Rudolf Balek

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.

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.

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.

Ultrasound and airflow induced thermal instability suppression of DC corona discharge: an experimental study

The effect of ultrasound waves, airflow and combined ultrasound with the airflow on the thermal instability suppression of a hollow needle-to-plate electrical discharge was studied experimentally. To evaluate the thermal instability suppression we used the V?A characteristics of the discharge in stationary air, with ultrasound applied in stationary air, and finally when the airflow was supplied into the discharge through the needle without and with ultrasound application. To illustrate the effect of ultrasound, airflow and combined ultrasound with airflow on the discharge thermal instability suppression we also studied the discharge ozone production.We found that in stationary air the application of ultrasound only slightly suppresses thermal instability. A substantial increase of ozone generation was not detected. Application of the airflow through the needle suppressed development of the thermal instability and resulted in a substantial increase of the discharge current. The ozone generation was strongly increased in comparison with the preceding case. The combined application of ultrasound and airflow through the needle caused further instability suppression and consequently an increase of the discharge current. The effect of ultrasound on the current?voltage range of the discharge was, however, smaller than the effect of the airflow through the needle. Nevertheless production of ozone was still substantially increased.

Hartmann ultrasound generator combined with electrical discharge

Environmental applications such as volatile organic compounds decomposition, destruction of nitrogen oxides or ozone generation utilize different chemical reactions. The efficiency of these reactions depends among others on the temperature, on the residence time (mixing of reactant medium) and also on the pressure in reaction volume. The increase of pressure in this volume can be achieved by application of power ultrasound. At the same time many reactions can be enhanced by ionization of the reactant medium, which is most frequently performed by electrical discharges. The synergy of power ultrasound with electrical discharges therefore opens new unique perspectives for many applications. The simple and reliable way which reflects these requirements is offered by the combination of Hartmann type ultrasound generators with non-thermal electrical discharges. We describe a new setup of the Hartmann gas-jet ultrasonic generator combined with the electrical discharge with reduced gas flow rate. Our idea involves a rod along the nozzle-resonator axis. New nozzle construction also enables to control electric field in the nozzle-resonator gap and creates high power ultrasoud field in volume in front of the resonator. Apart of the fact that the discharge is stabilized along the resonator orifice circumference, it becomes more uniform and it increases its volume.

5E-5 Numerical Simulation and Schlieren Visualization of the Ultrasonic Field Generated by a Piston Transducer in Gas

Numerically simulated ultrasonic density and pressure fields are compared with the schlieren pictures of the same field. The validation of the schlieren visualization is presented. From the experiment in which the gas jets through the nozzle against the piston surface, it can be concluded that in the case without ultrasound application, the gas flow leaving the nozzle does not significantly diverge and forms a boundary layer along the piston surface. In the presence of an ultrasonic field, the gas flow pattern is changed in a very similar way to the simulated ultrasonic pressure maximum gradient distribution. The results are used to explain the interaction of the gas flow with the ultrasonic field generated by the piston. Knowledge of these density distributions is required for the description of more complex processes, which take place if an electrical discharge is formed between the nozzle and the ultrasonic transducer. Such combined electrical and aerodynamic phenomena are presented in environmental applications such as production of ozone, VOC decomposition and de-NOx processes enhancement.