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- Numerical investigation of AC arc ignition on cold electrodes in atmospheric-pressure argonPublication . Santos, D. F. N.; Lisnyak, M; Almeida, N.; Benilova, L. G.; Benilov, M. S.Since experiments cannot clarify the mechanism of current transfer to non-thermionic arc cathodes, this can only be done by means of numerical modelling based on first principles and not relying on a priori assumptions. In this work, the first quarter-period after the ignition of an AC arc on cold electrodes in atmospheric-pressure argon is investigated by means of unified one-dimensional modelling, where the conservation and transport equations for all plasma species, the electron and heavy-particle energy equations, and the Poisson equation are solved in the whole interelectrode gap up to the electrode surfaces. Results are compared with those for DC discharges and analysed with the aim to clarify the role of different mechanisms of current transfer to non-thermionic arc cathodes. It is found that the glow-to-arc transition in the AC case occurs in a way substantially different from the quasi-stationary glow-to-arc transition. The dominant mechanisms of current transfer to the cathode during the AC arc ignition on cold electrodes are, subsequently, the displacement current, the ion current, and thermionic emission current. No indications of explosive emission are found. Electron emission from the impact of excited atoms can hardly be a dominant mechanism either. The introduction of the so-called field enhancement factor, which is used for description of field electron emission from cold cathodes in a vacuum, leads to computed cathode surface temperature values that are appreciably lower than the melting temperature of tungsten even in the quasi-stationary case. This means that pure tungsten cathodes of atmospheric-pressure argon arcs can operate without melting, in contradiction with experiments.
- Physics of the intermediate layer between a plasma and a collisionless sheath and mathematical meaning of the Bohm criterionPublication . Almeida, N. A.; Benilov, M. S.A transformation of the ion momentum equation simplifies a mathematical description of the transition layer between a quasi-neutral plasma and a collisionless sheath and clearly reveals the physics involved. Balance of forces acting on the ion fluid is delicate in the vicinity of the sonic point and weak effects come into play. For this reason, the passage of the ion fluid through the sonic point, which occurs in the transition layer, is governed not only by inertia and electrostatic force but also by space charge and ion-atom collisions and/or ionization. Occurrence of different scenarios of asymptotic matching in the plasma-sheath transition is analyzed by means of simple mathematical examples, asymptotic estimates, and numerical calculations. In the case of a collisionless sheath, the ion speed distribution plotted on the logarithmic scale reveals a plateau in the intermediate region between the sheath and the presheath. The value corresponding to this plateau has the meaning of speed with which ions leave the presheath and enter the sheath; the Bohm speed. The plateau is pronounced reasonably well provided that the ratio of the Debye length to the ion mean free path is of the order of 10 3 or smaller. There is no such plateau if the sheath is collisional and hence no sense in talking of a speed with which ions enter the sheath.
- Transition from a fully ionized plasma to an absorbing surfacePublication . Almeida, N. A.; Benilov, M. S.; Franklin, R. N.; Naidis, G. V.The ionization layer (presheath) separating a fully ionized low-temperature thermal plasma from the space-charge sheath adjacent to a solid surface is described by means of a (multi)fluid model. The character of the solution is governed by α, the ratio of the ionization length to the mean free path for ion–atom collisions. If α 1, the solution is determined by physically transparent boundary conditions, namely, by the Bohm criterion at the sheath edge and the condition of full ionization on the plasma side of the ionization layer. If α<1, the latter condition becomes ineffective. An alternative boundary condition is found for a certain range of α below unity, αcr α<1. An approximate approach which spans the whole range of α is suggested. While being incomplete theoretically, this approach is sufficient for practical purposes and gives results that are in agreement with experiment. On the other hand, the question of what is the lacking boundary condition in the range 0 <α<α cr remains open and challenging.
- Novel non-equilibrium modelling of a DC electric arc in argonPublication . Baeva, M.; Benilov, M. S.; Almeida, N. A.; Uhrlandt, D.A novel non-equilibrium model has been developed to describe the interplay of heat and mass transfer and electric and magnetic fields in a DC electric arc. A complete diffusion treatment of particle fluxes, a generalized form of Ohm’s law, and numerical matching of the arc plasma with the space-charge sheaths adjacent to the electrodes are applied to analyze in detail the plasma parameters and the phenomena occurring in the plasma column and the near-electrode regions of a DC arc generated in atmospheric pressure argon for current levels from 20 A up to 200 A. Results comprising electric field and potential, current density, heating of the electrodes, and effects of thermal and chemical non-equilibrium are presented and discussed. The current–voltage characteristic obtained is in fair agreement with known experimental data. It indicates a minimum for arc current of about 80 A. For all current levels, a field reversal in front of the anode accompanied by a voltage drop of (0.7–2.6) V is observed. Another field reversal is observed near the cathode for arc currents below 80 A.
- Near-cathode plasma layer on CuCr contacts of vacuum arcsPublication . Almeida, Nelson A.; Benilov, Mikhail S.; Benilova, Larissa G.; Hartmann, Werner; Wenzel, NorbertA model of near-cathode layers in vacuum arcs is developed. The model relies on a numerical solution of the problem of near-cathode space-charge sheath with ionization of atoms emitted by the cathode surface, and allows the selfconsistent determination of all parameters of the near-cathode layer, including the ion backflow coefficient. The dependence of the density of energy flux from the plasma to the cathode surface on the local surface temperature is nonmonotonic with a maximum, a feature that plays an important role in the physics of plasma–cathode interaction. The developed model may be used for a variety of purposes, including as a module of complex nonstationary multidimensional numerical models of plasma– cathode interaction in vacuum arcs. As a simple example, an analytical evaluation of parameters of stationary spots on copper and chromium is given. In the case of composite CuCr contacts with large grains, spots with current of several tens of amperes burning on the copper matrix coexist with spots with currents of the order of 1 A burning on the chromium grains.
