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Power ultrasound interaction with DC atmospheric pressure electrical discharge
Ultrasonics 44 (2006) e549–e553 www.elsevier.com/locate/ultras
Power ultrasound interaction with DC atmospheric pressure electrical discharge Rudolf Ba´lek *, Stanislav Peka´rek, Zuzana Barta´kova´ Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 166 27 Prague 6, Technicka 2, Czech Republic Available online 6 June 2006
Abstract The effect of power ultrasound application on DC hollow needle to plate atmospheric pressure electrical discharge enhanced by the flow of air through the needle electrode was studied experimentally. It was found that applying ultrasound increases discharge volume. In this volume take place plasmachemical processes, used in important ecological applications such as the production of ozone, VOC decomposition and de-NOx processes enhancement. In our experiments we used a negatively biased needle electrode as a cathode and a perpendicularly placed surface of the ultrasonic resonator – horn – as an anode. To demonstrate the effect of ultrasound waves on electrical discharge photographs of the discharge for the needle to the ultrasonic resonator at distances of 4, 6 and 8 mm are shown. By varying the distance between needle and the surface of the transducer, we were able to create the node or the antinode at the region around the tip of the needle, where the ionization processes are effective. In our experimental arrangement the amplitude of acoustic pressure at antinode exceeded 104 Pa. The photographs reveal that the diameter of the discharge on the surface of the ultrasonic horn is increased when ultrasound is applied. The increase of discharge volume caused by the application of ultrasound can be explained as a combined effect of the change of the reduced electric field E/n (E is electric field strength and n is the neutral particles density), strong turbulence of the particles in the discharge region caused by quick changes of amplitudes of the standing ultrasonic wave and finally by the boundary layer near the ultrasonic transducer perturbations due to vibrations of the transducer surface. 2006 Elsevier B.V. All rights reserved. Keywords: Power ultrasound; Ultrasound resonator cell; Change of discharge volume and voltage shift
1. Introduction Electrical discharges are influenced by a wide range of different factors, such as applied voltage, electrode geometry, gas pressure, type of gas and the gas flow pattern and pressure distribution in the discharge region. The last two factors can be influenced by the application of ultrasound waves. In our previous work, we studied the application of ultrasound waves on hollow needle to plate electrical discharge with the aim of increasing ozone generation [1] or influencing basic electrical parameters of the discharge in the mixture of air with volatile organic compounds [2].
*
Corresponding author. Tel.: +420 224352384; fax: +420 233337031. E-mail address: [email protected] (R. Ba´lek).
0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.121
The best results were obtained with the usage of high-intensity ultrasound or acoustic waves in resonators [3,4]. The application of ultrasound on electrical discharges for the above-mentioned purposes is connected with many physical phenomena. This paper describes the effect of power ultrasound in a full-wave ultrasonic resonator on hollow-needle to plate DC electrical discharge at atmospheric pressure. To contribute to the explanation of the observed phenomena we simulated the effect of ultrasound by the back airflow applied to the discharge against the flow of air through the needle electrode. Our results are analyzed from two different points of view. First of all we studied the change of the shape of the luminous part of the discharge (spread width of discharge) and then we studied the change of volt-ampere (V-A) characteristics (discharge voltage shift).
