Axial mass transport and distribution of particles in D.C. arc plasma

A model is proposed to establish the axial distribution of added substances in an arc plasma. Particles with a higher ionization degree in the plasma are retained by the cathode. This causes a decrease in the axial transport velocity of particles newly arriving in the vicinity of the cathode. Consequently, this decrease of transport velocity causes an increase in the density of particles and their radiation density. Such an assumption is confirmed by measurement of the axial transport velocities. The theoretical consideration here is based on the works of Boumans and Krinberg and Smirnova, and takes into account the stated phenomena of decrease of axial transport velocities near the cathode. Using the results of the experimentally determined axial radiation density distribution, the axial distribution of particle transport velocities was calculated. The proposed model to establish the axial distribution of added substances contributes to the explanation of cathode layer enrichment of radiation density.     

On the carrier effect mechanism in relation to mass transport processes in the arc discharge

The time of residence of impurity atoms in the arc discharge is calculated. Diffusion, ion motion in the electric field, and ambipolar diffusion are taken into account. It is shown for the first time that ambipolar diffusion contributes significantly to the total particle flow from the arc discharge zone. The effect of charge exchange on the speed of ion motion in the arc is estimated. The influence of a carrier on the residence time of atoms in the arc discharge zone is calculated. Attention is paid to the peculiarities of the mechanism of the carrier effect associated with halide compounds. An attempt is made to explain the influence of halide compounds on the residence time of atoms in the arc using the considered model of mass transport. The velocities of mass transport and the time of atoms in the discharge zone are calculated for the are with and without halide containing substances. The initial parameters of calculation (discharge temperature, electron density, degree of ionization, and coefficient of atom diffusion) are partly measured and partly taken from the literature. The results of the calculation are compared with experimental data published in the literature. The adopted mass transport model adequately accounts for the influence of a carrier on the residence time of atoms in the arc discharge zone.     

A theoretical model of the spatial density distribution of traces in a d.c. arc

A theoretical model of the spatial particle density distribution of traces in a d.c. arc plasma is established. The basic model is grounded on realistic estimations of transport velocities which indirectly include the influence of cathode layer phenomena on the particle distribution.     

Interactions between two bubbles on a hot or cold wall

A temperature gradient normal to a planar wall produces two-dimensional motion and aggregation or separation of bubbles on the hot or cold wall, respectively. The origin of the motion is fluid convection driven by the thermal Marangoni stress on the surface of the bubbles. Previous theories for the dynamics of two or more bubbles have been based on an analysis of flow about a single bubble and the resulting convection that entrains its neighbors. Here we extend the theory by solving the quasi-steady equations for the temperature and velocity fields for two bubbles. The result is a quantitative model for the relative velocity between two bubbles as a function of both the distance between them and the gap between each bubble and the surface. Interactions between the bubbles strongly increase the approach velocity, which is counter-intuitive because the hydrodynamic resistance increases as the bubbles approach each other. An asymptotic analysis indicates the thermocapillary force bringing them together or pushing them apart is singular in the separation when the bubbles are close to each other. The two-bubble theory agrees reasonably well with the experimentally measured velocities of pairs of bubbles on hot or cold surfaces, though it slightly overestimates the velocities.     

Nuclear magnetic resonance studies of convection in the 1,4-cyclohexanedione-bromate-acid reaction

The manifestation and development of convection during pattern formation in the 1,4-cyclohexanedione-acid-bromate reaction was investigated using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) experiments. An apparatus was devised that enabled convection to be probed inside an NMR spectrometer and prevented hydrodynamic motion arising from extraneous sources, such as poor mixing or temperature gradients imposed by the experimental setup. PGSE experiments were performed concurrently with magnetic resonance imaging (MRI) experiments to show that convection arose spontaneously from inhomogeneities associated with the chemical patterns. Quantitative data on diffusion coefficients and hydrodynamic velocities are reported.     

