Patrick Vanraes

Direct current plasma jet at atmospheric pressure operating in nitrogen and air

An atmospheric pressure direct current (DC) plasma jet is investigated in N2 and dry air in terms of plasma properties and generation of active species in the active zone and the afterglow. The influence of working gases and the discharge current on plasma parameters and afterglow properties are studied. The electrical diagnostics show that discharge can be sustained in two different operating modes, depending on the current range: a self-pulsing regime at low current and a glow regime at high current. The gas temperature and the N2 vibrational temperature in the active zone of the jet and in the afterglow are determined by means of emission spectroscopy, based on fitting spectra of N2 second positive system (C3Π-B3Π) and the Boltzmann plot method, respectively. The spectra and temperature differences between the N2 and the air plasma jet are presented and analyzed. Space-resolved ozone and nitric oxide density measurements are carried out in the afterglow of the jet. The density of ozone, which is formed...

Influence of helium mole fraction distribution on the properties of cold atmospheric pressure helium plasma jets

The influence of helium mole fraction distribution in air on the cold atmospheric plasma jets excited by 1.5 kHz rectangular high voltage pulse is studied in this work. Computational fluid dynamics (CFD) with incorporation of large eddy simulation (LES) model is used to simulate the helium mole fraction distribution in air under the helium flow from laminar to turbulent regime with increasing helium outlet velocity. Numerical simulation results are combined with experimental results in order to determine the influence of helium distribution on the cold plasma jets. It reveals that the structure of the helium distribution caused by diffusion or by turbulent mixing in turbulent regime determines the characteristics of the cold plasma jets. On the other hand, the curves of plasma jet length (L) versus helium outlet velocity (V) at different jet diameters (D) are unified in a map of jet Reynolds number (Re = ρHe·V·D/μHe, where μHe is the helium viscosity constant) versus dimensionless plasma jet length (l = L...

Electrical and spectroscopic characterization of underwater plasma discharge inside rising gas bubbles

The generation of high-energetic species in plasma by discharge in liquids makes it suitable for applications such as water treatment. Effective reduction of input power and an increase in the process efficiency can be achieved by externally generated gas bubbles introduced in the liquid. Pulsed discharge in nitrogen, helium and argon bubbles in between a pin-to-plate electrode system submerged in a water solution has been investigated by electrical characterization and emission spectroscopy. A dynamic model for a bubble discharge, based on previously reported experiments and our measurements, is reported. Two types of bubble discharge are observed: a direct bubble discharge, with an immediate onset of a spark discharge inside the bubble, and a delayed bubble discharge, where spark discharge occurs after a delay time of typically several microseconds. The width of the H? Balmer line in the emitted spectra is dominated by Stark broadening and implies electron densities in the order of 1024 to 1025?m?3, which is relatively high in comparison with other low-temperature atmospherical plasmas. Also, the emitted spectra contain a continuum, originating from black-body radiation of the heated bubble gas, with black-body temperatures from 6000 to 9000?K. The dependence of the measured plasma characteristics on the experimental parameters is in agreement with our proposed model.

Removal of atrazine in water by combination of activated carbon and dielectric barrier discharge

Efficiency of modern wastewater treatment plants to remove or decompose persistent contaminants in low concentration is often insufficient to meet the demands imposed by governmental laws. Novel, efficient and cheap methods are required to address this global issue. We developed a new type of plasma reactor, in which atrazine decomposition by atmospheric dielectric barrier discharge (DBD) in dry air is combined with micropollutant adsorption on activated carbon textile and with extra bubbling of generated ozone. Investigation of reaction kinetics and by-product analysis shows that increasing input power with a factor 3.5 leads to deeper atrazine oxidation without significantly changing energy yield of atrazine removal. By-products of first and later generations are detected with HPLC-MS analysis in water and adsorbed on the activated carbon textile. Our reactor is compared in energy efficiency with reactors described in literature, showing that combination of plasma discharge with pollutant adsorption and ozone recycling is attractive for future applications of water treatment.

Influence of air diffusion on the OH radicals and atomic O distribution in an atmospheric Ar (bio)plasma jet

Treatment of samples with plasmas in biomedical applications often occurs in ambient air. Admixing air into the discharge region may severely affect the formation and destruction of the generated oxidative species. Little is known about the effects of air diffusion on the spatial distribution of OH radicals and O atoms in the afterglow of atmospheric-pressure plasma jets. In our work, these effects are investigated by performing and comparing measurements in ambient air with measurements in a controlled argon atmosphere without the admixture of air, for an argon plasma jet. The spatial distribution of OH is detected by means of laser-induced fluorescence diagnostics (LIF), whereas two-photon laser-induced fluorescence (TALIF) is used for the detection of atomic O. The spatially resolved OH LIF and O TALIF show that, due to the air admixture effects, the reactive species are only concentrated in the vicinity of the central streamline of the afterglow of the jet, with a characteristic discharge diameter of ~1.5 mm. It is shown that air diffusion has a key role in the recombination loss mechanisms of OH radicals and atomic O especially in the far afterglow region, starting up to ~4 mm from the nozzle outlet at a low water/oxygen concentration. Furthermore, air diffusion enhances OH and O production in the core of the plasma. The higher density of active species in the discharge in ambient air is likely due to a higher electron density and a more effective electron impact dissociation of H2O and O2 caused by the increasing electrical field, when the discharge is operated in ambient air.

