Characterization of d.c. plasma torch voltage fluctuations

The arc root fluctuations at the anode-nozzle of a d.c. plasma spray torch with a special configuration of the electrodes allowing to work with the same gas flowrate with nozzle diameters ranging from 6 to 10 mm were systematically studied. The plasma gas was Ar/H2 (25 vol % H2), the current was varied between 200 and 600 A and the plasma gas flowrate between 24 and 80 slm. After 30–60 mn working the nozzle wall started to be sufficiently eroded to have a stagnant arc spot which lived until arcing created another one. It was shown that the life time of the upstream arc spots were 30–40 % longer than the downstream ones which could play an important role in the electrode erosion. Dimensional analysis allowed to find a relationship between the nozzle diameter D, the arc current I and gas flow rate G and the mean spot lifetime which is closely connected with the difference between D and the electrical diameter of the arc column. The comparison of voltage signal and light emission at a point of the plasma jet close to the nozzle exit on its axis allowed to determine the mean electrical field within the plasma column and the mean position of the arc root. The comparison with the electrode erosion area for well defined conditions showed a good correlation with the calculated arc root position.     

Degradation of Phenol in Aqueous Solutions by Gas–Liquid Gliding Arc Discharges

The efficiency of a gliding arc reactor designed with the aim to degrade aqueous phenol solutions is studied as a function of supply voltage, electrode gap distance, and gas–liquid flow properties. This efficiency, which steeply increases when increasing the supply voltage, can reach 96% when the minimum electrode distance is fixed at 3 mm. Experiments show that phenol degradation efficiency also depends on solution pH, Fe²⁺ addition, gas nature and gas flow rate. Furthermore, degradation pathways of phenol in aqueous solutions are proposed.     

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.     

Effect of Cathode Nozzle Geometry and Process Parameters on the Energy Distribution for an Argon Transferred Arc

The influence of two nozzle geometries and three process parameters (arc current, arc length and plasma sheath gas flow rate) on the energy distribution for an argon transferred arc is investigated. Measurements are reported for a straight bore cylindrical and for a convergent nozzle, with arc currents of 100 A and 200 A and electrode gaps of 10 mm and 20 mm. These correspond to typical operating parameters generally used in plasma transferred arc cutting and welding operations. The experimental set up consisted of three principal components: the cathode-torch assembly, the external, water-cooled anode, and the reactor chamber. For each set of measurements the power delivered to each system component was measured through calorimetric means, as function of the arc’s operating conditions. The results obtained from this study show that the shape of the cathode torch nozzle has an important influence on arc behaviour and on the energy distribution between the different system components. A convergent nozzle results in higher arc voltages, and consequently, in higher powers being generated in the discharge for the same applied arc current, when compared to the case of a straight bore nozzle. This effect is attributed to the fluidynamic constriction of the arc root attachment, and the consequential increase in the arc voltage and thus, in the Joule heating. The experimental data so obtained is compared with the predictions of a numerical model for the electric arc, based on the solution of the Navier–Stokes and Maxwell equations, using the commercial code FLUENT©. The original code was enhanced with dedicated subroutines to account for the strong temperature dependence of the thermodynamic and transport properties under plasma conditions. The computational domain includes the heat conduction within the solid electrodes and the arc-electrode interactions, in order to be able to calculate the heat distribution in the overall system. The level of agreement achieved between the experimental data and the model predictions confirms the suitability of the proposed, “relatively simple” model as a tool to use for the design and optimization of transferred arc processes and related devices. This conclusion was further supported by spectroscopic measurements of the temperature profiles present in the arc column and image analysis of the intensity distribution within the arc, under the same operating conditions.     

Steam Torch Plasma Modelling

Numerical modelling of physical properties and processes in an electric arc stabilized by a water vortex (steam torch) has been summarized in this review paper. One-fluid MHD equations are numerically solved for an axisymmetric thermal plasma flow inside a discharge chamber of the steam plasma torch. The steady state solution results are discussed for the range of currents 300–600 A with relatively low steam flow rate of about 0.3 g s^?1. The maximum obtained velocities and temperatures—8500 m s^?1, 26,300 K, are reported at the centre of the nozzle exit for 600 A. The evaporation of water, i.e. mass flow rate of steam, was predicted from a comparison between the present simulation and experiments. The generated plasma is mildly compressible (M < 0.7) with the inertial forces overwhelming the magnetic, viscous, centrifugal and Coriolis forces with the factor of 10³. Our calculations showed that the most significant processes determining properties of the arc are the balance of the Joule heat with radiation and radial conduction losses from the arc. Rotation of plasma column due to the tangential velocity component has a negligible effect on the overall arc performance, however, the rotation of water induces fluctuations in the arc and in the plasma jet with characteristic frequency which is related to the frequency of rotation of water. Reabsorption of radiation occurs at the radial position higher than 2.5 mm from the arc axis. The amount of reabsorbed radiation is between 17 and 28%. LTE conditions are satisfied in the arc column with the 2 mm radius. Comparison between the present simulations and experiments shows good agreement with the current–voltage characteristics, radial velocity and temperature profiles, as well as with the other related numerical simulation.     

