Stokes equations in two or three space dimensions. Turbulence is represented by subgrid-scale and k-ε models. Species diffusion is calculated by an effective binary diffusion approximation, generalized to allow /or ambipolar diffusion of charged species. Ionization, dissociation, recombination, and other chemical reactions are computed by general kinetic and equilibrium chemistry algorithms. Radiation heat loss is currently modeled as a temperature-dependent energy sink. Finite-difference approximations to the governing equations are solved on a rectangular spatial mesh using explicit temporal differencing. Computational inefficiency at low Mach number is avoided br reducing the effective sound speed. The overall computational model is embodied in a new computer code called LAVA. Computational results and comparisons with experimental data are presented Jbr LAVA simulations of a steady-stare axisymmetric argon plasma jet flowing into cold argon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01447028
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  • 9
    Electronic Resource
    Electronic Resource
    A computerized technique for measuringin-situ concentrations during miscible displacements in porous media (1992)
    Springer
    Transport in porous media 7 (1992), S. 127-145 
    Details
    ISSN: 1573-1634
    Keywords: Miscible displacement ; dispersion ; in-situ concentration measurement ; computerized data acquisition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A technique for measurement of thein-situ concentration in an unconsolidated porous medium has been developed. The method involves measurement of electrical conductivityin-situ, under dynamic conditions, for flow involving brine of differing concentrations, at selected locations along the porous medium and relating it to the brine strength. Data acquisition and analysis is carried out using a Hewlett — Packard micro-computer and its interface. A user-friendly software was designed and developed for the system. The measurement technique was evaluated by studying the effect of brine concentration, brine flow rate, and by conducting miscible displacements experiment. The experimentally measured dispersion coefficients for the porous medium agreed closely with the value predicted by the correlation available in the literature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00647393
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  • 10
    Electronic Resource
    Electronic Resource
    Waves in partially saturated porous media (1992)
    Springer
    Transport in porous media 9 (1992), S. 25-37 
    Details
    ISSN: 1573-1634
    Keywords: Waves ; dispersion ; shock tube ; gas bubbles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The propagation of compressional waves in a porous medium is investigated in case the pore liquid contains a small volume fraction of gas. The effect of oscillating gas bubbles is taken into account by introducing a frequency-dependent fluid bulk modulus, which is incorporated in the Biot theory. Using a shock tube technique, new experimental data are obtained for a porous column subjected to a pressure step wave. An oscillatory behaviour is observed, consisting of two distinct frequency bands, which is predicted by the theoretical analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01039623
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  • 11
    Electronic Resource
    Electronic Resource
    Heat transfer from a transferred-arc plasma to a cylindrical enclosure (1991)
    Springer
    Plasma chemistry and plasma processing 11 (1991), S. 57-79 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; transferred arc ; plasma reactor ; radiated power ; heat transfer to enclosure ; voltage gradient ; electric field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract Hear-transfer rates from an axially enclosed transferred arc to a surrounding water-cooled cylindrical sleeve, 15 cm high, were measured. The arc (argon or nitrogen) was struck between a movable cathode within the sleeve and a bath of molten copper below the sleeve, serving as anode. The distance from the bottom of the sleeve to the surface of the molten copper (L o) was constant. Variables studied were the diameterD of the sleeve (5, 7.5. and 10 cm), the length of the arc within the sleeveL (5, 10, and 15 cm), the currentI (200, 250, and 300 A) and a tangential flow of gas or vortex within the sleeve (0, ?0, and 50 liters/min). The total power transferred to the sleeve,P s was measured caloronetrically and was the sure ofP r the effective power radiated by the arc of lengthL within the sleeve.P a, the power radiated into the sleeve from the arc of length Lo below the sleeve, andP o , the power radiated from the melt surface (a constant of small value), minusP a , the power lost by convection from the sleeve (negligible, except for a strong vortex). BothP r andP o were found to be equal to the product of the Joule heat released within their respective arc lengths, IVgL and IVg0L0 (where Vg and Vg0 are the voltage gradients), and dimenonless efliciency factors, ηr and η0. which are functions ofL/D andL 0 /D, respectively, for each gas, regardless of the geometry of the sleeve, the current, and the strength of the vortex.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01447034
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  • 12
    Electronic Resource
    Electronic Resource
    Scaling mixing during miscible displacement in heterogeneous porous media (1991)
    Springer
    Transport in porous media 6 (1991), S. 607-626 
    Details
    ISSN: 1573-1634
