Search Results (12)

Quasi-direct current (Q-DC) filamentary electrical discharges are used to control the shock train in a back-pressured Mach 2 duct flow. The coupled interaction between the plasma filaments and the shock train leading edge (STLE) is studied for a variety of boundary conditions. Electrical parameters associated with the discharge are recorded during actuation, demonstrating a close correlation between the STLE position and dynamics. High-speed self-aligned focusing schlieren (SAFS) and high frame-rate color camera imaging are the primary optical diagnostics used to study the flowfield and plasma morphology. Shock tracking and plasma characterization algorithms are employed to extract time-resolved quantitative data during shock–plasma interactions. Four distinct shock–plasma interaction types are identified and outlined, revealing a strong dependence on the spacing between the uncontrolled STLE and discharge electrodes and a moderate dependence on flow parameters. Full article

This work demonstrates the possibility of electrochemical formation of Ge-Sn-O nanostructures from aqueous solutions containing germanium dioxide and tin (II) chloride at room temperature without prior deposition of fusible metal particles. This method does not require complex technological equipment, expensive and toxic germanium precursors, or binding additives. These advantages will make it possible to obtain such structures on an industrial scale (e.g., using roll-to-roll technology). The structural properties and composition of Ge-Sn-O nanostructures were studied by means of scanning electron microscopy and X-ray photoelectron spectroscopy. The samples obtained represent a filamentary structure with a diameter of about 10 nm. Electrochemical studies of Ge-Sn-O nanostructures were studied by cyclic voltammetry and galvanostatic cycling. Studies of the processes of lithium-ion insertion/extraction showed that the obtained structures have a practical discharge capacity at the first cycle ~625 mAh/g (specific capacity ca. 625 mAh/g). However, the discharge capacity by cycle 30 was no more than 40% of the initial capacity. The obtained results would benefit the further design of Ge-Sn-O nanostructures formed by simple electrochemical deposition. Full article

In this work, a jet of cold plasma is generated in a device supplied in helium and powered with a high-voltage nanopulse power supply, hence generating guided streamers. We focus on the interaction between these guided streamers and two targets placed in a series: a metal mesh target (MM) at floating potential followed by a metal plate target (MP) grounded by a 1500 Ω resistor. We demonstrate that such an experimental setup allows to shift from a physics of streamer repeatability to a physics of streamer self-organization, i.e., from the repetition of guided streamers that exhibit fixed spatiotemporal constants to the emergence of self-organized guided streamers, each of which is generated on the rising edge of a high-voltage pulse. Up to five positive guided streamers can be self-organized one after the other, all distinct in space and time. While self-organization occurs in the capillary and up to the MM target, we also demonstrate the existence of transient emissive phenomena in the inter-target region, especially a filamentary discharge whose generation is directly correlated with complexity order Ω. The mechanisms of the self-organized guided streamers are deciphered by correlating their optical and electrical properties measured by fast ICCD camera and current-voltage probes, respectively. For the sake of clarity, special attention is paid to the case where three self-organized guided streamers (α, β and γ) propagate at v_α = 75.7 km·s^–1, v_β = 66.5 km·s^–1 and v_γ = 58.2 km·s^–1), before being accelerated in the vicinity of the MM target. Full article

The problem considered in this work is shock wave (SW) positioning control in shock-dominated flows. Experiments are conducted to investigate the triggering effect of patterned near-surface electrical discharges on SW reflection from plane walls. In the wind tunnel, $M=4$, $P_0$ = 4 bar, a solid wedge SW generator is mounted on the upper wall. Q-DC filamentary electrical discharges were arranged on the opposite wall, so that the SW from the wedge impinged on the plasma filaments that are arranged flow-wise in either a row of three or a single central filament. Within the supersonic flow, narrow subsonic areas are actuated by electrical discharge thermal deposition, resulting in pressure redistribution, which, in turn, relocates the reflection of impinging SW to a predefined position. Mie scattering, schlieren imaging, and wall pressure measurements are used to explore the details of plasma-SW interaction. Using Mie scattering, the three-dimensional shape of the SW structure is mapped both before and after electrical discharge activation. Plasma-based triggering mechanisms are described in terms of the physical principles of flow control and a criterion for determining the effectiveness of the flowfield control. Full article

