Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

International audienceThis paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and ε r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300 to 600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when ε r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease of the tube radius and ε r. Then, we have compared the relative influence of the value of tube radius and ε r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge

Simulation numérique des décharges nanosecondes répétitives pulsées dans l'air sous pression atmosphérique : Application à la combustion assistée par plasma

In this Ph.D. thesis, we have carried out numerical simulations to study nanosecond repetitively pulsed discharges (NRPD) in a point-to-point geometry at atmospheric pressure in air and in H2-air mixtures. Experimentally, three discharge regimes have been observed for NRPD in air at atmospheric pressure for the temperature range Tg = 300 to 1000 K: corona, glow and spark. To study these regimes, first, we have considered a discharge occurring during one of the nanosecond voltage pulses. We have shown that a key parameter for the transition between the discharge regimes is the ratio between the connection-time of positive and negative discharges initiated at point electrodes and the pulse duration. In a second step, we have studied the dynamics of charged species during the interpulse at Tg = 300 and 1000 K and we have shown that the discharge characteristics during a given voltage pulse remain rather close whatever the preionization level (in the range 109-1011 cm��3) left by previous discharges. Then, we have simulated several consecutive nanosecond voltage pulses at Tg = 1000 K at a repetition frequency of 10 kHz. We have shown that in a few voltage pulses, the discharge reaches a stable quasi-periodic glow regime observed in the experiments. We have studied the nanosecond spark discharge regime. We have shown that the fraction of the discharge energy going to fast heating is in the range 20%- 30%. Due to this fast heating, we have observed the propagation of a cylindrical shockwave followed by the formation of a hot channel in the path of the discharge that expands radially on short timescales (t < 1 _s), as observed in experiments. Then we have taken into account an external circuit model to limit the current and then, we have simulated several consecutive pulses to study the transition from multipulse nanosecond glow to spark discharges. Finally the results of this Ph.D. have been used to find conditions to obtain a stable glow regime in air at 300 K and atmospheric pressure. Second we have studied on short time-scales (t_ 100_s) the ignition by a nanosecond spark discharge of a lean H2-air mixture at 1000 K and atmospheric pressure with an equivalence ratio of _ = 0:3. We have compared the relative importance for ignition of the fast-heating of the discharge and of the production of atomic oxygen. We have shown that the ignition with atomic oxygen seems to be slightly more efficient and has a completely different dynamics.Dans cette thèse, nous avons étudié des décharges nanosecondes répétitives pulsées dans une géométrie pointe-pointe à la pression atmosphérique dans l’air et dans des mélanges hydrogène-air. Expérimentalement, trois régimes de décharges ont été observés dans l’air à pression atmosphérique entre 300 et 1000 K : couronne, diffus et arc. Pour étudier ces différents régimes, nous avons tout d’abord simulé une décharge ayant lieu pendant un des pulses de tension nanosecondes. Nous avons montré qu’un paramètre clé pour la transition entre les régimes est le rapport entre le temps de connexion entre les décharges positives et négatives initiées aux pointes et la durée du pulse de tension. Dans une seconde étape, nous avons étudié la dynamique des espèces chargées entre les pulses de tension à 300 et 1000 K et nous avons montré que les caractéristiques de la décharge pendant un pulse de tension dépendaient très peu du niveau de préionisation (dans la gamme 109-1011 cm��3) laissé par les décharges précédentes. Nous avons ensuite simulé plusieurs pulses de tensions consécutifs à Tg=1000 K à une fréquence de 10 kHz. Nous avons montré que, en quelques pulses de tension, la décharge atteint un régime diffus "stable", observé dans les expériences. Nous avons ensuite étudié le régime de décharge de type arc nanoseconde. Nous avons montré que la fraction d’énergie de la décharge allant dans le chauffage rapide de l’air est de 20-30 %. A cause de ce chauffage rapide, nous avons observé la propagation d’une onde de choc cylindrique suivie par la formation d’un canal chaud, sur le passage initial de la décharge, qui se dilate radialement sur des temps courts (t 6 1 _s), comme observé dans les expériences. Ensuite nous avons pris en compte un modèle de circuit externe pour limiter le courant et ainsi nous avons simulé plusieurs pulses consécutifs pour étudier la transition entre les régimes diffus et d’arc nanoseconde. Pour finir, les résultats de cette thèse ont été utilisés pour trouver des conditions d’obtention d’un régime diffus stable à 300 K et à la pression atmosphérique. Puis nous avons étudié l’allumage sur des temps courts (t 6 100 _s) d’un mélange pauvre H2-air par une décharge de type arc nanoseconde à 1000 K et à pression atmosphérique avec une richesse de 0.3. Nous avons comparé les importances relatives pour l’allumage du chauffage rapide et de la production d’oxygène atomique. Nous avons montré que l’allumage par l’oxygène atomique semble être légèrement plus efficace et a une dynamique complètement différente de celle initiée par le chauffage rapide

