Particle melting in high temperature supersonic low pressure plasma jets

A numerical model has been developed to predict the temperature history of particles injected into a low pressure dc plasma jet. The temperature and velocity fields of the plasma jet are predicted as a free jet by solving the parabolized compressible Navier-Stokes equations using a spatial marching scheme. Particle trajectories and heat transfer characteristics are calculated using the predicted plasma jet temperature and velocity fields. Correction factors have been introduced to take into account noncontinuum effects encountered in the low pressure environment. The exchange of energy and momentum between the injected particles and plasma flow was treated by considering the source terms in the governing equations. The plasma jet profiles as well as the particle/plasma interactions with different jet pressure ratios (from underexpanded to overexpanded) have been investigated. The effect of particle loading on the resulting jet profiles, particle trajectories, and temperature profiles is presented and discussed. This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

The role of transport phenomena in the plasma synthesis of fine particles: The production of silicon powder

A mathematical formulation has been developed to describe heat and fluid flow phenomena in a complex nontransferred are plasma jet system, and this information is then used to calculate the rate at which an injected silane stream decomposes. Problems of this type are important in the plasma synthesis of fine ceramic particles. The most important finding of the work is that swirl of the plasma jet plays a key role in determining the intermixing of the various streams and, hence, the overall process kinetics. The theoretical predictions were found to be in good agreement with measurements regarding the temperature fields. Furthermore, the capability of predicting temperature fields, residence times, and process kinetics should provide useful guidelines regarding reactor design such that the formation of monodispersed particles is favored. A.H. DILAWARI, formerly with the Massachusetts Institute of Technology This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

Plasma-particle interactions in plasma spraying systems

A mathematical formulation is presented to represent the interactions between the plasma jet exiting a nontransferred arc plasma torch and injected solid particles. This is a generic problem in plasma spraying operations. The calculations are based on the solution of the two-dimensional equation of motion and the thermal energy balance for the particles. Additionally, the plasma temperatures and velocities in the torch and plume are calculated using a mathematical model based on a simplified set of conservation equations. In the formulation, we allow for departure from continuum conditions, particle vaporization, and temperature gradients within the particles. The calculations are compared with previously published experimental measurements of alumina particles injected into a room-temperature, turbulent air jet and into the plume of a commercial plasma torch operating in a turbulent mode. The second set of experiments provides simultaneous measurement of particle temperature, size, and velocity and so form an excellent basis for testing our model. The comparison of the model and the measurements brings new insight into the behavior of particles in plasma jets. This article is based on a presentation made in the symposium “Spray Processing Fundamentals: Coating and Deposition” presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee.     

Pyrometric temperature and size measurements of chalcopyrite particles during flash oxidation in a laminar flow reactor

A fast two-color pyrometer has been constructed and applied to simultaneous temperature and size measurements of individual chalcopyrite particles (CuFeS₂) during flash oxidation in a laminar flow reactor. The influence of different oxygen concentrations (0, 10, 20, 50, and 75 vol pct), gas temperatures (1073 and 1273 K), particle size fractions (53 to 74 μm and 74 to 150 μm), and reaction times (up to 0.17 seconds) on chalcopyrite particle temperature and size distributions was studied. The particle reaction rates were characterized by the sulfur mass in the solid reaction product. Particle temperatures ranged from the ambient gas temperature up to 2700 K. A strong correlation between the oxygen concentration and the median particle temperature was observed.     

Modelling of the temperature and the velocity of ceramic powder particles in a plasma flame—I. Alumina

A theoretical model has been developed to evaluate the temperature and velocity of a spherical particle at any instant during its flight in a plasma flame. Computations, which have been made for an alumina particle in this part (Part I) of the study, showed that neglecting temperature gradient in liquid film or solid core will lead to error in calculation of particle temperature and velocity. But this error is comparatively less for larger Al₂O₃ particles (about 50 μm) than for smaller ones (below 25 μ). This has been explained in terms of the negligible extent of size reduction of the particles due to the large values of heat of vaporisation and thermal conductivity of alumina. This vaporisation aspect has also been verified experimentally.     

