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.     

Effects of the Length of a Cylindrical Solid Shield on the Entrainment of Ambient Air into Turbulent and Laminar Impinging Argon Plasma Jets

When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed. Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet is to be completely avoided.     

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.     

Numerical Study of a Free-Burning Argon Arc with Copper Contamination from the Anode

The present modeling of a free-burning argon arc accounts for copper vapor contamination from the anode. Simulations are made for an atmospheric arc that has a length of 10 mm and an electric current of 200 amps. Predicted results for two different anode evaporation rates are compared to those from a pure argon arc with no copper vapor contamination. Copper vapor concentration, temperature, electric potential, and current density profiles are presented. Included in this analysis are radiation losses from both the argon and copper by using recently calculated net emission coefficients. It was found that evaporation of copper from the anode results in a cooling of the arc in a region close to the anode, but has an insignificant influence on the arc close to the cathode. Due to the arc flow characteristics most of the copper vapor tends to be confined to the anode region.     

Modeling on the Momentum and Heat/Mass Transfer Characteristics of an Argon Plasma Jet Issuing into Air Surroundings and Interacting with a Counter-Injected Argon Jet

Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different.     

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.     

Numerical Modeling of an Impinging and Compressible Dusted Plasma Jet Controlled by a Magnetic Field

The present study has been conducted to clarify the magnetic control characteristics of a particle-laden compressible plasma jet impinging on a substrate for the improvement of a low-pressure plasma spraying process and its controllable optimization. The plasma jet is described by an Eulerian approach and each injected particle is described by a Lagrangian approach, respectively, taking into account the compressible effect, variable transport properties, and plasma–particle interactions, coupled with the Maxwell's equations. The effects of the location of an applied rf electromagnetic field and of the injected particle size on the plasma jet characteristics and the particle behavior are clarified by numerical simulation. It is concluded that the particle dispersion is effectively influenced and, furthermore, the plasma temperature and the particle temperature can be strongly controlled by applying the rf electromagnetic field to the nozzle. The reasonable agreement of particle velocity between calculation and experiment is obtained.     

Insights into the Aggregation Behaviour of a Benzoselenadiazole-Based Compound and Generation of White Light Emission

We have designed and synthesized the benzoselenadiazole (BDS) based donor-acceptor-donor (D-A-D) π-conjugated compound 4,7-di((E)styryl)benzo[2,1,3]selenadiazole ( 1 ). A single-crystal study of 1 shows J-type molecular aggregation in the solid state. The crystal packing of 1 shows head-to-head dimeric intermolecular assembly via Se⋅⋅⋅N interactions while staircase-type interlock molecular packing has occurred via Se⋅⋅⋅π interaction. The staircase-type interlock packing of dimeric molecular arrangement induces sheet-type, herringbone type architecture along crystallographic a axis and ab plane via CH⋅⋅⋅π interactions. Interestingly, the J-type aggregation of 1 in solid state changes to H-type aggregation upon UV-irradiation. Moreover, our spectroscopic findings in solution state reveal H-type of aggregation of 1 in 90 % aqueous THF. We have further demonstrated white light emission in the binary mixture of 1 and 1-pyrenemethanol ( 2 ) in 90 % aqueous THF. Our study reveals solvent specific co-assembly of H-aggregated 1 and 2 in 90 % aqueous THF solution, which shows white light emissive properties with the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (0.32, 0.31). The observed white light emission arises mainly due to the combination of red light from H-aggregated 1 , blue light from monomeric 2 and green light from excimers of 2 .     

Three-Dimensional Modeling of the Turbulent Plasma Jet Impinging upon a Flat Plate and with Transverse Particle and Carrier-Gas Injection

Modeling results are presented concerning the turbulent thermal plasma jet impinging normally on a substrate and with transverse injection of feedstock particles and their carrier gas from a single injection tube. The k- two-equation model is employed to model the turbulence, and particle dispersion is studied considering the interaction between the moving particles and turbulent eddies and considering the effect on particle trajectories of the random variation of the turbulent fluctuating velocities in their magnitude and direction. A well-validated three-dimensional (3-D) computer code is used in the modeling. The 3-D effects due to the carrier gas injection on the jet flow field and thus on the particle trajectories and heating histories are shown to be appreciable. The radial location of the injection tube with respect to the plasma jet is shown to be a critical parameter for the study of 3-D effects, besides the carrier-gas/plasma stream mass flux ratio. Particle dispersion considerably widens the distribution of the particle trajectories and heating histories. In addition, although pertinent swirl number is often rather small, swirling may also affect the modeling results.     

