Gliding arc in tornado using a reverse vortex flow

Transcription

1 REVIEW OF SCIENTIFIC INSTRUMENTS 76, Gliding arc in tornado using a reverse vortex flow Chiranjeev S. Kalra, Young I. Cho, a Alexànder Gutsol, and Alexander Fridman Department of Mechanical Engineering and Mechanics, Drexel Plasma Institute, Drexel University, Philadelphia, Pennsylvania Tecle S. Rufael ChevronTexaco Energy Research and Technology Company, Houston, Texas Received 18 February 2004; accepted 22 November 2004; published online 21 January 2005 The present article reports a new gliding arc GA system using a reverse vortex flow tornado in a cylindrical reactor gliding arc in tornado, or GAT, as used to preserve the main advantages of traditional GA systems and overcome their main drawbacks. The primary advantages of traditional GA systems retained in the present GAT are the possibility to generate transitional plasma and to avoid considerable electrode erosion. In contrast to a traditional GA, the new GAT system ensures much more uniform gas treatment and has a significantly larger gas residence time in the reactor. The present article also describes the design of the new reactor and its stable operation regime when the variation of GAT current is very small. These features are understood to be very important for most viable applications. Additionally the GAT provides near-perfect thermal insulation from the reactor wall, indicating that the present GAT does not require the reactor wall to be constructed of high-temperature materials. The new GAT system, with its unique properties such as a high level of nonequilibrium and a large residence time, looks very promising for many industrial applications including fuel conversion, carbon dioxide conversion to carbon monoxide and oxygen, surface treatment, waste treatment, flame stabilization, hydrogen sulfide treatment, etc American Institute of Physics. DOI: / I. INTRODUCTION gree of nonequilibrium to support selective chemical processes. That is why one of the vital challenges of modern discharge physics is to combine the advantages of both thermal and nonthermal plasma systems by developing powerful and high-pressure discharges that can generate a nonequilibrium plasma, which in turn can be applied to large-scale exhaust gas cleaning, pollution control, fuel conversion, hydrogen gas production, and various surface treatments. One of the possible ways to create powerful nonequilibrium plasma is to use the transient gliding arc GA discharge. 3 This periodic discharge evolves during a cycle from arc to transitional discharge with a relatively high level of electron density. Thus, the transient gliding arc can be very effective for the above-mentioned industrial applications. The concept of a gliding arc in a flat geometry was investigated by several researchers during the last decade and showed promising potential. 4,5 However, this GA technology has drawbacks, which include: initial equilibrium regime: flat two-dimensional 2D electrode geometry incompatible with most industrial systems; extremely nonuniform gas treatment because most of the incoming gas flows around the arc; low gas residence time in the reactor because of high gas velocity demand. All these drawbacks reduce the efficiency and the degree of conversion in plasma processes. The objective of the present study was to develop an innovative method to generate transitional gliding-arc plasma in a cylindrical three-dimensional 3D geometry using a reverse vortex flow RVF. It was found that the properties of the electrical discharges in the reverse vortex flows i.e., tornado differ significantly from the properties of disa Electronic mail: choyi@drexel.edu Plasma-chemical systems are traditionally divided into two major categories: thermal and nonthermal, both of which are characterized by their specific advantages and disadvantages. 1 Thermal plasma systems include, first of all, arcs and radio frequency inductively coupled plasma rf ICP, and are associated with Joule heating and thermal ionization. These systems enable the delivery of high power i.e., over 50 MW at high operating pressures. However, thermal systems generate plasmas without excitation selectivity and at very high temperatures, which cause serious quenching requirements and electrode erosion problems for arcs ; these features in turn result in limited energy efficiency and applicability of the thermal plasma sources. 2 Nonthermal plasma systems include, first of all, low pressure glow, rf, and microwave discharges and offer high selectivity and energy efficiency of plasma chemical reactions. These systems are able to operate effectively at low temperatures without any special quenching. However, operating pressures and power levels of the nonthermal discharges are usually limited, restricting the production rates for large-scale industrial processes. 2 Conventional thermal and nonthermal discharges cannot simultaneously provide a high level of nonequilibrium, high electron temperature and high electron density. However most prospective plasma chemical applications require both high power for efficient reactor productivity and a high de /2005/76 2 /025110/7/$ , American Institute of Physics