Flame holders are widely used in ramjet combustors. We propose using surface nanosecond-pulsed surface-dielectric-barrier-discharge (NS-DBD) to manipulate the flame-holder flow field experimentally. The electrical characteristics, induced flow performance, and temperature distribution of NS-DBD were investigated via the electrical and optical measurement system. In the filamentary discharge mode, the discharge energy rose with decrease of the ambient pressure. The discharge pattern of NS-DBD changed from filamentous to uniform around 5 kPa. Starting-vortex intensity and jet-flow angle relative to the wall increased at low pressure. The recirculation zone was asymmetrical at pressures above 60 kPa. The recirculation zone’s area and length were smaller at lower pressures, but when the actuator was operating, the recirculation zone was nearly 11.8% longer. The vorticity increased with pressure. When the pulse width was 300 ns, the actuator had the greatest effect, and the low velocity region (LVR) area and the fuel–air-mixture residence time (FMRT) could be increased by 31.8% and 20.5%, respectively. The actuator had a smaller widening effect on the LVR area at lower pressure. Rising-edge time should increase with pressure to optimize LVR increase; it should be above 300 ns to optimize FMRT increase. We conclude that NS-DBD is a viable method of controlling flame-holder airflow at low pressure. Full article

In this work, a new plasma reactor operating with a butane/propane (C₄H₁₀/C₃H₈) gaseous mixture, designed for hydrogen molecule production, was experimentally studied. This reactor is based on a rotating electrode, biased by an AC high voltage. The plasma discharge was investigated for different AC voltages, rotational frequencies, and gas flow rates. A discharge in the filamentary mode was produced as proved by the electrical characterization. Gas Chromatography (GC) was applied to study the LPG remediation. The maximum conversion factors of 70% and 60% were found for the C₃H₈ and C₄H₁₀, respectively, with an H₂ selectivity of 98%. Hydrogen atomic lines from the Balmer series and various molecular bands were detected by optical emission spectroscopy (OES). The stark broadening of the H_α Balmer line was used for the determination of the electron density. The spectra simulation of the C₂ band was permitted to obtain the gas temperature while the first five lines of hydrogen atoms were used to calculate the electron temperature. A non-equilibrium plasma with two very different temperatures for electrons and heavy particles was found. The spectroscopic study allowed us to explain the experimental results of the LPG conversion and its dependence on the plasma conditions, resulting in optimizing the H₂ formation. Full article

In the present work, a single dielectric barrier discharge (SDBD)-based actuator is developed and experimentally tested by means of various diagnostic techniques. Flexible dielectric barriers and conductive paint electrodes are used, making the design concept applicable to surfaces of different aerodynamic profiles. A technical drawing of the actuator is given in detail. The plasma is sustained by audio frequency sinusoidal high voltage, while it is probed electrically and optically. The consumed electric power is measured, and the optical emission spectrum is recorded in the ultraviolet–near infrared (UV–NIR) range. High-resolution spectroscopy provides molecular rotational distributions, which are treated appropriately to evaluate the gas temperature. The plasma-induced flow field is spatiotemporally surveyed with pitot-like tube and schlieren imaging. Briefly, the actuator consumes a mean power less than 10 W and shows a fair stability over one day, the average temperature of the gas above its surface is close to 400 K, and the fluid speed rises to 4.5 m s⁻¹. A long, thin layer (less than 1.5 mm) of laminar flow is unveiled on the actuator surface. This thin layer is interfaced with an outspread turbulent flow field, which occupies a centimeter-scale area. Molecular nitrogen-positive ions appear to be part of the charged heavy species in the generated filamentary discharge, which can transfer energy and momentum to the surrounding air molecules. Full article

