Investigation of Anode Attachment Area in Water/Argon Stabilized Plasma Arc

Transcription

1 WDS'15 Proceedings of Contributed Papers Physics, , ISBN MATFYZPRESS Investigation of Anode Attachment Area in Water/Argon Stabilized Plasma Arc P. Ondáč, 1,2 A. Mašláni, 1 and M. Hrabovský 1 1 Institute of Plasma Physics AS CR, Prague, Czech republic. 2 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Arc-anode attachment in a DC plasma arc stabilized by water vortex and argon, influences power distribution in the plasma, lifetime of the anode and also flow structure of the plasma jet. A movement of the attachment and the surrounding plasma has been directly observed by using a high-speed camera (max. 1,080,000 fps). The observations were compared with cathode anode voltage measurements (sample rate 80 MHz). More than one attachment was observed in one frame with exposure time 0.25 µs or 0.29 µs. We have directly measured average velocities of the attachments and hydrodynamic waves above them, as well as the characteristic dwell times and dwell frequencies of the attachments. Introduction Hybrid water gas dc arc plasma torch [Brezina et al., 2001] is used for plasma spraying [Kavka et al., 2011] or bio-waste gasification [Hlina et al., 2014]. In general, the plasma torches are used also for plasma arc cutting [Kavka et al., 2013] or for spheroidization of particles. The electric arc burns between an internal cathode and an external anode which is copper disk located downstream of the exit nozzle. The arc anode processes affect properties of generated plasma jet (visible part of the plasma flow) and lifetime of the anode. The lifetime of the anode determines the lifetime of the whole torch. In this paper, the so called restrike anode mode is studied. During this mode the anode attachment moves periodically downstream along the upper anode surface. This anode mode was described in gas stabilized plasma arc [Eckert et al., 1967; Zhukov et al., 1975] as well as in the water/gas stabilized plasma arc [Hrabovsky et al., 1997; Hrabovsky et al., ; Chumak et al., 2012], where the anode attachment region is well visible. However, the motion of the anode attachment is still not a well understood phenomena. Especially, mechanisms that control the motion of the anode attachment and process of reigniting the anode attachment in the upstream part of the anode surface. It is assumed that the position of the anode attachment is determined by the balance of the gas dynamic drag force acting in the downstream direction and the Lorentz force acting in the upstream direction due to the double curvature of the anode attachment [Wutzke, 1967; Russ et al., 1993]. The purpose of this work is to show the new detailed experimental observations of the anode attachment motion in the restrike mode. For this purpose, we have used a high-speed camera. For the first time, the anode attachment area was observed by using so high frame rate (300,000 fps and 1,080,000 fps) with an exposure time of 0.29 µs and 0.25 µs respectively. The description of the experimental setup and diagnostics used for our measurements is given in the second section. In the third section, the results are presented and in the last section, we discuss these results and also announce our future plans for further investigation. Experiments We have performed our measurements on a plasma torch (Fig. 1) with dc electric arc stabilized by water vortex and argon [Brezina et al., 2001]. The plasma was created from the water and the argon. The rotating copper anode and 2 % thoriated tungsten cathode are cooled down by a separate water circuit. During rotation, the anode surface moves perpendicular to the main stream of the plasma jet with the velocity around 47 m/s. The thickness of the anode 245

2 Figure 1. Sketch of the plasma torch with dc electric arc stabilized by the water vortex and argon. Figure 2. The arc-anode attachment area detail. We see 2 attachments and radiation has wavelength of (810 ± 12) nm. Exposure time was 0.29 µs. is (16 ± 1) mm and the vertical distance between the bottom edge of the gap (with diameter about 6 mm) in the exit nozzle and the upper anode surface was around 0.8 mm. The horizontal distance between the exit nozzle and the anode was about 4.5 mm. We have filmed the arc-anode attachment area by using the monochromatic high speed camera Photron FASTCAM SA-X2 (Model 1080K) with maximum frame rate 1,080,000 fps. Because of strong radiation and the brightness of the plasma jet, an optical filter in front of the camera objective was needed. We have used the optical filter with CWL = 809 nm and FWHM = 12.5 nm. The detailed image of the anode attachment area taken by this camera with the optical filter is shown in Fig. 2. The electric current between the cathode and anode is typically in the range of A, the maximum plasma temperature around K, the water consumption around 3 l/h and the argon consumption in the range of around 6 40 slm. The arc current is kept constant and the cathode anode voltage periodically changes in saw-tooth form, mainly because of the periodical movement of the anode attachment. In all cases mentioned in this work, the plasma jet was at atmospheric pressure. We have also measured the cathode anode voltage with sampling rate 80 MHz, by using the high voltage probe Tektronix P6015 (1000, 3 pf, 100 MΩ) connected with the PC Oscilloscope (PicoScope). The videos were synchronized with the cathode anode voltage measurements with the help of the synchronized signal from the camera to the PicoScope. The whole experimental setup is illustrated in Fig. 3. Three isolation transformers were used in order to reduce the electric noise in the used cables. Results We have investigated the arc-anode attachment area under 6 combinations of the plasma torch parameters: three values of the arc current (300 A, 400 A and 500 A) and two values of the argon flow rate (12.5 slm and 22.5 slm). 246

