Acta Polytechnica Acta Polytechnica 53(2):213–218, 2013 © Czech Technical University in Prague, 2013 available online at http://ctn.cvut.cz/ap/ LONG-LIVED PLASMA PROCESS, CREATED BY IMPULSE DISCHARGE IN MICRO-DISPERSE DROPLET ENVIRONMENT Serge Olszewski∗, Tamara Lisitchenko, Vitalij Yukhymenko Taras Shevchenko National University of Kyiv, Radio Physics Faculty, Pr. A. Glushkova 4G, Kyiv, Ukraine ∗ corresponding author: olszewski.serge@gmail.com Abstract. The processes of organic compound (phenol and cation-active surfactants) destruction in water solutions, which stay under the influence of plasma treatment have been investigated in different dynamic plasma-liquid systems (PLS) with discharges in droplet micro-disperse environments (DMDE). The long-lived plasma process with separate spectral properties has been observed for pulsed discharge in DMDE. The approximate computer model is being proposed for a description of this effect. According to the introduced model this long-lived process is the aggregation of correlated discharges between charged droplets. Keywords: dynamic plasma–liquid system, plasma-chemical processing, ultrasonic nebulization, droplet micro-disperse environment. 1. Introduction Water is a valuable natural resource. With metabolic processes in all aspects, forming the base of hu- man living, water plays an exclusive role. Methods based on plasma-chemical processes in the liquid-gas environments for water treatment and purification of highly polluted wastewater are the most promising among the others. Unlike the regenerative methods which remove the collected impurities from the water into the solid (absorption), gas (desorption) or non- aqueous liquid (extraction) phase, the destructive method (technology of water and industrial waste plasma-chemical processing) is based on conversion of the chemical structure of molecules and impurities. The problem of complete cleaning of the indus- trial wastewaters from organic highly active and toxic substances (HATS) is important enough and simul- taneously difficult to apply. However this problem can not be solved completely. Apparently, plasma- chemical technologies are represented by the most perspective ones, as allowing achieving quick speed of substances destruction under the conditions of ex- pense of high-energy concentration. However, it is necessary to take into account that chemical reac- tions occurring in the liquid phase can be stimulated only by particles that penetrate from plasma into the liquid. They can be active radicals created as a re- sult of recombination processes in the plasma phase. They could also be excited molecules that are gener- ated as a result of the bombardment of liquid surface by charged particles of plasma. Therefore the effec- tiveness of plasma-liquid systems (PLS) as applica- tions for liquid treatment is primarily determined by the size of the interaction zone between liquid phase and plasma. The integral feature for all systems with plasmaliq- uid interaction is their little active volume as com- pared with total liquid volume. As a matter of fact, the active volume of plasma-liquid systems is de- fined as the contact area multiplied by the thickness of the diffusion layer of active plasma particle in liq- uid. The contact area is the area of surface of contact plasma and liquid. One of the directions to increase the ratio of active volume to total liquid volume is the using of PLS with liquid solutions in micro-disperse state. For ex- ample, PLS are based on discharges in fog that con- tains drops with size of order diffusion layer thick- ness. Using of the pulsed discharges as sources of de- caying plasma in droplet environment can generate the flows of chemical-active particles onto extended surface of liquid. At the same time, the small size of discrete droplets provides more complete treatment of the total liquid volume. Some features of plasma-liquid systems based on pulsed discharges in the droplets micro-disperse environments have been studied in the present work. 2. Material and methods The scheme and photo of experimental setup are rep- resented in Fig. 1. In this setup the distilled water (4) was sprayed by ultrasound field into inside volume of working vessel (3) and transformesd to monodis- perse fog (10). The quartz cylinder with inside diameter 28 mm and height 150 mm was used as a working vessel. The ultra- sound field was created by quartz crystal (8). The fric- tion of an ultrasound field was 800 Hz and acoustic power ∼ 60 W. For initiating a spark discharge be- tween copper electrodes (1), high voltage ∼ 10 kV was input onto them. The discharge gap between electrodes was 3 mm. The gap between high-voltage