Nonequilibrium effects in thermal plasma chemistry

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.     

The sound velocity in ideal liquid mixtures from thermal volume fluctuations.

The selection of the correct mixing rule for sound velocity in ideal liquid mixtures determines the interpretation of the sound velocity in real mixtures. This is especially important for the determination of apparent properties of solutes, such as their apparent compressibility. There are different approaches reported in the literature, and this article presents a new derivation of the mixing rule based on statistical mechanics. It is shown that the correlation of volume fluctuations between adjacent components has a crucial influence on the ideal mixing rule.     

Ultrasonic velocity and apparent isentropic compressibilities in mixtures of nonelectrolytes

Ultrasonic velocities have been determined for binary mixtures of pyridine + n-alkanol (C₁-C₁₀) over the whole composition range at 25‡C. The excess isentropic compressibilities K _S ^E and apparent molar isentropic compressibilities K_Φ,s are estimated from these measurements. The K _S ^E values are negative for all the systems over the complete mole fraction range except pyridine + decanol for which small positive values are obtained. The standard partial molar isentropic compressibilitiesK‡ of the alkanols are positive and increase linearly with the chain length of the alkanol molecules. It indicates that a methylene functional group makes a positive contribution to the expansion coefficient of a solute in these mixtures.     

Generation and behavior of fine particles in thermal plasmas—A review

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.     

Particle velocity measurements in induction plasma spraying

Laser Dopple anemometry (LDA) measurements of the particle velocity are carried out during an induction plasma spraying operation. The velocity of nickel alloy particles, or molten droplets, at the exit of an induction plasma torch prior to impact on the substrate is shown to vary with the plasma and powder injection conditions. Plasma spraying under soft vacuum (150–450 Torr) gives rise to substantially higher particle velocities (40–60 m/sec) compared to those attained at atmospheric pressure (10–20 m/sec).     

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part II. Particle heat and mass transfer in thermal plasmas

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.     

Additivity of Sound Velocity in Binary Liquid Mixtures

Ideality and additivity of sound velocity in liquid mixtures are discussed. The methods of calculation of deviations of sound velocity from theoretically predicted values are analyzed using literature data for 24 different binary liquid systems. Calculations of such deviations, assuming linearity with mole fraction of a component, were found to be wrong. It is also shown that the Nomoto relation predicting the sound velocity in liquid mixtures yields results similar to those of the equation of Ernst et al., while the Van Dael model often fails. The validity of Rao's hypothesis on additivity of molar sound velocities (Rao constant) has been confirmed.     

Isentropic compressibilities of the mixture chlorobenzene + n-hexane + (n-heptane or n-octane) at 298.15 K

In this work, the applicability of the free length and the collision factor theories (FLT and CFT, respectively) to predict multicomponent changes of isentropic compressibilities is analysed and compared. To this end, appropiate expansions for ternary mixtures were derived from the original works, and then applied to a mixture containing unlike compounds in terms of functional molecular groups. Experimental data of excess molar volumes from open literature and new experimental isentropic compressibilities of the mixture chlorobenzene?+?n-hexane?+?(n-heptane or n-octane) were used to compute the parameters. A good accuracy was obtained when ternary prediction is attempted in this partially soluble mixture at different temperatures by the collision factor theory. These results show the versatility of this model for estimation in complex multicomponent mixtures with phases splitting.     

Modelling particle formation and growth in a plasma synthesis reactor

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.     

Effects of metallic vapor on the properties of an argon arc plasma

During thermal plasma processing of materials, vapor generated from injected particulate matter will enter the plasma. Even traces of metallic vapors may have a strong effect on the properties and the behavior of the plasma and on the associated heat flux to the injected particles. In this paper a model system is considered in which an argon plasma at atmospheric pressure is contaminated by small amounts of copper vapor. By using the Chapman-Enskog approximation for a multicomponent gas mixture the transport properties are calculated for such a contaminated argon plasma. The results show that there is a drastic effect on the electrical properties. For temperatures below 10⁴ K, the electrical conductivity, for example, increases by more than an order of magnitude if metallic vapor is present. The presence of metallic contaminants is also somewhat felt by the reactional thermal conductivity. In contrast, there is no effect on the heavy-particle properties as long as the percentage of the contaminants remains small.     

