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.     

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.     

Influence of the Gas Injection Angle on the Jet Characteristics of a Non-transferred DC Plasma Torch

The non-transferred direct current (DC) plasma torch has been widely used in various industrial applications due to its special jet characteristics. The jet characteristics are determined by different factors, including the working parameters, the torch construction, the gas injection angle (GIA) etc. As there is little study on the influence of the GIA on the jet characteristics, experimental study on the GIA’s effects on the jet characteristics has been carried out on a specially designed non-transferred DC plasma torch, whose GIA can be changed by replacing a gas injection component. The arc voltages and thermal efficiencies of the plasma torch, the specific enthalpies and jet lengths of the plasma jets at different working conditions were obtained and analyzed. It has been found that the GIA greatly affects the arc voltage, the thermal efficiency, the specific enthalpy and the jet length. Based on these findings, plasma torch with appropriate GIA could be used to help generating the plasma jet with desired characteristics.     

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.     

Direct and Indirect Bactericidal Effects of Cold Atmospheric-Pressure Microplasma and Plasma Jet

The direct and indirect bactericidal effects of dielectric barrier discharge (DBD) cold atmospheric-pressure microplasma in an air and plasma jet generated in an argon-oxygen gas mixture was investigated on Staphylococcus aureus and Cutibacterium acnes. An AC power supply was used to generate plasma at relatively low discharge voltages (0.9–2.4 kV) and frequency (27–30 kHz). Cultured bacteria were cultivated at a serial dilution of 10⁻⁵, then exposed to direct microplasma treatment and indirect treatment through plasma-activated water (PAW). The obtained results revealed that these methods of bacterial inactivation showed a 2 and 1 log reduction in the number of survived CFU/mL with direct treatment being the most effective means of treatment at just 3 min using air. UV–Vis spectroscopy confirmed that an increase in treatment time at 1.2% O₂, 98.8% Ar caused a decrease in O₂ concentration in the water as well as a decrease in absorbance of the peaks at 210 nm, which are attributed NO₂⁻ and NO₃⁻ concentration in the water, termed denitratification and denitritification in the treated water, respectively.     

A Study of the Influence of the Surrounding Gas on the Plasma Jet and Coating Quality During Plasma Spraying

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of H_α emission was found to become gradually stronger while H_β and H_γ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.     

Electrical,Thermal and Optical Diagnostics of an Atmospheric Plasma Jet System

Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.     

Optimization of a Direct Sample Insertion Into a Stabilized Capacitive Plasma for Optical Emission Spectrometry (SCP-OES)

 A new sample insertion device for the stabilized capacitive plasma (SCP) has been developed, which enables it to analyze dry residues of micro amounts of liquid samples. Insertion was applied into an SCP as plasma source because of its good stability and excitation properties as well as its low instrument and operation costs. The plasma is sustained at a frequency of 27.12 MHz and an RF power of 150 W. For analysis the liquid samples are positioned at the tip of a quartz rod with the aid of a μL syringe. Then the sample is dried and the sampling rod inserted into the plasma. After optimization of the carrier gas flow (5 L/h) and the sample volume (20 μL) the detection limit for Pb with Ar as plasma gas is 200 pg. By further improving the guidance of the insertion detection limits for Pb, Cu, Cd and Mg in the 1 to 30 ng/mL range or 20 to 600 pg range absolute were obtained. It was found that the detection limits in the case of He are better than those obtained with Ar. The matrix interferences caused by changes in the concentration of the easily ionizable element Na were found to be below 10% for Na concentrations of up to 0.45 μg/mL. Ethanol concentrations of up to 14% in the analyte solutions did not cause any interferences. Received December 17, 1998. Revision June 4, 1999.     