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2. Experimental arrangement Experiments dealing with the study of the change of the shape and V-A characteristics of the discharge were performed in two different set-ups. 2.1. Application of ultrasound The experimental set-up together with the distribution of pressure in the ultrasound resonator (sine shaped curve – p) is shown in Fig. 1. The hollow needle to plate electrode system is situated in the full wave cylindrical air-filled ultrasonic resonator cell. The cell consists of the glass discharge tube G and two resonator wall surfaces TR and RF. The stainless steel hollow needle N passes through the reflector RF situated against the plane surface of the ultrasound high power transducer TR. The needle is used as a cathode and the transducer surface serves as an anode. The circular surface of the horn of the ultrasound transducer TR, made from titanium, was electrically grounded. A hypodermic needle (Terumo, Belgium) of an outer and inner diameter of 1.2 and 0.7 mm respectively was used. The radius of the needle tip curvature was 17 lm. The distance between the tip of the needle and the surface of the ultrasonic transducer varied between d = 4–8 mm. The transducer consisted of a circular stepped shape horn driven on one side by a piezoelectric vibrator. The vibrator consisted of piezoceramic rings in a sandwich arrangement. The transducer elements were calculated to be resonant at the working frequency of 20.3 kHz. The vibrating system was driven by a power generator system, which incorporated a feedback circuit to automatically adjust the excitation frequency to the transducer working frequency. The length of the ultrasonic cell (the distance between RF and TR surfaces) could be changed by moving the reflector RF. For our experiments, this length was adjusted to h = 17.5 mm. The maximum amplitude of the
ultrasound horn surface was 42 lm. The ultrasound resonator amplitude of pressure at antinode (position A – Fig. 1) reached 104 Pa; consequently, the acoustic velocity at the pressure node (position B – Fig. 1) was 23 m/s. The temperature in the ultrasonic resonator cell, which was cooled by a fan, was measured by a thermocouple T. The experiments were carried out with the ambient air that was supplied into the needle through water and oil separator WOS by a compressor. The airflow through the needle, which was measured in slm units (standard litre per minute), was adjusted by a mass flow controller Bronkhorst. A DC power supply provided voltage up to 30 kV. The needle was ballasted by resistors R = 1–7.4 MX. 2.2. Application of the back airflow In this case the experimental set up shown in Fig. 1 was modified in the following way. The ultrasound high power transducer TR was replaced by a hollow tube ended by metallic mesh at the position A (see Fig. 1). This mesh, substituting the surface of the ultrasound high power transducer TR at the distance d from the needle served as a grounded electrode. Through the tube and the mesh we supplied so-called ‘‘back airflow’’ in the direction opposite to the airflow from the needle electrode. The back airflow, supplied by another compressor, was adjusted by a second mass flow controller. Mesh R 0.60, made by M-Metall Italy, constituted a rhombus with cell dimensions 0.60 · 0.50 mm and thickness 0.15 mm. 3. Experimental results and discussion Experiments described in this chapter were performed either without the application of ultrasound on the discharge or, when ultrasound was applied, the amplitude of the horn surface was 42 lm and the velocity amplitude was 5.3 m/s. The ultrasound resonator amplitude of pressure at antinode reached 104 Pa and the ultrasound velocity at the pressure node was 23 m/s. The airflow through the needle was Q = 5 slm. 3.1. Spread width of the discharge
Fig. 1. Experimental set-up.
Fig. 2 shows photograph of the discharge without ultrasound application (left part) and the discharge if ultrasound is applied (right part) for three distances: d = 4, 6 and 8 mm between the needle and the surface of the ultrasonic horn. When ultrasound is applied, the discharge width varies in proportion to the ultrasound pressure and to the distance d. Placing the tip of the needle to the ultrasonic pressure node d = 4 mm (position B in Fig. 1), antinode d = 8 mm (position D in Fig. 1), or to position C in Fig. 1 does not result in any qualitative differences from a cone shape. The luminous sphere at the tip of the needle (see Fig. 2 top left) is missing if ultrasound is applied (see Fig. 2 top right).
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Fig. 4. Photograph of the discharge. I = 1.8 mA, Q = 5 slm, d = 8 mm. Left – without back air flow, right – with back air flow, QB = 96 slm. Exposition time 1/125 s.
Fig. 2. Photograph of the discharge. I = 1.8 mA, Q = 5 slm, d = 4-8 mm. Left – without ultrasound, right – with ultrasound. Exposition time 1/30 s.
