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Analyse du comportement et des caractéristiques des arcs thermiques soumis à des champs externes et internes

Abstract : This PhD thesis aims at understanding and analysing the behaviour of plasma arcs and their interactions with magnetic and dynamic field. The various methods that we have developed and the different case studies correspond to direct applications of thermal plasmas in industrial processes. The study of the arc’s dynamic and its characteristics is at the heart of every plasma technology upgrade or development. In the wake of the energy transition, plasma systems turn out to be very promising for many reasons, the most important of which are: They are ecological given the fact that make use of clean energy (electricity). Technologically, they allow for a tremendous rise in temperature that exceeds by far the temperature that can be reached in conventional combustion processes.We propose two different approaches to deal with the mathematical model that correspond to thermal plasmas: An analytical and a numerical approach. The analytical approaches encompass multiple computation methods that are relatively easy to implement and very practical for basic design. They constitute an extension of various analytical methods already broached in the 60’s and 70’s by American and Soviet researchers, but later abandoned in favor of numerical modelling with the advent of advanced computational machines.One section is dedicated to the study of a fixed spots AC or DC arc exposed to cross fields (magnetic or dynamic). Stability criteria employing dimensionless numbers have been established. At high currents, radiation also plays a key role in stabilisation. The analytical results are compared with the results of numerical simulations. A good agreement is observed.Another part deals with the study of a plasma arc, moving between 2 parallel electrodes under the effect of an external or electrode-induced magnetic field. The properties of the arc's dynamic strongly depend on the arc radius. The latter is obtained from an analytical 2D resolution of the heat equation. The results are validated by comparison with previous analytical and numerical works.The radiative exchange is also addressed in this thesis. Given the fact that radiation is hard to implement even in a numerical setting because of its dependence on numerous variables (specter frequency, temperature, pressure, geometry, gas mixture and species, etc.), the isothermal sphere approximate method is commonly used. An algorithm, whose aim is to seek the best value of “Rs”, is built based on a comparison between approximate and exact calculation for a wall stabilised arc of H2 at .The part concerning numerical modelling presents all the numerical approached that are currently used in thermal plasma modelling. It provides the good boundary conditions for the magnetic potential , if a transport model (TADR) is employed in a steady-state case or when the magneto-quasi static (MQS) assumption is made. Two hybrid finite-volume and finite-element (FV-FE) methods are proposed in order to improve the arc modelling, in particular for AC transient cases where the MQS fails to remain valid especially when electrodes are accounted for. Flow, energy and transport equation are solved using the FV approach whereas the electromagnetic equations are solved by means of the FE method. Comparisons with benchmark cases are done and a very good agreement is observed.Other numerical methods used for the numerical simulation of large scale industrial plasma reactors are also presented. A separation between the different physical phenomena occurring at small and large scales, is made. The electromagnetic phenomena are analytically modelled and averaged for an AC (mono, three or multi-phase) and are then inserted as source terms in source domains representing the arc region. As consequence, only the flow and energy equations are solved in order to obtain the most important characteristics in the reactor (velocity field, temperature distribution, etc.).
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Submitted on : Wednesday, May 27, 2020 - 8:53:16 AM
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Youssef Abdo. Analyse du comportement et des caractéristiques des arcs thermiques soumis à des champs externes et internes. Thermique [physics.class-ph]. Université Paris sciences et lettres, 2018. Français. ⟨NNT : 2018PSLEM040⟩. ⟨tel-02612506v2⟩



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