S. Pekárek

Hollow needle-to-plate electrical discharge at atmospheric pressure

Ecological applications dealing with the cleaning of flue gases, the decomposition of volatile hydrocarbons and the destruction of toxic pollutants require, in order to reach high efficiency, the use of non-thermal plasma sources. Typical sources of such non-equilibrium plasmas are barrier discharge, direct current (DC) or alternating current (AC) gliding arc, pulsed or DC corona and DC atmospheric pressure discharge stabilized by a fast gas flow (APD-GFS). In case of APD-GFS the gas flows in a rectangular channel, the top wall of which serves as the anode and the multi-needle cathode is built into the bottom wall of the channel. In order to prevent the transition to a spark and to stabilize this type of discharge the velocity of the gas should be about 100-200 m s-1 or the discharge current must be limited. To avoid the problem connected with the acceleration of the primary (polluted) gas at such a velocity, the external flow of the primary gas around the needle electrodes can be superimposed by a flow of a secondary gas through the needles. Thus the primary gas need not be accelerated to high velocity and in order to stabilize the discharge a relatively small amount of a secondary gas supplied through the needle is required. This work is therefore focused on the study of the DC APD-GFS in hollow needle-to-plane geometry. The basic electrical characteristics, magnetic noise and integral emission spectra of this type discharge with the flow of nitrogen or air through the needle are given.

Ozone generation by hollow-needle to plate electrical discharge in an ultrasound field

The effect of ultrasound waves on the production of ozone by dc electrical discharge in air, at atmospheric pressure, with a single hollow-needle to plate electrode enhanced by the flow of air through the needle for both polarities of the needle, different airflow rates and currents has been studied experimentally. It was found that the application of ultrasound increases ozone generation for the discharge, with the needle negatively biased, and has no effect on ozone production for the discharge when the needle is biased positively. For the discharge with the needle biased negatively, the ozone generation increases with the increase of the ultrasound transducer surface amplitude and decreases with the increased airflow rate through the needle.

Multi hollow needle to plate plasmachemical reactor for pollutant decomposition

Modification of the classical multipin to plate plasmachemical reactor for pollutant decomposition is proposed in this paper. In this modified reactor a mixture of air and pollutant flows through the needles, contrary to the classical reactor where a mixture of air and pollutant flows around the pins or through the channel plus through the hollow needles. We give the results of comparison of toluene decomposition efficiency for (a) a reactor with the main stream of a mixture through the channel around the needles and a small flow rate through the needles and (b) a modified reactor. It was found that for similar flow rates and similar energy deposition, the decomposition efficiency of toluene was increased more than six times in the modified reactor. This new modified reactor was also experimentally tested for the decomposition of volatile hydrocarbons from gasoline distillation range. An average efficiency of VOC decomposition of about 25% was reached. However, significant differences in the decomposition of various hydrocarbon types were observed. The best results were obtained for the decomposition of olefins (reaching 90%) and methyl-tert-butyl ether (about 50%). Moreover, the number of carbon atoms in the molecule affects the quality of VOC decomposition.

Ozone production of hollow-needle-to-mesh negative corona discharge enhanced by dielectric tube on the needle electrode

For the hollow-needle-to-mesh negative corona discharge in air, we studied the effect of placing the dielectric tube on the needle electrode and the effect of various positions of the end of this tube with respect to the tip of the needle electrode on the concentration of ozone produced by the discharge, the ozone production yield and the discharge V–A characteristics. We found that the placement of the dielectric tube on the needle electrode with a suitable position of this tube end with respect to the tip of the needle electrode for a particular discharge power led to a more than fourfold increase in the concentration of ozone produced by the discharge and also, for a constant airflow, the ozone production yield.