Electrical discharge in water treatment technology for micropollutant decomposition

Hazardous micropollutants are increasingly detected worldwide in wastewater treatment plant effluent. As this indicates, their removal is insufficient by means of conventional modern water treatment techniques. In the search for a cost-effective solution, advanced oxidation processes have recently gained more attention since they are the most effective available techniques to decompose biorecalcitrant organics. As a main drawback, howev‐ er, their energy costs are high up to now, preventing their implementation on large scale. For the specific case of water treatment by means of electrical discharge, further optimi‐ zation is a complex task due to the wide variety in reactor design and materials, dis‐ charge types, and operational parameters. In this chapter, an extended overview is given on plasma reactor types, based on their design and materials. Influence of design and ma‐ terials on energy efficiency is investigated, as well as the influence of operational parame‐ ters. The collected data can be used for the optimization of existing reactor types and for development of novel reactors.

Degradation of AB25 dye in liquid medium by atmospheric pressure non-thermal plasma and plasma combination with photocatalyst TiO2

Abstract In this work, degradation of the anthraquinonic dye Acid Blue 25 by non-thermal plasma at atmospheric pressure with and without photocatalyst is investigated. Titanium dioxide (TiO2) is used as a photocatalyst. The dye degradation by plasma in the presence of TiO2 is investigated as a function of TiO2 concentration, dye concentration and pH. The degradation rate is higher in acidic solutions with pH of 2 to 4.3, especially at pH 2, and decreases to 0.38 mg L-1 min-1 with the increase of pH from 2 to 5.65. A similar effect is observed in basic media, where a higher degradation rate is found at pH = 10.3. The degradation rate increases in the presence of TiO2 compared to the discharge without photocatalysis. The results show that the degradation of the dye increases in the presence of TiO2 until the catalyst load reaches 0.5 g L-1 after which the suppression of AB25 degradation is observed. The results indicate that the tested advanced oxidation processes are very effective for the degradation of AB25 in aqueous solutions. Graphical Abstract

Characteristics of an underwater direct current discharge in bubbles and the temperature distribution in the bubbles

An underwater direct current (DC) discharge in artificially produced air bubbles is investigated. Electrical and optical emission properties of the plasma and temperature distribution in bubbles evaluated by using computational fluid dynamics (CFD) are presented. The behavior of plasma inside a bubble significantly depends on the bubble size. The discharge with water as a cathode is characterized by streamer nature, whereas the plasma with water as an anode appears diffuse and homogenous. The gas temperature is estimated from emission of the plasma, and it is much higher when water is a cathode. Bubble dynamics is investigated by CFD simulation, and results are in good agreement with experimental data. It shows the temperature distribution in bubbles strongly depends on the bubble dynamics, and gas-water interface has a sharp temperature gradient and acts as an efficient heat sink.

Hydrogen peroxide generation by DC and pulsed underwater discharge in air bubbles

Abstract The generation of H2O2 in underwater discharge in air bubbles is studied with consideration of the influence of electrodes polarity, input power, solution conductivity and the inter-electrode distance. The efficiency of hydrogen peroxide generation strongly depends on the polarity, input power and the inter-electrode distance. Discharges in air bubbles with water as a cathode have significantly higher energy yield of hydrogen peroxide in comparison with negative DC or pulsed discharges. The generation of hydrogen peroxide by DC discharge increases with decrease in the inter-electrode distance, but it is opposite for pulsed discharges. Different efficiency of H2O2 production is explained based on physical processes which result to formation of OH radicals.

Study of an AC dielectric barrier single micro-discharge filament over a water film

In the last decades, AC powered atmospheric dielectric barrier discharges (DBDs) in air with a liquid electrode have been proposed as a promising plasma technology with versatile applicability in medicine, agriculture and water treatment. The fundamental features of the micro-discharge filaments that make up this type of plasma have, however, not been studied yet in sufficient detail. In order to address this need, we investigated a single DBD micro-discharge filament over a water film in a sphere-to-sphere electrode configuration, by means of ICCD imaging and optical emission spectroscopy. When the water film temporarily acts as the cathode, the plasma duration is remarkably long and shows a clear similarity with a resistive barrier discharge, which we attribute to the resistive nature of the water film and the formation of a cathode fall. As another striking difference to DBD with solid electrodes, a constant glow-like plasma is observed at the water surface during the entire duration of the applied voltage cycle, indicating continuous plasma treatment of the liquid. We propose several elementary mechanisms that might underlie the observed unique behavior, based on the specific features of a water electrode.