Enhancement of thin air jets produced by ring-shaped dielectric barrier discharges using an auxiliary electrode

A ring-shaped dielectric barrier discharge (DBD) was explored as a small form factor ionic wind device. Using a concentric ring electrode geometry, the DBD produced a converging ionic wind that leads to a vertical flow away from the DBD electrodes. The vertical flow was channeled through an outlet nozzle to produce a thin air jet, and a grounded auxiliary electrode was placed at the nozzle to enhance the exit velocity. The inner diameter of the ring-shaped DBD electrode and the auxiliary electrode ranged 3.18–9.54 mm and 1.0–4.0 mm, respectively. Results showed that the auxiliary electrode generated an ionic wind velocity up to 3.7 m/s and increased the conversion efficiency from discharge to flow power by a factor of 30 by strengthening the electric field where the ions are accelerated.     

Self-organized filaments in dielectric barrier glow discharges

The filamentation of a plasma created by a dielectric barrier discharge in conditions of low pd products (i.e., Townsend breakdown and not streamer breakdown) is investigated both experimentally and with a two-dimensional numerical discharge model. Complex stationary and dynamical domains and filaments are observed experimentally. Some of the properties of these systems are reproduced by the model     

Generation of excimer emission in dielectric barrier discharges

Dielectric barrier discharges (silent discharges) are used to excite a large number of excimers radiating in the VUV, UV or visible spectral range. The excited species include rare-gas dimers, halogen dimers as well as rare-gas halogen excimers and mercury halogen excimers. In many cases narrow-band UV radiation of typically 1–17 nm halfwidth and remarkable efficiency (1–10%) could be generated. Thus, dielectric barrier discharges provide a simple, versatile arrangement to study the basic reaction kinetics of excimer formation and also bear a substantial potential for large-scale industrial UV processes.     

Polytetrafluoroethylene surface modification by filamentary and homogeneous dielectric barrier discharges in air

In this paper, polytetrafluoroethylene (PTFE) films are modified using non-equilibrium plasma generated by homogeneous DBD in air at medium pressure, and the results are compared to those treated by using filamentary DBD in air at atmospheric pressure. The surface properties of PTFE films before and after the treatments are studied using contact angle and surface energy measurement, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It is found that the plasma treatments modify the PTFE surface in both morphology and composition. The PTFE films modified in both treatments show a remarkable decrease in water contact and a remarkable increase in surface energy. XPS analysis reveals that oxygen-containing polar groups are introduced onto the PTFE surface, and SEM analysis shows that the surfaces of the films are etched after both the treatments. It is found that homogeneous DBD is more effective in PTFE surface modification than filamentary DBD as it can make the contact angle decline to a lower level by introducing more oxygen-containing groups, and the possible reason for this effect is discussed.     

Rapid breakdown mechanisms of open air nanosecond dielectric barrier discharges

The discharge initiation mechanism of nanosecond dielectric barrier discharges in open air has been clarified with time-dependent measurement of the discharge electric field by electric-field-induced coherent Raman scattering and optical emission. Our experimental observations have revealed that, in the prebreakdown phase of a nanosecond dielectric barrier discharge, the externally applied fast-rising electric field is strongly enhanced near the cathode due to large accumulation of space charge, which then strongly enhances ionization near the cathode. Once a sufficiently large number of ionizations take place, the location of peak ionization forms a front and propagates toward the cathode with strong optical emission, which establishes the discharge. This process is essentially different from the well-known Townsend mechanism for slower discharges.     

介质阻挡放电等离子体与激波相互作用实验研究

主要针对介质阻挡放电等离子体改变激波系结构展开了实验研究。验证了介质阻挡放电等离子体气动激励能够对边界层施加影响,顺气流放电时能减小激波强度,逆气流放电时能增大激波强度。逆气流放电时,3组电极放电与4组电极放电比较,3组电极放电时压比更高。由于在该实验中放电区域比边界层小得多,介质阻挡放电产生的体积力远小于高速来流条件下的气动力,因此对激波的作用效果十分微弱。     

介质阻挡放电等离子体与激波相互作用实验研究

 主要针对介质阻挡放电等离子体改变激波系结构展开了实验研究。验证了介质阻挡放电等离子体气动激励能够对边界层施加影响,顺气流放电时能减小激波强度,逆气流放电时能增大激波强度。逆气流放电时,3组电极放电与4组电极放电比较,3组电极放电时压比更高。由于在该实验中放电区域比边界层小得多,介质阻挡放电产生的体积力远小于高速来流条件下的气动力,因此对激波的作用效果十分微弱。     