electrode and inside surface of quartz vessel was 5 mm. The current of spark was measured by Rogowski coil. 213 http://ctn.cvut.cz/ap/ Serge Olszewski, Tamara Lisitchenko, Vitalij Yukhymenko Acta Polytechnica Figure 1. Experimental setup: a) scheme, b) photo; 1 – metal electrodes, 2 – quartz insulator, 3 – quartz vessel, 4 – up flanges, 5 – side metal walls, 6 – rubber seals, 7 – ultrasonic sprayer, 8 – quartz piezocrystal, 9 – water cooler, 10 – ultrasonic fog, 11 – ultrasonic fountain, 12 – work liquid. Figure 2. Diagram of optical measurements; 1 – copper electrodes, 2 – droplet environment, 3 – pulsed discharge, 4 – filter wheels, 5 – optical-fiber waveguide, 6 – CCD-spectrometer, 7 – PC-recorder. The value of current was ∼ 1 kA. All processes were observed in atmospheric pressure. The emission spectra were registered by CCD- spectrometer SOLAR TII. The scheme of optical mea- surements is shown in Fig. 2. The coordinate axis z was directed along the geo- metrical axis of system down. The point z = 0 was lo- cated in the horizontal plane contained discharge gap. The optical radiation was collected along axis directed perpendicularly to axis z through the filter wheel (4) into optical-fiber lightwaveguide (5). The spec- tra registered by CCD-spectrometer (6) were stored by specialized software in personal computer (7). The typical lead time for slow processes in the vol- ume of system was investigated by analysis of im- ages from video-frames. Video filming was performed by video camera EAN850A with duration of video- frame 30 ms. The video-frame duration itself deter- mined the interval between time readings in experi- ment. According to the result of test-drives for differ- ent video-cameras [7, 9, 8] the typical time interval of CCD-matrix perception is ∼ 1 µs. This time in- terval determined the experiment accuracy in time for slow processes. Figure 3. Photo of plasma processes in micro-disperse droplet environment, a) spark discharge between metal electrodes, b) sliding discharge along inside surface working vessel between high-voltage electrode and wa- ter surface, c) long-lived plasma process with spectral properties different from the previous discharges types. 3. Results and discussion 3.1. Experiment In the course of experiment three different processes in working volume of system were being observed (Fig. 3). Fig. 3a shows the trivial spark discharge between metal electrodes. The breakdown voltage of this dis- charge in water fog was ∼ 7 ÷ 10 kV. The pulsing current had amplitude ∼ 1.5 kA. This discharge had a single channel. The visual lengthwise size of spark channel was ∼ 10 mm. The visual diameter of spark channel was ∼ 1 mm. The duration current impulse of spark discharge was ≤ 10 µs. Fig. 3b is shows the sliding discharge between high-voltage metal electrode and liquid surface. The breakdown voltage of this discharge was ∼ 10 kV. The pulsing current of sliding discharge had ampli- tude ∼ 1.5 kA. This discharge had a multiple chan- nels located near inside dielectric surface of work- ing vessel. The visual lengthwise size of these chan- nels was ∼ 50 mm. The visual diameter of channels was ∼ 1 mm. The duration of current impulse of slid- ing discharge was ∼ 10 µs. Fig. 3c shows the volume process with glow color distinct from the spark and gliding discharges. On the video the evolution of this process was placed on sev- eral consecutive video-frames. As a rule the number of these video-frame was ≥ 4. In some cases their num- ber attained 18. The typical live time of this process was determined by calculation of these video-frame number. This live time was ∼ 120÷540 ms. This pro- cess did not have channels and had a look of transpar- ent luminous cloud. As systematical observations have shown, the necessary condition for start-up of long- lived process is fog existence in area of this process. For the cases of impulsing discharges the breakdown voltage was measured by high-voltage voltmeter, built in power source for reservoir capacitor. The reservoir capacitor was used for energy supply into discharge and in times of discharge breakdown power source was detached from this capacitor automatically. However 214 vol. 53 no. 2/2013 Long-Lived Plasma Process, Created by Impulse Discharge this on-board voltmeter did not register the voltage in system during the time of volume process exis- tence. The discharge current was not registered ei- ther. The maximum output current of power source was 150 mA. Therefore during the time of volume pro- cess existence the current in system was ≤ 150 mA. The results of spectroscopic investigations of impuls- ing discharges in PLS with droplet micro-disperse envi- ronment are shown on