Physikalische und emissionsspektroskopische Untersuchungen am Lichtbogen unter Helium-Drücken bis zu 50 at

This study deals with the application of the d.c. arc as a spectrochemical source of light at high helium pressures, viz. power reactor conditions, for identifying spherical graphitic fuel elements which had been deliberately contaminated with defined impurities. Tipped tungsten tacks and graphite electrodes of cylindrical or spherical shape were used: the tacks served as cathodes, the graphite electrodes as anodes. The He-gas pressure during discharge was varied between 1 atm and 50 atm. The runs at high He-pressure show that the arc changes at pressures > 30 atm into a form of discharge governed by convection. At 10 atm however the arc was still found stabilised by the electrodes. At pressures > 30 atm we got distinct evidence for vaporisation. The change in characteristics of the gas type arc into a vapour arc is marked by an increase in operating voltage at currents greater than 20 amps. Not unexpectedly the operating voltage increases rapidly with increasing gas pressure during discharge. The lines of the spectrum are broadening proportionally to the gas pressure. A homogeneous magnetic field, simultaneously applied along the axis of the arc, centralises and stabilises the arc discharge and reduces the material transport from the anode. With increasing magnetical induction the line broadening reduces. The good thermal conductivity of the He and the increasing heat convection with rising pressure both result in rapid increase of power and radiation density with the consequence of higher temperature. Temperature measurements with two pairs of Mg II lines using a 20 amps arc discharge at He pressure of 40 atm yielded about 13000 K. In an example for a pair of ion lines in the Co/Ni element combination a satisfactory spectral-analytical calibration curve was obtained at helium pressure. It is possible to increase the reproducibility and narrow down the lines of the spectrum with the consequence of greater accuracy of the resulting analytical curve if a homogeneous magnetic field is applied.     

The influence of non-easily ionized elements on emission intensities in a d.c. arc plasma

Higher concentrations of non-easily ionized elements (NEIE) increase the spectral line intensity of trace elements in the arc plasma. It was found that these additives do not greatly affect the evaporation processes of the trace elements. Changes in temperature, electron density and transport velocities together can optimize the excitation for particular atomic species. The importance of the investigations is to underline the possible application of NEIE in spectrochemical practice.     

Thermal onset chemically reactive Oldroyd-B nanofluid with immersion of microorganism in three-dimensional accelerating frame

The current research effort focuses on the employment of nanoparticles for 3-D chemically reactive flow of an Oldroyd-B nanofluid caused by a bidirectional accelerating surface. The implication of thermal radiation is also taken into account. The main characteristics of nanofluids, such as Brownian motion and thermophoresis are investigated using classic Buongiorno nanofluid design. The suitable transformation has been used to decrease the relevant equation for the defined theoretical model, for which the exact method is determined using the method of homotopic. Following that, a comprehensive graphic assessment of dimensionless velocities, concentration, and temperature distribution, as well as their physical significance, is considered. Furthermore, interesting physical quantities such as local Nusselt and Sherwood numbers are calculated and determined mathematically. The study stresses that increasing relaxation time reduces variation in both components of velocities, but the effect of continuous retardation time is exactly the reverse. For a while, larger combined convection and floating proportion parameters, the velocity distribution is said to have a rising movement in the horizontal plane. Furthermore, increasing the thermophoresis parameter improves temperature and centration distributions.     

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.     

Modeling of thermal processes in high pressure liquid chromatography: I. Low pressure onset of thermal heterogeneity

Heat due to viscous friction is generated in chromatographic columns. When these columns are operated at high flow rates, under a high inlet pressure, this heat causes the formation of significant axial and radial temperature gradients. Consequently, these columns become heterogeneous and several physico-chemical parameters, including the retention factors and the parameters of the mass transfer kinetics of analytes are no longer constant along and across the columns. A robust modeling of the distributions of the physico-chemical parameters allows the analysis of the impact of the heat generated on column performance. We developed a new model of the coupled heat and mass transfers in chromatographic columns, calculated the axial and radial temperature distributions in a column, and derived the distributions of the viscosity and the density of the mobile phase, hence of the axial and radial mobile phase velocities. The coupling of the mass and the heat balances in chromatographic columns was used to model the migration of a compound band under linear conditions. This process yielded the elution band profiles of analytes, hence the column efficiency under two different sets of experimental conditions: (1) the column is operated under natural convection conditions; (2) the column is dipped in a stream of thermostated fluid. The calculated results show that the column efficiency is remarkably lower in the second than in the first case. The inconvenience of maintaining constant the temperature of the column wall (case 2) is that retention factors and mobile phase velocities vary much more significantly across the column than if the column is kept under natural convection conditions (case 1).     