A comparison of experimental measurements and theoretical predictions regarding the behavior of a turbulent argon plasma jet discharging into air

Computed results are presented describing the temperature and concentration fields obtained when an argon plasma jet is being discharged into ambient air. A previously published mathematical model for turbulent plasma plumes is used for the calculations. These predictions are compared with recent), published experimental measurements by Brossa and Pfender, performed with an enthalpy probe. The theoretical predictions appear to agree reasonably well with the measurements of both the temperature and concentration profiles, with a maximum deviation in the 10–20% range.Notation A _max maximum temperature or velocity in the torch exit profile - C ₁ C ₂ C _D constants inK- model - h enthalpy - I torch current - K turbulent kinetic energy per unit mass - m mass concentration of plasma p pressure - Q How rate of argon through the torch - r radial coordinate - r _n nozzle radius (inside) - S source term for dependent variable      

An overall mechanism for the deposition of plasma polymers from methane in a low-pressure argon plasma jet

An overall mechanism for plasma polymer deposition from a methane-seeded argon plasma jet was established from experimental measurements and a simplified model of reaction kinetics within the plasma jet. Total mass deposition rates were obtained at various substrate positions and methane flow rates. Methane consumption was estimated from residual gas analysis. The influence of substrate coolant temperature on deposition rate was evaluated. The model was based on particle densities, jet temperature, and jet velocity data published previously, and reaction rate constants from the literature were used. No adjustable parameters were employed in this model. Experimental results for total deposition rate and methane consumption were in good agreement with model predictions. The overall deposition mechanism consists of three steps: Penning ionization of methane by excited argon neutrals, followed by dissociative recombination of CN _x ⁺ to yield CH, followed by incorporation of CH into the growing film upon impact. Contributions of species other than CH to the total deposition rate are minor, and adsorption is not a prerequisite for incorporation into the growing film.     

Validation of a Method for the Determination of Total Chromium in Rat Feces by Inductively Coupled Plasma Optical Emission Spectrometry

Abstract

The validation of a method for the determination of total chromium in Fischer-344 rat feces by inductively coupled plasma optical emission spectrometry following a rapid, atmospheric-pressure microwave digestion is described. The performance of the method was evaluated over the concentration range of 5.00 to 200 µg Cr/g feces. Data for method linearity, accuracy, precision, digest stability, and storage stability are presented along with limit of detection and limit of quantitation data. Data from a cross-validation method for B6C3F1 mouse feces are also presented. Following validation, the method was applied to analyze samples collected in support of two chronic toxicological investigations.     

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.     

盐酸浸提-电感耦合等离子体发射光谱法测定烟草中的多元素

研究了用5mol/L盐酸煮沸提取烟叶中的部分金属元素，用ICP－AES测定浸提液，K、Ca、Mg、Cu、B、Ba、Zn获得了较满意的结果，P、Fe、Mn的测定结果稍微偏低。此方法操作简便、快速安全，特别适合大批量烟草样的快速检测。与硝酸－高氯酸湿消化法进行对照，结果吻合得较好。并测定了国家烟草标准样品GBW08514和GBW08515，其结果与标准值相符。     

Numerical simulation of the spatiotemporal evolution of a nanoparticle–plasma system

A one-dimensional nanodusty plasma was modeled by self-consistently coupling a plasma model with nanoparticle growth, charging, and transport models. As nanoparticles grow from subnanometer to tens of nm in diameter, the numerical results predict a rich spatiotemporal structure, including four distinct temporal phases: a charge-limited phase, a charge accumulation phase, an early ion drag phase, and a sheath interaction phase.     

State-crossing from a Locally Excited to an Electron Transfer State(SLEET) Model Rationalizing the Aggregation-induced Emission Mechanism of (Bi)piperidylanthracenes

Luminogens with aggregation-induced emission(AIE)characteristics(or AIEgens)have been widely used in various applications due to their excellent luminescent properties in molecular aggregates and the solid state.A deep understanding of the AIE mechanism is critical for the rational development of AIEgens.In this work,the“state-crossing from a locally excited to an electron transfer state”(SLEET)model is employed to rationalize the AIE phenomenon of two(bi)piperidylanthracenes.According to the SLEET model,an electron transfer(ET)state is formed along with the rotation of the piperidyl group in the excited state of(bi)piperidylan-thracene monomers,leading to fluorescence quenching.In contrast,a bright state exists in the crystal and molecular aggregates of these compounds,as the intermolecular interactions restrict the formation of the dark ET state.This mechanistic understanding could inspire the deployment of the SLEET model in the rational designs of various functional AIEgens.     