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part I. The motion of a single particle without thermal effects

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.     

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     

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.     

Density and Speed of Sound for Binary Mixtures of a Cyclic Ether with a Butanol Isomer

Densities and speeds of sound of the binary mixtures 1,3-dioxolane + 1-butanol, 1,3-dioxolane + 2-butanol, 1,4-dioxane + 1-butanol, and 1,4-dioxane + 2-butanol have been measured at 25 and 40°C. The excess molar volumes and excess isentropic compressibility coefficients were calculated from experimental data and fitted to a Redlich–Kister polynomial function. Results were analyzed in terms of molecular interactions and compared with literature data.     

Computational fluid dynamics modeling of multicomponent thermal plasmas

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- 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.     

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.     

Thermal plasma synthesis of transition metal nitrides and alloys

Applications of arc plasma processing to high-temperature chemistry of Group V nitrides and Si and Ge alloys are studied. The transition metal nitrides -VN, -NbN, and -TaN are directly synthesized in a dc argon-nitrogen plasma from powders of the metals. A large excess of N₂ is required to form stoichiometric -VN, while the Nb and Ta can only be synthesized with a substoichiometric N content. In a dc argon plasma the alloys V₃Si, VSi₂, NbSi₂, NbGe₂, Cr₃Si, and Mo₃Si are obtained from powder mixtures of the corresponding elements. The compounds are identified by x-ray diffraction patterns and particle shape and size are studied by electron microscopy.     

Use of the velocity of sound in predicting the PVT relations

Thermodynamic properties of fluids are generally calculated from the PVT relations through equations of state. The majority of existing equations of state require the critical properties or intermolecular potential energy parameters as their input data. In many cases, such properties are neither available nor they can be accurately estimated.

One accessible and accurately measurable property of substances is the velocity of sound. In this report a method is introduced through which one can predict the PVT behavior of fluids using the velocity of sound data. A general mathematical relationship,

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expressing the velocity of sound, c, in terms of the hard-core velocity of sound, c_hs, and thermodynamic properties is derived. One may use this equation to extract PVT data from cVT data, or vice versa. As an example the virial coefficients, the Lennard-Jones intermolecular potential parameters, and the constants of the van der Waals equation of state for a number of pure fluids are calculated using the velocity of sound data. Utility of this method is particularly attractive for such compounds as heavy hydrocarbons, unstable fluids, and newly designed molecules for which intermolecular parameters and critical properties are not available.     

Excess volumes and isentropic compressibilities of binary mixtures of di- n -butylamine with iso-alkanols at 303.15 K

Volumes of mixing and deviations in isentropic compressibility for binary mixtures of di-n-butylamine with iso-propanol, iso-butanol and iso-pentanol were measured at 303.15 K. The excess volumes and deviations in isentropic compressibility are negative over the entire range of mole fractions in all the systems. The results point out that there is a strong H-bond interaction between unlike molecules     

声速匹配技术对甲状腺癌的诊断价值

本研究的目的是分析声速匹配技术对甲状腺癌的诊断价值。选取了159例甲状腺病变患者设为甲状腺病变组,健康志愿者40例设为甲状腺正常组,行声速匹配技术检查,分析特征并对比ZSI值,评估了声速匹配技术对甲状腺癌的诊断价值。结果显示,甲状腺癌组ZSI值较高(P<0.05)。ZSI值预测甲状腺癌的ROC曲线下面积(AUC)为0.867(0.729~1.004),灵敏度为86.67%,特异度为60.00%,似然比为2.167,ZSI值临界值为50.42 m/s。预后不良组甲状腺癌患者ZSI值高于预后良好组(P<0.05)。由本文结果可知声速匹配技术对不同类型甲状腺癌的诊断准确性均较高,可准确测出患者甲状腺声速值,稳定性良好,可用于临床甲状腺癌的诊断和预后预测。