Velocity Measurements for Arc Jets Produced by a DC Plasma Spray Torch

An optical method was used to determine the axial velocity of plasma jets produced by a DC plasma spray torch. Different experimental conditions were tested in order to systematically study the influence of the working parameters on the plasma velocity. In this way, the arc current ranged between 200 and 600 A, the gas flow rate between 30 and 80 slm, and the internal nozzle diameter between 6 and 10 mm; the plasma gases were either an Ar–H ₂ mixture or N ₂ . Rather well defined tendencies were observed and at the same time it appeared that the arc stability greatly influenced the fluctuations of the velocity.     

High-Speed Large Scale Chromatographic Purification of Plasmid DNA with a Novel Giant-Pore Stationary Phase

A rigid spherical giant-pore poly (glycidyl methacrylate-co-ethylene dimethacrylate) matrix has been prepared by radical suspension–polymerization on the basis of a novel porogenic mode using superfine particles of calcium carbonate as a solid porogen. Scanning electron microscopy reveals that the bead has pores as large as 10 μm. The hydrodynamic properties show that this polymeric material has good strength and a low back pressure of 1.0 MPa at a flow velocity of 3,000 cm h⁻¹. After being modified to be an anion-exchange material, high dynamic binding capacity of plasmid DNA of above 1,000 μg plasmid per mL of bed by a column of this material, could be obtained comparing to the 150 μg plasmid per mL of bed with a Q-Sepharose FF column at the same flow rate. Large-scale preparative plasmid separations (2–20 mL) from cell lysate were investigated. A 75% yield and 94.9% purity of SC plasmid DNA were obtained by a 20 mL column of giant-pore beads at a flow rate of 600 cm h⁻¹.     

W-C Synthesis in a Pulsed Arc Submerged in Liquid

Pulsed arc production of tungsten carbide (W-C) powders in deionized water and analytical (99.8%) ethanol was studied. The arc was ignited between two submerged electrodes: one of 99.99% graphite (C) and the other of 99.5% W. The pulse energy and duration were in the ranges of 7.7–192 mJ and 25–65 μs, respectively. The WC_1−x production rate was maximized by configuring the C electrode as the anode and the W electrode as the cathode. The rate was greater in ethanol than in water. The rate of producing ∼10 nm particles in ethanol was two orders of magnitude greater when using W anode -C cathode configuration, than with the opposite polarity.     

Comparative Study of Flow Characteristics Inside Plasma Torch with Different Nozzle Configurations

A numerical analysis of the influence of different nozzle configurations on the plasma flow characteristics inside D.C plasma torches is presented to provide an advanced nozzle design basis for plasma spraying torches. The assumption of steady-state, axis-symmetric, local thermodynamic equilibrium, and optically thin plasma is adopted in a two-dimensional modeling of plasma flow inside the plasma torch. The PHOENICS software is used for solving the governing equations, i.e. the conservation equations of mass, momentum, and energy along with the equations describing the K-epsilon model of turbulence. The calculated arc voltages are consistent with the experimental results when arc current, gas inflow rate, and working gas are the same as the experimental parameters. Temperature, axial velocity contours inside plasma torches, profiles along the torch axis and profiles at the outlet section are presented to show the plasma flow characteristics. Comparisons are made among those torches. The results show that torches with different anode nozzle configurations produce different characteristics of plasma flows, which suggest some important ideas for the advanced nozzle design for plasma spraying. In order to validate the model and to show its level of predictivity, a comparison of the model with experimental results encountered in the literature is presented in the last part.     

Characterization of volatile organic compounds and odors by in-vivo sampling of beef cattle rumen gas, by solid-phase microextraction, and gas chromatography–mass spectrometry–olfactometry

Volatile organic compounds (VOCs) and odors in cattle rumen gas have been characterized by in-vivo headspace sampling by solid-phase microextraction (SPME) and analysis by gas chromatography–mass spectrometry–olfactometry (GC–MS–O). A novel device enabling headspace SPME (HS-SPME) sampling through a cannula was designed, refined, and used to collect rumen gas samples from steers. A Carboxen–polydimethylsiloxane (PDMS) fiber (85 μm) was used for SPME sampling. Fifty VOCs from ten chemical groups were identified in the rumen headspace. The VOCs identified had a wide range of molecular weight (MW) (34 to 184), boiling point (−63.3 to 292 °C), vapor pressure (1.05 × 10⁻⁵ to 1.17 × 10² Pa), and water solubility (0.66 to 1 × 10⁶ mg L⁻¹). Twenty-two of the compounds have a published odor detection thresholds (ODT) of less than 1 ppm. More than half of the compounds identified are reactive and have an estimated atmospheric lifetime of <24 h. The amounts of VFAs, sulfide compounds, phenolic compounds, and skatole, and the odor intensity of VFAs and sulfide compounds in the rumen gas were all higher after feeding than before feeding. These results indicate that rumen gases can be an important potential source of aerial emissions of reactive VOCs and odor. In-vivo sampling by SPME then GC–MS–O analysis can be a useful tool for qualitative characterization of rumen gases, digestion, and its relationship to odor and VOC formation. Figure Modified cannula for rumen gas sampling with SPME     

