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- Modelling self-organization on electrodes of DC glow dischargesPublication . Bieniek, Matthew Simon; Benilov, Mikhail; Almeida, Pedro Jorge Gomes Camacho deO trabalho apresentado nesta dissertaªo refere-se modelaªo de padrıes de autoorganizaªo de densidade de corrente em elØtrodos de descargas DC luminescentes. Padrıes de manchas andicas foram modelados de forma auto-consistente pela primeira vez e os fenmenos nas manchas foram investigados. As soluıes que descrevem as manchas foram encontradas num intervalo de correntes com mœltiplas soluıes. Foi descoberta uma inversªo da densidade de corrente local do nodo no centro de cada uma das manchas, isto Ø, mini-cÆtodos sªo formados dentro das manchas; poder-se-ia dizer, as manchas do nodo funcionam como uma descarga luminescente unipolar. As soluıes nªo se enquadram no padrªo convencional de auto-organizaªo em sistemas dissipativos nªo-lineares biestÆveis; por exemplo, as transiıes de um modo para outro nªo se realizam atravØs de bifurcaıes. Padrıes auto-organizados de manchas catdicas em descargas luminescentes foram modelados na camada de plasma junto ao cÆtodo, numa geometria igual utilizada na maioria das experiŒncias descritas na literatura. O efeito da geometria da cmara de descarga nas manchas foi investigado. Os padrıes de manchas modelados sªo idŒnticos aos observados nas experiŒncias e similares aos calculados na conguraªo de elØtrodos planos em paralelo. Uma tentativa foi feita para modelar quantitativamente a descarga DC luminescente com manchas catdicas. Uma descriªo detalhada desta modelaªo, a mais precisa deste fenmeno atØ data, Ø apresentada. Em geral, os padrıes calculados sªo semelhantes aos observados nas experiŒncias, mas as CVC sªo qualitativamente diferentes. CenÆrios de transiıes entre modos com diferentes padrıes de manchas em elØtrodos de descargas DC luminescentes e em cÆtodos de descargas de arco sªo investigados. No caso de transiıes entre padrıes em cÆtodos de descargas DC luminescentes, foram encontradas as transiıes observadas nas experiŒncias que podem estar diretamente relacionados a bifurcaıes de soluıes estacionÆrias, e as bifurcaıes correspondentes foram modeladas. Os padrıes encontrados na modelaªo numØrica estªo em conformidade com os observados no decurso das transiıes nas experiŒncias. No caso dos cÆtodos de descargas de arco, mostra-se que qualquer transiªo entre diferentes modos de transferŒncia de corrente estÆ relacionada a uma bifurcaªo de soluıes estacionÆrias.
- Self-consistent modeling of self-organized patterns of spots on anodes of DC glow dischargesPublication . Bieniek, M. S.; Almeida, P. G. C.; Benilov, M. S.Abstract Self-organized patterns of spots on a flat metallic anode in a cylindrical glow discharge tube are simulated. A standard model of glow discharges is used, comprising conservation and transport equations for a single species of ion and electrons, written with the use of the drift-diffusion and local-field approximations, and the Poisson equation. Only processes in the near-anode region are considered and the computation domain is the region between the anode and the discharge column. Multiple solutions, existing in the same range of discharge current and describing modes with and without anode spots, are computed for the first time. A reversal of the local anode current density in the spots was found, i.e. mini-cathodes are formed inside the spots or, as one could say, anode spots operate as a unipolar glow discharge. The solutions do not fit into the conventional pattern of self-organization in bistable nonlinear dissipative systems; In particular, the modes are not joined by bifurcations.