- Modelização das camadas junto aos eléctrodos em descargas de arco de alta pressãoPublication . Almeida, Nelson Alexandre Pereira da RochaNesta dissertação foi realizada a modelização numérica das camadas de não-equilíbrio junto aos cátodos e ânodos em descargas de arco de alta pressão. Ao contrário de trabalhos anteriores, a modelização é uni cada, i.e., baseia-se num único conjunto de equações escritas sem suposições simpli cativas tais como equilíbrio térmico, equilíbrio de ionização (Saha) e quaseneutralidade, e não utiliza uma divisão a priori do domínio de cálculo em sub- camadas com propriedades físicas diferentes. Processos de transporte de iões e electrões são descritos no âmbito do modelo de difusão. Foi também desenvolvida uma teoria da camada de ionização junto ao cátodo no âmbito do modelo multi uido. Os resultados obtidos são comparados com dados experimentais. Os resultados da tese elucidam a física de transferência de corrente através das camadas junto aos eléctrodos em descargas de arco de alta pressão, e servem para melhor compreen- der os fenómenos que ocorrem em vários dispositivos industriais, como as lâmpadas de descarga de muito alta pressão e as tochas de plasma.
- Computing anode heating voltage in high-pressure arc discharges and modelling rod electrodes in dc and ac regimesPublication . Almeida, N. A.; Cunha, M. D.; Benilov, M. S.Numerical modelling of near-anode layers in arc discharges in several gases (Ar, Xe and Hg) is performed in a wide range of current densities, anode surface temperatures, and plasma pressures. It is shown that the density of energy flux to the anode is only weakly affected by the anode surface temperature and varies linearly with the current density. This allows one to interpret the results in terms of anode heating voltage (volt equivalent of the heat flux to the anode). The computed data may be useful in different ways. An example considered in this work concerns the evaluation of thermal regime of anodes in the shape of a thin rod operating in the diffuse mode. Invoking the model of nonlinear surface heating for cathodes, one obtains a simple and free of empirical parameters model of thin rod electrodes applicable to dc and ac high-pressure arcs provided that no anode spots are present. The model is applied to a variety of experiments reported in the literature and a good agreement with the experimental data found.
- Unified modelling of near-cathode plasma layers in high-pressure arc dischargesPublication . Almeida, N. A.; Benilov, M. S.; Naidis, G. V.A model of a near-cathode region in high-pressure arc discharges is developed in the framework of the hydrodynamic (diffusion) approximation. Governing equations are solved numerically in 1D without any further simplifications, in particular, without explicitly dividing the near-cathode region into a space-charge sheath and a quasi-neutral plasma. Results of numerical simulation are reported for a very high-pressure mercury arc and an atmospheric-pressure argon arc. Physical mechanisms dominating different sections of the near-cathode region are identified. It is shown that the near-cathode space-charge sheath is of primary importance under conditions of practical interest. Physical bases of simplified models of the near-cathode region in high-pressure arc discharges are analysed. A comparison of results given by the present model with those given by a simplified model has revealed qualitative agreement; the agreement is not only qualitative but also quantitative in the case of an atmospheric-pressure argon plasma at moderate values of the near-cathode voltage drop. The modelling data are compared with results of spectroscopic measurements of the electron temperature and density in the near-cathode region.
- Investigating near-anode plasma layers of very high-pressure arc dischargesPublication . Almeida, N. A.; Benilov, M. S.; Hechtfischer, U.; Naidis, G. V.Numerical and experimental investigation of near-anode layers of very high-pressure arcs in mercury and xenon is reported. The simulation is performed by means of a recently developed numerical model in which the whole of a near-electrode layer is simulated in the framework of a single set of equations without simplifying assumptions such as thermal equilibrium, ionization equilibrium and quasi-neutrality and which was used previously for a simulation of the near-cathode plasma layers. The simulation results support the general understanding of similarities and differences between plasma–cathode and plasma–anode interaction in high-pressure arc discharges established in preceding works. In particular, the anode power input is governed primarily by, and is approximately proportional to, the arc current. In the experiment, the spectral radiance from the electrodes and the near-electrode regions in xenon arcs was recorded. The derived total anode power input and near-anode plasma radiance distribution agree reasonably well with the simulation results.
- Account of near-cathode sheath in numerical models of high-pressure arc dischargesPublication . Benilov, M. S.; Almeida, N. A.; Baeva, M.; Cunha, M. D.; Benilova, L. G.; Uhrlandt, D.Three approaches to describing the separation of charges in near-cathode regions of highpressure arc discharges are compared. The first approach employs a single set of equations, including the Poisson equation, in the whole interelectrode gap. The second approach employs a fully non-equilibrium description of the quasi-neutral bulk plasma, complemented with a newly developed description of the space-charge sheaths. The third, and the simplest, approach exploits the fact that significant power is deposited by the arc power supply into the near-cathode plasma layer, which allows one to simulate the plasma–cathode interaction to the first approximation independently of processes in the bulk plasma. It is found that results given by the different models are generally in good agreement, and in some cases the agreement is even surprisingly good. It follows that the predicted integral characteristics of the plasma–cathode interaction are not strongly affected by details of the model provided that the basic physics is right.