Modeling Study on the Flow,Heat Transfer and Energy Conversion Characteristics of Low-Power Arc-Heated Hydrogen/Nitrogen Thrusters

A modeling study is conducted to investigate the effect of hydrogen content in propellants on the plasma flow, heat transfer and energy conversion characteristics of low-power (kW class) arc-heated hydrogen/nitrogen thrusters (arcjets). 1:0 (pure hydrogen), 3:1 (to simulate decomposed ammonia), 2:1 (to simulate decomposed hydrazine) and 0:1 (pure nitrogen) hydrogen/nitrogen mixtures are chosen as the propellants. Both the gas flow region inside the thruster nozzle and the anode-nozzle wall are included in the computational domain in order to better treat the conjugate heat transfer between the gas flow region and the solid wall region. The axial variations of the enthalpy flux, kinetic energy flux, directed kinetic-energy flux, and momentum flux, all normalized to the mass flow rate of the propellant, are used to investigate the energy conversion process inside the thruster nozzle. The modeling results show that the values of the arc voltage, the gas axial-velocity at the thruster exit, and the specific impulse of the arcjet thruster all increase with increasing hydrogen content in the propellant, but the gas temperature at the nitrogen thruster exit is significantly higher than that for other three propellants. The flow, heat transfer, and energy conversion processes taking place in the thruster nozzle have some common features for all the four propellants. The propellant is heated mainly in the near-cathode and constrictor region, accompanied with a rapid increase of the enthalpy flux, and after achieving its maximum value, the enthalpy flux decreases appreciably due to the conversion of gas internal energy into its kinetic energy in the divergent segment of the thruster nozzle. The kinetic energy flux, directed kinetic energy flux and momentum flux also increase at first due to the arc heating and the thermodynamic expansion, assume their maximum inside the nozzle and then decrease gradually as the propellant flows toward the thruster exit. It is found that a large energy loss (31–52%) occurs in the thruster nozzle due to the heat transfer to the nozzle wall and too long nozzle is not necessary. Modeling results for the NASA 1-kW class arcjet thruster with hydrogen or decomposed hydrazine as the propellant are found to compare favorably with available experimental data.     

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.     

Effects of Nozzle Length and Process Parameters on Highly Constricted Oxygen Plasma Cutting Arc

The influence of nozzle length and two process parameters (arc current, mass flow rate) on the plasma cutting arc is investigated. Modeling results show that nozzle length and these two process parameters have essential effects on plasma arc characteristics. Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase with the nozzle length. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage and power, therefore increases plasma velocity, temperature and heat flux.     

Flow Rate and Velocity of Ideal Gas in a Capillary Column with Uniform Permeability

When the column pressure drop is high, the average velocity of a carrier gas is proportional to the square root of the outlet velocity and the flow rate. Characteristic velocity, flow rate and pressure – the boundary conditions between low and high pressure drop regions – are introduced. Previously derived equations for average velocity vs. outlet velocity were modified to include the flow rate and to become more suitable for the separate studies of the low and high pressure drop regions.     

Influence of the Shroud Gas Injection Configuration on the Characteristics of a DC Non-transferred Arc Plasma Torch

The characteristics of the plasma jet emanating from a dc non-transferred plasma torch is affected by many factors including arc current, type of gas, gas flow rate, gas injection configuration and torch geometry. The present work focuses on experimental investigation of the influence of shroud gas injection configuration on the I–V characteristics and electro-thermal efficiency of a dc non-transferred plasma torch operated in nitrogen at atmospheric pressure. The plasma gas is injected into the torch axially and shroud gas is injected through three different nozzles such as normal, sheath and twisted nozzles. The effects of flow rates of plasma/axial gas and arc current on I–V characteristics and electro-thermal efficiency of the torch holding different nozzles are investigated. The I–V characteristics and electro-thermal efficiency of the torch are found to be strongly influenced by the shroud gas injection configuration. The effect of arc current on arc voltage decreases with increasing the axial gas flow rate. At higher axial gas flow rate (>?45 lpm), the I–V characteristics of the plasma torch are similar irrespective of the nozzle used. The variation of electro-thermal efficiency with arc current is almost similar to that of arc voltage with arc current. As expected, the electro-thermal efficiency is increased when the axial gas flow rate is increased and at higher axial gas flow rate, it is not influenced by the arc current and shroud gas configuration. The plasma torch with normal nozzle may be better in the range of operating conditions used in this study.     