The discharge-spread width from ultrasound application is the result of several complex processes, which are also caused by electrical and aerodynamic interactions. One of these processes is the interaction of air leaving the needle and vibrations of the horn surface. The horn’s surface vibrates with frequency 20.3 kHz (period approxi-
mately 50 ls) with the amplitude 42 lm and velocity 5 m/ s. The airflow leaving the needle after the impact on the horn’s surface creates a thin boundary layer see Fig. 3, taken from [5]. Due to the horn vibrations, this boundary layer is perturbated, which can contribute to the radial spread of the discharge. Another factor, which may explain the discharge spread may be connected with the transfer of mass due to the pressure gradients in the ultrasonic standing wave field near the horn surface. Velocities of air molecules are given by the vector sum of velocities caused by the airflow pattern from the needle and velocities caused by transitions from compressions to rarefactions of the ultrasonic standing wave. These velocities have a strong radial component. This idea is in agreement with the results presented in [4], where, under the influence of transversal movement of air in the acoustic pressure node, a similar discharge spread width was obtained. To confirm the idea that the discharge spread is caused by summing up vectors of different velocities, we tried to substitute the ultrasound field with the flow of air in a direction opposite to that in which the air is leaving the needle. We used the experimental arrangement, described in Section 2.2. This back airflow directed towards the needle removes any possible radial flows. This result is nicely demonstrated in Fig. 4 for the mesh electrode to needle distance d = 8 mm. The discharge without the back flow is in the left part of this figure. With increasing back airflow, the width of the discharge decreases to the shape, shown in the right side of Fig. 4 (back airflow 96 slm or the air velocity at the mesh 32 m/s). 3.2. Discharge voltage shift DU
Fig. 3. Formation of thin air layer close to horn electrode surface [5].
The next effect connected with the application of ultrasound on the discharge is the change of its V-A characteristics. To quantify this effect we introduced the discharge voltage shift DU, the difference between the discharge voltage if ultrasound is applied and the voltage without ultrasound application for the same discharge current. The dependence of the discharge voltage shift DU as a function of current I is shown in Fig. 5.
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Fig. 5. The discharge voltage shift versus current for the distance d = 4, 6 and 8 mm and Q = 5 slm.
This figure shows that the maximum DU occurs for the tip of the needle at the distance d = 4 mm, which corresponds to the node of ultrasound pressure (position B – Fig. 1). Position B at the same time corresponds to the antinode of ultrasound velocity. Particles contained in the ionization region, which surrounds the tip, are exposed to the superposition of velocities due to the flow through the needle and ultrasound velocity. The ionization region is characterized by a high gradient of electric field. If we apply ultrasound in this region, we can influence the electrical parameters of the discharge, which is represented by DU. The change of discharge electrical parameters can be also partially attributed to the variation of the reduced electric field E/n (E is electric field strength and n is the neutral particles density), because it influences ionization rate. Electrical parameters of the discharge are associated with the discharge shape. As was shown in Section 3.1 the discharge shape strongly depends on the back flow
through the mesh. We tried to influence the electrical parameters of the discharge by the back flow with the aim of producing the same voltage shift DU as when ultrasound was applied. To do this, we used an experimental arrangement with the back airflow through the mesh for distance d = 8 mm. Fig. 6 shows the dependence of the discharge voltage shift versus current for back airflows 24, 32, 40, 48, and 96 slm and the discharge voltage shift caused by ultrasound application only. It is seen that applying ultrasound (squares) or applying a back airflow slightly higher than QB = 32 slm (up triangles) produces a similar effect on the discharge voltage shift. However the mechanism of both processes is different. In the case of back airflow, we can influence motion of heavy particles (neutrals and negative ions-mainly oxygen) in the whole discharge region, while in the case of ultrasound application; we mainly influence the processes taking place at the ionization region. 4. Conclusions The effect of power ultrasound application on DC hollow needle to plate or mesh atmospheric pressure electrical discharge enhanced by the flow of air through the needle electrode was studied experimentally. To clarify the role of ultrasound on the discharge we also used the back airflow to induce similar effects to those induced by ultrasound. It was found that ultrasound application increases discharge shape. The discharge shape is affected by back airflow. We also found that ultrasound application influences V-A characteristics of the discharge. We were able to obtain the same change of the discharge voltage as in the case of ultrasound application by the application of the back airflow. The above-mentioned findings are the result of complex processes and can be used for environmental applications such as production of ozone, VOC decomposition and de-NOx processes enhancement. Acknowledgement This research has been supported by the research program No. MSM6840770015 ‘‘Research of Methods and Systems for Measurement of Physical Quantities and Measured Data Processing ’’ of the CTU in Prague sponsored by the Ministry of Education, Youth and Sports of the Czech Republic. References
Fig. 6. Discharge voltage shift versus current for back airflows QB = 24, 32, 40, 48, and 96 slm. Discharge voltage shift caused by ultrasound application. Needle to electrode distance d = 8 mm.