2 Kalra et al. Rev. Sci. Instrum. 76, FIG. 1. Conventional gliding arc Ref. 10. a Schematic of gliding arc: i point of gliding arc ignition, ii point of developed gliding arc when maximum energy is transferred, and iii point of total extinction. b picture of the flat gliding arc discharge. charges in the linear axial flows. 6,7 The most significant difference was for the nonequilibrium discharges, as the RVF provides an intense convective cooling of the discharge zone and a perfect thermal insulation for the discharge zone from the walls of the reactor. 7,8 This difference should make the gliding arc in tornado practical and attractive for many industrial applications. II. TRANSITIONAL GLIDING ARC DISCHARGES AND REVERSE VORTEX FLOW High-pressure nonthermal gas discharges are considered cold discharges when the gas temperature does not change significantly or thermal ionization does not take place, for example, in corona or dielectric barrier discharges. On the other hand, gas becomes hot in thermal discharges where the thermal effect in ionization is predominant as in the case of thermal arc plasmas or in rf ICP. There are also discharges with plasma parameters between those of the thermal and nonthermal discharges. Such discharges should be considered as transitional nonthermal discharges, where the gas temperature increases considerably but the discharges still are not in the thermal regime. Gas temperature in the case of transitional discharges is on the order of K, which is much less than the electron temperature K. The ionization of gasses in transitional discharges is defined by direct electron impact or stepwise electron ionization. 2 Direct electron impact involves the interaction of an incident high-energy electron and a neutral atom or molecule. In the stepwise mechanism electron energy for ionization may be significantly lower as molecules are primarily electronically excited. As mentioned earlier, transitional plasma parameters can be realized in the transient gliding arc discharge. 9 Figure 1 shows a sketch of a conventional gliding arc system, generating a nonstationary discharge. 9,10 The arc starts in a narrow gap at the low end marked i in Fig. 1 between two diverging flat electrodes in a gas flow. It starts immediately after breakdown, a process that takes place when the electric field in the gap is high enough to ignite the arc. The current of the arc increases very fast and accordingly the voltage on the arc drops rapidly. If the gas velocity is strong enough to push the arc downstream, it forces the arc to move along the diverging electrodes i.e., to glide and elongate. The elongated arc demands more power to sustain itself. It continues to elongate until the power supply can no longer compensate the energy lost through heat transfer to the surrounding gas. At this point the arc cools down and finally extinguishes. Immediately after the extinction, the next cycle starts as the voltage across the narrow gap between the two electrodes reaches the breakdown value, an event which usually occurs just after the fading of the previous arc. 3 The GA can exist in two different regimes: the highcurrent GA HCGA, with current, J 10 A and the lowcurrent GA LCGA, J 10 A. The former starts as an equilibrium discharge and is associated with thermal ionization effects, whereas the latter is a nonequilibrium transitional discharge during the whole cycle of the evolution. As a simplification, HCGA can be considered a conventional thermal arc with strong convective cooling by a fast transversal gas flow and with specific boundary conditions on the electrodes. On the other hand, LCGA can be considered a high voltage atmospheric-pressure discharge HVAPD, 11 also with strong convective cooling by fast transversal gas flow, and with specific boundary conditions on the electrodes. In general, if intensive cooling of any electrical discharge is compensated by the increase in the electric field strength, the role of nonequilibrium mechanisms of ionization increases. Thus, in order to get more nonequilibrium conditions in the arc discharge, it is necessary to increase the cooling of the discharge without an increase in the current strength. This is the reason why the GA with intensive convective cooling is more nonequilibrium than a HVAPD with the same current. The LCGA is thus a good example of a transitional discharge, providing benefits of both thermal and nonthermal discharges: high plasma density, high power and high operating pressure typical for thermal plasma systems and a high level of nonequilibrium, high electron temperature, intermediate gas temperature, 9 resulting in the possibility of stimulating selective chemical processes without the need for quenching typical for nonthermal plasma systems. These properties of transitional GA discharges make it attractive for many industrial applications. The present work utilized a RVF in a cylindrical volume, the flow which is similar to the natural tornado. To obtain the RVF, pressurized gas entered the cylindrical volume tangentially and the gas flow exited from the top of the cylindrical volume, i.e., the same side as the gas entry was placed see Fig. 2. The diameter of the exit was considerably smaller than that of the cylindrical vessel. Figure 2 shows the flow direction of gas in the RVF used in the present study: i three-dimensional sketch of flow and ii sketch of streamlines on the axial plane. The RVF had been used in atmospheric-pressure electrical discharges such as microwave MW discharge, 7 and rf ICP. 12 The tornado geometry was also applied to gaseous flame, 13 which could be also considered a low-temperature plasma. In the cases of MW discharge and rf ICP plasma generators and gas combustion chamber, the RVF was compared experimentally through calorimetric investigations and numerically with conventional forward vortex flow.