Fig. 4. The emission spectrum of spark discharge is represented on plot a) and spec- trum of sliding discharge on b). The spark spectrum was registered along axis with coordinate z = 0 mm. The radiation of sliding discharge was registered along axis with coordinate z = 15 mm (Fig. 2). This axis was distanced from place of spark purposely to avoid the overlapping of spectra of different discharges. Also this method was used for registration of spectra of long- lived process too. The atomic lines of hydrogen was λ = 657.8 nm, of oxygen λ = 778.8 nm and material of electrodes λ = 229.2; 327.8; 524.8 nm are present in spectra for both cases. In these experiments it was the atomic lines of copper. The molecular bands of hydroxyl (286.3 ÷ 343.7 nm) were not registered in radiation of impulsing discharges. The probably cause of absence of hydroxyl bands in emission spectra of both pulsed discharges can be related with presence of metal vapor. The metals have the most low excitation potentials of electron states. Therefore the high concentration of metal atoms can to increase appreciably probability of radiationless deexcitation of hydroxyl molecules. But this supposi- tion requires of screening and goes beyond the scope of this work. The emission spectrum of long-lived process is rep- resented on Fig. 5. Optical radiation of long lived process was registered along axis with coordinate z = 15 mm. The atomic lines of hydrogen and oxygen were also present in this spectrum. However unlike cases of impulsing dis- charges the power band of hydroxyl was present and atomic lines of electrodes material were absent in emis- sion spectrum of long-lived process. The absence of atomic lines of copper in emission spectra of long- lived process can to explain by hypothetical nature of this process. The long-lived process can be related with discharge between charged drops. If it is true then the material of electrodes in this case is the wa- ter and atomic lines of copper can not be present in emission spectra. The spreading of long-lived process in working vol- ume is shown by serial video-frames on Fig. 6. As a result of processing of images from these video- frames the estimation of linear speed of glow area spreading was made. Estimated rate of glow area boarder spreading was ∼ 0.7 m s−1. Such a small value of this speed specifies that the mentioned process cannot be propagation of a combustion wave. Figure 4. Emission spectra of impulsing discharges in PLS with micro-disperse droplet environment, a) spark discharge, z = 0 mm, b) gliding discharge z = 15 mm. Figure 5. Emission spectra of long-lived processes in PLS with micro-disperse droplet environment z = 15 mm. 3.2. Physical model Statistical analysis of experimental data has shown that the necessary conditions of durable process ap- pearing are obligatory presence of water fog in the vol- ume with discharge. The hypothesis, that long-lived process for given conditions is the correlated multi- ple spark (CMS) discharge between charged and un- charged fog drops during their approaching has been introduced. To verify this hypothesis, the approximate model of long-lived process was created. According to the model, fog drops located in the area impulsing discharge channel gain an electric charge due to their contact with plasma. In the result of the Brownian motion they are mixed in the volume with the un- charged fog drops. Chaotic motion of aerosol particles 215 Serge Olszewski, Tamara Lisitchenko, Vitalij Yukhymenko Acta Polytechnica Figure 6. Spreading of long-lived process in volume on serial video-frames. leads to the approaching of single drops on the lengths in order of magnitude of their radii. In the case of such approaching between the fog particles with different charges, or between charged and uncharged parti- cles, the electric field appears with the magnitude that can be much larger (according to [5, 3]) then the one calculated using the Coulomb’s law. Electric fields’ value between the single particles is also related to self-consistent electric field, formed by the ensemble of charged drop, chaotically distributed in the work- ing space. Spatial redistribution of charged particles in time is defined by the Langevin equation with the additional determinate force, which has an elec- trostatic nature, ξxtr dx dt = fx(t) + Fx , (1) where ξxtr is drag coefficient for a ellipsoidal drop, fx(t) is random force and Fx is external electrostatic force. According to [5], the drop can be split into the parts as a result of Rayleigh or Taylor instability. As far as Taylor instability depends on the outer electric field and can be developed even for the electro neutral drops, in case of charged