3 Figure 3. The experimental setup that we have used for our measurements. First we have calculated the velocities of the anode attachments defined like bright connection between the main stream of the plasma and the anode surface (see Fig. 2). They were calculated from the images by recording their horizontal positions of the upstream attachment edges (closer to the exit nozzle) frame by frame (frame rate 300,000 fps, pixel resolution), with respect to the time. The typical position vs. time graphs for the attachments are shown in Fig. 4b. The position vs. time graphs were approximated by the linear trend line and the average attachment velocities were calculated like average value of the slopes of these linear trend lines. We were omitting the position vs. time data in the anode edge area (more than 14 mm in position), because in this area the anode surface is curved, and we were interested only in the movement of the attachments on the straight anode surface. The corresponding velocity vs. time graphs (Fig. 4d) were obtained from the position vs. time graphs by the numerical differentiation using the symmetric difference quotient (we used the gradient Matlab function). From the velocity vs. time graph in Fig. 4d, we can see the typical behaviour of the anode attachments. They periodically move and stop at certain positions of the anode surface. We also calculated characteristic dwell time and characteristic dwelling frequencies for the attachments. The characteristic dwell time is meant to be the average time during which the attachment dwells/stops at a certain position of the anode surface. The average frequency with which the attachments come to a stop is called the characteristic dwelling frequency, in this work. The calculated values for 6 mentioned combinations of the plasma torch parameters are shown in Table 1. Every value of the average attachment velocity was obtained from 6 attachments (totally 36 attachments were investigated), and every value of the characteristic dwell time (average value) and dwelling frequency was obtained from the time interval 1 ms. The uncertainties in the Table 1 are total standard uncertainties obtained like combination (by the summation in quadrature) of the standard uncertainty of the mean with the average standard uncertainty of each measured/computed value. In the case of the arrachment velocities the standard uncertainty of each computed value is computed uncertainty of each slope with known uncertainty on position data (due to image resolution). In the case of the characteristic dwell times the uncertainty of each measured value was estimated like 2 µs. The total uncertainty of the dwelling frequencies was estimated. As we can see in Fig. 4a the dwelling of the attachments 247

4 Figure 4. In the graphs (b), (d), the typical position time dependences and corresponding velocity time dependences for two attachments are shown. The graph (a) is the synchronized cathode anode voltage time dependence. The image of the arc-anode attachment area (c) was taken in the time of µs of the video with exposure time 0.29 µs. Figure 5. The hydrodynamic wave in the plasma jet in 2 consecutive frames with exposure time 0.29 µs. The downstream direction is to the right. Visible radiation has wavelength of (810 ± 12) nm. at a certain position is also visible in the cathode anode voltage time dependence like a constant value of the voltage for a short time. In addition, the sampling rate for the cathode anode voltage measurements (80 MHz) was much greater than in the case of the position time dependence (300 khz). Therefore, we calculated the characteristic dwell time and dwelling frequencies from these voltage time dependences. We have also directly measured wavelengths and velocities of the hydrodynamic waves in the plasma jet. They are result of Kelvin Helmholtz instability and arise because of the velocity and density gradient between the low density, fast plasma jet and high density, steady 248