    Keywords: dispersion ; dispersivity ; heterogenity ; miscible ; porous media ; scaling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract This paper discusses scaling of mixing during miscible flow in heterogeneous porous media. In large field systems dispersivity appears to depend on system length due to heterogeneities. Three types of scaling are discussed to investigate the heterogeneous effects. Dimensional analysis of mixing during flow through geometerically scaled heterogeneous models is illustrated using measured dispersion. Fractal analysis of mixing in statistically scaled heterogeneous porous media is discussed. Analog scaling of pressure transients in heterogeneous porous media is suggested as an in-situ method of estimating dispersion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00137852
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  • 13
    Electronic Resource
    Electronic Resource
    Hydrodynamics and heat transfer in a plasma spouted bed reactor (1990)
    Springer
    Plasma chemistry and plasma processing 10 (1990), S. 71-85 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; spouted bed ; hydrodynamics ; heat transfer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract 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 (Hm) decreases with increasing particle diameter and decreasing mean bed temperature. A relation between the inlet plasma velocity and Hm 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 Dz and Dr are proposed on the basis of an identification procedure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01460448
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  • 14
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    One-dimensional analysis of the wall region for a multiple-temperature argon plasma (1990)
    Springer
    Plasma chemistry and plasma processing 10 (1990), S. 189-206 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; modeling ; heat and mass transfer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01460455
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  • 15
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    Thermal plasma composition in the presence of easily and noneasily ionized components (1989)
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 445-463 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; equilibrium composition ; metal halide discharge ; easily and noneasily ionized components
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract The influence of easily and noneasily ionized components on equilibrium particle density ratios in thermal plasmas has been investigated. Particular emphasis is given to the modeling of metal halide discharges with iodine as a halide. The calculations were done for “standard” air with lithium or iodine, “standard” argon with aluminum, and with a mixture of aluminum and iodine. The system setup is in agreement with spectroscopic results for an open-chamber gas-stabilized d.c. arc, with a pressure of 1×105 Pa and temperatures corresponding to the radial distribution of temperature for this type of discharge. It is shown that the behavior of the plasma core is dominated by the easily ionized component (Li or Al) while the periphery is characterized by the noneasily ionized component (iodine).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01023913
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  • 16
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    Nonequilibrium effects in thermal plasma chemistry (1989)
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 435-443 
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    ISSN: 1572-8986
    Keywords: Thermal plasmas ; nonequilibrium ; finite rates ; dissociation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract Numerical calculations have been performed to assess the potential significance of nonequilibrium effects on chemical reactivity in thermal plasmas The calculations consider situations in which the electron temperature and/or the electron density are elevated above their equilibrium values corresponding to the local gas temperature. Such nonequilibrium may occur in the plasma torch itself or could be purposefully imposed by a controlled hybrid discharge in a downstream reactor region so as to augment reactivity over a longer residence time. The calculations account for finite ionization/recombination rates of atomic and molecular species, electron-impact dissociation, dissociative recombination, dissociative attachment, and predissociation effects, as well as thermal reactions between neutral chemical species. As an example of the possible nonequilibrium enhancement of molecular decomposition, initial consideration has focused on the dissociation rates of diatomic species where heavy particle reaction rates and cross sections can be reasonably estimated. The results show that for O2 or H2 in argon at moderate temperatures, electron-temperature elevation can give rise to a notable enhancement of the dissociation rate, in comparison with the equilibrium case. Depending on the situation, it is found that either relatively energetic electron-impact dissociation or dissociative attachment (for O2) can dominate the enhanced dissociation rate—which can be more than a factor of 2 greater than in the absence of a discharge. Similar effects would be expected for the decomposition of more complicated molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01023912
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  • 17
    Electronic Resource
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    Some characteristics of transferred-Arc plasmas (1989)