Influence of temperature on the glow regime of a discharge in air at atmospheric pressure between two point electrodes

International audienceThis paper presents simulations of the dynamics of air discharges between two point electrodes at atmospheric pressure for two different gas temperatures 300 and 1000 K. Simulation results show that in the early stages of the glow regime, two streamer discharges propagate in the gap and form after their connection a conducting channel between electrodes. In a recent experimental work on nanosecond repetitively pulsed (NRP) discharges at 1000 K between two point electrodes with an interelectrode gap distance of 5 mm, it was found that a glow regime is obtained if the average electric field in the gap is at least equal to the breakdown field. Simulation results show that for the conditions studied in the experiments, the time of connection of both discharges is close to the 10 ns duration of the voltage pulse if the average electric field in the gap in the conduction phase is equal to the breakdown field. Under these conditions, a glow regime is obtained as a conducting channel has just the time to form between electrodes during the voltage pulse and no significant gas heating may occur. At 300 K, we found that a minimal value of the Laplacian electric field of 8-9 kV cm−1 at atmospheric pressure is necessary to have a stable propagation of the positive streamer without branching in the point-to-point geometry. Then, based on simulation results, we discuss the conditions of existence of the glow regime in NRP discharges at atmospheric pressure and 300 K in a 1 cm interelectrode gap

Simulation numérique des décharges nanosecondes répétitives pulsées dans l'air sous pression atmosphérique (Application à la combustion assistée par plasma)