Mercury discharge lamps are often used because of their high efficiency; however, the usage of mercury lamps will be restricted or forbidden for safety and environmental purposes. Finding alternative solutions to suppress mercury is of major interest. The aim of this work is to increase the luminous efficacy of a commercial-free mercury flat dielectric barrier discharge lamp (Planilum, St Gobain) in order to reach the necessary conditions for the lamp to be used as a daily lighting source. The lamp is made of two glass plates separated by a gap of 2 mm. The gap is filled by a neon xenon mixture. The external electrodes made of transparent ITO (indium tin oxide) are deposited on the lamp glass plates. The electrical signal applied to the electrodes generates a UV-emitting plasma inside the gap. Phosphors deposited on the glass allow the production of visible light. The original electrode geometry is plane-to-plane; this induces filamentary discharges. We show that changing the plane-to-plane geometry to a coplanar geometry allows the plasma to spread all over the electrode surface, and we can reach twice the efficacy of the lamp (32 lm/W) as compared to the original value. Using this new electrode geometrical configuration and changing the electrical signal from sinusoidal to a pulsed signal greatly improves the visual uniformity of the emitted light all over the lamp. Electrical and optical parametric measurements were performed to study the lamp characteristics. We show that it is possible to develop a free mercury lamp with an efficacy compatible with lighting purposes. Full article

Nonthermal pulsed filamentary microplasma treatment for drying is a nonthermal technology with promising perspectives to dehydrate plant agricultural materials. The modified set of Luikov’s equations for heat, mass and pressure transfer, has been used to analyze nonthermal pulsed filamentary microplasma treatment effects. The finite element method in combination with the step-by-step finite-difference method for a coupled system of differential equations in partial derivatives was used for numerical simulation of heat, humidity and pressure potentials transfer. The drying time of samples treated by nonthermal pulsed filamentary microplasma treatment assisted by thermionic emission was reduced up to 20.6% (5 kV/cm; 1200 discharges) in comparison to intact tissue. The effect of the obtained approach is very useful for studying process mechanisms and for explaining nonthermal pulsed filamentary microplasma treatment effects. Refined transfer kinetic coefficients from a set of equations based on experimental drying curve can be used for the quantitative determination of thermodynamic coefficients. The agreement of the simulation data with the analytical equation and experimental results is satisfactory (discrepancy less than 3%). Obtained results showed that the proposed model with the refined transfer kinetic coefficients adequately describe the experimental data. Full article

In the last few years, due to the large amount of greenhouse gas emissions causing environmental issue like global warming, methods for the full consumption and utilization of greenhouse gases such as carbon dioxide (CO₂) have attracted great attention. In this study, a packed-bed dielectric barrier discharge (DBD) coaxial reactor has been developed and applied to split CO₂ into industrial fuel carbon monoxide (CO). Different packing materials (foam Fe, Al, and Ti) were placed into the discharge gap of the DBD reactor, and then CO₂ conversion was investigated. The effects of power, flow velocity, and other discharge characteristics of CO₂ conversion were studied to understand the influence of the filling catalysts on CO₂ splitting. Experimental results showed that the filling of foam metals in the reactor caused changes in discharge characteristics and discharge patterns, from the original filamentary discharge to the current filamentary discharge as well as surface discharge. Compared with the maximum CO₂ conversion of 21.15% and energy efficiency of 3.92% in the reaction tube without the foam metal materials, a maximum CO₂ decomposition rate of 44.84%, 44.02%, and 46.61% and energy efficiency of 6.86%, 6.19%, and 8.85% were obtained in the reaction tubes packed with foam Fe, Al, and Ti, respectively. The CO₂ conversion rate for reaction tubes filled with the foam metal materials was clearly enhanced compared to the non-packed tubes. It could be seen that the foam Ti had the best CO₂ decomposition rate among the three foam metals. Furthermore, we used density functional theory to further verify the experimental results. The results indicated that CO₂ adsorption had a lower activation energy barrier on the foam Ti surface. The theoretical calculation was consistent with the experimental results, which better explain the mechanism of CO₂ decomposition. Full article

For the use of natural fibers in composite materials it is often necessary to improve the compatibility between fiber (sizing) and polymer matrix systems, e.g., by increasing the number of functional groups on the fiber surfaces. In this work, a dielectric barrier discharge (DBD) source in plane configuration is used to treat flax fabrics in ambient air. It is examined whether it is possible to increase the functionality on both fabric sides, which is achieved by simple changes in the DBD setup. After evaluating the treatment homogeneity of the filamentary plasma, an explanation for the treatment mechanism on the fiber surfaces is developed. It is shown that waxy substances, which naturally occur on natural fibers, play an important role in the wettability of the fabric. Full article

The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers. Full article