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 65S 
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    ISSN: 1572-8986
    Keywords: Thermal plasmas ; transferred arcs ; plasma radiation ; plasma heat transfer ; plasma columns ; plasma stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract Although the theoretical foundations of the characteristics of transferred arcs were established more than three decades ago, the design of reator systems based on this method of plasma are generation is still in an early stage of development. Although deceptively simple in concept, attempts to use transferred arcs im practical applications at scales of arc length and power somewhat larger than those previously studied in the laboratory soon revealed that considerably more information was required on the arc's unique properties before a sound reactor design could be carried out. This was principally due to the much higher temperature levels (up to 20,000 K) with consequent much higher radiative power achieved in this type of arc, as well as the fluid mechanical effects of the high-velocity plasma gas flowing in the latter. These, in turn, had profound effects on the various modes of heat transfer occurring in the system, on the stability of the arc, on the injection of the material to be treated, and on the overall efficiency of energy utilization in the whole system. The objective of this paper is to summarize the experimental evidence concerning transforred arc characteristics which have been obtained by the author and his colleageus during recent years.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01015874
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  • 18
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    Particle behavior in thermal plasmas (1989)
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 167S 
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    ISSN: 1572-8986
    Keywords: Thermal plasmas ; heat, mass, and momentum transfer ; material processing ; overview
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract In this overview, effects exerted on the motion and on heat and mass transfer of particulates injected into a thermal plasma are discussed, including an assessment of their relative importance in the context of thermal plasma processing of materials. Results of computer experiments are shown for particle sizes ranging from 5–50 μm, and for alumina and tungsten as sample materials. The results indicate that (i) the correction terms required for the viscous drag and the convective heat transfer due to strongly varying properties are the most important factors; (ii) noncontinuum effects are important for particle sizes 〈10 μm at atmospheric pressure, and these effects will be enhanced for smaller particles and/or reduced pressures; (iii) the Basset history term is negligible, unless relatively large and light particles are considered over long processing distances; (iv) thermophoresis is not crucial for the injection of particles into thermal plasmas; (v) turbulent dispersion becomes important for particle 〈10 μm in diameter; and (vi) vaporization describes a different particle heating history than that of the evaporation process which, however, is not a critical control mechanism for interphase mass transfer of particles injected into thermal plasmas.
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    URL: http://dx.doi.org/10.1007/BF01015878
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  • 19
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    Silicon nitride synthesis in an atmospheric pressure convection—Stabilized arc (1989)
    Springer
    Plasma chemistry and plasma processing 9 (1989), S. 277-289 
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    ISSN: 1572-8986
    Keywords: Thermal plasmas ; synthesis of Si3N4 ; experiments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract Submicron α-, β-, and amorphous-phase silicon nitride particles have been synthesized in an experimental plasma reactor, using metallic silicon and ammonia as reactants. Injection of ammonia at different locations of the reactor results in different yields. A maximum yield of 85 wt% has been achieved by injecting NH3 at both downstream and upstream locations of the reactor. The powders synthesized in this way contained approximately 60 wt% silicon nitride in crystal form with equal amounts of α and β phases. The remainder consisted of the amorphous phase. The average size of the particles ranged from 50 to 90 nm, with a standard deviation of 1.47–1.87 depending on the location of ammonia injection. Seeding with 1 and 10 wt% of preexisting silicon nitride particles for fostering heterogeneous nucleation did not improve the yield, but it changed the particle size distribution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01054286
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  • 20
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    The effect of dispersion model on the linear stability of miscible displacement in porous media (1989)
    Springer
    Transport in porous media 4 (1989), S. 85-96 
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    ISSN: 1573-1634
