Zhenghao He

Dielectric barrier discharge in a two-phase mixture

This paper reports the experimental investigation of the dielectric barrier discharge in which the gap area is filled with a two-phase mixture (TPM), air and solid particles. We found that there are two kinds of discharges in the TPM. One is the surface discharge generated on the surface of the solid particles and the other is the filament discharge generated in the air void. For the case of low volume fraction of solid particles, the surface discharge starts to occur when the applied voltage is higher than the onset voltage. At a further voltage increase, the filament discharge takes place at the same time. For the case of high volume fraction, such as the packed-bed reactor, only the surface discharge exists. Under the condition of the same volume fraction, the larger the diameter of the solid particles, the lower the surface discharge onset voltage. As a conclusion, we think that the plasma reactor using the form of low volume fraction of solid particles may be a better choice for waste-gas treatment enhanced by catalysts.

Effect of the Trigger Circuit on Delay Characteristics of a Triggered Vacuum Switch With a Six-Gap Rod Electrode System

Nowadays, the triggered vacuum switch (TVS) is widely used in electromagnetic launcher systems. An important performance of TVS with the six-gap rod electrode system, which has long operating life and maximal switching capacity, has been used in experimental research. The delay time is an important feature of TVS. Therefore, this paper focuses on the effect of trigger circuit and trigger current on the delay characteristics of the TVS. The result shows that, in an indirect trigger circuit, by changing the parameters of the pulse transformer and capacitance C1, the trigger current can be increased, and the delay time can be decreased simultaneously. In a freewheeling circuit, the rate of the trigger current rise can reach 150 A/μs, and the delay time is 2-4 μs. Yet, in the trigger circuit, in which the capacitor C2 is in parallel between the second side of the pulse transformer and the trigger electrode, the rate of the trigger current rise can reach 20 kA/μs, and the delay time is 2 μs. In a direct trigger circuit without the pulse transformer, the rate of the trigger current rise is greater than 4 kA/μs, and the delay time shortens to 500 ns, which is the minimum. The delay time can be shortened obviously by changing the trigger circuit.

An investigation on two-phase mixture discharges: the effects of macroparticle sizes

A two-phase mixture (TPM) is a mixture of gas and macroparticles of high concentration, and there has been significant interest in many technical applications and natural phenomena concerning two-phase mixture discharges (TPMDs), but until now there has been no widely accepted analysis for the propagation of discharges in TPMs. In this paper, 21 kinds of different dielectric materials are used to investigate the effects on TPMD. The diameters of macroparticles in 21 kinds of TPMs are measured by microscope, laser particle size analyzer, etc, and the volume fractions are measured by a video camera and particle image velocimetry system. Based on a direct comparison of the breakdown voltages and the percentages of the discharge path in TPMs with those in air, this work reveals that whether TPMs promote the discharge development or not depends mainly on the macroparticle sizes. These macroparticles in TPMs distort the electric field, interact with ions, electrons or photons, and produce corresponding enhancements or decreases in ionization and excitation as the streamer front encounters them, but the details of alterations on the discharge development are highly correlated with the macroparticle sizes.

Research on the Interaction of Primary Plasma and Main Electrode for Laser Triggered Vacuum Switch

In this paper, the basic structure and working mechanism of laser triggered vacuum switch (LTVS) are briefly introduced, the processes of primary plasma production and expansion are also analyzed. A series of experiments have been carried out under different conditions of LTVS, such as laser energy levels, voltages, and polarity configuration modes. The relationship of conduction and jitter time delays among the laser energy, voltage, and polarity are described, and the effect of main electrode and the expansion of primary plasma are analyzed. The theoretical basis for structure optimization design of LTVS is provided. Under positive polarity configuration that is beneficial to generate the cathode spots, the high working reliability and small time delay of LTVS are verified by the experiments. The results prove that the expansion of primary plasma is the growth of plasma sheath near cathode.

Experimental Investigation of Two-Phase Mixture Discharges Under DC Voltage From Effects of Macroparticle Sizes

There has been a significant interest in many technical applications and natural phenomena concerning two-phase mixture discharges (TPMDs). Few studies have focused on the influences of different macroparticles on TPMDs. The dielectric constant is usually considered to be the only factor affecting the discharges, but in this paper, 21 kinds of different dielectric materials were used to investigate the effects on TPMDs. By the experimental results, this paper reveals the effects of macroparticle sizes, and the dielectric constant has little influence. When the diameter of TPMs is less than about 0.01 mm, the percentage of the selection of TPMs is always smaller than 50%, the discharge path selects the air but TPM, and the breakdown voltage is higher than that in air. However, when the diameter is greater than about 0.1 mm, the conclusion is opposite. When the diameter is in the range of 0.01-0.1 mm, the TPMDs have a polarity effect. The dipole-enhanced approximation model is applied to explain the experimental phenomena. It is very important in TPMDs that the macroparticle dielectric constant, the saturation charge, and the adsorption/desorption process on the material, except the effects of TPMDs, are highly correlated with the macroparticle sizes.