[1] S. Peka´rek, R. Ba´lek, Ozone generation by hollow needle to plate electrical discharge in ultrasound field, J. Phys. D: Appl. Phys. 37 (2004) 1214–1220. [2] S. Peka´rek, R. Ba´lek, M. Pospı´sˇil, Effect of ultrasound waves on electrical characteristics of a hollow needle to plate electrical
R. Ba´lek et al. / Ultrasonics 44 (2006) e549–e553 discharge in air or mixture of air with VOC, B. Am. Phys. Soc. 49 (2004) 21. [3] R.Ba´lek, S. Peka´rek, Z. Sˇlegrova´, Ultrasonic resonator with electrical discharge cell for ozone generation, Ultrasonics, submitted for publication.
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[4] T. Nakane, Discharge phenomenon in a high-intensity acoustic standing wave field, IEEE Trans. Plasma Sci. 33 (2005) 356–357. [5] J. Mizeraczyk, J. Podlin´ski, M. Dors, J. Dekowski, Velocity field in flow stabilized hollow needle-to-plate electrical discharge in atmospheric pressure Air, Czech. J. Physics 52 (Suppl. D) (2002) 769–776.
Power ultrasound interaction with DC atmospheric pressure electrical discharge Rudolf Ba´lek *, Stanislav Peka´rek, Zuzana Barta´kova´ Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 166 27 Prague 6, Technicka 2, Czech Republic Available online 6 June 2006
Abstract The effect of power ultrasound application on DC hollow needle to plate atmospheric pressure electrical discharge enhanced by the flow of air through the needle electrode was studied experimentally. It was found that applying ultrasound increases discharge volume. In this volume take place plasmachemical processes, used in important ecological applications such as the production of ozone, VOC decomposition and de-NOx processes enhancement. In our experiments we used a negatively biased needle electrode as a cathode and a perpendicularly placed surface of the ultrasonic resonator – horn – as an anode. To demonstrate the effect of ultrasound waves on electrical discharge photographs of the discharge for the needle to the ultrasonic resonator at distances of 4, 6 and 8 mm are shown. By varying the distance between needle and the surface of the transducer, we were able to create the node or the antinode at the region around the tip of the needle, where the ionization processes are effective. In our experimental arrangement the amplitude of acoustic pressure at antinode exceeded 104 Pa. The photographs reveal that the diameter of the discharge on the surface of the ultrasonic horn is increased when ultrasound is applied. The increase of discharge volume caused by the application of ultrasound can be explained as a combined effect of the change of the reduced electric field E/n (E is electric field strength and n is the neutral particles density), strong turbulence of the particles in the discharge region caused by quick changes of amplitudes of the standing ultrasonic wave and finally by the boundary layer near the ultrasonic transducer perturbations due to vibrations of the transducer surface. 2006 Elsevier B.V. All rights reserved. Keywords: Power ultrasound; Ultrasound resonator cell; Change of discharge volume and voltage shift
1. Introduction Electrical discharges are influenced by a wide range of different factors, such as applied voltage, electrode geometry, gas pressure, type of gas and the gas flow pattern and pressure distribution in the discharge region. The last two factors can be influenced by the application of ultrasound waves. In our previous work, we studied the application of ultrasound waves on hollow needle to plate electrical discharge with the aim of increasing ozone generation [1] or influencing basic electrical parameters of the discharge in the mixture of air with volatile organic compounds [2].