3 Gliding arc in reverse vortex flow Rev. Sci. Instrum. 76, FIG. 4. Flow visualization for reverse vortex flow with dye injection to air stream before inlet nozzles. FIG. 2. Reverse vortex tornado flow in a cylindrical reactor. i Threedimensional sketch of flow. ii Stream lines sketch on axial plane. The thermal efficiency of a reverse vortex was found to be much better than that of the forward vortex flow system. 8 The tornado flow obtained in the RVF ensures high gas velocities necessary for the gliding arc and very effective heat and mass exchange at the central zone of the plasma inside the cylindrical volume because of the fast radial migration of the turbulent micro-volumes and deceleration near the tube walls. 8,14 Thus, development of the GAT makes most use of the properties of the RVF for enhancing nonequilibrium LCGA plasma parameters. III. EXPERIMENTAL SETUP FOR A NEW GA REACTOR IN A CYLINDRICAL GEOMETRY A. Gas flow Figure 3 shows the present plasma reactor system used to produce a gliding arc in reverse vortex flow inside a quartz tube i.d.=40 mm; L=150 mm indicated by 1 in Fig. 3. The tube enclosed a cylindrical volume 2. At one end of the tube, a swirl generator 3 with tangential inlet holes to the cylindrical volume injected air or reacting gasses. Gas outlet 5 in the plasma reactor was on the same side as the swirl generator, setting up reverse vortex. An axial inlet 4 was located at the bottom end of the cylindrical reactor providing the flexibility to introduce additional air or other reacting gases as desired. Typical swirl velocities inside this reactor FIG. 3. Experimental setup with a ring electrode, b with spiral and ring electrode. varied from 10 to about 60 m/s, and axial velocity at the outlet 5 ranged from 2 to 10 m/s in cold conditions. Residence time varied according to the flow rate. B. Electrode setup inside the reverse vortex flow Gliding arc requires electrodes to be in the plane of the gas flow. In case of the reverse vortex this plane of flow is a cylindrical surface parallel to the walls of the cylindrical volume Circular electrode Figure 3 a shows the present GA generation system, which consists of a circular disk-shaped electrode 7 as the anode at the top-end of the reactor and a circular ring electrode 8 as the cathode. Note that although the circular disk was used for the anode, the sharp inner edge of the circular disk, which formed the top-end of the cylindrical reactor, was located where the arc ignited and remained. Thus, the present reactor essentially used two circular electrodes. This setup required the ability to move the electrodes apart to increase the distance between them and consequently increase the length of the gliding arc. This was accomplished by a crank and a screw-jack mechanism 9 to get the linear motion of the ring electrode. The two electrode systems in Fig. 3 a could be considered parallel electrodes in a circular geometry with the ability to increase the distance between the electrodes. 2. Spiral electrode Figure 3 b shows another interesting GA generating system, which had a more complex design for the electrodes but without moving parts. A spiral electrode 6 was used as cathode, which was placed inside the cylindrical volume coaxially with the tube. It was important to design the spiral cathode such that it caused minimum disturbance to the flow inside. Thus the helix angle of the spiral electrode was made identical to the pattern of the reverse vortex flow inside the cylindrical reactor. Therefore, the spiral electrode was parallel to the streamlines of the RVF and caused no or very little disturbance to the flow. The angle of the spiral electrode shape for the present reverse vortex flow setup was determined using flow visualization techniques as demonstrated in Fig. 4 and numerical modeling of the RVF. 6 In the spiral electrode arrangement, the initial breakdown started between the top circular disk electrode and the tip of the spiral electrode, which was at the top and closest to the anode. This arrangement can be considered diverging electrodes in a circular geometry. At the opposite end of this spiral, a ring