    Keywords: Stability ; linear stability analysis ; miscible displacement ; aspect ratio ; mobility control ; dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Chang and Slattery (1986, 1988b) introduced a simplified model of dispersion that contains only two empirical parameters. The traditional model of dispersion (Nikolaevskii, 1959; Bear, 1961; Scheidegger, 1961; de Josselin de Jong and Bossen, 1961; Peaceman, 1966; Bear, 1972) has three empirical parameters, two of which can be measured in one-dimensional experiments while the third, the transverse dispersivity, must be measured in experiments in which a two-dimensional concentration profile develops. It is found that nearly the same linear stability behavior results from using either model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00134743
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  • 21
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    Modelling particle formation and growth in a plasma synthesis reactor (1988)
    Springer
    Plasma chemistry and plasma processing 8 (1988), S. 145-157 
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    ISSN: 1572-8986
    Keywords: Thermal plasmas ; particle nucleation and growth ; modelling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract A model for particle nucleation and growth in a thermal plasma reactor is discussed. A nondimensional form of the aerosol general dynamic equation is derived under a set of simplifying assumptions which are appropriate to plasma powder synthesis, and the resulting set of equations is solved numerically. The results are converted to dimensional form for the case of iron powder, for which experimental data are available, and for silicon carbide. Calculated particle sizes increase significantly with increasing reactant concentrations and with decreasing cooling rate, although the influence of cooling rate is mainly a residence time effect.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01016154
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  • 22
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    Two-dimensional adaptive Eulerian-Lagrangian method for mass transport with spatial velocity distribution (1988)
    Springer
    Transport in porous media 3 (1988), S. 473-489 
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    ISSN: 1573-1634
    Keywords: Advection ; dispersion ; particles ; characteristics ; finite-element ; continuous fluid velocity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A Eulerian-Lagrangian scheme is used to solve the two-dimensional advection-dispersion equation. Concentration and its partial differential operator are decomposed into advection and dispersion terms. Thus, advection is formally decoupled from dispersion and solved by continuous forward particle tracking. Dispersion is handled by implicit finite elements on a fixed Eulerian grid. Translation of steep gradients of concentration in advection-dominated flow regimes, is done without numerical distortion. Continuous spatial distribution of velocities are evaluated by using Galerkin's approach in conjunction with Darcy's law based on hydraulic input data from each element. The method was implemented on coarse FE grid with linear shape functions, demonstrating no over/under shooting and practically no numerical dispersion. Simulations, covering a wide range of Peclet numbers, yield high agreement with analytic and practical results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00138612
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    Saltwater transport in a heterogeneous formation: A case study (1988)
    Springer
    Transport in porous media 3 (1988), S. 217-256 
    Details
    ISSN: 1573-1634
    Keywords: Salinity ; advection ; dispersion ; aquifers ; flow model ; transport model ; simulation ; sea water ; connate water ; leakage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Historical information of the hydraulic and salinity aspect, detailed geological information, and information on the physical characteristics of the different layers comprising the formation, are needed for simulating the saltwater transport process in aquifers. In most simulation studies of field situations, there is an inadequacy of data and the modeller has to make justifiable assumptions to analyze a particular situation in order to provide an insight into the problem. A quasi-three-dimensional solute transport model is used to analyze the saltwater encroachment phenomena in aquifers underlying the City of Bangkok; first by calibrating the model's performance with available historical data and then by assessing the extent of future saltwater encroachment with the implementation of the regulatory pumpage to be followed in order to restrict the alarming rate of land subsidence. Model simulation indicates a substantial reduction in the rate of encroachment of the saltwater front with a reduction of pumpage after 1987.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00235330
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    An adaptive domain decomposition method for simulation of transport in porous media (1988)
    Springer
    Transport in porous media 3 (1988), S. 549-562 
    Details
    ISSN: 1573-1634
    Keywords: Adaptive mesh ; finite element method ; dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A Galerkin finite element method is used along with a self-adaptive strategy of domain discretisation to model dispersion in an axisymmetric cylindrical porous medium. A solution strategy is proposed based on the use of a Gear scheme for the time stepping and partial vectorisation of the code. The domain is highly discretised in the area of the sharp transient front, while the remainder is coarsely discretised. The area covered by the fine mesh is determined by the value of the local concentration gradients. Numerical results are presented for the one and two dimensional cases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00959102