Melting metal powder particles in an inductively coupled r.f. plasma torch

A numerical model is developed for the prediction of melting metal powder particles in an inductively coupled r.f. plasma torch. The model is developed for dilute spray conditions where the gas phase flow is not affected by the loading condition. The governing equation for the gas phase flow contains the source terms from the electromagnetic field. The theoretical calculations have shown that particle thermal history and its velocity are greatly affected by the plasma operating conditions (i.e., carrier gas flow rate, injector location, and power level,etc.). Without the proper control of particle trajectories, particles may bounce around the fireball and exit the torch as unmelted or resolidified solid particles. With the insertion of an injector or injecting particles with a high carrier gas flow rate, the predictions show that even relatively small size particles can be directed into the fireball and maintained in the molten state before they impact on the substrate. Consequently, more uniform and dense deposits can be achieved.     

Melting metal powder particles in an inductively coupled r.f. plasma torch

A numerical model is developed for the prediction of melting metal powder particles in an inductively coupled r.f. plasma torch. The model is developed for dilute spray conditions where the gas phase flow is not affected by the loading condition. The governing equation for the gas phase flow contains the source terms from the electromagnetic field. The theoretical calculations have shown that particle thermal history and its velocity are greatly affected by the plasma operating conditions (i.e., carrier gas flow rate, injector location, and power level,etc.). Without the proper control of particle trajectories, particles may bounce around the fireball and exit the torch as unmelted or resolidified solid particles. With the insertion of an injector or injecting particles with a high carrier gas flow rate, the predictions show that even relatively small size particles can be directed into the fireball and maintained in the molten state before they impact on the substrate. Consequently, more uniform and dense deposits can be achieved.     

高超音速火焰喷涂粒子飞行行为研究

以高超音速火焰喷枪为研究对象,采用计算流体力学软件Fluent对高超音速火焰喷涂(HVOF)过程中的焰流流场以及粒子飞行过程进行数值模拟。HVOF系统以氧气为助燃气体,煤油为燃料。研究了加入粒子前喷枪内火焰焰流温度、速度和压力分布规律,采用离散相模型计算喷涂粒子的动力学飞行行为,探究了粒子大小、注入速度、球形度对粒子飞行行为的影响。发现最佳粒子粒径范围应为30～50 μm,在此范围内粒子均匀的分布在焰流中心,且为熔融状态,更易形成结合强度较高的涂层;小粒径粒子最佳注入速度为10～15 m·s?1,中等粒径粒子最佳注入速度为5～10 m·s?1,大粒径粒子最佳注入速度为1～5 m·s?1;与球形颗粒相比,非球形颗粒具有较高的阻力系数,在飞行过程中获得更大的动能和更少的热量。      

Effects of particle loading on a reduced pressure inductively coupled radio frequency plasma torch

Our previous attempts in the modeling of the heat transfer and fluid flow in radio frequency (RF) plasma torches considered dilute particle-loading conditions. It was assumed that the injected particles have no effect on the plasma temperature and velocity profiles. However, in practice, the plasma deposition process is carried out under fairly high loading conditions to achieve high energy efficiency. The plasma gas experiences significant local cooling and deceleration due to high particle injection rates. To this end, a numerical model has been developed which considers the coupling effects between the plasma temperature and velocity fields and injected particles. In this study, effort has been focused on the particle-loading effect in an inductively coupled RF plasma torch under a reduced pressure environment. The temperature and flow fields in an inductively coupled RF plasma torch are solved using an axisymmetric, variable property formulation of the Navier-Stokes equations. Pseudo two-dimensional electrical and magnetic field equations were considered. In addition, an integral constraint condition is used to maintain a specified discharge power in the plasma torch. The interaction between the plasma gas and injected particles is considered using the well-known “Particle-Source-In-Cell” (PSI-Cell) method. The exchanged mass, momentum, and energy between the plasma gas and injected particles are accounted for through additional source terms in the governing equations. The effect of particle loading on the resulting torch flow, thermal profiles, and particle-melting characteristics are presented and discussed.     

