Chengkang Wu

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 times its diameter could be generated with a DC arc 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 arc channel geometry, gas flow rate, 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 arc jet.

What do we know about long laminar plasma jets

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

Application of similarity theory to the characterization of non-transferred laminar plasma jet generation

Laminar-flow non-transferred DC plasma jets were generated by a torch with an inter-electrode insert by which the arc column was limited to a length of about 20 mm. Current–voltage characteristics, thermal efficiency and jet length, a parameter which changes greatly with the generating parameters in contrast with the almost unchangeable jet length of the turbulent plasma, were investigated systematically, by using the similarity theory combined with the corresponding experimental examination. Formulae in non-dimensional forms were derived for predicting the characteristics of the laminar plasma jet generation, within the parameter ranges where no transfer to turbulent flow occurs. Mean arc temperature in the torch channel and mean jet-flow temperature at the torch exit were obtained, and the results indicate that the thermal conductivity feature of the working gas seems to be an important factor affecting thermal efficiency of laminar plasma generation.

Fluctuation characteristics of arc voltage and jet flow in a non-transferred dc plasma generated at reduced pressure

A torch with a set of inter-electrode inserts between the cathode and the anode/nozzle with a wide nozzle exit was designed to generate plasma jets at chamber pressures of 500?10?000?Pa. The variation of the arc voltage was examined with the change in working parameters such as gas flow rate and chamber pressure. The fluctuation in the arc voltage was recorded with an oscilloscope, and the plasma jet fluctuation near the torch exit was observed with a high-speed video camera and detected with a double-electrostatic probe. Results show that the 300?Hz wave originated from the tri-phase rectified power supply was always detected under all generating conditions. Helmholtz oscillations over 3000?Hz was detected superposed on the 300?Hz wave at gas flow rates higher than 8.8?slm with a peak to valley amplitude lower than 5% of the average voltage value. No appreciable voltage fluctuation caused by the irregular arc root movement is detected, and mechanisms for the arc voltage and jet flow fluctuations are discussed.

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.

Arcjet Thruster Operated With Different Propellants

As a simple and reliable propulsion system, arcjet thrusters have been used in multiple satellite missions. The performance of an arcjet thruster is controlled by the characteristics of the arc-discharge behavior and the arc-electrode interactions to a large extent. The nozzle of an arcjet thruster is where the input electric energy is converted into kinetic energy. The nozzle images of an arcjet thruster operated with different propellants are presented in this paper.

Energy fluctuations in a direct current plasma torch with inter-electrode inserts operated at reduced pressure

Direct current (dc) plasma torch with inter-electrode inserts has the merits of fixed arc length, relative high enthalpy and may show advantages in future plasma processes where stability and controllability are must-have. Energy fluctuations in the plasma may result from power supply ripple, arc length variation, and/or acoustic oscillation. Using an improved power supply with a flat waveform, the characteristics of an argon plasma energy instabilities under reduced pressure were studied by means of simultaneously monitoring the arc voltage and arc current spectrum. Dependence of the arc fluctuation behavior on the plasma generating parameters, such as the current intensity, the plasma gas flow rates and the vacuum chamber pressure were investigated and discussed. Results show that the plasma torch has a typical U-shaped voltage-ampere characteristic (VAC). The correlation between the VAC and the probability of energy distributions was studied. Through pressure measurements at the cathode cavity and the vacuum chamber, the existence of sonic flow in the inter-electrode insert channel was confirmed.

Supersonic Plasma Jets of Different Gases in Low-Pressure Environment

A low-power dc arc plasma generator with regeneratively cooled or natural-radiation-cooled anode is used to produce supersonic plasma jets of pure helium, argon, hydrogen, nitrogen, hydrogen-nitrogen mixture, and helium-nitrogen mixture in a vacuum chamber. Plasma plume images of different characteristics were observed, and are discussed briefly along with the measurement and analysis results.

Images of Arc-Heated Supersonic Argon Plasma Jet Impinging on a Flat Plate

Low-density flow fields are usually difficult to visualize using ordinary optical methods. However, plasma flows, having high temperature or excited species in them, may show up some features of the flow field in a simple photograph. A dc arc-heated supersonic argon plasma jet impinging on a flat plate under low ambient pressure was photographed. The varying brightness shows features of the flow field which bear some resemblance to the results by numerical simulation of a cold argon jet impinging on a flat plate.

Underexpanded Supersonic Plasma Jet Generated by a Small Arc-Heated Facility

As a flexible and efficient heat source to generate high enthalpy flows, a small arc-heated facility is unique in its ability to reproduce local aerodynamic heating environments for a long time period, to be used for validating the survivability of thermal protection materials used in hypersonic flights. Images of an underexpanded supersonic plasma jet generated by a small arc-heated facility and its interaction with some typical material samples are presented.