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    Stability of vertical miscible displacements with developing density and viscosity gradients (1988)
    Springer
    Transport in porous media 3 (1988), S. 277-297 
    Details
    ISSN: 1573-1634
    Keywords: Stability ; linear stability analysis ; miscible displacement ; aspect ratio ; mobility control ; dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Viscous fingering and gravity tonguing are the consequences of an unstable miscible displacement. Chang and Slattery (1986) performed a linear stability analysis for a miscible displacement considering only the effect of viscosity. Here the effect of gravity is included as well for either a step change or a graduated change in concentration at the injection face during a downward, vertical displacement. If both the mobility ratio and the density ratio are favorable (the viscosity of the displacing fluid is greater than the viscosity of the displaced fluid and, for a downward vertical displacement, the density of the displacing fluid is less than the density of the displaced fluid), the displacement will be stable. If either the mobility ratio or the density ratio is unfavorable, instabilities can form at the injection boundary as the result of infinitesimal perturbations. But if the concentration is changed sufficiently slowly with time at the entrance to the system, the displacement can be stabilized, even if both the mobility ratio and the density ratio are unfavorable. A displacement is more likely to be stable as the aspect ratio (ratio of thickness to width, which is assumed to be less than one) is increased. Commonly the laboratory tests supporting a field trial use nearly the same fluids, porous media, and displacement rates as the field trial they are intended to support. For the laboratory test, the aspect ratio may be the order of one; for the field trial, it may be two orders of magnitude smaller. This means that a laboratory test could indicate that a displacement was stable, while an unstable displacement may be observed in the field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00235332
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    An analysis of the viability of polymer flooding as an enhanced oil recovery technology (1988)
    Springer
    Transport in porous media 3 (1988), S. 591-618 
    Details
    ISSN: 1573-1634
    Keywords: Polymer flooding ; viscous fingering ; multiphase flow ; dispersion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The concept of improving oil recovery through polymer flooding is analysed. It is shown that while the injection of a polymer solution improves reservoir conformance, this beneficial effect ceases as soon as one attempts to push the polymer solution with water. Once water injection begins, the water quickly passes through the polymer creating a path along which all future injected water flows. Thus, the volume of the polymer slug is important to the process and an efficient recovery would require that the vast majority of the reservoir be flooded by polymer. It is also shown that the concept of grading a polymer slug to match the mobilities of the fluids at the leading and trailing edges of a polymer slug does not work in a petroleum reservoir. While this process can supply some additional stability to the slug, it is shown that for the purposes of enhanced oil recovery this additional stability is not great enough to be of any practical use. It is found that in this case the instability has simply been hidden in the interior of the slug and causes the same sort of instability to occur as was the case for the uniform slug.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00959104
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    A linear stability analysis for miscible displacements (1986)
    Springer
    Transport in porous media 1 (1986), S. 179-199 
    Details
    ISSN: 1573-1634
    Keywords: Stability ; linear stability analysis ; miscible displacement ; dispersion ; mobility control
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A linear stability analysis has been performed for a miscible displacement in a semi-infinite system of finite thickness and unbounded width. A more general description of dispersion has been adopted than those used by previous workers. It is shown that, when there is a step change in concentration and the mobility ratio is unfavorable, the displacement can be unstable at the injection boundary. But, if the concentration is changed sufficiently slowly with time at the entrance to the system, the displacement is stable to infinitesimal perturbations, no matter how unfavorable the mobility ratio. When the mobility ratio is favorable, the displacement is unconditionally stable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00714691
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    Issues in flow and transport in heterogeneous porous media (1986)
    Springer
    Transport in porous media 1 (1986), S. 319-338 
    Details
    ISSN: 1573-1634
    Keywords: heterogeneity ; dispersion ; saturated flow ; unsaturated flow
