Probe measurements in argon plasma jets operated in ambient argon

Measurements of local enthalpies and velocities have been performed in plasma jets generated by a DC plasma spray torch, using an enthalpy probe. The torch has been operated in an argon confined atmosphere at different currents and argon flow rates.⁽¹⁾ The validity of the measured enthalpy and velocity profiles has been checked by performing energy flux and mass flux balances, which show reasonable agreement between the input quantities, measured independently, and those obtained by integrating over the experimental profiles. The data are compared with those obtained by operating the same torch in ambient air. The results show that temperatures and velocities measured in pure argon are substantially higher than those in air, and consequently, the jets in argon appear wider and substantially longer.     

Entrainment of cold gas into thermal plasma jets

There is increasing evidence that the entrainment of cold gas surrounding a turbulent plasma jet is more of an engulfment type process rather than simple diffusion. A variety of diagnostic techniques have been employed to determine the development of turbulence in a plasma jet and to measure concentration and temperatures of the cold gas entrained into atmospheric-pressure argon plasma jets in ambient argon or air. The results indicate that the transition to turbulence causes a rapid drop of the axial jet velocity due to entrainment of the cold gas surrounding the plasma jet. Dissipation of the cold engulfed gas bubbles by molecular diffusion is relatively slow if molecular gases (for example air) are entrained, as indicated by conditional sampling and CARS measurements. Temperature measurements using emission spectroscopy and enthalpy probes show strong discrepancies in the jet fringes.     

Modeling of plasma jets with superimposed vortex flow

This work is concerned with analytical studies of thermal plasma jets, which are finding increasing interest for thermal plasma processing. A two-dimensional model for turbulent plasma jets with superimposed vortex flow has been developed, incorporating multiple time scales for velocity and temperature fluctuations and a density-weighted averaging for the density fluctuation effect. Results show that adding swirl to the flow field for confined and free jets induces strong axial and radial pressure gradients near the nozzle exit, causing a rapid decay of the axial velocity with increasing distance from the nozzle. Comparisons with cold flow show similar trends close to the nozzle exit, but further downstream, the axial velocities increase again, especially for larges swirl numbers. Comparisons of theoretical predictions based on the present model with available experimental data are, in general, in reasonable agreement.     

Unsteadiness and mixing in thermal plasma jets

This study was undertaken to examine the mechanisms which produce the large entrainment measured at the exit of thermal plasma torches. The experiments studied a Metco 7MB plasma torch with a 706 (6.35 mm diameter) anode nozzle and swirled argon gas injection. The vortex structure produced in the shear layer of the plasma jet was visualized using a laser shadowgraph system with a short exposure lime (10^–4 s). A high-speed video system provided information on the structure and unsteadiness of the hot potential core of the plume. Tile shear layer visualizations were compared to previous measurements of acoustical power spectra and indicate coherent vortex structure formation at low gas flowrates. At higher gas flowrates the shear layer rapidly broke down, producing relatively small scale turbulence. The visualizations of the hot potential core were compared to previous measurements of the torch voltage fluctuations caused by arc instabilities. At low flowrates the arc-produced voltage fluctuations were guile low card the phone was very steady. At higher flowrates the voltage fluctuations increased and produced surging and whipping in the hot potential core.     

Experimental investigation of powder vaporization in thermal plasma jets

An experimental study of the vaporization of metallic and ceramic particles in a thermal do plasma jet has been initiated and two series of experiments have been performed: (1) measurement of the vapor concentration within the plasma jet by absorption spectroscopy. (2) Investigation of the vapor cloud surrounding a single particle in flight by emission spectroscopy. The temperature within this cloud is determined by the intensity ratio of two lines which are simultaneously measured. The cloud radius is deduced /torn measurement of the particle velocity by laser doppler anemometry, and the vapor concentration is calculated from the line intensity profile, once the temperature is known. Results on iron and alumina particles injected in argon or argon-hydrogen plasma jets are given and discussed.     

The temperature profiles in an argon plasma issuing into an argon atmosphere: A comparison of measurements and predictions

Experimental measurements and computed results are reported on a nontransferred argon plasma discharging into an argon environment in a laminar regime. The experimental data provide information on the temperature profiles, particularly those close to the torch exit. The mathematical representation of the system involves the simultaneous statement of the equations of continuity, motion, and thermal energy balance for an axisymmetric system, but for fully temperature-dependent property values. On the whole, the theoretical predictions are in very good agreement with the measurements, with the maximum discrepancy being of the order of 5–10%. This augurs well for the extension of this work to more complex systems, also including gas mixtures.     