丙烯和氧等离子体直接气相合成环氧丙烷

在室温和大气压下,在针板式反应器中,通过脉冲电晕放电等离子体对分子氧和丙烯直接合成环氧丙烷的活化作用,考察了放电电极间距、电晕放电脉冲电压以及电晕放电重复频率参数对丙烯转化率、环氧丙烷产率和其选择性的影响,反应物及各产物通过在线色谱法进行分析.实验结果表明,在室温和大气压下,用脉冲电晕等离子体法可转化丙烯和氧气直接生成环氧丙烷,适当调节上述各参数可提高环氧丙烷的收率.当反应气总流速为200mL/min,极间距为4mm,脉冲放电电压为18kV,放电频率为120Hz时,环氧丙烷的收率、丙烯的转化率及环氧丙烷的选择性分别为5.15%,19.15%和26.89%.     

Abnormal Optical Emissions in a Condensing Monosilane–Argon Gas Jet Activated by Electron Beam Plasma

Emissions from Si, SiH, H, Ar, and Ar⁺ are investigated for a free supersonic jet of a mixture of monosilane 5% (or 10%) with argon; the jet is activated by electron- beam plasma, and all the emissions are measured as a function of pressure, temperature in the gas source, the nozzle-to-electron beam distance, and the beam current. It is found that for certain parameters a process initiates which inhibits radiation of the spectral lines of Si, SiH, and H, and stimulates radiation of some spectral lines of argon atoms. It is shown that these anomalies are connected with a condensation process in the jet and, apparently, they are caused by electron-induced fluorescence of monosilane-containing clusters. The mechanisms for abnormal emissions of argon atoms and electron-excited fragments of monosilane molecule are suggested.     

直流等离子体离子化检测器的研究：管道煤气中可燃气体的测定

本文提出一种改进的直流等离子体离子化检测器（ＤＣＰＩＤ），用于管道煤气中可燃气体Ｈ２、ＣＨ４和ＣＯ的测定。采用氢气作工作气体，对该检测器的工作参数和响应特性进行了优化。本法结果与微波诱导等离子体离子化检测器（ＭＩＰＩＤ）的结果作了比较，结果满意。     

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.     

Synthesis of Ultrafine Carbon Black by Pyrolysis of Polymers Using a Direct Current Thermal Plasma Process

A direct current thermal plasma system was developed and applied to synthesize ultrafine carbon black by using PS (polystyrene) and HDPE (high density polyethylene) as carbon sources. The precursors were pyrolyzed at different temperatures and the pyrolysis products were employed to investigate the actual synthesis of carbon black through the plasma jet. Spherical carbon nanoparticles with a high degree of turbostratic structure were obtained, showing a fingerprint graphite structure with a large surface area and ultrafine particle size. The carbon black was characterized by transmission and scanning electron microscopy, powder X-ray diffraction, Raman spectroscopy, and nitrogen physisorption. Particle size distribution and dispersion stability in solvents were also investigated.     

Ambient Species Density and Gas Temperature Radial Profiles Derived from a Schlieren Technique in a Low-Frequency Non-thermal Oxygen Plasma Jet

A quantitative interpretation of the schlieren technique applied to a non-thermal atmospheric-pressure oxygen plasma jet driven at low-frequency (50 Hz) is reported. The jet was operated in the turbulent regime with a hole-diameter based Reynolds number of 13,800. The technique coupled to a simplified kinetic model of the jet effluent region allowed deriving the temporally-averaged values of the gas temperature of the jet by processing the gray-level contrast values of digital schlieren images. The penetration of the ambient air into the jet due to turbulent diffusion was taken into account. The calibration of the optical system was obtained by fitting the sensitivity parameter so that the oxygen fraction at the nozzle exit was unity. The radial profiles of the contrast in the discharge off case were quite symmetric on the whole outflow, but with the discharge on, relatively strong departures from the symmetry were evident in the near field. The time-averaged gas temperature of the jet was relatively high, with a maximum departure of about 55 K from the room temperature; as can be expected owing to the operating molecular gas. The uncertainty in the temperature measurements was within 6 K, primarily derived from errors associated to the Abel inversion procedure. The results showed an increase in the gas temperature of about 8 K close to the nozzle exit; thus suggesting that some fast-gas heating (with a heating rate ~0.3 K/μs) still occurs in the near field of the outflow.     