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract This invited lecture enumerates various categories of flow and transport in heterogeneous media with particular reference to this Symposium. Specific attention is given to five topics within these categories. (1) We explore the compounding of spatially variable local permeability K to produce an apparent permeability K * on a scale large compared with that of variation of K. An inverse method generates and analyzes flow systems with K spatially periodic in two and three dimensions. (2) Physical arguments indicate that apparent hydraulic properties of an unsaturated composite medium may not represent any conceivable mean of the properties of the component media. (3) Conventionally, buried holes are thought to stay empty during generally unsaturated soil-water flow. The hole, however, acts as an obstacle to flow so that water may seep through it. The larger the hole the more likely this will happen. (4) Dispersion in heterogeneous porous systems with no maximum scale of variation is explored using a Lagrangian mode of analysis. (5) Comments are offered on ‘geostatistics’ and its application to heterogeneous soils and aquifers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208041
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    Particle dynamics and particle heat and mass transfer in thermal plasmas. Part I. The motion of a single particle without thermal effects (1985)
    Springer
    Plasma chemistry and plasma processing 5 (1985), S. 211-237 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; particle motion ; computer experiments ; review
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract A particle injected into a thermal plasma will experience a number of effects which are not present in an ordinary gas. In this paper effects exerted on the motion of a particle will be reviewed and analyzed in the context of thermal plasma processing of materials. The primary purpose of this paper is an assessment of the relative importance of various effects on particle motion. Computer experiments are described, simulating motion of a spherical particle in a laminar, confined plasma jet or in a turbulent, free plasma jet. Particle sizes range from 5 to 50 µm, and as sample materials alumina and tungsten are considered. The results indicate that (i) the correction term required for the viscous drag coefficient due to strongly varying properties is the most important factor; (ii) non-continuum effects are important for particle sizes 〈10 µm at atmospheric pressure and these effects will be enhanced for smaller particles and/or reduced pressures; (iii) the Basset history term is negligible, unless relatively large and light particles are considered over long processing distances; (iv) thermophoresis is not crucial for the injection of particles into thermal plasmas; (v) turbulent dispersion becomes important for particle 〈10 µm in diameter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00615122
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    Particle dynamics and particle heat and mass transfer in thermal plasmas. Part II. Particle heat and mass transfer in thermal plasmas (1985)
    Springer
    Plasma chemistry and plasma processing 5 (1985), S. 391-414 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; heat and mass transfer ; review
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract This paper is concerned with a review of heat and mass transfer between thermal plasmas and particulate matter. In this situation various effects which are not present in ordinary heat and mass transfer have to be considered, including unsteady conditions, modified convective heat transfer due to strongly varying plasma properties, radiation, internal conduction, particle shape, vaporization and evaporation, noncontinuum conditions, and particle charging. The results indicate that (i) convective heat transfer coefficients have to be modified due to strongly varying plasma properties; (ii) vaporization, defined as a mass transfer process corresponding to particle surface temperatures below the boiling point, describes a different particle heating history than that of the evaporation process which, however, is not a critical control mechanism for interphase mass transfer of particles injected into thermal plasmas; (iii) particle heat transfer under noncontinuum conditions is governed by individual contributions from the species in the plasma (electrons, ions, neutral species) and by particle charging effects.
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    URL: http://dx.doi.org/10.1007/BF00566011
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    Generation and behavior of fine particles in thermal plasmas—A review (1985)
    Springer
    Plasma chemistry and plasma processing 5 (1985), S. 1-37 
    Details
    ISSN: 1572-8986
    Keywords: Thermal plasmas ; fine particles ; review
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract The generation and processing of fine powders in thermal plasmas has attracted increasing interest over the past years, precipitated by a growing awareness that conservation of materials is no longer an option but rather a necessity. Plasmaspheroidization, densification, fuming, metallurgical reduction, and the production of refractory oxides, carbides, nitrides, and borides in thermal plasmas are fast developing technologies which, in some cases, have already reached industrial production scale. In this survey, pertinent literature (198 references) will be reviewed with emphasis on basic studies in this field, reported over the past 20 years. The first part of this review covers powder handling, quenching characteristics, nucleation and growth, and modeling of plasma-particle interactions. The second part is concerned with plasma furnaces for the production of fine particles, including RF induction plasmas, DC anode furnaces, DC plasma jets, DC transferred arcs, cathode pump fed arcs, hybrid induction-DC plasmas, and three-phase AC furnaces.
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    URL: http://dx.doi.org/10.1007/BF00567907
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