Action of corona discharges on bacteria and spores

We have measured the E.coli bactericidal efficiency of DC and AC point-to-plane coronas in air and in argon in the corona modes positive glow, positive streamer, negative Trichel pulse and negative glow. Negative Trichel pulse and positive streamer coronas are the most efficient for production of bactericidal agents in air. Less than 5 minute exposure to the products from 30 A of these coronas will sterilize the E.coli covered agar surfaces in a 3l volume to a survival ratio of 10^–5. Positive glow coronas are two orders of magnitude less efficient. 50 Hz AC coronas seem to mainly work during the negative half cycles. The bactericidal agent(s) produced by the coronas have not all been identified, but ozone O₃ with concentration 35–40 ppm is probably the most important one. We show that charged particles and photons can play only a minor role, so the agent(s) must be neutral particles. The main part of them must have a lifetime in the vessel of more than 3 minutes. The much smaller, and volume/distance dependent, efficiency of coronas in humid argon indicates that metastable neutrals or radicals from H₂O can play only a minor role. A preliminary test showed negligible effects of the corona treatment on the spores of Bacillus subtilis.     

Diagnostics of dielectric barrier discharges in noble gases: atmospheric pressure glow and pseudoglow discharges and spatio-temporal patterns

We present experimental results of atmospheric pressure glow discharges (APGD) in a dielectric barrier discharge reactor. These are examined in different noble gases and in the N/sub 2//O/sub 2/ system (air and pure N/sub 2/), under varying experimental conditions (frequency f; gap length d; and electric field intensity E). Discharge diagnostics have been carried out using ultrahigh speed imaging, and synchronous dual-detection of light emission and current-voltage measurements, the former using a photomultiplier . The time evolutions of the discharges and of columnar patterns in regular geometric arrangements at atmospheric pressure under different experimental conditions are reported for all of the noble gases studied here. We present evidence that columnar patterns and APGD are manifestations of the same discharge physics, which is discussed with reference to recent work reported by others.     

使用斯塔克展宽计算大气压射频介质阻挡放电氮气等离子体的电子密度

大气压射频等离子体是近几年发展起来的一种新型非平衡等离子体。以氮气掺杂少量氩气为放电气体,实现了大气压射频介质阻挡放电。利用发射光谱对放电进行在线诊断研究,并分析谱线线型,从中分离出谱线的Stark线型,从而计算出放电通道的电子密度。研究了单个放电通道中电子密度的空间分布并测量了通道同一位置的电子密度随放电输入功率的变化。结果显示,在放电通道中部,当放电输入功率由138W增加至248W,电子密度由4.038×1021 m-3升高至4.75×1021 m-3。     

Analysis of geometric scaling of miniature,multi-electrode assisted corona discharges for ionic wind generation

The assisted corona discharge is a unique discharge configuration that utilizes multiple collecting electrodes to minimize the voltage required to initiate a corona discharge and to generate an ionic wind. In this work, the geometric parameters that govern the formation of the assisted corona discharge and subsequent ionic wind are evaluated. Flow velocity measurements suggest that the geometry of the electrode spacings is optimized for ionic wind generation when the current flowing to the collector electrode is maximized, and that as the electrode gap is decreased to microscale dimensions, ionic wind production is inhibited.     

Numerical study on the discharge characteristics and nonlinear behaviors of atmospheric pressure coaxial electrode dielectric barrier discharges

The discharge characteristics and temporal nonlinear behaviors of the atmospheric pressure coaxial electrode dielectric barrier discharges are studied by using a one-dimensional fluid model. It is shown that the discharge is always asymmetrical between the positive pulses and negative pulses. The gas gap severely affects this asymmetry. But it is hard to acquire a symmetrical discharge by changing the gas gap. This asymmetry is proportional to the asymmetric extent of electrode structure, namely the ratio of the outer electrode radius to the inner electrode radius. When this ratio is close to unity, a symmetrical discharge can be obtained. With the increase of frequency, the discharge can exhibit a series of nonlinear behaviors such as period-doubling bifurcation, secondary bifurcation and chaotic phenomena. In the period-doubling bifurcation sequence the period-n discharge becomes more and more unstable with the increase of n. The period-doubling bifurcation can also be obtained by altering the discharge gas gap. The mechanisms of two bifurcations are further studied.It is found that the residual quasineutral plasma from the previous discharges and corresponding electric field distribution can weaken the subsequent discharge, and leads to the occurrence of bifurcation.     