Kinetics of deoxidation of liquid copper by graphite particles during submerged injection

Liquid copper can be deoxidized by submerged injection of inert gas in the presence of graphite particles. This paper describes the results of experiments in which approximately 20 kg copper melts were deoxidized by injection of N₂, CO₂, and air in the presence of low sulfur graphite particles. The apparent kinetics of the deoxidation process are first order with respect to the concentration of dissolved oxygen, and the concentration of oxygen in the melt could be reduced to less than 10 ppm by weight after less than 30 minutes of injection. The kinetics are consistent with mixed rate control with both mass transfer and chemical reaction rate affecting the rate of deoxidation. In these experiments, the rate of deoxidation under a graphite covering was slower when particles were injected with the gas stream than when gas was injected alone, although this result may have been influenced by the small size of the melt. Y. W. Chang, formerly Graduate Student with the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

Modeling of oxidation of molybdenum particles during plasma spray deposition

An oxidation model for molybdenum particles during the plasma spray deposition process is presented. Based on a well-verified model for plasma chemistry and the heating and phase change of particles in a plasma plume, this model accounts for the oxidant diffusion around the surface of particles or splats, oxidation on the surface, as well as oxygen diffusion in molten molybdenum. Calculations are performed for a single molybdenum particle sprayed under Metco-9MB spraying conditions. The oxidation features of particles during the light are compared with those during the deposition. The result shows the dominance of oxidation of a molybdenum particle during the flight, as well as during deposition when the substrate temperature is high (above 400 °C).     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

Plastic deformation of metal particles in gas-dynamic spraying

The main stage of the gas-dynamic spraying process is the deformation and heating of particles upon impact with the substrate. A solution of the impact deformation problem by use of the finite element method is given. The model developed enables determination of the extent of particle deformation and the time and temperature of impact as functions of the material and particle properties and the initial velocity and temperature of particles. Ukraine State Metallurgical Academy, Dnepropetrovsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 10–15, July–August, 1998.     

Numerical studies of the motion of particles in current-carrying liquid metals flowing in a circular pipe

A mathematical model has been developed to describe the motion of particles in current-carrying liquid metals flowing through a cylindrical pipe. The fluid velocity field was obtained by solving the Navier-Stokes equations, and the trajectories of particles were calculated using equations of motion for particles. These incorporate the drag, added mass, history, electromagnetic, and fluid acceleration forces. The results show that particle trajectories are affected by the magnetic pressure number R _H, the Reynolds number Re, the blockage ratio k, and the particle-fluid density ratio γ according to the relative importance of associated force terms. In the axial direction, the particles follow the fluid velocity closely and will move further axially before reaching the wall as the fluid velocity (Re) increases. In the radial direction, the outwardly directed electromagnetic force on the particle increases with radial distance from the axis, with increasing electric current (R _H), and increasing size (k) of particle. The competition between the electromagnetic force and the radial fluid acceleration force in the entrance region results in particle movement toward the central axis before moving toward the wall for small electric current (low R _H) and directly toward the wall for large current (high R _H). The low inertia (γ) bubbles move faster toward the wall than heavier particles do. The radial velocity of the particle movement as it approaches the wall is predicted to decrease due to wall effects. This model has been applied to the movement of inclusions within the electric sensing zone (ESZ) of the liquid metal cleanliness analyzer (LiMCA) system in molten aluminum, and it was proved that LiMCA system could be used in aluminum industries.     

Transport phenomena and penetrability of solid particles in hot metal during lance injection

Abstract

The transport phenomena in injection lance and the penetrability of solid particles into liquid metal at the lance tip during injection treatment was analysed by a one-dimensional mathematical model developed in this work. Mechanic interactions and heat transfers between a solid particle, carrier gas, lance and/or hot metal have been incorporated in the model. Temperatures and velocities of carrier gas and solid particles were examined for a typical hot metal desulphurisation process by granulated magnesium injection. The temperature of gas increases by several hundred degrees, while that of solid magnesium particles only by several degrees in the lance. The gas velocity is increased by thermal expansion in lance. At the lance tip, the magnesium particle velocity is slower than the gas velocity. The penetrability of a magnesium particle into the hot metal at the lance tip was analysed.     

