Dissertations / Theses on the topic 'Plasma arc melting'

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 1999
The simulation of Plasma Arc Cutting is presented in this study. The plasma arc cutting process employs a plasma torch with a very narrow bore to produce a transferred arc to the workpiece. A technique for modelling plasma arc cutting has been developed by applying the thermo-metallurgical model to the process and integrating a model of material removal to this model. The model is solved using the finite element method using the FE package SYSWORLD, more specifically SYSWELD. The objective is to determine the minimum energy required to cut a plate of some thickness using this virtual model. The characteristics of the cut need to exhibit the characteristics of a "high quality cut". The model presented can predict the kerf size given certain process variable settings. The numerical results obtained are assessed by conducting experiments. By maintaining Ill1rumum energy input cost savings can be made through energy savings, limiting additional finishing processes and reducing expense of shortening the electrode and nozzle lifetimes. The modelling of the PAC process using virtual design techniques provides a cost-effective solution to the manufacturing industries with respect to process specification development. This plays an important role in South Africa's transition into a competitive global market. It is envisaged that the model will provide an alternative more efficient, non-destructive means of determining the optimum process variable settings for the plasma arc cutting process.

The thermal plasma production of fumed silica in a transferred arc consists of the decomposition of quartz to SiO (g) and oxygen followed by an oxidizing quench back to SiO2. The particles formed have diameters of the order of 10 to 20 nm and are linked in a three dimensional branched chain aggregate. Previous work by Addona and Munz (1999) demonstrated the technical feasibility of producing fumed silica using this method, but was unable to demonstrate the special rheological properties of the powder. The most important characteristic of fumed silica is the presence of hydroxyls on the surface of the particles, in the form of isolated hydroxyl groups, hydrogen-bonded hydroxyl groups and siloxane groups.
In the present work, we studied the changes in the powder quality by varying the quench conditions used for the production of the powder and by agglomerating the obtained particles. The fumed silica was agglomerated by conveying in a length of tubing with sharp bends. The powder was characterized using BET, Viscosity tests, FT-IR, TEM, SEM and XRD. The product was compared to both a commercial product (Aerosil 200) and the material previously produced by Addona. Tests were done before and after the agglomeration experiments.
The experimental results showed that the agglomeration had no effect on the powder's rheological properties. We concluded that the smaller viscosity values obtained for the plasma produced fumed silica were due to the lack of the free hydroxyl groups from the surface of the particles.

In this thesis we mainly focus on the study of the turbulent transport of impurity particles in the plasma due to the electrostatic drift wave microinstabilities. In a fusion reactor, the helium produced as a result of the fusion process is an internal source of impurity. Moreover, impurities are released from the material surfaces surrounding the plasma by a variety of processes: by radiation from plasma, or as a result of sputtering, arcing and evaporation. Impurities in tokamak plasmas introduce a variety of problems. The most immediate effect is the radiated power loss (radiative cooling). Another effect is that the impurity ions produce many electrons and in view of the operating limits on density and pressure, this has the effect of replacing fuel ions. For example, at a given electron density, $n_{e}$, each fully ionized carbon ion (used in the wall materials in the form of graphite) replaces six fuel ions, so that a 7\\% concentration of fully ionized carbon in the plasma core, would reduce the fusion power to one half of the value in a pure plasma. Therefore, for all tokamaks it become an immediate and continuing task to reduce impurities to acceptably low concentrations. However, the presence of impurities, with control, can be beneficial for the plasma performance and reduction of strong plasma heat loads on the plasma facing walls. The radiative cooling effect which was mentioned above can be used at the edge of the plasma in order to distribute the plasma heat more evenly on the whole surface of the vessel walls and therefore, reduce significantly plasma heat bursts on the small regions on the divertor or limiter tiles. The experiments at TEXTOR show that the presence of the impurities at the plasma edge can also improve the performance and reduce the turbulent transport across the magnetic field lines. The observed behavior was explained trough the proposed mechanism of suppression of the most important plasma drift wave microinstability in this region, namely, the Ion Temperature Gradient mode (ITG mode) by the impurities. The impurity's positive impact on the plasma performance offered a possibility to better harness the fusion power, however, it is vital for a fusion reactor to have feedback controls in order to keep impurities at the plasma edge and limit their accumulation in the plasma core where the fusion reactions are happening. In order to have control over the impurity transport we first need to understand different mechanisms responsible for its transport.

One of the least understood areas of the impurity transport and indeed any plasma particle or heat transport in general, is the turbulent transport. Extensive efforts of the fusion plasma community are focused on the subject of turbulent transport. Motivated by the fact that impurity transport is an important issue for the whole community and it is an area which needs fundamental research, we focused our attention on the development of turbulent transport models for impurities and their examination against experiments. In a collaboration effort together with colleagues (theoreticians as well as experimentalist) from different research institutes, we tried to find, through our models, physical mechanisms responsible for experimental observations. Although our main focus in this thesis has been on the impurity transport, we also tried a fresh challenge, and started looking at the problem of drift wave turbulent transport in a different framework all together. Experimental observation of the edge turbulence in the fusion devices show that in the Scrape of Layer (SOL: the layer between last closed magnetic surface and machine walls) plasma is characterized with non-Gaussian statistics and non-Maxwellian Probability Distribution Function (PDF). It has been recognized that the nature of cross-field transport trough the SOL is dominated by turbulence with a significant ballistic or non-local component and it is not simply a diffusive process. There are studies of the SOL turbulent transport using the 2-D fluid descriptions or based on probabilistic models using the Levy statistics (fractional derivatives in space). However, these models are base on the fluid assumptions which is in contradiction with the non-Maxwellian plasmas observed. Therefore, we tried to make a more fundamental study by looking at the effect of the non-Maxwellian plasma on the turbulent transport using a gyro-kinetic formalism. We considered the application of fractional kinetics to plasma physics. This approach, classical indeed, is new in its application. Our aim was to study the effects of a non-Gaussian statistics on the characteristic of the drift waves in fusion plasmas.