Dans cette thèse, nous avons étudié des décharges nanosecondes répétitives pulsées dans une géométrie pointe-pointe à la pression atmosphérique dans l air et dans des mélanges hydrogène-air. Expérimentalement, trois régimes de décharges ont été observés dans l air à pression atmosphérique entre 300 et 1000 K : couronne, diffus et arc. Pour étudier ces différents régimes, nous avons tout d abord simulé une décharge ayant lieu pendant un des pulses de tension nanosecondes. Nous avons montré qu un paramètre clé pour la transition entre les régimes est le rapport entre le temps de connexion entre les décharges positives et négatives initiées aux pointes et la durée du pulse de tension. Dans une seconde étape, nous avons étudié la dynamique des espèces chargées entre les pulses de tension à 300 et 1000 K et nous avons montré que les caractéristiques de la décharge pendant un pulse de tension dépendaient très peu du niveau de préionisation (dans la gamme 109-1011 cm 3) laissé par les décharges précédentes. Nous avons ensuite simulé plusieurs pulses de tensions consécutifs à Tg=1000 K à une fréquence de 10 kHz. Nous avons montré que, en quelques pulses de tension, la décharge atteint un régime diffus "stable", observé dans les expériences. Nous avons ensuite étudié le régime de décharge de type arc nanoseconde. Nous avons montré que la fraction d énergie de la décharge allant dans le chauffage rapide de l air est de 20-30 %. A cause de ce chauffage rapide, nous avons observé la propagation d une onde de choc cylindrique suivie par la formation d un canal chaud, sur le passage initial de la décharge, qui se dilate radialement sur des temps courts (t 6 1 _s), comme observé dans les expériences. Ensuite nous avons pris en compte un modèle de circuit externe pour limiter le courant et ainsi nous avons simulé plusieurs pulses consécutifs pour étudier la transition entre les régimes diffus et d arc nanoseconde. Pour finir, les résultats de cette thèse ont été utilisés pour trouver des conditions d obtention d un régime diffus stable à 300 K et à la pression atmosphérique. Puis nous avons étudié l allumage sur des temps courts (t 6 100 _s) d un mélange pauvre H2-air par une décharge de type arc nanoseconde à 1000 K et à pression atmosphérique avec une richesse de 0.3. Nous avons comparé les importances relatives pour l allumage du chauffage rapide et de la production d oxygène atomique. Nous avons montré que l allumage par l oxygène atomique semble être légèrement plus efficace et a une dynamique complètement différente de celle initiée par le chauffage rapide.In this Ph.D. thesis, we have carried out numerical simulations to study nanosecond repetitively pulsed discharges (NRPD) in a point-to-point geometry at atmospheric pressure in air and in H2-air mixtures. Experimentally, three discharge regimes have been observed for NRPD in air at atmospheric pressure for the temperature range Tg = 300 to 1000 K: corona, glow and spark. To study these regimes, first, we have considered a discharge occurring during one of the nanosecond voltage pulses. We have shown that a key parameter for the transition between the discharge regimes is the ratio between the connection-time of positive and negative discharges initiated at point electrodes and the pulse duration. In a second step, we have studied the dynamics of charged species during the interpulse at Tg = 300 and 1000 K and we have shown that the discharge characteristics during a given voltage pulse remain rather close whatever the preionization level (in the range 109-1011 cm 3) left by previous discharges. Then, we have simulated several consecutive nanosecond voltage pulses at Tg = 1000 K at a repetition frequency of 10 kHz. We have shown that in a few voltage pulses, the discharge reaches a stable quasi-periodic glow regime observed in the experiments. We have studied the nanosecond spark discharge regime. We have shown that the fraction of the discharge energy going to fast heating is in the range 20%- 30%. Due to this fast heating, we have observed the propagation of a cylindrical shockwave followed by the formation of a hot channel in the path of the discharge that expands radially on short timescales (t < 1 _s), as observed in experiments. Then we have taken into account an external circuit model to limit the current and then, we have simulated several consecutive pulses to study the transition from multipulse nanosecond glow to spark discharges. Finally the results of this Ph.D. have been used to find conditions to obtain a stable glow regime in air at 300 K and atmospheric pressure. Second we have studied on short time-scales (t_ 100_s) the ignition by a nanosecond spark discharge of a lean H2-air mixture at 1000 K and atmospheric pressure with an equivalence ratio of _ = 0:3. We have compared the relative importance for ignition of the fast-heating of the discharge and of the production of atomic oxygen. We have shown that the ignition with atomic oxygen seems to be slightly more efficient and has a completely different dynamics.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Simulation of the hydrodynamic expansion following a nanosecond pulsed spark discharge in air at atmospheric pressure

International audienceThis paper presents 2D simulations of the dynamics of formation of a nanosecond spark discharge between two point electrodes in air at atmospheric pressure at 300 and 1000 K, the induced air heating and the following hydrodynamic expansion. As a first step, we have considered that 30% of the discharge energy instantaneously heats the ambient air. At the end of the voltage pulse, it is shown that the energy density and the air temperature distributions are non-uniform in the interelectrode gap. Rapidly after the nanosecond voltage pulse, a cylindrical shock wave is formed and propagates with a velocity very close to the speed of sound of the surrounding ambient air. Furthermore, the rapid dilatation of the hot channel formed on the discharge path is observed for t ≤ 1 µs, as in experiments. Then we have carried out a parametric study on the influence of the value of the fraction of discharge energy going to fast heating on the hydrodynamic expansion at 1000 K, assuming an instantaneously fast gas heating. For all values in the range of 15% to 60% studied in this work, we have observed a very similar gas dynamics. Then, we have considered that the nanosecond spark discharge heats the ambient air at 1000 K with a longer relaxation time of 1 µs, and in this case we have observed the propagation of a weak pressure wave and no dilatation of the hot channel on the discharge path. Finally, the comparison with experiments seems to validate the hypothesis that the 10 ns spark discharges studied in this work at 300 and 1000 K, significantly heat the ambient air on very short time-scales

Influence of the external electrical circuit on the regimes of a nanosecond repetitively pulsed discharge in air at atmospheric pressure