aerosol drops ensemble it is more probable. According to [6], charged liquid drop always acquires an ellipsoidal form. An instability criterion for ellipsoid drops is given by the inequality ε0E 2 SR 4αS ≤ 1.54 (2) according to Taylor [4], wherein R is the drops’ radius, αS is coefficient of surface tension, ES is the value of outer electric field and ε0 is absolute dielectric permittivity. The conditions of spark ignition dis- charge between the drops, according to [2], are defined by the electric field value in kV/cm as ES = 27.2 ( 1 + 0.54 √ R ) , (3) wherein R is radius of drops. The conditions of spark breakdown require more intensity of the electric field, comparing to the case of drops’ break-up according to the capillary surface instability of Taylors’ criterion. In cases of quasi- static systems, spark breakdown between micro-drops of electro conductive liquid is low probable. But ac- cording to [1] the typical time of capillary instability development can be estimated as τ ∼ R2ρ1/2α1/2S , (4) wherein ρ is liquids’ density. This time range is more than in 6 times of magnitude greater than the time range of spark discharge. Hence, in dynamic systems the conditions can be carried out when the electric field between neighbor drops can increase to the value enough for spark discharge ignition during period of time less than the typical time of instability devel- opment. Such an increasing of the electric field can be provided either by rate of charged and uncharged drops approaching, or by the superposition of charged aerosol drops’ self-consistent field and vortex electric field produced by the alternating current of spark dis- charge between neighbor pair of drops. The latter mechanism is also an extra discharges correlation fac- tor between the approaching drops’ pairs, because it relieves the breakdown conditions due to the photo- electric effect and charged particles diffusion, and leads to the impulse increasing of the electric field value. 3.3. Simulation According to presented physical model, 3D computer model has also been developed. Due to the orbital sym- metry of ellipsoidal drops, the number of dimensions can be reduced to 2D, so the working space was chosen more like the experiment only for two coordinates, and the third dimension was chosen as contracted in two orders of magnitude: 0.02 × 2 × 10 cm. Fogs density was chosen as 5 × 103 cm−3. On the first step of calculation the ensemble of fog drops was created, with Gamma-distributed character- istic sizes and random coordinates inside the working space, n(a) = µµ+1 Γ(µ + 1) aµ r µ+1 m exp ( −µ a rm ) , (5) wherein Γ(µ + 1) is gamma function, rm is the most probable drops radius and µ is half-width of distribu- tion. Initial velocities of particles were generated accord- ing to the Maxwell distribution. For each N-th par- ticle random charge was specified, so that value 1/N could be distributed in space according to the Gaus- sian law. On the next step self-consistent electric field in in- stant coordinate of each particle was calculated. Then, for each pair of particles the correction to the electric fields’ intensity [3, 6], criterion of elec- trical break-down – Eq. 3, and particles’ break-up criterion were calculated – Eq. 2. For the pairs of drops, which conformed to the breakdown criterion, the break-down impulse discharge current was calcu- lated. For the drops conform to the break-up criteria, 216 vol. 53 no. 2/2013 Long-Lived Plasma Process, Created by Impulse Discharge the initial time point of instability development was fixated. If in the process of further evolution integral time, during which the break-up criteria was fulfilled, exceeded the time of instability development, such pair of drops was replaced by the ensemble of drops with typical sizes and integral charge according to [4]. For the particles conform to the criterion of corona discharge   τapproach ≤ R2ρ1/2α 1/2 S , ES ≥ 50 kV, r ≥ 10R, (6) the loss of charge took place, been calculated as the dis- charge current multiplied by the time step. The pair of drops, approaching at the distance equal to the sum of their radii, was replaced by the one particle with the integral volume and charge. On the next step, the Langevin equation Eq. 1 was numerically solved for each drop, and on the next step the recal- culation of particles ensemble spatial evolution was took place. After that, iteration was repeated. Cal- culations stopped when the linear velocity of glow boundary of CMS-discharges area was formed. As simulation result it was calculated how the ar- eas that contained multiple spark discharges between drops were propagated in space. The evolution of sim- ulated CMS-discharge is shown