Effect of magnetic field, airflow or combination of airflow with magnetic field on hollow needle-to-cylinder discharge regimes

For the hollow needle-to-cylinder dc discharge in air at atmospheric pressure we studied the effect of magnetic field, airflow and combination of airflow with magnetic field on transitions between various discharge regimes. We showed that application of magnetic field on the discharge in static air does not substantially change the discharge characteristics. In contrast with this finding, application of airflow through the needle increases the range of current of the discharge pulseless regime in comparison with the discharge in static air. A similar but more significant effect was obtained for combination of the airflow and magnetic field on the discharge. It was also shown that for the discharge in static air, discharge in static air in magnetic field, discharge with airflow and discharge in magnetic field with airflow for the filamentary streamer regime, the duration of the discharge voltage pulses decreases with increasing current. Using the combination of the airflow and magnetic field we were therefore able to stabilize the discharge and to increase the volume of the discharge chamber occupied by the non-equilibrium plasma.

Ozone generation in gas flow enhanced hollow needle to plate electrical discharge in air

The results of studies of ozone production for DC electrical discharge in air at atmospheric pressure with a single hollow needle to plate electrode configuration enhanced by the flow of air through the needle for both polarities of the needle, different airflow rates and currents are given in this paper.

Acoustic field effects on a negative corona discharge

For a negative corona discharge under atmospheric pressure in different regimes, we investigated the effects of an acoustic field both on its electrical parameters and on the change in its visual appearance. We found that the application of an acoustic field on the true corona discharge, for particular currents, decreases the discharge voltage. The application of an acoustic field on the discharge in the filamentary streamer regime substantially extends the range of currents for which the discharge voltage remains more or less constant, i.e. it allows a substantial increase in the power delivered to the discharge. The application of an acoustic field on the discharge causes the discharge to spread within the discharge chamber and consequently, a highly reactive non-equilibrium plasma is created throughout the inter-electrode space. Finally, our experimental apparatus radiates almost no acoustic energy from the discharge chamber.

Ozone production by a dc corona discharge in air contaminated by n-heptane

Beneficial purposes of ozone such as elimination of odours, harmful bacteria and mildew can be used for transportation of food, fruits and vegetables with the aim to extend their storage life. To date the main technique used for this purpose in the transportation of these commodities, e.g. by trucks, was cooling. Here a combination of cooling together with the supply of ozone into containers with these commodities is considered. For these purposes we studied the effect of air contamination by n-heptane (part of automotive fuels) and humidity on ozone production by a dc hollow needle to mesh corona discharge.We found that, for both polarities of the needle electrode, addition of n-heptane to air (a) decreases ozone production; (b) causes discharge poisoning to occur at lower current than for air; (c) does not substantially influence the current for which the ozone production reaches the maximum. Finally the maximum ozone production for the discharge in air occurs for the same current as the maximum ozone production for the discharge contaminated by n-heptane. We also found that humidity decreases ozone production from air contaminated by n-heptane irrespective of the polarity of the coronating needle electrode. This dependence is stronger for the discharge with the needle biased positively.

Ultrasonic Field Effects on Corona Discharge in Air

The application of ultrasonic waves on corona discharge causes the existence of pressure gradients in the discharge gap. According to Meeks criterion, formation of streamers in the discharge and therefore the discharge profile is affected by these pressure gradients. We calculated the distribution of ultrasonic pressure in the interelectrode gap and we demonstrated experimentally that the discharge profile is affected by this ultrasonically induced pressure distribution.

Effect of TiO2 on Various Regions of Active Electrode on Surface Dielectric Barrier Discharge in Air

For surface dielectric barrier discharge in air, we examined the effect of titanium dioxide on various regions of the active electrode on the electrical parameters of discharge, on its emission spectra, and for demonstration of the obtained results also on the concentration of ozone produced by the discharge. We used the active electrode in the form of nine interconnected parallel strips and a square counter electrode. The TiO2 layer covered either only the strips, the region between the strips, or all active electrode. As reference discharge we used the discharge without any layer of TiO2. We found that direct application potential has a version when the strips of the active electrode are covered with a layer of TiO2, because the concentration of ozone produced by the discharge is the highest in all investigated cases. This finding could therefore be used for construction of more efficient ozone generators.