*
Corresponding author. Tel.: +420 224352384; fax: +420 233337031. E-mail address: [email protected] (R. Ba´lek).
0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.121
The best results were obtained with the usage of high-intensity ultrasound or acoustic waves in resonators [3,4]. The application of ultrasound on electrical discharges for the above-mentioned purposes is connected with many physical phenomena. This paper describes the effect of power ultrasound in a full-wave ultrasonic resonator on hollow-needle to plate DC electrical discharge at atmospheric pressure. To contribute to the explanation of the observed phenomena we simulated the effect of ultrasound by the back airflow applied to the discharge against the flow of air through the needle electrode. Our results are analyzed from two different points of view. First of all we studied the change of the shape of the luminous part of the discharge (spread width of discharge) and then we studied the change of volt-ampere (V-A) characteristics (discharge voltage shift).
R. Ba´lek et al. / Ultrasonics 44 (2006) e549–e553
e550
2. Experimental arrangement Experiments dealing with the study of the change of the shape and V-A characteristics of the discharge were performed in two different set-ups. 2.1. Application of ultrasound The experimental set-up together with the distribution of pressure in the ultrasound resonator (sine shaped curve – p) is shown in Fig. 1. The hollow needle to plate electrode system is situated in the full wave cylindrical air-filled ultrasonic resonator cell. The cell consists of the glass discharge tube G and two resonator wall surfaces TR and RF. The stainless steel hollow needle N passes through the reflector RF situated against the plane surface of the ultrasound high power transducer TR. The needle is used as a cathode and the transducer surface serves as an anode. The circular surface of the horn of the ultrasound transducer TR, made from titanium, was electrically grounded. A hypodermic needle (Terumo, Belgium) of an outer and inner diameter of 1.2 and 0.7 mm respectively was used. The radius of the needle tip curvature was 17 lm. The distance between the tip of the needle and the surface of the ultrasonic transducer varied between d = 4–8 mm. The transducer consisted of a circular stepped shape horn driven on one side by a piezoelectric vibrator. The vibrator consisted of piezoceramic rings in a sandwich arrangement. The transducer elements were calculated to be resonant at the working frequency of 20.3 kHz. The vibrating system was driven by a power generator system, which incorporated a feedback circuit to automatically adjust the excitation frequency to the transducer working frequency. The length of the ultrasonic cell (the distance between RF and TR surfaces) could be changed by moving the reflector RF. For our experiments, this length was adjusted to h = 17.5 mm. The maximum amplitude of the
ultrasound horn surface was 42 lm. The ultrasound resonator amplitude of pressure at antinode (position A – Fig. 1) reached 104 Pa; consequently, the acoustic velocity at the pressure node (position B – Fig. 1) was 23 m/s. The temperature in the ultrasonic resonator cell, which was cooled by a fan, was measured by a thermocouple T. The experiments were carried out with the ambient air that was supplied into the needle through water and oil separator WOS by a compressor. The airflow through the needle, which was measured in slm units (standard litre per minute), was adjusted by a mass flow controller Bronkhorst. A DC power supply provided voltage up to 30 kV. The needle was ballasted by resistors R = 1–7.4 MX. 2.2. Application of the back airflow In this case the experimental set up shown in Fig. 1 was modified in the following way. The ultrasound high power transducer TR was replaced by a hollow tube ended by metallic mesh at the position A (see Fig. 1). This mesh, substituting the surface of the ultrasound high power transducer TR at the distance d from the needle served as a grounded electrode. Through the tube and the mesh we supplied so-called ‘‘back airflow’’ in the direction opposite to the airflow from the needle electrode. The back airflow, supplied by another compressor, was adjusted by a second mass flow controller. Mesh R 0.60, made by M-Metall Italy, constituted a rhombus with cell dimensions 0.60 · 0.50 mm and thickness 0.15 mm. 3. Experimental results and discussion Experiments described in this chapter were performed either without the application of ultrasound on the discharge or, when ultrasound was applied, the amplitude of the horn surface was 42 lm and the velocity amplitude was 5.3 m/s. The ultrasound resonator amplitude of pressure at antinode reached 104 Pa and the ultrasound velocity at the pressure node was 23 m/s. The airflow through the needle was Q = 5 slm. 3.1. Spread width of the discharge
Fig. 1. Experimental set-up.