Effective corona discharge and ozone generation from a wire-plate discharge system with a slit dielectric barrier

A new wire-plate-type discharge system, with a slit dielectric barrier, has been proposed and investigated by focusing on the generation of ozone and the discharges on the slit barrier. This wire-plate-type discharge system, with a slit barrier, can generate a corona discharge twice, once from the corona wire electrode and again from the surface and the slits of the slit barrier. As a result, this proposed wire-plate-type discharge system, with the slit barrier, can produce much more ozone than without the slit barrier. The discharge mechanism of the wire-plate-type discharge system, with the slit barrier, would be hard to arc discharge and to break the system down directly because of the blocking action of the slit dielectric barrier. This proposed wire-plate-type discharge system, with the slit barrier, may be useful as an effective means for removing pollutant gases.     

Influence of corona on strike probability of grounded electrodes by high voltage discharges

This paper presents the results of an investigation into the influence of corona formation processes at the tips of grounded rod electrodes on the probability of those rods being “struck” by high-voltage discharges. Experiments simulating the final stage of the attachment process were carried out with a composite voltage comprising a simultaneously applied impulse and DC potential of different levels to grounded rod electrodes simulating lightning rods, featuring either a spherical or pointed (conical) tip. The experiments show that corona does not influence the probability of the electrode being struck until a critical electric field (EF) strength is reached.     

Physical and chemical studies of corona discharges in air

This paper is devoted to a physical and chemical approach of DC coronas in air from a few Torr to 150 Torr. In the first part, the physical results obtained using a positive point-to-plane corona discharge are presented and described. In the second part, the ionic chemical species detected by mass spectrometry at the same conditions are introduced and commented. Finally, these results will be analysed in order to point-out correlations between the physical and chemical phenomena.     

Theoretical investigation of dielectric corona pre-ionization TEA nitrogen laser based on transmission line method

This paper models the dielectric corona pre-ionization, capacitor transfer type of flat-plane transmission line traveling wave transverse excited atmospheric pressure nitrogen laser by a non-linear lumped RLC electric circuit. The flat-plane transmission line and the pre-ionizer dielectric are modeled by a lumped linear RLC and time-dependent non-linear RC circuit, respectively. The main discharge region is considered as a time-dependent non-linear RLC circuit where its resistance value is also depends on the radiated pre-ionization ultra violet (UV) intensity. The UV radiation is radiated by the resistance due to the surface plasma on the pre-ionizer dielectric. The theoretical predictions are in a very good agreement with the experimental observations. The electric circuit equations (including the ionization rate equations), the equations of laser levels population densities and propagation equation of laser intensities, are solved numerically. As a result, the effects of pre-ionizer dielectric parameters on the electrical behavior and output laser intensity are obtained.     

UV and VUV excilamps excited by glow, barrier and capacitive discharges

An investigation was made into the characteristics of XeCl (5~308 nm), KrCl (5~222 nm) and XeI (5~253 nm)capacitive HF discharge and glow discharge excilamps. High fluorescence efficiency of the exciplex molecules has been obtained from excilamps with simple design under capacitive HF discharge excitation. A fluorescence efficiency with respect to the electrical power deposition of up to ~12% has been measured. The study of XeCl, KrCl and XeI excilamps has shown that it is possible to create sealed-off excilamps with gas mixture lifetimes of over 1000 hours.     

Unmanned air vehicle flow separation control using dielectric barrier discharge plasma at high wind speed

The present paper described an experimental investigation of separation control of an Unmanned Aerial Vehicle(UAV)at high wind speeds.The plasma actuator was based on Dielectric Barrier Discharge(DBD)and operated in a steady manner.The flow over a wing of UAV was performed with smoke flow visualization in theΦ0.75 m low speed wind tunnel to reveal the flow structure over the wing so that the locations of plasma actuators could be optimized.A full model of the UAV was experimentally investigated in theΦ3.2 m low speed wind tunnel using a six-component internal strain gauge balance.The effects of the key parameters,including the locations of the plasma actuators,the applied voltage amplitude and the operating frequency,were obtained.The whole test model was made of aluminium and acted as a cathode of the actuator.The results showed that the plasma acting on the surface of UAV could obviously suppress the boundary layer separation and reduce the model vibration at the high wind speeds.It was found that the maximum lift coefficient of the UAV was increased by 2.5%and the lift/drag ratio was increased by about 80%at the wind speed of 100 m/s.The control mechanism of the plasma actuator at the test configuration was also analyzed.     

Compared ionic wind measurements on multi-tip corona and DBD plasma actuators

Two sets of actuators with triangular tips on their active electrodes (13 corona and 15 DBD) are studied in the laboratory. Far field ionic wind velocity, mass flow and efficiency are measured for all the actuators. The best electrode shape as function of tip sharpness and tips number/unit length is determined for each of the above measurements. The gas velocity increases downstream of the tips in all the actuators, but the DBD flow has a three-dimensional structure more complicated than for coronas. The tips improve the efficiency of all the actuators, and the stability of coronas is remarkably improved.