Numerical simulations of argon plasma jets flowing into cold air

Computational results and comparisons with experimental data are presented for simulations of axisymmetric turbulent argon plasma jets flowing into a cold air environment. The calculations were performed using the LAVA code [J. D. Rarnshaw and C. H. Chang,Plasma Chem. Plasma Process. 12, 299 (1992)], and were designed to simulate experiments performed by Brossa and Pfender (Plasma Chem. Plasma Process. 8, 75 (1988)) (BP) and by Finckeet al (private communication, 1992] (FSH). To our knowledge, these are the first such simulations in which multicomponent diffusion and interactions between dissociation and ionization of different species are consistently, accounted for. Turbulence effects were represented by a standard- model, both with and without an axisymmetric jet correction term and for several different choices of the turbulent Prandil and Schmidt numbers Pr_t and Sc_l. Simulations were performed for one FSH experiment and two BP experiments at different values of torch powerP and argon flow rateW. The inflow profiles in the FSH simulations were adjusted to matchP,W, and the experimental data slightly downstream of the torch exit as closely as possible. The same profile shapes were then used to matchP andW for the BP simulations, for which data near the torch exit were not available. Swirl was neglected except in one of the FSH calculations, where it was found to have negligible effect, as expected. Best results were obtained with the axisymmetric jet correction term omitted and with Pr_t = Sc_l = 0.7. Agreement with the experimental data was then lair overall, but still showed systematic deviations and cannot be regarded as fully satisfactory. Possible reasons for the discrepancies are discussed.     

Diagnostics of a thermal plasma jet by optical emission spectroscopy and enthalpy probe measurements

An accurate determination of electron density, temperature, and velocity distributions is of primary interest for the characterization of steady-state thermal plasma spray jets. Our diagnostic capabilities based on optical emission spectroscopy include measurements of absolute emission coefficients and Stark broadening. In addition, enthalpy probe diagnostics has also been used for temperature and velocity measurements. Observation of large discrepancies between temperatures derived from absolute emission coefficients, Stark broadening, and from enthalpy probe measurements indicate that severe deviations from LTE (local thermal equilibrium) exist in various regimes of plasma spray jets. Nonequilibrum characterization of such turbulent thermal plasma jets suggests that diffusion of high-energy electrons into the fringes of plasma jets and deviations from chemical equilibrium due to high velocities in the core of plasma jets and entrainment of cold gas, are the main reasons for these discrepancies. The establishment of a reliable data base, taking these nonequilibrium effects into account, is a prerequisite for meaningful modeling of real plasma jets.     

Velocity measurement of dc plasma jets based on arc root fluctuations

The axial component of the radial velocity distribution of a plasma flow generated by a dc plasma spray torch was measured by using a nonintrusive optical method, based on the propagation of the plasma jet luminosity fluctuations. In contrast to the simplicity of the experimental set-up, a special effort was made in the data processing, namely by using numerical techniques defined in the context of signal theory. Both centerline and radial profiles of the axial velocity were obtained for pure Ar and Ar–H₂ plasma flows. These experimental results were satisfactorily validated by calculating enthalpy and mass balances.Notation 1 refers to the reference signal - 2 refers to the shifted signal - a dimensionless adjustment factor - A ₁,_{f 1} values used for the estimation ofs ₁(t) energy spectrum - B ₁ frequency cutoff (Hz) - f frequency (Hz) - F ₁(f),F ₂(f) optimum low-pass filters - h mass enthalpy (J/kg) - h° enthalpy flowrate (J/s) - I(y) wavelength integrated radiation intensity - n ₁(t),F ₂(t) white noise - N ₁(t),N ₂() n ₁(t),n ₂(t)Fourier transforms - P _i probability - r radial coordinate - R plasma radius - s ₁(t),s ₂(t) signal components to be correlated - S ₁(f),S ₂(f) s ₁(t),s ₂(t) Fourier transforms - t time - T temperature (K) - , _i velocity (m/s) - x ₁(t),x ₂(t) recorded signals - X ₁(f),X ₂(f) x ₁(t),x ₂(t) Fourier transforms - x horizontal distance from the jet axis - y vertical distance from the jet axis - average velocity - standard deviation - z axial distance from the nozzle exit Greek Symbols dimensionless constant - r radial position uncertainty - velocity uncertainty - x width of the emission coefficient profile - z distance between the two sampling points - (r) radiative emission coefficient - time delay betweens ₁(t) ands ₂(t) - ₁ ^dg () x₁(t) autocorrelation function = x ₁(t)x ₁(t+)dt - ₁ ^dg (f) x₁(t) energy spectrum - _s1(), _s2() s ₁(t),s ₂(t) autocorrelation functions - _xt(f) s ₁(t) energy spectrum - ₁ ² , ₂ ² energy spectrum of white noisen ₁(t),n ₂(t) - ₁ time of correlation - ₁₂() cross-correlation function - coherence factor     

Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

The kinetics of decomposition of plastics are of interest from different points of view, i.e. evolution of harmful substances during fires or waste incineration, recovering of chemical raw materials from plastic refuses and designing of recycling procedures. To measure the formal kinetic parameters of the degradation of polymers isothermal and dynamic methods are applied in this work. Dynamic measurements are performed by combined thermogravimetry mass spectrometry (TG-MS), the isothermal measurements are carried out with a new closed loop-type reactor. To evaluate consistent kinetic data from isothermal and dynamic measurements, the energy balance for the sample in dynamic measurements has to be considered to obtain the true sample temperature and heating rate. Subject of this investigation is the exploitation of dynamic and isothermal methods for measuring and interpreting the kinetics of thermal decomposition of plastics. Results for commodity plastics polyethylene and poly(vinyl chloride) (PVC) are presented. The combined application of TG–MS, isothermal experiments in the closed loop-type reactor and DSC leads to new results for the decomposition kinetics of PVC. The dehydrochlorination mechanism at moderate temperature can be distinguished in an endothermal and exothermal part. The benzene formation is identified as a second order reaction. A great advantage of the isothermal method is, that changes in the mechanisms are detectable, i.e. changes in the apparent order of the reaction and the apparent activation energy. From that, new mechanistic aspects of the decomposition kinetics of polyethylene were obtained.     

Temperature and density measurements in a recombining argon plasma with diluent

Spectroscopic and callorimetric measurements of temperature arid number density have been made using a 50-kW radio-frequency inductively coupled plasma (RFICP) torch operated at atmospheric pressure with maximum temperatures and electron densities near 8,1000 K and 2 x 10²¹ m³, respectively These measurements enabled the determination o/ the stale o/ equilibrium and of the corresponding applicability of rarious diagnostic techniques in hoth a recombining argon plasma and a recombining plasma with hydrogen or nitrogen. Results indicate that the Pure argon plasma is well described by u partial equilibrium model in which the free and bound-excited electrons are in mutual equilibrium irespective of possible departures from equilibrium with the ground state. The addition of just tenths of a percent of either atomic Hydrogen or nitrogen, however, disturbs this partial equilibrium hr argon plasmas with electron densities roughly less than 10²¹ m³ such that only diagnostic techniques which are independent o/ partial equilibrium assumptions can be reliably implemented.     

Nonequilibrium modeling of low-pressure argon plasma jets; Part I: Laminar flow

This paper presents a modeling attempt related to low-pressure plasma spraying processes which find increasing applications for materials processing. After a review of the various models for ionization and recombination processes, a two-temperature model for argon plasmas in chemical (ionization) nonequilibrium is established using finite rate chemistry. Results of sample calculations manifest departures from kinetic as well as chemical equilibrium, demonstrating that the conventional models based on the LTE (local thermodynamic equilibrium) assumption cannot provide proper prediction for low-pressure plasma jets.     

Behavior of particulates in thermal plasma flows

Injection of particulate matter into a thermal plasma represents one of the approaches used in thermal plasma processing. The injected particles are usually treated as a dispersed phase, governed by the equation of motion and the rate equations for heat and mass transfer in Lagrangian coordinates. A stochastic approach is introduced to take particle dispersion into account due to turbulent fluctuations by randomly sampling instantaneous flow fields. Three-dimensional effects are also considered which are mainly due to particle injection and the presence of a swirl component. A modified approach for investigating noncontinuum effects on plasma-particle heat transfer is proposed, incorporating both electric and aerodynamic effects on the boundary layer around a particle immersed into a thermal plasma. Comparisons of theoretical predictions based on the present model with available experimental data are, in general, in reasonable agreement.     

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part III. Thermal plasma jet reactors and multiparticle injection