Study of Mechanism for Hexane Decomposition with Gliding Arc Gas Discharge

The mechanism of hexane decomposition under gliding arc gas discharge conditions is studied from both qualitative and quantitative analyses of its products for various hexane initial concentrations and different background atmospheres : nitrogen, argon, air (O₂ 21% N₂ 79% vol.) and N₂–O₂ mixtures. The decomposition rate, which decreases with increasing hexane initial concentration, can reach 94% when the carrier gas is air. Due to the electron energy consumed by the dissociation of nitrogen, the decomposition rate of hexane in nitrogen is lower than in argon. The radical channel plays a predominant role in the hexane decomposition process. With increasing oxygen concentration in the carrier gas, the hexane decomposition rate increases and promotes the conversion of CO– CO₂, but it also leads to the formation of NO₂.     

Temperature and density profiles of magnetically rotating arcs burning in contaminated argon

Spectroscopic measurements on copper vapors emitted by the cathode are presented for magnetically rotating arcs in a coaxial copper electrode geometry. The maximum temperature of a 100-A arc column burning in contaminated argon is shown to be lower than 8000 K. A maximum Cu density of 5 × 10²¹ in m^–3 is observed when argon is contaminated with 1% CO, while it is larger than 10²² m^–3 with 1% nitrogen contamination. The copper vapors emitted by the cathode explain the low temperatures observed. Cases of surface control of the arc velocity at the cathode and radial losses of copper vapors out of the arc column are observed front specific parameters describing the arc profiles. Evidence is given for a copper ion recombination zone extending 2 mm from the cathode in the nitrogen contamination case.     

循环气流床反应器甲醇合成研究

1975年美国Chem Systems Inc公司提出液相法甲醇合成工艺的概念,采用导热性能优良的液体作为热载体,床层温度均匀易控,从而可提高原料气的单程转化率,出口甲醇质量分数可达到15％。传统浆态床反应器相对固定床虽然具有良好的传热性能,温度分布比较均匀,但是浆态床由于液体溶剂的存在以及气体分布不均匀,给传质过程带来了负面影响,增加了体系的复杂性。尽管优化气体分布器结构,可以起到改善传质的效果,但是还是无法明显改善传统浆态床内气液传质性能,反应器内固体沉积团聚严重。循环气流床通过循环泵的强制循环和喷嘴的雾化作用,相比如传统浆态床反应器,具有良好的传质能力、固体悬浮,并且能有效地减少轴向返混,具有相间接触充分、气液比（气固比）调节灵活、催化剂利用率高等特点。本实验研究了空速、循环量、喷嘴个数、催化剂浓度对甲醇合成的影响,为以后反应器的放大优化提供基础数据。     

Features of the Current Sustainment in a Low-Current Discharge in Airflow

The paper relates to the investigations of a low-current discharge in a vortex airflow with the electrode configuration corresponding to classical coaxial plasmatron. The gas flow rate is varied from 0.1 to 0.3 g/s at an inner diameter of the plasmatron nozzle of 5 mm. The discharge is powered by dc voltage via a ballast resistor. Typical averaged current is changed from 0.06 to 0.15 A so that a maximum averaged power dissipated in the discharge amounts to 160 W. In these conditions, a luminous gas region at the plasmatron exit, which in most publications is associated with a plasma jet, is observed. The method for the jet diagnostics based on a usage of the additional electrodes at the plasmatron exit has been proposed. The main idea of the experiments is the elucidation of the problem whether the jet actually represents the plasma area or we have to apply the term “plasma” with care. In particular, in the case under discussion the main charged particles in the jet are electrons that are emitted from a plasma column located in the plasmatron nozzle. The model that describes the formation of electron flow in the jet has been proposed. Typical electron density in the jet estimated with a usage of the model is at a level of 10⁹ cm^?3.

     

Galvanostatic investigation of electrochemical processes at the C/RbCu4Cl3I2 interface

The kinetics of electrochemical processes at a vitreous carbon electrode in contact with the solid electrolyte RbCu₄Cl₃I₂ has been investigated. In the range of potentials from zero up to the decomposition potential of the electrolyte (0.57 V), the rate of the reversible electrode reaction Cu⁺–e^–↔Cu²⁺ is controlled by slow diffusion of the electronic defects (Cu²⁺ ions or holes) in the electrolyte. The diffusion coefficient of these defects is equal to 1.5×10^–8 cm² s^–1. The exchange current density of the electrode reaction is equal to 20×10^–6 A cm^–2. Electronic Publication