Fig. 2 shows photograph of the discharge without ultrasound application (left part) and the discharge if ultrasound is applied (right part) for three distances: d = 4, 6 and 8 mm between the needle and the surface of the ultrasonic horn. When ultrasound is applied, the discharge width varies in proportion to the ultrasound pressure and to the distance d. Placing the tip of the needle to the ultrasonic pressure node d = 4 mm (position B in Fig. 1), antinode d = 8 mm (position D in Fig. 1), or to position C in Fig. 1 does not result in any qualitative differences from a cone shape. The luminous sphere at the tip of the needle (see Fig. 2 top left) is missing if ultrasound is applied (see Fig. 2 top right).
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Fig. 4. Photograph of the discharge. I = 1.8 mA, Q = 5 slm, d = 8 mm. Left – without back air flow, right – with back air flow, QB = 96 slm. Exposition time 1/125 s.
Fig. 2. Photograph of the discharge. I = 1.8 mA, Q = 5 slm, d = 4-8 mm. Left – without ultrasound, right – with ultrasound. Exposition time 1/30 s.
The discharge-spread width from ultrasound application is the result of several complex processes, which are also caused by electrical and aerodynamic interactions. One of these processes is the interaction of air leaving the needle and vibrations of the horn surface. The horn’s surface vibrates with frequency 20.3 kHz (period approxi-
mately 50 ls) with the amplitude 42 lm and velocity 5 m/ s. The airflow leaving the needle after the impact on the horn’s surface creates a thin boundary layer see Fig. 3, taken from [5]. Due to the horn vibrations, this boundary layer is perturbated, which can contribute to the radial spread of the discharge. Another factor, which may explain the discharge spread may be connected with the transfer of mass due to the pressure gradients in the ultrasonic standing wave field near the horn surface. Velocities of air molecules are given by the vector sum of velocities caused by the airflow pattern from the needle and velocities caused by transitions from compressions to rarefactions of the ultrasonic standing wave. These velocities have a strong radial component. This idea is in agreement with the results presented in [4], where, under the influence of transversal movement of air in the acoustic pressure node, a similar discharge spread width was obtained. To confirm the idea that the discharge spread is caused by summing up vectors of different velocities, we tried to substitute the ultrasound field with the flow of air in a direction opposite to that in which the air is leaving the needle. We used the experimental arrangement, described in Section 2.2. This back airflow directed towards the needle removes any possible radial flows. This result is nicely demonstrated in Fig. 4 for the mesh electrode to needle distance d = 8 mm. The discharge without the back flow is in the left part of this figure. With increasing back airflow, the width of the discharge decreases to the shape, shown in the right side of Fig. 4 (back airflow 96 slm or the air velocity at the mesh 32 m/s). 3.2. Discharge voltage shift DU
Fig. 3. Formation of thin air layer close to horn electrode surface [5].
The next effect connected with the application of ultrasound on the discharge is the change of its V-A characteristics. To quantify this effect we introduced the discharge voltage shift DU, the difference between the discharge voltage if ultrasound is applied and the voltage without ultrasound application for the same discharge current. The dependence of the discharge voltage shift DU as a function of current I is shown in Fig. 5.
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Fig. 5. The discharge voltage shift versus current for the distance d = 4, 6 and 8 mm and Q = 5 slm.