Metal content in mosses from the Metropolitan Area of the Toluca Valley: a comparative study between inductively coupled plasma optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectrometry (TXRF)

The comparison between inductively coupled plasma optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectrometry (TXRF) for simultaneous determination of metal content (Cr, Cu, Fe, Mn, Pb and Zn) in mosses from the Metropolitan Area of the Toluca Valley was performed. Epiphytic mosses (Fabriona ciliaris and Leskea angustata) were collected in two sampling campaigns and were digested with HNO₃, HCl and HF for ICP-OES method and HNO₃ and HCl for TXRF method. The certified reference material (IAEA-336, Lichen) and the Standard Reference Material (SRM-1573, tomato leaves) were used for the quality control and to evaluate trueness and precision. Linearity, detection and quantification limits were also determined. Results show an ICP-OES and TXRF trueness mean of 101 ± 5% and 97 ± 9%, respectively; the relative standard deviation (RSD percent) was less than 17% in both methods. The moss samples exhibit a satisfactory precision (RSD ≤ 20%), because the RSD percent for ICP-OES, from 2% to 15%, and that for TXRF, from 1% to 17%, were obtained. One factor experimental design and simple regression analysis (α = 0.05) were used to compare the ICP-OES and TXRF metal concentrations. The statistical results do not show significantly different values for Cu, Mn, Pb and Zn in both the sample campaigns. In addition, the average results for Cr in the first sample campaign (30.3 ± 11.4 mg/kg for ICP-OES and 18.6 ± 9.8 mg/kg for TXRF) and Fe values in the second sample campaign (10,810 ± 2980 mg/kg for ICP-OES and 8380 ± 2350 mg/kg for TXRF) were significantly different in both methods. These differences are attributed to an incomplete sample digestion in the TXRF sample preparation. The results of the simple regression analyses show p-value less than 0.05, which indicates an equivalent and significant relation between ICP-OES and TXRF.     

Synthesis, characterization and application of a novel sorbent, glucamine-modified MCM-41, for the removal/preconcentration of boron from waters

A novel sorbent was prepared by the functionalization of an inorganic support material, MCM-41, with N-methylglucamine for the uptake of boron from aqueous solutions prior to its determination by inductively coupled plasma optical emission spectrometry (ICP-OES). Characterization of the newly synthesized material was performed using BET, XRD, TEM, SEM and DRIFTS techniques, in addition to its C and N elemental content. Sorption behavior of the novel sorbent for boron was also investigated and found to obey Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The maximum amount of B (as H₃BO₃) that can be sorbed by the sorbent was calculated from the D-R isotherm and was found to be 0.8 mmol B g⁻¹ of sorbent. The applicability of the new sorbent for the removal/preconcentration of boron from aqueous samples was examined by batch method. It was found that the sorbent can take up 85% of boron in 5 min whereas quantitative sorption is obtained in 30 min. Any pH greater than 6 can be used for sorption. Desorption from the sorbent was carried out using 1.0 M HNO₃. The sorption efficiency of the new sorbent was also compared to that of Amberlite IRA 743, a commercial resin with N-methylglucamine functional groups. Within the experimental conditions employed, the new sorbent was found to have higher sorption efficiency than the commercial resin. For method validation, spike recovery tests were performed at various concentration levels in different water types and were found to be between 83-95 and 75-92% for ultra pure water and geothermal water, respectively.     

Comparison of the cold vapor generation using NaBH4 and SnCl2 as reducing agents and atomic emission spectrometry for the determination of Hg with a microstrip microwave induced argon plasma exiting from the wafer

A 2.45 GHz low power microwave microstrip plasma (MSP) exiting the wafer and operated with Ar at atmospheric pressure was used for the optical emission spectrometric determination of Hg with the aid of a miniaturized optical fiber spectrometer with a CCD detector and the cold vapor (CV) generation technique using NaBH₄ and SnCl₂ as reductants. The experimental conditions were optimized with respect to the relative intensity of the Hg I 253.6 nm line and its signal-to-background intensity ratio (SBR). So as to understand the results of the optimization experiments, the excitation temperatures as measured from Ar I lines (T _exc) and the electron number densities (n _e) for the Ar MSP loaded with Hg vapors were determined and found to be in the range from 5500 to 6300 K and from 1.4 to 2.0 × 10¹⁴ cm⁻³, respectively. Under the optimized conditions, the detection limit for Hg of the CV-MSP-OES using SnCl₂ as the reducing agent was found to be much lower (0.11 ng mL⁻¹) than in the case where NaBH₄ was used (9 ng mL⁻¹). The linearity range was found to be up to 1 μg mL⁻¹ while the precision was of the order of 0.7–5%. The procedure with SnCl₂ as reductant was used for the determination of Hg at a concentration of 0.2 μg mL⁻¹ in synthetic water samples containing 1 to 4% (m/v) of NaCl with an accuracy of 3% as well as in a solution of the domestic sludge standard reference material (NIST SRM 2781) with a certified concentration for Hg of 3.64 ± 0.25 μg g⁻¹ for which 3.55 ± 0.41 μg g⁻¹ was found. Correspondence: J. A. C. Broekaert, Institut für Anorganische und Angewandte Chemie, Universit?t Hamburg, 20146 Hamburg, Germany