A novel Hydrodynamic Cavitation-Assisted Oxygen Plasma (HCAOP) process, which employs a venturi tube and oxygen injection, has been developed for enhancing the production and utilization of hydroxyl radicals (·OH) in the degradation of organic pollutants. This study has systematically investigated the fluid characteristics and discharge properties of the gas-liquid two-phase body in the venturi tube. The hydraulic cavitation two-phase body discharge is initiated by the bridging of the cavitation cloud between the electrodes. The discharge mode transitions from diffuse to spark to corona as the oxygen flow rate increases. The spark discharge has the highest current and discharge energy. Excessive oxygen results in the change of the flow from bubbly to annular and a subsequent decrease in discharge energy. The effects of cavitation intensity, oxygen flow rate, and power polarity on discharge characteristics and ·OH production were evaluated using terephthalic acid as a fluorescent probe. It was found that injecting 3 standard liter per minute (SLPM) of oxygen increased the ·OH yield by 6 times with only 1.2 times increase in power, whereas<0.5 SLPM of oxygen did not improve the ·OH yield due to lower breakdown voltage. Negative polarity voltage increased the breakdown voltage and ·OH yield due to asymmetric density and pressure distribution in the throat tube. This polarity effect was explained by numerical simulation. Using indigo carmine (E132) as a model pollutant, the HCAOP process degraded 20 mg/L of dye in 5 L water within 2 min following a first-order reaction. The lowest electric energy per order (E_EO) was 0.26 (kWh/m³/order). The HCAOP process is a highly efficient flow-type advanced oxidation process with potential industrial applications.

The breakdown and recovery characteristics of the highly repetitive (≤100 kHz) and short nanosecond (∼10 ns) pulsed discharge were investigated. It was found that if one pulse of 40 kV is applied, the air gap between two conical electrodes for a reliable breakdown is only 1 mm; however, if 1000 same pulses are repetitively applied, the longest gap that can be broken down dramatically increases with the increase in the repetitive rate and it reaches 90 mm at 100 kHz. During the preceding pulses that do not cause the breakdown of the gap, long streamers develop from both conical electrodes and nearly bridge the gap, which provides the residual and decaying plasma channels for the rapid breakdown of 90-mm gap in the following pulses, which was experimentally proved to be unique in the extremely non-uniform electric field. The voltage recovery was determined with the two-pulse method and found that the physical processes of the voltage recovery after the short nanosecond pulsed breakdown are similar to that after the microsecond pulsed breakdown, but the time needed for a full recovery in the gas density, in between 50 and 100 μs, is shorter by three orders than that after the microsecond pulsed breakdown. The faster recovery in the gas density may be understood from the small energy of 36 mJ in one nanosecond pulse, which does not induce a high perturbation in the gas density. The evolution in the discharge mode in a 15-mm gap, from corona to diffuse and finally to multichannel spark, with the applied voltage and the repetitive rate was observed. The formation of the multichannel spark is due to the limited energy per pulse that is not enough to heat the primary channel to high conductivity for the collapse of the gap voltage.

The initial stage of the gas breakdown with the generation of runaway electrons was investigated using particle-in-cell/Monte Carlo collision simulations. The parameters of the solved problem are a 1 mm long atmospheric air gap between tip-plane electrodes applied with a nanosecond pulse voltage. The pulse is 5.2 ns in rising time (10%–90%), 10 ns in pulse width (FWHM) and 40 kV in amplitude. The cathode is a cone-shaped electrode, the tip of which is defined by the elliptic equation (the major axis is 4 mm and the minor axis is 1 mm), and the minimum radius of curvature is 0.125 mm. As it is found in the simulation that the development of the discharge channel from the cathode to the anode only takes about tens to hundreds of picoseconds, especially at high overvoltages with runaway electrons, it is assumed that the gap voltage applied in such a short time is nearly constant. Depending on the voltage at the breakdown, different behavior of the energetic electrons is observed. When the voltage is low, about 12 kV, energetic electrons are only produced in the tip cathode layer, where the electric field is the highest; no runaway electrons leading the discharge channel are observed. When the voltage is higher, about 15 kV, the energetic electrons begin to run away at the head of the discharge channel, where the electric field is high enough. When the voltage is even higher, the energetic electrons run away, even at the beginning of the discharge, and from the cathode to the anode. Pre-ionization of the gas ahead of the discharge channel by the runaway electrons is observed, which may play an important role in the fast breakdown of the gas under the nanosecond short pulse.

Direct current (DC) gas insulated transmission lines (GILs) have been widely used in power transmission but might be threatened by partial discharge due to the presence of floating impurities (e.g., dust and metal particles) inside the sealed chamber. In this Letter, by using a 2D fluid model, we characterize the microscopic properties of the partial discharge induced by a floating linear metal particle in SF6 (both the discharge propagation and interaction between space charge and metal particle) under negative high voltage direct current conditions. Due to the strong electronegativity of SF6, the spatiotemporal distributions of the charged species (electrons, positive ions, and negative ions), space charge, and reduced electric field are rather different from those in air. Notably, a negative ion region is observed around the top tip of the metal particle, and it plays an important role in the generation and propagation of primary and secondary streamers in SF6, which may lead to severe motion characteristics of the particle and aliasing of partial discharge signals. Additionally, we analyze the charging process and electric force reversal phenomenon, which may provide a more precise understanding of the underlying mechanisms of the firefly motion previously reported for DC GILs.