International audienceThis paper presents 2D simulations of nanosecond repetitively pulsed discharges in air at atmospheric pressure coupled with a model of the external electrical circuit used in experiments. Then, during the pulsed discharge, the voltage applied to the electrodes varies in time as a function of the time dependent value of the plasma channel conductivity. In this work, we have simulated several consecutive nanosecond pulsed discharges between two point electrodes in air initially at 1000 K at a frequency of 10 kHz. First, we have simulated three consecutive nanosecond spark discharges. We have shown that the air temperature increases significantly pulse after pulse in the discharge channel. As a consequence, for the three consecutive simulated nanosecond spark discharges, we have put forward a decrease in the discharge radius, pulse after pulse. Then, to further limit the discharge current, a ballast resistance R has been added into the electrical circuit and the results are presented for seven consecutive nanosecond discharges. For a value of R = 1000 Ω in the conditions studied in this work, we have shown that the first nanosecond discharges are in the glow regime, with a small gas heating per pulse. However, as the number of pulses increases due to the gas heating by each pulse, the discharge may transit to a multipulse nanosecond spark regime. For a higher value of R = 10 000 Ω, we have put forward that the gas heating by each nanosecond discharge becomes negligible and then the multipulse nanosecond discharge remains in this case in a stable 'quasi-periodic' multipulse glow regime

Simulation of the stable 'quasi-periodic' glow regime of a nanosecond repetitively pulsed discharge in air at atmospheric pressure

International audienceThis paper presents simulations of the dynamics of nanosecond repetitively pulsed discharges between two point electrodes in atmospheric pressure air at 300 and 1000 K. At 300 K, the preionization left by successive discharges at the end of interpulses mainly consists of positive and negative ions with a density of about 10^9 cm−3 for a repetition frequency of 10 kHz. When photoionization is taken into account with a level of seed charges of about 10^9 cm−3, the dynamics and the characteristics of the discharge during a voltage pulse are shown to depend only weakly on the nature of negative seed charges (electrons or ions). At 1000 K, the preionization left by successive discharges at the end of interpulses consists of positive and negative ions and electrons with a density of about 10^10 cm−3 for a repetition frequency of 10 kHz. Simulation results show that the dynamics and the characteristics of the discharge during a voltage pulse remain rather close whatever the preionization level considered in the range 10^9-10^11 cm−3, corresponding to nanosecond repetitively pulsed discharges in the frequency range 1-100 kHz. The simulation of several consecutive nanosecond voltage pulses at a frequency of 10 kHz shows that, at 1000 K, the discharge can reach in a few voltage pulses a stable 'quasi-periodic' glow regime in agreement with experiments. Finally, the influence of an external air flow aligned with the electrode axis on the conditions to obtain a stable 'quasi-periodic' glow regime is discussed

Simulation of the Lightning Arc Root Interaction with Anisotropic Materials

International audienceThe interaction between lightning arcs and composite materials is a challenging issue for maintenance and safety considerations in the aerospace industry [1]. Composite materials exhibit a much lower conductivity than metallic materials, as well as an important anisotropy due to the orientation of the fibres and the insulating nature of the matrix. In this study, the impact of the anisotropy of composite materials on the lightning arc root dynamics during the pulsed arc phase has been focused. A finite-volume compressible MHD code including Joule heating, Laplace forces and radiative fluxes has been used in a simplified 2D configuration with a single layer composite material. The different current distributions in the arc root are examined up to 30 µs for different ratios between longitudinal and perpendicular conductivities

Influence of fast-heating processes and O atom production by a nanosecond spark discharge on the ignition of a lean –air premixed flame

International audienc

Modelling of an electric contact under lightning current

International audienceLightning stroke on aircrafts induce high current levels in aeronautic assemblies which electrical resistance is mainly concentrated in the contact interfaces between the different parts. As a consequence, the maximum Joule effects, electric fields, and hence sparking probabilities take place in the electric contacts of the aeronautic assemblies [1]. Being able to predict the behaviour of electric contacts under high current levels is then necessary to provide a better understanding on sparking and out-gassing phenomena induced by lightning stroke on aeronautic structures. The present work addresses the modelling at the microscopic scale of such electric contacts under high current levels, through a simplified geometric and physical description. 2D axisymmetric and 3D finite volume simulations are used to study simplified contact geometries and examine the current distribution dynamics, temperature increase, and phase transitions. Finally, a simple pseudo-analytical model is proposed that enables parametric studies on more complex and realistic electric contacts