Hervé Rabat

Plasma morphology and induced airflow characterization of a DBD actuator with serrated electrode

Plasma morphology and airflow induced by a dielectric barrier discharge (DBD) actuator, whose exposed electrode geometry is designed with a serrated configuration, are investigated in quiescent air and compared with a DBD actuator consisting of electrodes designed with a standard linear strip configuration. ICCD imaging, electrical measurements and three-component laser Doppler velocimetry were carried out to compare various features of these two actuators. With the serrated configuration, ICCD images of the discharge show that streamers are bent, whereas with the linear configuration they are straight. These curved streamers induce a three-dimensional flow topology, which is confirmed by friction line visualization and velocity measurements. Whereas a two-dimensional wall-jet is induced with the linear configuration, a transverse velocity component is measured with the serrated configuration, implying the creation of spanwise-periodic vorticity. Phase-averaged velocity measurements allow the temporal variation of this transverse velocity to be highlighted. On both sides of a tooth, it has qualitatively the same variation as the longitudinal velocity with respect to the negative or positive half-cycles of the high voltage signal. Moreover, with the same electrical operating parameters, the measured longitudinal velocity was higher, particularly at the tips.

Thermal Characterization of a DBD Plasma Actuator: Dielectric Temperature Measurements Using Infrared Thermography

Active flow control by plasma actuators is currentl y under investigation in order to improve the aerodynamic performance of vehicles. One of these actuators consists in using a surface dielectric barrier discharge (DBD) by creat ing a non-thermal plasma at the dielectric surface. The plasma discharge induces a low-velocity airflow, the so-called “ionic wind”, which can be used to modify external flows. In this study, we focus on the description of the thermal effect of a DBD actuator in order to contribute to a better understanding of the mechanisms of interaction with the flow. The te mperature of the dielectric surface was first determined with the plasma on and secondly af ter switching off the discharge. The measurements were conducted for several amplitudes and frequencies of the applied voltage. The study comprised two parts: in the first, measur ements were performed in quiescent air, and in the second, the influence of an external bou ndary layer over the discharge on the dielectric temperature was investigated.

Plasma propagation of a 13.56?MHz asymmetric surface barrier discharge in atmospheric pressure air

The propagation of an rf asymmetric surface barrier discharge in atmospheric pressure air has been investigated. Measurements of the pulse-modulated 13.56 MHz voltage and current together with ICCD images of the plasma were recorded to study the visible plasma structure with respect to the rf pulses, time within the pulses and the rf waveforms. When exposing images over full rf pulses, which comprise over 150 oscillations of the applied voltage, clearly defined filamentary structures are observed indicating a strong memory effect. The discharge intensity decreases exponentially with distance from the electrode edge, and the average propagation length increases linearly with the applied voltage. Similar to some lower frequency asymmetric surface dielectric barrier discharges, two distinct breakdown events occur during one period of the voltage waveform. The number of filaments is found to be the same for both breakdown events, and collective effects are observed in both discharges.

Experimental investigation of a surface DBD plasma actuator at atmospheric pressure in different N2/O2 gas mixtures

This paper presents an investigation of the influence of nitrogen and oxygen on the behavior of a surface dielectric barrier discharge (SDBD) used for active flow control. The SDBD operated in a controlled atmosphere under several N2/O2 gas mixture ratios. For each gas mixture, the consumed power was measured as a function of voltage amplitude. Then, for a given applied high voltage, the plasma morphology was recorded and commented and lastly, ionic wind velocity measurements were performed. Results show that the induced ionic wind velocity is mainly due to oxygen negative ions during the negative half-cycle. Nevertheless, the contribution of nitrogen to velocity is not negligible during the positive half-cycle. Moreover, the propagation of negative spark filaments during the negative half-cycle is linked to the proportion of O2 in the gas mixture. Increasing this proportion beyond 20% leads to a shift in the saturation effect to lower voltages and to a decrease in the maximum ionic wind velocity value.

Experimental Study of a Surface DBD Actuator Supplied by an Atypical Nanosecond Rising High-Voltage Pulse

This paper presents experimental studies of a surface discharge of an aerodynamic actuator produced by a high-voltage pulse with a nanosecond rise time and a millisecond decrease time. Time-resolved imaging of the plasma and interferometric imaging of the shock wave generated by a unique nanosecond ramp were performed. Interferometry enabled shock fronts to be visualized with a 1 μs time resolution and to experimentally deduce for the first time the associated overpressure values. The interaction of the shock wave with the ionic wind generated during consecutive millisecond-scale voltage decay is also reported from phase-averaged laser Doppler velocimetry measurements. The observed phenomena were correlated with time-resolved images of the plasma developing at the dielectric surface during discharge phases.