5 Table 1. In the green part of the table, the calculated values of the average attachment and wave velocities and characteristic dwell times and dwelling frequencies for the anode attachments are shown. They were calculated for the 6 combinations of the plasma torch parameters (first 2 columns of the table). ambient atmosphere. These waves have already been studied in the plasma jet with water/argon stabilization indirectly, by using an array of high frequency photodiodes [Kopecky et al., 2003; Kopecky et al., 2011; Hrabovsky et al., 2001]. In this work they were studied first time directly, by using the high speed camera. We have observed they start to be clearly visible above the anode. It is probably because of the clouds of the anode material that amplify them. Especially they are strongly amplified when they propagate just above the attachment. Above the anode their wavelength was (7 ± 1) mm. This wavelength seems to be independent of the arc current and argon flow rate and only slightly increases with the distance from the exit nozzle. The velocities of these waves were calculated directly from differences of horizontal positions of their crest in one frame and the next frames, as we can see in Fig. 5. Every value of the average wave velocity in Table 1 was calculated from 8 waves. The total standard uncertainty and average value of the wave velocities were computed exactly just like in the case of the attachment velocities. Discussion and Conclusions We have observed more than 1 attachment defined like bright connection between the plasma jet and the anode surface in one frame with exposure time 0.29 µs. These bright connections are probably always also the conductive connections between the plasma jet and the anode surface, but sometimes, for a very short time, they can be only afterglows of previous conductive connections. As there are sometimes more attachments together, often, the later one catches up with the earlier one. The later attachment is stopping by the earlier one and after catching up, they move together for a little. If there is long enough distance from them to the anode edge, they separate again. We have not observed the double curvature of the anode attachment, proposed in the previous publications [Wutzke, 1967; Russ et al., 1993]. It is possible, that the second curvature of the anode attachment is very close to the anode surface and is small. Therefore it is not easily visible, even in our detailed images of the anode attachment area. There is also possibility, that in the upstream direction, a different magnetic force, caused by electromagnetic induction, acts on the induced electric current. As the anode attachment moves downstream, the magnetic flux 249

6 in the area between the electric arc and the anode increases with time, and the opposite electric current is induced within the electric arc. This induced electric current has opposite direction to the main arc current and the total arc current is therefore decreased. The arc-anode attachments very often come to a stop at certain positions of the anode surface. We think that these positions are anode surface inhomogeneities, where anode material vaporizes faster than in the immediate vicinity. The clouds of the anode material are drifted downstream by the plasma flow, and the anode attachments incline downstream. The average anode attachment velocity and hydrodynamic wave velocity increases with the arc current as well as with the argon flow rate (see Table 1). For greater value of the arc current the water evaporation rate and consequently also a pressure in the plasma torch cavity is greater. Similarly, the faster argon flow rate causes greater pressure in the cavity and the greater pressure gradient between the cavity and the ambient atmosphere causes faster flowing of the plasma jet. The characteristic dwell time for the anode attachments is around µs and it seems to be independent of the arc current and the argon flow rate. The characteristic dwelling frequency for the attachments also increases with the arc current and with the argon flow rate. It means, the faster the attachment moves the more often it comes to a stop. This is well understandable if we consider that the faster movement does not prevent them from dwelling at all accessible anode inhomogeneities. We have confirmed our calculations of the attachment velocities also in the video with frame rate 1,080,000 fps and exposure time 0.25 µs. But because of the worse pixel resolution (128 8), the final velocity uncertainties were greater and we don t present these results here. The directly obtained velocities, wavelengths and corresponding frequencies of the waves, shown in this work, correspond with previously obtained velocities, wavelengths and frequencies of hydrodynamic waves, which were measured by using an array of high frequency photodiodes [Kopecky et al., 2003; Kopecky et al., 2004; Hrabovsky et al., 2001]. Also according to the theory of homogeneous incompressible fluid with triangle velocity profile, the wavelength of sinusoidal disturbances in the flow is independent of the flow velocity and is proportional only to the local thickness of the flow (the stream)[bejan, 2004]. For all examined combinations of the plasma torch parameters the anode attachments have been moving around 3 4 times slower than the plasma flow waves above them. The relatively great uncertainties of the calculated attachment and wave velocities result mainly from the bad resolution (256 80) of the images. A precise determination of the position of the attachments or of the crest of the waves was not possible. In the future, we would like to do better statistics for our measurements (to process more attachments and waves, and longer voltage time dependence). Especially the calculating of the characteristic dwell times and dwelling frequencies is fast and can be easily done for a longer time interval. We would also like to compare the directly obtained wave velocities with indirectly calculated plasma flow velocities for our 6 mentioned combinations of the plasma torch parameters. That indirect calculations of the plasma flow velocity, for hybrid plasma torch, were already made in the past [Kavka et al., 2007]. Acknowledgments. the project GA S. The work was supported by the Grant Agency of the Czech Republic under References Bejan, A., Convection heat transfer, John Wiley & sons, inc., 3rd edition, USA, Brezina, V., M. Hrabovský, M. Konrád, V. Kopecký, and V. Sember, New plasma spraying torch with combined gas-liquid stabilization of arc, 15th International Symposium on Plasma Chemistry, Orléans, France, 2001, Vol. III, pp Chumak, O. M., A. Mašláni, and M. Hrabovský, Experimental observations of arc-anode attachment in steam argon air environment, Journal of Physics: Conference Series, Vol. 406, conference 1, 2012,

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