Characterization of the Converging Jet Region in a Triple Torch Plasma Reactor

Enthalpy probe measurements were taken of the converging plasma plume in a triple torch plasma reactor and related to substrate heat flux measurements. Results show excellent entrainment of process gases injected into the converging plasma plume by way of the central injection probe. At lower pressures (40 kPa), the plasma volume is equivalent to at least a 3 cm diameter, 4 cm long cylinder, with relatively uniform temperature, velocity, and substrate heat flux profiles when compared to a typical dc arc jet. Converging plasma plume size, substrate heat flux, and enthalpy profiles are also shown to be a strong function of applied system power. Substrate heat flux measurements show smaller radial gradients than enthalpy probe measurements, because of the high radial velocity component of gases above the substrate boundary layer. Enthalpy probe measurements were also conducted for diamond deposition conditions and approximate temperature and velocity profiles obtained. Problems with the uniform gas mixture assumption prohibited more accurate measurements. Reproducibility of enthalpy measurement results was shown with an average standard deviation of 11.8% for the velocity and 7.6% for the temperature measurements.     

Numerical Prediction of the Noise Emission from a Turbulent Argon Thermal-Plasma Jet Issuing into Ambient Air

This paper attempts to predict the noise emission characteristics of a turbulent argon thermal-plasma jet issuing into ambient air. The flow, temperature and concentration fields and turbulence characteristics of the turbulent plasma jet are computed at first, and then the noise emission from the plasma jet to a sideline far-field observer is calculated using the approach proposed by Fortuné and Gervais (AIAA J. 37(1999)1055) for predicting the noise emission from a turbulent, hot but non-ionized, air jet after some modification. The diffusion of ambient air into the turbulent argon plasma jet is handled using the turbulence-enhanced combined–diffusion-coefficient method. Velocity fluctuation correlations (aerodynamic noise source) in the plasma jet are calculated still using the K-ɛ two-equation turbulence model, but the temperature-velocity fluctuation correlations (entropic noise source) within the jet are calculated by solving a second-order turbulent Reynolds heat-flux transport equation in order to better deal with the contribution of temperature fluctuation to the noise emission. It is shown that among the contributions of aerodynamic noise source, entropic noise source and their mixed effect, the entropic noise source (i.e. the temperature-velocity fluctuation correlations) is dominant for the noise emission from the turbulent plasma jet to the sideline observer. The noise intensity increases with increasing plasma jet temperature or velocity. The predicted noise frequency spectrum characteristics and noise intensity levels are shown to be reasonably consistent with available experimental data.     

Modeling of a DC Plasma Torch in Laminar and Turbulent Flow

A mathematical 2D representation is developed describing the temperature and the velocity profiles in a DC plasma torch and in the resulting plume. It is based on the resolution of conservation equations using the Simple method after Patankar. In the first part, we illustrate the effects of the turbulence, using, on the one hand, two Prandtl's mixing length models and, on the other hand, a standard k – model. We also show the influence of physical parameters like the inlet mass flow rate, the current intensity, and the kind of gas (argon or air) on the characteristics of the plasma. The second part of this study presents a comparison of the model with experimental results encountered in the literature. The profiles obtained at the exit of the torch are compared to the mathematical formulation used as boundary condition by the models taking into account only the plasma jet.     

Numerical Simulation of Downstream Kinetics of an Atmospheric Pressure Nitrogen Plasma Jet Using Laminar, Modified Laminar, and Turbulent Models

We present numerical simulation of the nitrogen atmospheric pressure plasma jet (APPJ) using three fluid models—namely, laminar model, modified laminar model, and turbulent (k-ε) model—coupled with gas-phase reaction kinetics. The spatial profiles of the light emission intensities, gas temperature, and NO density predicted by the turbulent model show a better agreement with the experimental observations, compared with laminar and modified laminar models. We have demonstrated that the turbulent model shows more oxygen entrainment, more mixing with the ambient air, and a lower axial velocity at the downstream. These allow the turbulent model to more precisely capture the APPJ characteristics than the laminar and modified laminar models do.