Thermal plasma processing involves complex interactions of particulates with plasmas. In previous studies (see Parts I and II of this series), an assessment of different effects has been made considering the dynamics and heat and mass transfer of a single particle immersed into a thermal plasma. The last paper of this sequence is concerned with the simulation of thermal plasma jet reactors and the effects caused by multiparticle injection.A mathematical model is proposed for the simulation of thermal plasma jet reactors, including the mixing phenomena between the jet and the surrounding gases by generalizing the governing equations for simple mixing flows. Also included is the density fluctuation effect by extending the K- model to a four-equation turbulence model combined with a probability density function. This model is internally consistent covering additional physical phenomena which are not covered by existing models. Unfortunately, its expected higher accuracy cannot be proven because of the present uncertainties associated with the input.For multiparticle injection, the simulation repeats calculations for single-particle injection, but with different initial conditions correcting the solutions by considering the coupling effects between particles and the plasma.The results indicate that (i) thermal plasmas show different mixing behavior in different gases; (ii) the density fluctuation effect is important since it causes large differences between the mass-weighted and unweighted time-averaged temperatures of thermal plasma jets; (iii) coupling effects become important when the particle loading rate exceeds half of the plasma mass flow rate; (iv) there are 16 constraints imposed on the modeling work which have to be considered for establishing a base for comparison with future experimental studies.     

The effect of gas injection on a thermal argon plasma flow in a water-cooled tube

The effect of gas injection on an atmospheric thermal argon plasma flow in a water-cooled tube was investigated experimentally and numerically. The injection gas is argon, helium, or nitrogen. The static pressure with helium injection increases greatly because of its high thermal conductivity, while little increase occurs for nitrogen injection because of the dissociation. The increasing rate of the static pressure depends on the ratio of the momentum term to the viscosity term. The heat flux to the tube wall with gas injection changes less than that without injection. The numerical results showed variations similar to the experimental ones.     

Atmospheric‐pressure spin plasma jets processing of polymethylmethacrylate surface using experimental design methodology

Atmospheric‐pressure spin plasma jets (APSPJs) have been developed to induce surface modifications on polymethylmethacrylate (PMMA). In this study, an experimental design methodology was used to investigate the influence of process parameters [such as radio frequency (RF) power, processing gap, and number of treatment cycles] on the characteristics of PMMA surface treated by APSPJs. It was observed from the atomic force microscope (AFM) and scanning electron microscope (SEM) results that the surface morphology of PMMA treated by direct plasma is much rougher than that treated by remote plasma. The direct plasma used in APSPJs processing created a substantial amount of nanostructure grains. Moreover, the measured XPS results showed that the O/C ratios of the PMMA surface were substantially increased and subsequently water contact angle decreased on direct plasma treatment. This decrease is due to an increase of oxygen‐containing functional groups on the PMMA surface by the APSPJs processing. From the statistical analysis, the RF power and the processing gap were found to play a major role in enhancing the hydrophilic properties of PMMA surface. In contrast, the number of treatment cycles played only a secondary role in this case. Finally, in this study the APSPJs processing was demonstrated to be an effective method for surface modification of PMMA by controlling processing parameters during the treatment process. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeling Study on the Entrainment of Ambient Air into Subsonic Laminar and Turbulent Argon Plasma Jets

Modeling study is performed to reveal the special features of the entrainment of ambient air into subsonic laminar and turbulent argon plasma jets. Two different types of jet flows are considered, i.e., the argon plasma jet is impinging normally upon a flat substrate located in atmospheric air surroundings or is freely issuing into the ambient air. It is found that the existence of the substrate not only changes the plasma temperature, velocity and species concentration distributions in the near-substrate region, but also significantly enhances the mass flow rate of the ambient air entrained into the jet due to the additional contribution to the gas entrainment of the wall jet formed along the substrate surface. The fraction of the additional entrainment of the wall jet in the total entrained-air flow rate is especially high for the laminar impinging plasma jet and for the case with shorter substrate standoff distances. Similarly to the case of cold-gas free jets, the maximum mass flow-rate of ambient gas entrained into the turbulent impinging or free plasma jet is approximately directly proportional to the mass flow rate at the jet inlet. The maximum mass flow-rate of ambient gas entrained into the laminar impinging plasma jet slightly increases with increasing jet-inlet velocity but decreases with increasing jet-inlet temperature.     

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10⁻⁴ kg/s.     

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.     

Effect of quenching conditions on particle formation and growth in thermal plasma synthesis of fine powders

The vapor-phase synthesis of ultrafine powders in reactive thermal plasma systems is studied. A mathematical model is developed to determine the effect of quenching conditions on the size characteristics of powders produced. The particle nucleation is considered to be due to both condensation of product vapor and surface reaction between adsorbed reactant species. The particle growth is considered to be exclusively due to further condensation of product vapor. Numerical predictions on powder formation are explored through a case study for the synthesis of zinc oxide powders from zinc vapor and oxygen carried in argon gas. The results of the present srudy indicate that the size characteristics of plasma-produced powders can be significantly enhanced by gradual, regulated quenching, as opposed to the rapid quenching conventionally used in the past. The results further indicate that distribution of the quench gas along the reactor provides an effective means to accomplish the much desired control over the powder properties.