Design and investigation of a multichannel laser-triggered vacuum switch

A laser-triggered vacuum switch (LTVS) is an advanced closing switch with nanosecond delay and jitter. In order to enhance hold-off voltage and extend the service lifetime of an LTVS, we designed a multichannel laser-triggered vacuum switch (MLTVS) utilizing a cone-shaped target electrode placed on the cathode platform. The fabrication and testing of the MLTVS is described in this paper. Experimental results show that the working voltage of the MLTVS with a gap distance of 12 mm is from 30 V to 20 kV. The threshold energy for triggering the switch is 0.4 mJ corresponding to a peak power density of 27.9 MW/cm(2). The triggering lifetime of a spot can reach up to 18,000 shots. In addition, the relationship between triggering lifetime and target materials is analyzed using a field emission scanning electron microscope. A hypothesis of the vacuum gaps triggering mechanism is discussed based on the measured results.

Research on the Time-Delay Characteristics of the Laser-Triggered Vacuum Switch

The basic structure and work mechanism of laser-triggered vacuum switch (LTVS) are introduced briefly in this paper, and the processes of primary plasma production and expansion are analyzed as well. A series of experiments has been carried out with different gap voltages, laser wavelengths, laser energies, polarity configuration modes, switch structures, focal lengths, and laser waveforms. The switch structure is modified to improve the heavy-current capability of the switch, and the gap distance is increased to improve the voltage capacity. The special design of the trigger structure can reduce the dependence on the laser energy of the switch and the time delay. A new laser waveform of LTVS multiple wave crests could further decrease the laser energy, and it is the research basis of laser trigger miniaturization and its application in the LTVS.

Initiation and Propagation of Discharge in Liquid Droplets: Effect of Droplet Sizes

Three kinds of water mist with different droplet sizes are applied to understand the effect of droplet sizes on the initiation and propagation of discharges in a nonuniform field at atmospheric pressure. Based on the measurement of droplet sizes in three mists and the volume fraction of water droplets, effect of droplet sizes is demonstrated by directly comparing the percentages of the discharge path in mist at the different positions with those in air. It can be concluded that the mist with smaller μm-droplets has the negative impact on the initiation and propagation of discharges; the mist with larger μm-droplets has the inhibited effect on the starting and growth of positive discharge but has the contrary effects on negative discharge.

The Research on Pulsed Arc Electrohydraulic Discharge With Discharge Electrode and Its Application to Removal of Bacteria

With the rapid expansion of urbanization, the technology of wastewater treatment has an increasingly crucial role in the field of environmental protection science and technology. The technology of pulsed arc electrohydraulic discharge (PAED) has been studied in depth by virtue of its signally treatment effect, low power consumption, and so on. The experiments were carried out with the wastewater from cyclic activated sludge system tank. The 10-L PAED reactor with eccentric electrodes is designed as industrialization mode. The PAED power supply is 3 kJ/pulse and the discharge rate is 2 Hz. The experiments on different distances, radii, and types of water gap electrodes were carried out. Three different electrode materials (titanium, tungsten, and titanium alloy) were used in the PAED system in the experiments. The results show that the sterilization effect on Escherichia coli is the highest when the material of the water gap electrodes is tungsten and the cumulative input electrical power is 1000 Wh/m3. Fundamental discharge characteristics linked with disinfection performance will be discussed in detail in this paper.

Effect of Two Phase Mixtures on the Selection of the Discharge Path

This paper investigates the development, the breakdown process, and the discharge path selection of the lightning discharges while there exist two phase mixtures. 13 kinds of solid-gas mixtures and 3 kinds of liquid-gas mixtures are employed to study effect of two-phase mixtures (TPM) on the selection of the discharge path under lightning impulses. Grain size effects are shown upon these experimental results. When the diameter of solid or liquid grain is less than about 10 mum, the discharge path selects not TPM but air. And the discharge path selects TPM when the diameter is greater than about 100 mum. And the diameter is between about 10 mum and 100 mum, the discharge path selects TPM under negative lightning impulses, but it has a greater selection of air than TPM under positive lightning impulses. Volume fraction and permittivity of solid/liquid can also influence the selection of the discharge path.