Branislav Pongrác

Electrospraying of Water With Streamer Corona Discharge

The effect of electrospraying of water [electrohydrodynamic atomization (EHDA)] was studied in combination with dc streamer corona discharge as a possible method of biodecontamination of water. This effect was investigated under various voltages and water flow rates. The best conditions when the water spray was localized directly in the relatively stable and intense corona active region were reached at 8 kV and 8.3 μl/s. Images of EHDA with streamer corona discharge are presented.

Fast imaging of intermittent electrospraying of water with positive corona discharge

The effect of the electrospraying of water in combination with a positive direct current (dc) streamer corona discharge generated in air was investigated in this paper. We employed high-speed camera visualizations and oscilloscopic discharge current measurements in combination with an intensified charge-coupled device camera for fast time-resolved imaging. The repetitive process of Taylor cone formation and droplet formation from the mass fragments of water during the electrospray was visualized. Depending on the applied voltage, the following intermittent modes of electrospraying typical for water were observed: dripping mode, spindle mode, and oscillating-spindle mode. The observed electrospraying modes were repetitive with a frequency of a few hundreds of Hz, as measured from the fast image sequences. This frequency agreed well with the frequency of the measured streamer current pulses. The presence of filamentary streamer discharges at relatively low voltages probably prevented the establishment of a continuous electrospray in the cone?jet mode. After each streamer, a positive glow corona discharge was established on the water filament tip, and it propagated from the stressed electrode along with the water filament elongation. The results show a reciprocal character of intermittent electrospraying of water, and the presence of corona discharge, where both the electrospray and the discharge affect each other. The generation of a corona discharge from the water cone depended on the repetitive process of the cone formation. Also, the propagation and curvature of the water filament were influenced by the discharge and its resultant space charge. Furthermore, these phenomena were partially influenced by the water conductivity.

On the mechanism of OH radical formation by nanosecond pulsed corona discharge in water

Summary form only given. Electrical discharge plasmas in liquids have been studied for a number of years for applications in different environmental, biological or medical applications. However, the physics underlying the complex phenomena of electric discharge formation in liquid media as well as the chemistry of plasma/liquid interactions induced by these discharges is not fully understood. OH radical is one of the most strongly oxidative species produced by plasma in water and is also building block of H2O2 which is an important agent in the chemical activity of plasma-liquid systems. So far, pulse durations applied for generation of discharge plasmas in liquids were typically in the range of microseconds. The average energy of electrons formed by streamer-like discharges in water was estimated to be 0.5-2 eV This would be sufficient to cause only vibration and rotational excitation rather than electron dissociative reactions of water. Therefore, metastable induced or thermal dissociation of water molecules and electron dissociative recombination of water ions are proposed as more likely pathways of plasmachemical production of OH radicals in water. Recently, a fundamentally different type of discharge generated in water by application of high-voltage pulses with nanosecond duration was reported by several authors. The observed discharge was reported to have a completely different nature from the discharges with microsecond pulse duration. This suggests, that it might be possible to vary electron energy distribution in the discharge and plasmachemical processes in water by pulse duration of pulsed power used for discharge in water.In this work, the formation of OH radicals by a nanosecond pulsed corona discharge in deionized water was studied by means of time resolved optical emission spectroscopy and by chemical methods. FID generator was used for high-voltage pulse generation of pulse length 5ns (FWHM) and amplitude +80-120 kV. Electronically excited radiative state of the OH (A-X) was not detected in emission spectrum of ns discharge in water, however, chemical methods proved formation of OH radicals in water. This is in contradiction to the results obtained for OH radical production in water analyzed under microsecond discharge conditions. These observations were evaluated in more detail and will be discussed with regard to the mechanisms of OH radical formation by plasma in water. * This work was supported by the Czech Science Foundation (project 15-12987S).

Bulk-phase chemistry induced by nanosecond discharge plasma in water

We have evaluated chemical activity of the nanosecond discharge in bulk liquid water by measuring the plasmachemical production of hydrogen peroxide and oxidation of phenol used as chemical probe of OH radicals. Nanosecond pulse generator was used as a source of high voltage pulses (6 ns FWHM) with adjustable amplitude of the positive polarity (60–110 kV) and pulse energy 50–150 mJ/pulse. Obtained results gave clear evidence about the plasmachemical production of OH radicals by nanosecond discharge in water.

Study of nanosecond pulsed corona-like discharge in deionized water by optical methods

Basic morphologic structure and emission fingerprints of micro-discharges produced in deionized water by short high voltage pulses (duration of 6 ns and amplitude of + 100 kV) are presented. Time-resolved ICCD images evidence typical streamer-like branched filamentary structure of the discharge with propagation velocity 2–4×10⁵ m/s. Emission spectra show a broad-band continuum evolving during the first few nanoseconds followed by the well-known H^I/O^I atomic lines. The electron densities were estimated of the order of 10¹⁹ cm⁻³.