Low energy plasma treatment of a proton exchange membrane used for low temperature fuel cells

A low energy (∼30 V) plasma treatment of Nafion, a commercial proton exchange membrane used for low temperature fuel cells, is performed in a helicon radiofrequency (13.56 MHz) plasma system. For argon densities in the 10 9 –10 10 cm −3 range, the water contact angle (hydrophobicity) of the membrane surface linearly decreases with an increase in the plasma energy dose, which is maintained below 5.1 J cm −2 , and which results from the combination of an ion energy dose (up to 3.8 J cm −2 ) and a photon (mostly UV) energy dose (up to 1.3 J cm −2 ). The decrease in water contact angle is essentially a result of the energy brought to the surface by ion bombardment. The measured effect of the energy brought to the surface by UV light is found to be negligible.

Radiation of long and high power arcs

The operators working on electrical installations of low, medium and high voltages can be accidentally exposed to short-circuit arcs ranging from a few kA to several tens of kA. To protect them from radiation, according to the exposure limits, we need to characterize the radiation emitted by the powerful arc. Therefore, we have developed a general experimental and numerical study in order to estimate the spectral irradiance received at a given distance from the arc. The experimental part was based on a very long arc (up to 2 m) with high ac current (between 4 and 40 kA rms, duration 100 ms) using 3 kinds of metallic contacts (copper, steel and aluminium). We measured the irradiance received 10m from the axis of the arc, and integrated on 4 spectral intervals corresponding to the UV, visible, IRA + B and IRC. The theoretical part consisted of calculating the radiance of isothermal plasmas in mixtures of air and metal vapour, integrated over the same spectral intervals as defined in the experiments. The comparison between the theoretical and experimental results has allowed the defining of three isothermal radiation sources whose combination leads to a spectral irradiation equivalent to the experimental one. Then the calculation allowed the deduction of the spectral description of the irradiance over all the wavelength range, between 200 nm and 20 μm. The final results indicate that the influence of metal is important in the visible and UVA ranges whereas the IR radiation is due to the air plasma and surrounding hot gas and fumes.

Control of diffuse and filamentary modes in an RF asymmetric surface barrier discharge in atmospheric-pressure argon

The controlled generation of diffuse and filamentary modes is demonstrated in a radio-frequency (RF, 13.56?MHz) asymmetric surface barrier discharge in atmospheric-pressure argon. For both continuous and pulsed input power, it is shown that the transition from a streamer-driven filamentary discharge at breakdown to a low-current, diffuse plasma can be attained. Fast imaging is used to visualize the structure of the discharge and examine the transition between three distinct modes: moving filaments, branching filaments and a filament-free plasma. The breakdown of the pulsed discharge is studied for pulse periods ranging from ?5??s to 1?ms to investigate the mechanism behind the generation of the diffuse mode.

Influence of the N2/O2 volumetric ratio on the behavior of a SDBD plasma actuator

This paper is focused on studying the in uence of nitrogen and oxygen on the behavior of a surface dielectric barrier discharge (SDBD) actuator. Experimental characterizations have been conducted under several ratio of N2/O2 gas mixtures and in ambient air. Ionic wind velocity pro les have been measured for di erent positions above the dielectric surface, electrical power measurements and ICCD imaging of plasma laments in oxygen, nitrogen and air are presented. Experimental results show that ionic wind velocity is higher in pure oxygen than in pure nitrogen and thus con rm the main role of oxygen ions. They also show that the presence of water molecules in ambient air induces a stronger ionic wind velocity than in dry air. ICCD imaging illustrates di erent shapes of plasma depending on the gas composition.

Relative Residual Charge Distribution and the Corresponding Discharge Image of a Surface DBD

The relative residual charge distribution (RCD) on a dielectric surface due to the discharge created by a single high-voltage pulse was measured using an electrostatic voltmeter equipped with a 2-D scanning system. This distribution was compared with the discharge image recorded with an intensified charge-coupled device camera. The comparison confirmed that the relative RCD reflects the discharge morphology composed of a main discharge channel and branching, while the discharge image provided the relative residual charge distribution on the dielectric surface.