in Fig. 7. This evolution is represented by space distribution of spark current in different time stations. The black points correspond to CMS-discharge state after 25 ms of charged cloud creation. The dark grey points cor- respond to time station 50 ms and light-grey points to 100 ms. The comparison of experiment and simulation re- sults is presented in Fig. 8. This plot represents the dependence of border lo- cation of long-lived process glow area upon time. The black curve with round points corresponds to re- sults of experimental measurement. The grey curve with triangular points corresponds to the results of simulation. The experimental estimated average value of linear speed of long-lived process spreading is 0.7 m s−1. The calculated rate of CMS-discharge area boundary spreading is ∼ 0.4 m s−1. This result is conformed to experimental measured values. The mean radius of drops that support CMS- discharge is ∼ 0.8 ÷ 1 µm. For drops’ size ≤ 10 µm the atomization probability is greater then probability of spark breakdown, because for this case drops have smaller mean velocity and capillary instability has enough time to progress. For drops’ size ≤ 0.5 µm the probability of lost charge in the result of corona discharge is greater then probabilities of the other con- sidered remainder elementary processes of the drops state transform. The increase of self-consistent field value reduces the probability of spark breakdown, since it re- lieves the conditions of capillary instability develop- ment. However the field gradient enhances condi- Figure 7. Space distribution of areas that contained pairs of drops with spark discharges between it. Figure 8. Dependence of position of glow border on time; the black curve represent results of experi- mental measurements; the grey curve represent results of simulation. tions of spark breakdown for sufficiently rapid drops. On the periphery of charged fog cloud the gradi- ent of self-consistent field mainly hinders the elec- trical field increase during drops approach. It is right for charged drops that are moved from charged cloud to periphery. But chaotic pattern of drops movement provides a grate drops number with velocity that is oriented to self-consistent field. Just the ensemble of these drops is the main source of CMS-discharge existence. 4. Conclusions Comparison of calculation and experimental results gives the following conclusions: • In terms of the introduced model, long-lived plasma process created by the impulsing discharge in micro-disperse droplet environment can be pre- sented as correlated multiple spark discharge (CMS- discharge) between discrete pairs of differently- charged drops. 217 Serge Olszewski, Tamara Lisitchenko, Vitalij Yukhymenko Acta Polytechnica • The self-consistent electrical field of charged droplets ensemble decreasei the probability of CMS- discharge initiation on periphery of charged fog cloud and increase the probability of droplet frag- mentations due to Taylor instability. • The calculated speed and experimentally measured speed of propagation of glow boundary of CMS- discharge match together within the order of magni- tude. This speed corresponds to submicron size of drops that predominately support the CMS- discharge. Acknowledgements The present work was partly supported by the Kyiv National Taras Shevchenko University, by the National Academy of Sciences of Ukraine, by the Ministry of Edu- cation and Science of Ukraine. References [1] D. F. Belonoszko, A. I. Grigoriev. The characteristic time of instability droplets charged to rayleigh limit. Technical Physics Letters 25(15):41–45, 1999. [2] E. D. Lozanskij, O. B. Firsov. The theory of the spark. Atomizdat, Moscow, 1975. [3] M. V. Mirolubov, et al. Methods for calculating electrostatic fields. Higher School, Moscow, 1963. [4] V. M. Muchnik, B. E. Fishman. The electrification of coarse aerosols in the atmosphere. Gidrometeoizdat, Moscow, 1982. [5] V. A. Saranin. Some of the effects of electrostatic interaction of water drops in the atmosphere. Technical Physics 65(12):12–17, 1999. [6] A. A. Shutov. The form of drops in a constant electric field. Technical Physics 72(12):15–22, 2002. [7] A. Zajtsev, et al. Testing of a network television camera Samsung SNP-5200H. PROSystem CCTV 1:28–45, 2012. [8] A. Zajtsev, et al. Testing of a network television camera Sony SNOEM52. PROSystem CCTV 2:60–72, 2012. [9] A. Zajtsev, et al. Testing of the network video registrar Softtera NVR-SR5Q24. PROSystem CCTV 1:46–53, 2012. 218 Acta Polytechnica 53(2):213–218, 2013 1 Introduction 2 Material and methods 3 Results and discussion 3.1 Experiment 3.2 Physical model 3.3 Simulation 4 Conclusions Acknowledgements References