This figure shows that the maximum DU occurs for the tip of the needle at the distance d = 4 mm, which corresponds to the node of ultrasound pressure (position B – Fig. 1). Position B at the same time corresponds to the antinode of ultrasound velocity. Particles contained in the ionization region, which surrounds the tip, are exposed to the superposition of velocities due to the flow through the needle and ultrasound velocity. The ionization region is characterized by a high gradient of electric field. If we apply ultrasound in this region, we can influence the electrical parameters of the discharge, which is represented by DU. The change of discharge electrical parameters can be also partially attributed to the variation of the reduced electric field E/n (E is electric field strength and n is the neutral particles density), because it influences ionization rate. Electrical parameters of the discharge are associated with the discharge shape. As was shown in Section 3.1 the discharge shape strongly depends on the back flow
through the mesh. We tried to influence the electrical parameters of the discharge by the back flow with the aim of producing the same voltage shift DU as when ultrasound was applied. To do this, we used an experimental arrangement with the back airflow through the mesh for distance d = 8 mm. Fig. 6 shows the dependence of the discharge voltage shift versus current for back airflows 24, 32, 40, 48, and 96 slm and the discharge voltage shift caused by ultrasound application only. It is seen that applying ultrasound (squares) or applying a back airflow slightly higher than QB = 32 slm (up triangles) produces a similar effect on the discharge voltage shift. However the mechanism of both processes is different. In the case of back airflow, we can influence motion of heavy particles (neutrals and negative ions-mainly oxygen) in the whole discharge region, while in the case of ultrasound application; we mainly influence the processes taking place at the ionization region. 4. Conclusions The effect of power ultrasound application on DC hollow needle to plate or mesh atmospheric pressure electrical discharge enhanced by the flow of air through the needle electrode was studied experimentally. To clarify the role of ultrasound on the discharge we also used the back airflow to induce similar effects to those induced by ultrasound. It was found that ultrasound application increases discharge shape. The discharge shape is affected by back airflow. We also found that ultrasound application influences V-A characteristics of the discharge. We were able to obtain the same change of the discharge voltage as in the case of ultrasound application by the application of the back airflow. The above-mentioned findings are the result of complex processes and can be used for environmental applications such as production of ozone, VOC decomposition and de-NOx processes enhancement. Acknowledgement This research has been supported by the research program No. MSM6840770015 ‘‘Research of Methods and Systems for Measurement of Physical Quantities and Measured Data Processing ’’ of the CTU in Prague sponsored by the Ministry of Education, Youth and Sports of the Czech Republic. References
Fig. 6. Discharge voltage shift versus current for back airflows QB = 24, 32, 40, 48, and 96 slm. Discharge voltage shift caused by ultrasound application. Needle to electrode distance d = 8 mm.
[1] S. Peka´rek, R. Ba´lek, Ozone generation by hollow needle to plate electrical discharge in ultrasound field, J. Phys. D: Appl. Phys. 37 (2004) 1214–1220. [2] S. Peka´rek, R. Ba´lek, M. Pospı´sˇil, Effect of ultrasound waves on electrical characteristics of a hollow needle to plate electrical
R. Ba´lek et al. / Ultrasonics 44 (2006) e549–e553 discharge in air or mixture of air with VOC, B. Am. Phys. Soc. 49 (2004) 21. [3] R.Ba´lek, S. Peka´rek, Z. Sˇlegrova´, Ultrasonic resonator with electrical discharge cell for ozone generation, Ultrasonics, submitted for publication.
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[4] T. Nakane, Discharge phenomenon in a high-intensity acoustic standing wave field, IEEE Trans. Plasma Sci. 33 (2005) 356–357. [5] J. Mizeraczyk, J. Podlin´ski, M. Dors, J. Dekowski, Velocity field in flow stabilized hollow needle-to-plate electrical discharge in atmospheric pressure Air, Czech. J. Physics 52 (Suppl. D) (2002) 769–776.