- Plasma-enhanced catalysis for the upgrading of methane: a review of modelling and simulation methodsPublication . Maitre, Pierre-André; Bieniek, Matthew S.; Kechagiopoulos, Panagiotis N.The direct upgrading of methane to higher hydrocarbons via plasma-catalysis has received particular attention due to the high valorisation potential of the molecule and methane's wide availability in both fossil and sustainable sources. Extensive experimental work has focused on the study of the performance of plasma-catalytic systems. However, it is recognised that most plasma-surface interactions are still not fully understood, and that the combination of experimental, theoretical and modelling studies is essential to gain a fundamental mechanistic insight, leading to advancements in the field. The current review presents the state-of-the-art of simulation methodologies for plasma-phase kinetics, plasma fluid modelling and plasma-catalyst interactions as these have been applied for methane plasmas. Focus is placed on the species that should be considered during modelling, ranging from stable molecules and radicals to ions and excited states. The types of plasma-chemical processes these species participate in are discussed and the methods for calculating their rates are presented. Modelling approaches spanning from spatially homogeneous and computationally inexpensive zero-dimensional codes to more complex fluid modelling approaches are covered. First principles calculations concerning plasma relevant species interactions with metal surfaces are finally summarised.
- Modeling the thermalization of electrons in conditions relevant to atmospheric pressure He-O2 nanosecond pulsed dischargesPublication . Bieniek, M. S.; Walsh, J. L.; Hasan, M. I.The electron thermalization process is significant in nanosecond pulsed discharges due to the applied voltage pulse’s short duration and rapid rise and fall times. In this contribution a comparison was made between two approaches to modeling the electron kinetics of electron thermalization in atmospheric pressure helium plasma with an oxygen admixture. Modeling based on the direct solution of the local time-dependent electron Boltzmann equation was compared with modeling based on the commonly used but less general local mean energy approximation. For modeling based on the local time-dependent electron Boltzmann equation, a temporary faster decay in the population of electrons in the high energy tail, and a slower decay in the population of intermediate energy electrons was observed while the electron swarm cooled from an average energy of above 8 eV, without an electric field present. During that period, the electron impact reaction rate coefficients of helium direct ionization and electronic excitation decreased by more than 3 orders of magnitude as compared to the modeling based on the local mean energy approximation. Global modeling of the evolution of plasma species densities in response to an electric field typical of atmospheric pressure pulsed discharges was performed with the two approaches to electron kinetics. Differences in the species densities were observed between the two approaches, with an 100% increase in the maximum density of electrons found with the modeling based on the local mean energy approximation.
- Modelling cathode spots in glow discharges in the cathode boundary layer geometryPublication . Bieniek, M. S.; Almeida, P. G. C.; Benilov, M. S.Self-organized patterns of cathode spots in glow discharges are computed in the cathode boundary layer geometry, which is the one employed in most of the experiments reported in the literature. The model comprises conservation and transport equations of electrons and a single ion species, written in the drift-diffusion and local-field approximations, and Poisson’s equation. Multiple solutions existing for the same value of the discharge current and describing modes with different configurations of cathode spots are computed by means of a stationary solver. The computed solutions are compared to their counterparts for plane-parallel electrodes, and experiments. All of the computed spot patterns have been observed in the experiment.
- Bifurcations in the theory of current transfer to cathodes of dc discharges and observations of transitions between different modesPublication . Bieniek, M. S.; Santos, D.; Almeida, P. G. C.; Benilov, M. S.General scenarios of transitions between different spot patterns on electrodes of dc gas discharges and their relation to bifurcations of steady-state solutions are analyzed. In the case of cathodes of arc discharges, it is shown that any transition between different modes of current transfer is related to a bifurcation of steady-state solutions. In particular, transitions between diffuse and spot modes on axially symmetric cathodes, frequently observed in the experiment, represent an indication of the presence of pitchfork or fold bifurcations of steady-state solutions. Experimental observations of transitions on cathodes of dc glow microdischarges are analyzed and those potentially related to bifurcations of steady-state solutions are identified. The relevant bifurcations are investigated numerically and the computed patterns are found to conform to those observed in the course of the corresponding transitions in the experiment.