Ce travail de thèse porte sur le transport turbulent d'impuretés dans les plasmas de fusion

par confinement magnétique. C'est une question de la plus haute importance pour le développement

de la fusion comme source d'énergie. En effet, une accumulation d'impuretés au coeur

du plasma impliquerait des pertes d'énergie par radiation, conduisant par refroidissement à

l'extinction des réactions de fusion. Il est par contre prévu d'injecter des impuretés dans le

bord du plasma, afin d'extraire la chaleur par rayonnement sans endommager les éléments de

la première paroi. Ces contraintes contradictoires nécessitent un contrôle précis du transport

d'impuretés, afin de minimiser la concentration d'impuretés au coeur du plasma tout en la

maximisant au bord. Une très bonne connaissance de la physique sous-jacente au transport

est donc indispensable. L'effet de la turbulence, principal mécanisme de transport, sur les impuretés

est alors une question centrale. Dans cette thèse, un code numérique, AFC-FL, a été développé sur la base d'une approche ``fluide' linéaire pour la turbulence d'ondes de dérive. Il calcule les taux de croissance qui caractérisent la rapidité de l'amorçage des instabilités. L'analyse de stabilité est complétée par l'évaluation des taux de croissance en présence d'un gradient de densité, un cisaillement magnétique ou un nombre arbitraire de différentes espèces d'impureté. Les formules complètes du flux turbulent d'impuretés pour ces taux de croissance calculés des instabilités des ondes de dérive ont été dérivées. Un modèle de transport anormal qui nous permet d'étudier la dépendence du transport en fonction de la charge d'impureté a été développé. Ce modèle prend en compte les effets collisionnels entre les ions, l'impureté et les particules principales de plasma. Une telle dépendence du transport anormal en fonction de la charge de l'impureté est observée dans les expériences et il a été montré que les résultats obtenus sont en bon accord avec les observations expérimentales. Nous avons également étudié l'effet des impuretés sur le confinement de l'énergie dans les plasmas du tokamak JET. La modélisation de transport a été exécutée pour des plasmas avec injection de néon dans la périphérie du tokamak. Cette technique est utilisée afin d'extraire la chaleur par rayonnement sans endommager la paroi et pour réduire certaines instabilités (ELM). Des simulations du code RITM ont été comparées à des mesures effectuées lors d'expériences au JET. Il a été montré que l'injection de néon mène toujours à une dégradation du confinement par rapport aux décharges sans néon. Cependant, l'augmentation de la charge effective, en raison du presence du néon peut diminuer le taux de croissance d'autres instabilité (ITG) et amèliorer le confinement du coeur du plasma. Ce confinement amélioré du coeur peut alors compenser la dégradation au bord et le confinement global du plasma peut s'améliorer.

Doctorat en sciences, Spécialisation physique
info:eu-repo/semantics/nonPublished

博士
國立成功大學
材料科學(工程)研究所
84
The purpose of this study is to overlay an alloy with high wear and corrosion resistant on SG cast irons in order to widen its application. Stellite 1,6 and 21 alloys were overlaid on SG cast irons with various carbon and silicon contents by plasma trans- ferred arc process. Furthermore, tearing property of the speci- mens after overlaying were also invertigated. Results revealed that the variation of the carbon and silicon contents has no effect on the solidification structure of the overlayers with low overlaying current. The solidification structure of the Stellite 1 overlayer is hypereutectic while the Stellite 6and 21 are den- dritic. An interfacial layer which contain large amount of car- bides formed at the interface region between three overlayers and high carbon content substrate. The occurrence of interfacial layer is detrimental to the bonding strength of overlaid speci- mens. The formation of interfacial layer cannot be inhibited by increasing silicon content of substrate. On the other hand, the amount of interfacial layer decreases with decreasing carbon con- tent of the substrate. Tearing strength increased as the amount of the interfacial layer decreased. The overlayers of the high overlaying current specimens are iron rich, this indicated that the overlayers were highly diluted by the substrate, and the tearing strength of these specimens is low. Tearing strength in- creased when the carbon content of substrate decreased.

Іванов Ю. О. Розробка технології виплавки високоякісних сталей за допомогою процесів плазмено-дугового відновлення : кваліфікаційна робота магістра спеціальності 136 «Металургія» / наук. керівник Ю. В. Мосейко. Запоріжжя : ЗНУ, 2020. 65 с.
UA : Розглянуто способи виробництва конструкційних сталей, проведено аналіз існуючих методів виплавки високоякісних сталей, фізико-хімічні основи виплавки конструкційної високоякісної сталі, конструкція основного пічного устаткування. Проаналізовано вплив розташування плазмотрона на кінцевий вміст азоту у металі.
EN : Methods of production of structural steels, review of existing methods of smelting of high-quality steels, physical and chemical bases of smelting of structural high-quality steel, design of the main furnace equipment are considered. The influence of the plasmatron location on the final nitrogen content in the metal is analyzed.