High Speed Video Camera and Electrical Signal Analyses of Arcs Behavior in a 3-Phase AC Arc Plasma Torch

A 3-phase AC plasma torch has been developed and aims at overcoming some limits of the classical DC torches in terms of efficiency, cost and reliability. However, the arc behavior in 3-phase plasma torch remains poorly explored. This paper is dedicated to the high speed video camera at 100,000 frames per second and electrical signal analyses of arcs behavior in a 3-phase AC arc plasma torch. First, a reference case at 150 A, in nitrogen as working gas, has been deeply analyzed. Afterwards, a parametric study based on current and inter-electrode gap has been carried out. Results show that only one arc can exist at a given time and arcs rotate by switching from a pair of electrodes to another one, following the maximal electrical gap potential. However, a particular “abnormal” arc behavior was sometimes observed. Indeed, the arc motion within the inter-electrode gap increases the heat exchange and stabilizes the 3-phase discharge whereas the system is unbalanced when the arc is in the periphery. The analysis highlights that the arc motion is strongly influenced by the electrode jet velocity and repulsive Lorentz forces. The parametric study shows that the current increases both jet velocity and arc discharge stability. Elsewhere, the increase of the inter-electrode gap can also stabilizes the electrical 3-phase arc discharge. Furthermore, the correlation between arc motion and current waveform is highlighted. This work is likely to open the way toward a better understanding of 3-phase discharges in the perspective of their further optimization.     

A new equipment for mass transport study in a free-burning d.c. arc in air

A method for injection of liquid substances into the arc plasma is proposed. This method, which is also applicable to other plasmas at atmospheric pressure, seems to be very convenient for the study of the transport properties of the injected elements. The results of time-resolved measurements of the intensity of characteristic radiation emitted by the injected substances are presented in terms of residence times of particles in the plasma. The effect of the polarity of the arc was also studied. The cathode retains the substance in the vicinity of this electrode.     

Effect of intermittent convection movements on voltammogram and current transients

The influence of intermittent convection movements on electrochemical voltammograms is investigated. When the bath temperature rises to 315 K, the voltammograms exhibit irregular plateaus that differ for independent voltammetry scans, even when the setup is maintained under exactly the same conditions. In this paper, we show that such behavior can be caused by convection movements that develop in the electrolytic cell as a consequence of velocity fluctuations, since no bubbles or regular convective patterns are observed at this temperature. Theoretical current-potential curves for the heterogeneous deposition of metals on silicon electrodes is derived from a model consisting of a one-dimensional balance equation that includes diffusion, convection, and reaction through a time-dependent boundary condition. We obtain the current density associated with the adsorption of particles on the surface and, through this expression, we consider the effect of constant convective velocities on voltammograms. Finally, we examine the effect of random convective movements, described by a Monte Carlo algorithm that takes into account the random temporal fluctuations around a null convective current. The model predicts accentuated fluctuations on the current profiles, especially on the current plateaus that correspond to a stationary current regime. The validity of the theoretical model is checked against experimental data.     

Spectroscopic study of a magnetically rotating arc between opper electrodes in contaminated argon

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.     

Heat Generation and Particle Injection in a Thermal Plasma Torch

The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions.     

