Study of Gliding Arc Discharge Plasma

本文研究了滑动弧放电过程中电参数的变化，并对滑动弧等离子体中的非平衡度和各参数之间关系进行了讨论。应用了双通道电弧模型 ，对电弧在气流作用下的运动规律进行了数值模拟。模拟的结果有助于分析滑动弧非平衡等离子体的产生机理。 The elelctric parameters change during discharge is studied and the relationship between non-equilibrium degree and parameters is discussed for gliding arc discharge.Using two-channel model, the rules of arc moving due to effect of the airflow is simulated.The numerical simulation results can help analyzing the generation mechanism of gliding arc non-equilibrium plasma.

Three-Dimensional Modeling of Heat Transfer and Fluid Flow in Laminar-Plasma Material Re-Melting Processing

Modeling study is performed concerning the heat transfer and fluid flow for a laminar argon plasma jet impinging normally upon a flat workpiece exposed to the ambient air. The diffusion of the air into the plasma jet is handled by using the combined-diffusion-coefficient approach. The heat flux density and jet shear stress distributions at the workpiece surface obtained from the plasma jet modeling are then used to study the re-melting process of a carbon steel workpiece. Besides the heat conduction within the workpiece, the effects of the plasma-jet inlet parameters (temperature and velocity), workpiece moving speed, Marangoni convection, natural convection etc. on the re-melting process are considered. The modeling results demonstrate that the shapes and sizes of the molten pool in the workpiece are influenced appreciably by the plasma-jet inlet parameters, workpiece moving speed and Marangoni convection. The jet shear stress manifests its effect at higher plasma-jet inlet velocities, while the natural convection effect can be ignored. The modeling results of the molten pool sizes agree reasonably with available experimental data.

Carbon transition efficiency and process cost in high-rate, large-area deposition of diamond films by DC arc plasma jet

A new DC plasma torch in which are jet states and deposition parameters can be regulated over a wide range has been built. It showed advantages in producing stable plasma conditions at a small gas flow rate. Plasma jets with and without magnetically rotated arcs could be generated. With straight are jet deposition, diamond films could be formed at a rate of 39 mu m/h on Mo substrates of Phi 25 mm, and the conversion rate of carbon in CH4 to diamond was less than 3%. Under magnetically rotated conditions, diamond films could be deposited uniformly in a range of Phi 40 mm at 30 mu m/h, with a quite low total gas flow rate and high carbon conversion rate of over 11%. Mechanisms of rapid and uniform deposition of diamond films with low gas consumption and high carbon transition efficiency are discussed.

Effects of Natural Convection on the Characteristics of a Long Laminar Argon Plasma Jet Issuing Horizontally into Ambient Air

Three-dimensional modeling results show that the appearance of the long laminar plasma jet is less influenced by natural convection even as it is issuing into ambient air horizontally. However, plasma parameter distributions may deviate from axi-symmetry

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.

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.

Preliminary Investigations on Low-Pressure Laminar Plasma Spray Processing

The usual plasma spraying methods often involve entrainment of the surrounding air into the turbulent plasma core and result in coated materials having relatively high porosity and low adhesive strength. Therefore, exploration of new plasma spraying methods for fabricating high quality coatings to meet the requirement of special applications will be quite important. In this study, an alternative plasma spraying method, i.e. the low-pressure laminar plasma spraying process, is investigated and used in an attempt for spraying thermal barrier coatings (TBCs). Investigations on the characteristics of the laminar plasma jets, feeding methods for the ceramic powder and the formation process of the individual quenched splats have been carried out. The properties of the TBCs sprayed by laminar plasma jet process, such as the adhesive strength at the interface of the ceramic coating/bond coat, the surface roughness and microstructure, are examined by tensile tests and scanning electron microscope (SEM) observations.

Experimental Study on Fluctuation Characteristics of Laminar/Turbulent Plasma Jets

应用电压传感器、光电倍增管及水冷皮托管，对产生射入空气中的纯氩层流和湍流等离子体射流的弧电压波动、发生器出口处的射流光强波动以及沿射流轴线的滞止压力波动进行了测量。结果显示层流等离子体射流各参数的波动幅度远小于湍流射流的对应值；弧电压的波动幅度随气流量的变化明显，但随电流的变化很小；弧电压的波动幅度与其平均值之比随电流增加呈下降的趋势。

Laminar Plasma Spraying of Metal Wire in Atmosphere

利用层流等离子体射流，以普通工程铁丝为喷涂材料，在Q235基体表面制备金属涂层，并利用喷涂系统的参数可调性研究工艺参数对涂层质量的影响。结果表明，利用层流等离子体射流喷涂可以得到具有典型层状结构、氧化较少的致密涂层。

Propagation Properties of Electromagnetic Wave in a Plasma Slab

In this paper, the calculated results about the propagation properties of electromagnetic wave in a plasma slab are described. The relationship of the propagation properties with frequencies of electromagnetic wave, and parameters of plasma (electron temperature, electron density, dimensionless collision frequency and the size of the plasma slab) is analyzed.

Investigation of Laminar Plasma Remelting/Cladding Processing

Investigation of remelting and cladding processing with laminar plasma jets on several metals has been conducted looking for possible development of a new surface modification technique. The remelting tests illustrated that the new method could evidently improve the material microstructure and properties of cast iron. The cladding was done with Al2O3 ceramic powder on stainless steel. The energy dispersive spectra (EDS) analysis was used to determine the distribution of the major cladding element in the plasma-processed layers, for which the microstructure observations and hardness measurements were also performed.

Generation of long, laminar plasma jets at atmospheric pressure and effects of flow turbulence

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 fi,Hes its diameter could be generated with a DC are torch by! restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including are channel geometry gas flow I ate, and feeding methods, etc. Influence of flow turbulence at the torch,nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet, with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent are let.

Effect of diffusion on coating microstructure and oxidation resistance of aluminizing steel

The effect of diffuse treatment on coating microstructure and oxidation resistance at high-temperature of hot-dip aluminum were studied by means of TEM, SEM and XRD. The results show that, the diffusion temperature has significant effect on structure of coatings and its oxidation resistance. After diffusion at 750 degreesC, the coating consists of thick outer surface layer (Fe2Al5+ FeAl2), thin internal layer (FeAl + stripe FeAl2), and its oxidation resistance is poor. After diffusion at 950 degreesC, the outer surface layer is composed of single FeAl2 phase, the internal layer is composed of FeAl phase, and its oxidation resistance declines due to the occurrence of early stage internal oxidation cracks in the coating. After diffusion at 850 degreesC, the outer surface layer becomes thinner and consists of FeAl2 Fe2Al5(small amount), the internal layer becomes thicker and consists of FeAl+spherical FeAl2, and the spheroidized FeAl2 phase in the internal layer and its existing in FeAl phase steadily improve the oxidation resistance of the coating.