Plasma-arc spray-coating of powders of self-fluxing iron-base alloys 1. Estimation of the temperature and velocity of powder particles in the plasma flow

Mathematical simulation has been used to estimate the effect of technological factors on the variation of the temperature and velocity of particles of self-fluxing iron-base powder in a plasma flow of propane-butane combustion products. The influence of the plasma generator arc current, the flow rates of the plasma-forming gases and their relations, the powder particle size, the diameter of the plasma generator nozzle, the powder flow rate, and the spraying distance are analyzed. Optimal spraying conditions are determined for various powder fractions.Kiev Polytechnic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(377), pp. 35–40, January–February, 1995.     

Solidification of Pb particles embedded in Al

Hypermonotectic alloys of Al-5 wt% Pb and Al-5 wt% Pb-0.5 wt% X where X = Mn, Cu, Zn, Fe and Si have been manufactured by chill-casting and melt-spinning. The resulting microstructures have been examined by a combination of optical microscopy, scanning and transmission electron microscopy, and electron probe microanalysis. The as-solidified hypermonotectic alloys exhibit a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1–2 μm and 5–50 μm, and melt-spun Pb particle sizes of 5–10 nm and 50–100 nm. The larger Pb particles are formed during cooling through the region of liquid immiscibility while the smaller Pb particles are formed during monotectic solidification of the Al matrix. The Pb particles exhibit a cube-cube orientation relationship with the Al matrix, and a truncated octahedral shape with {111} and {100} facets. The as-solidified Pb particle distributions are resistant to coarsening during post-solidification heat treatment. The equilibrium Pb particle shape and therefore the anisotropy of solid Al-solid Pb and solid Al-liquid Pb surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 550°C. The anisotropy of solid Al-solid Pb surface energy is constant between room temperature and the Pb melting point, with the {100} surface energy 14% greater than the {111} surface energy, in good agreement with geometric near-neighbour bond energy calculations. The {100} facets disappear when the Pb particles melt, and the anisotropy of solid Al-liquid Pb surface energy decreases gradually with increasing temperature above the Pb melting point, until the Pb particles become spherical at about 550°C. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. Pb particle solidification is nucleated catalytically by the Al matrix on the {111} facet surfaces, with an undercooling of 22K and a contact angle of 21°C. Ternary additions of Mn, Cu, Zn and Fe do not influence the Pb particle solidification behaviour, but Si is a potent catalyst and stimulates the Pb particles to solidify close to the equilibrium Pb melting point.     

Physical chemistry of the carbothermic reduction of alumina in the presence of a metallic solvent: Part II. Measurements of kinetics of reaction

The carbothermic reduction of alumina was studied in the temperature range of 1700°C to 1850°C in the presence of either tin or copper as the metallic solvent. The total pressure in the smelting system was controlled at pressures between 0.08 and 0.20 atm. The overall reaction is Al₂O₃(s)+3C(s)=2Al+3CO(g). The rate of reduction of alumina was found to depend strongly on temperature, increasing by three orders of magnitude between 1700°C and 1850°C. Total pressure and activity of aluminum in the solvent bath also affected the rate of reduction. Changes in the alumina particle type and size, in the carbon type, in the carbon-to-oxygen ratio, and in the pellet size had little effect on the rate of reduction of alumina. The kinetics of reduction are shown to follow a pseudo-first order kinetic model. CHARLES W. FINN, formerly Research Scientist, Department of Materials Science and Engineering, MIT This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

Behavior of ceramic particles at the solid- liquid metal interface in metal matrix composites

Directional solidification experiments have been conducted to document SiC particle behavior at the solid-liquid interface in Al-2 pct Mg (cellular interface) and Al-6.1 pct Ni (eutectic interface) alloys. Particle size ranged from 20 to 150 μm diameter. Although predictions based on the thermodynamic approach suggest that no engulfment is possible, it was demonstrated that particles can be entrapped in the solid if adequate solidification rates and temperature gradients are used. The main factors responsible for this behavior are considered to be the difference between the thermal conductivities of particles and metal, the buildup of volume fraction of particles at the interface, and the morphological instability of the interface induced by the particles. A model including the contribution of drag and thermal conductivity has been proposed. Calculation with this model produced numbers for the critical velocity slightly higher than those evaluated experimentally. Various factors which can account for this discrepancy are discussed.