Microgravity laser-photophoresis of high density microparticles in water

Laser-photophoresis is a new technique, which can be used to characterize and separate microparticles in liquids. The photophoretic migration of high density solid particles in water has been observed experimentally for the first time by experiments under microgravity conditions. The photophoretic velocity was measured under microgravity conditions, in order to minimize the effects of density difference and convection. Furthermore, by using an optical cylindrical cell, we could observe the precise photophoretic velocities without the wall-induced drag effect. The apparatus consisted of a cw Nd:YAG laser (532 nm), a microscope, a CCD system, and a remote controlled sample stage and was set in a capsule which was used for a free-fall experiment. All the experimental operations were made externally by using a personal computer. The photophoretic velocities for the particles of carbon, stainless steel, gold plated nickel, and polystyrene in water were determined under microgravity. It was found that the photophoretic efficiencies of the photo-absorbing carbon particles and the photo-reflecting metal particles were much larger than those of transparent particles. The order of magnitude of the observed photophoretic efficiency was carbon>stainless steel>gold plated nickel>polystyrene. The photophoretic efficiencies were compared with those calculated by a Mie scattering theory. It was proved that the Mie scattering theory was useful for the prediction of the photophoretic efficiency of various kinds of particles in water.     

The Role of Metal Vapour in Gas Metal Arc Welding and Methods of Combined Experimental and Numerical Process Analysis

Gas metal arc welding (GMAW) processes are characterized by a high number of simultaneously running physical processes. The process capability is mainly determined by the properties of a metal vapour influenced arc and the material transfer. In recent years, experimental as well as numerical methods are being used increasingly in order to understand the complex interactions between the arc and material transfer. In this paper, we discuss the influence of metal vapour on GMAW processes in spray as well as pulsed material transfer mode. With respect to the high complexity of the process, experimental and numerical methods are combined in a targeted manner in order to obtain a high level of expressive capability with moderate numerical and experimental effort. The results illustrate the high influence of the changing vaporization rate not only on the arc properties but on the arc attachment at the filler wire. It could be shown, that in many cases the metal vapour concentration in the arc region has a greater influence on the arc properties and the material transfer than different shielding gas components like oxygen, hydrogen or helium.     

Hydrodynamics of nearly smooth granular gases

Hydrodynamic equations of motion for a monodisperse collection of nearly smooth homogeneous spheres have been derived from the corresponding Boltzmann equation, using a Chapman-Enskog expansion around the elastic smooth spheres limit. Because in the smooth limit the rotational degrees of freedom are uncoupled from the translational ones, it turns out that the required hydrodynamic fields include (in addition to the standard density, velocity, and translational granular temperature fields) the (infinite) set of number densities, n(s,r, t), corresponding to the continuum of values of the angular velocities. The Chapman-Enskog expansion was carried out to high (up to 10th) order in a Sonine polynomial expansion by using a novel computer-aided method. One of the consequences of these equations is that the asymptotic spin distribution in the homogeneous cooling state for nearly smooth, nearly elastic spheres, is highly non-Maxwellian. The simple sheared flow possesses a highly non-Maxwellian distribution as well. In the case of wall-bounded shear, it is shown that the angular velocity injected at the boundaries has a finite penetration length.     

The effect of iodine in spectrochemical analysis of trace elements in the arc plasma

The addition of iodine increases the spectral-line intensity of elements present in traces in the arc plasma. This increase is particularly significant for elements having high ionization potentials. Besides the effect of iodine on the evaporation from the electrode, iodine exerts a certain effect on processes taking place in the arc plasma. The mechanism of the effect should be primarily related to decreased transport velocities of particles in the presence of iodine.     

Motion of a colloidal sphere with interfacial self-electrochemical reactions induced by a magnetic field

The motion of a spherical colloidal particle with spontaneous electrochemical reactions occurring on its surface in an ionic solution subjected to an applied magnetic field is analyzed for an arbitrary zeta potential distribution. The thickness of the electric double layer adjacent to the particle surface is assumed to be much less than the particle radius. The solutions of the Laplace equations governing the magnetic scalar potential and electric potential, respectively, lead to the magnetic flux and electric current density distributions in the particle and fluid phases of arbitrary magnetic permeabilities and electric conductivities. The Stokes equations modified with the Lorentz force contribution for the fluid motion are dealt by using a generalized reciprocal theorem, and closed-form formulas for the translational and angular velocities of the colloidal sphere induced by the magnetohydrodynamic effect are obtained. The dipole and quadrupole moments of the zeta potential distribution over the particle surface cause the particle translation and rotation, respectively. The induced velocities of the particle are unexpectedly significant, and their dependence on the characteristics of the particle-fluid system is physically different from that for electromagnetophoretic particles or phoretic swimmers.