Olivier Ducasse

Experimental analysis and modelling of positive streamer in air: towards an estimation of O and N radical production

This paper is mainly devoted to the comparison between the calculation and experimental results of primary and secondary streamer development in a point-to-plane positive corona discharge in dry air at atmospheric pressure. The qualitative agreement between experimental and calculation results based on the hydrodynamics approximation shows that the O radical is mainly produced in the secondary streamer which is in good agreement with the recent literature measurements using TALIF diagnostics. However, the O radical production yield (in terms of radicals produced per energy injected) is more efficient in the primary streamer than in the secondary one. The main positive corona discharge characteristics are revisited using fast electrical and optical ICCD and streak camera measurements. The calculation shows two streamer radii of, respectively, 10 µm (associated with the radial extension of a high electron density region) and 200 µm (corresponding to the extension of the radial space charge electric field).

Critical Analysis on Two-Dimensional Point-to-Plane Streamer Simulations Using the Finite Element and Finite Volume Methods

Two different categories of high-order numerical methods are adopted for the electro-hydrodynamic modeling of the streamer: the finite-element method (FEM) with flux-corrected-transport (FCT) technique and the finite-volume method (FVM) with monotonic upwind-centered scheme for conservation-law (MUSCL) algorithm. Specific numerical tests on the transport equation are used to investigate the efficiency of the two methods to propagate sharp density gradients in stationary uniform and nonuniform velocity fields. The streamer simulations are performed in a positive point-to-plane electrodes filled with air at room temperature and atmospheric pressure. The influence of the numerical schemes on the 2-D streamer modeling is analyzed. Both FVM-MUSCL and FEM-FCT accurately describe the streamer propagation and the morphology of the discharge channel, thereby giving similar axial and radial density profiles. Furthermore, the computational cost is higher for the FEM-FCT solver as compared to the FVM-MUSCL one, which is at least twice as fast. However, the unstructured-grid approach adopted by FEM-FCT proves to be very efficient in describing nonuniform geometries.

Simulation of Expansion of Thermal Shock and Pressure Waves Inducaed by a Streamer Dynamics in Positive DC Corona Discharges

This paper is devoted to the simulation of the thermal shock and the induced pressure-waves expansion, generated by a dc pin-to-plan corona discharge in the air at ambient temperatures and under atmospheric pressure. The positive dc voltage applied to the tip generates a monofilamentary streamer that crosses the gap from the tip toward the plan. The simulation models are based on the coupling of a 2-D dynamics streamer model with the hydrodynamics conservation equations of a compressible gas. The source term for the gas dynamics equations takes into account the fast-energy relaxation from excited molecules to the random thermal energy. The simulation shows that the streamers generate a thermal shock near the anodic tip, which induces high pressure gradients and finally the gas expansion. The thermal shock is located just in front of the anodic tip, where the injected energy density is the highest. After 0.3 μs, the mean gas temperature increases up to around 800 K in a small volume just in front of the anodic tip while the maximum temperature reaches 1200 K. In addition, two pressure waves, a spherical and a cylindrical one, are induced with a propagation velocity of 370 m s-1 i.e., close to the speed of sound in air.

Primary and Secondary Streamer Dynamics in Pulsed Positive Corona Discharges

Preliminary comparisons between numerical simulation and experimental results are obtained under pulsed applied voltages in the case of positive point to plane corona discharges in dry air at atmospheric pressure. The experimental conditions correspond to 7 mm interelectrode distances with a tip radius of 20

Effects of numerical and physical anisotropic diffusion on branching phenomena of negative-streamer dynamics

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Electro-Hydrodynamics and Kinetic Modeling of Dry and Humid Air Flows Activated by Corona Discharges

. The calculations are based on a first-order 2-D electrohydrodynamic model including the main charged-particle/gas reactions occurring in dry air discharge. The development of the corona discharge is analyzed using streak camera pictures. The transition from the branching structure to the monofilamentary one is described. An interesting comparison is performed between the streak camera pictures and the spatiotemporal evolution of the calculated ionization rate. This shows a qualitative agreement in the development of both primary and secondary streamers.

Benchmarks of 3D Laplace Equation Solvers in a Cubic Configuration for Streamer Simulation

This paper is a contribution to the fluid modelling and simulation of the spontaneous branching of an initial mono-filamentary negative streamer propagating in molecular nitrogen at atmospheric pressure. The effects of both numerical diffusion and physical anisotropic diffusion on the branching structure are studied. We used MUSCL-type flux limiters where an artificial amount of numerical diffusion can be introduced through the choice of the value of a characteristic slope parameter. It was shown that a small amount of numerical diffusion can inhibit the spontaneous streamer branching. This means that the use of a high-order numerical scheme preventing the numerical diffusion and dispersion is a major parameter that must be taken into account in the interpretation of the simulated streamer development and splitting. This paper also clearly shows that the consideration of the anisotropy of electron diffusion affects the streamer head structure in comparison with the isotropic diffusion case. This especially occurs for electrons in gases presenting a large difference between the longitudinal and transversal diffusion coefficients as in N2 or in air.

Finite Volume Method for Streamer and Gas Dynamics Modelling in Air Discharges at Atmospheric Pressure

The present work is devoted to the 2D simulation of a point-to-plane Atmospheric Corona Discharge Reactor (ACDR) powered by a DC high voltage supply. The corona reactor is periodically crossed by thin mono filamentary streamers with a natural repetition frequency of some tens of kHz. The study compares the results obtained in dry air and in air mixed with a small amount of water vapour (humid air). The simulation involves the electro-dynamics, chemical kinetics and neutral gas hydrodynamics phenomena that influence the kinetics of the chemical species transformation. Each discharge lasts about one hundred of a nanosecond while the post-discharge occurring between two successive discharges lasts one hundred of a microsecond. The ACDR is crossed by a lateral dry or humid air flow initially polluted with 400 ppm of NO. After 5 ms, the time corresponding to the occurrence of 50 successive discharge/post-discharge phases, a higher NO removal rate and a lower ozone production rate are found in humid air. This change is due to the presence of the HO2 species formed from the H primary radical in the discharge zone.

Ion swarm data of N4+ in N2, O2 and dry air for streamer dynamics simulation

The aim of this paper is to test a developed SOR R&B method using the Chebyshev accelerator algorithm to solve the Laplace equation in a cubic 3D configuration. Comparisons are made in terms of precision and computing time with other elliptic equation solvers proposed in the open source LIS library. The first results, obtained by using a single core on a HPC, show that the developed SOR R&B method is efficient when the spectral radius needed for the Chebyshev acceleration is carefully pre-estimated. Preliminary results obtained with a parallelized code using the MPI library are also discussed when the calculation is distributed over one hundred cores.

2D simulation of active species and ozone production in a multi-tip DC air corona discharge

Electrical discharges in air at atmospheric pressure like corona or dielectric barrier discharges are generally crossed by thin ionized filament called streamers (about 100μm diameter). The streamer develops and propagates inside the background gas with a high velocity (around 106 m/s) higher than the electron drift velocity (around 105 m/s). During the transport of charged particles within the filaments under the action of the electric field, the energetic charged particles undergo many collisions with the background gas (neutral particles). The interactions between charged and neutral particles generate in turn a gas dynamics characterized by gas temperature and density gradients. The variation of density, momentum transfer and energy of the different particles, present within the ionized filaments, are governed by the fluid conservation laws (or continuity equations) coupled, in the charged particles case, to the electric field or Poisson equation.