4 Kalra et al. Rev. Sci. Instrum. 76, FIG. 6. Elongation of gliding arc along parallel electrodes due to difference in gas velocity near the two electrodes. 1,2,3,4, and 5 represent sketches of one arc at different times as the gas flow drags the arc along parallel electrodes. FIG. 5. A series of photographs of GAT on a ring electrode. Photographs taken with different exposition times varying from in a to 0.2 in b. electrode was placed Fig. 8 and 10 in Fig. 3, which was smaller in diameter than the spiral itself, providing a stabilization substrate when the arc was fully elongated and could not elongate further. C. High voltage connections and power supply The spiral and the ring electrodes see 6, 8, and 10 in Fig. 3 were connected to the high-voltage power supply Universal Voltronics, and the flat circular disk electrode 7 was used as the ground. It was possible to change the resistance applied in series to the plasma channel from 12 to 200 k in order to have the flexibility to vary the power characteristics of the gliding arc. The initial breakdown for 10 kv occurred in air at an approximate 3 mm gap between the anode and cathode. Subsequently the arc was elongated according to the reactor geometry. IV. RESULTS A. Operation with a ring electrode When the high potential of 10 kv dc was applied between the electrodes and the distance between electrodes was small enough i.e., electric field reached about 3 kv/mm in air, electrical breakdown ignited an arc. The strong vortex flow forced this arc to move between the ring electrode and the circular disk electrode around the cylindrical volume axis, thereby creating a GAT. The electrodes were separated using the aforementioned crank mechanism. As the length of the separation between the two electrodes was increased, the length of the gliding arc also increased. At this point, the arc continued to elongate between the ring electrode and parallel disk electrode. At the distance beyond which the arc could no longer be maintained i.e., extinguished, the ring electrode was stopped. Figure 5 shows a series of photographs of GAT obtained with the ring electrode taken at different exposition times from a short time of s for the top left photograph marked a to 0.02 s for the bottom right photograph marked as b. One could see a single arc for the exposition time of s Fig. 5 a. With increasing exposition time, one could see photographs of a traveling arc along the ring electrode. The elongation of the arc in this case was not due to the diverging electrode geometry as in the case shown in Fig. 1, and not only because of the increasing distance between electrodes, but also because of the difference in the gas flow velocity in the regions near the two electrodes. Figure 6 describes the elongation mechanism of the gliding arc in tornado on two parallel circular electrodes, where the top line represents a portion of the disk electrode with a large circumferential gas flow velocity and the bottom line represents a portion of the ring electrode with a small circumferential gas flow velocity. In other words, the gas velocity near the disk electrode was greater than that near the ring electrode. Thus, the arc attachment at the disk electrode moved faster than the one at the ring electrode, resulting in elongation, cooling, and eventually extinguishing at the point when power loss could no longer be compensated by the power supply. The faster movement of the arc on the disk electrode becomes clear after careful examination of pictures in Fig. 5. In Fig. 5 a, for example, we can see the large displacement of the arc on the disk electrode compared to the ring electrode in the given exposition time of s. Almost immediately after the extinction of the arc, reignition took place in the present GAT. This reignition process cannot occur in the manner of breakdown in cold air across electrodes as seen in flat geometry, 3 because the electrodes are separated by distances greater than the initial breakdown limit. Instead the ignition was plasma assisted, and with this assistance, a new arc was initiated. Thus, the elongation of the arc, reignition after extinction, and continuous cooling of the two electrodes were achieved. In a reverse vortex flow with high velocity gas rotating along the cylindrical wall, the lowest pressure occurred near the cylinder axis, and radial velocity of gas was always directed to this axis see Fig. 2. It was in this low-pressure zone where the gliding arc was stabilized i.e., existed for most of the time when it remained between the two parallel electrodes flat circular disk electrode 7 and ring electrode 10, Fig. 3. There was an intense cooling of the arc column because of the reverse vortex flow, but simultaneously, a near-perfect thermal insulation of the discharge zone from the reactor walls was achieved such that one could touch the reactor wall with the naked hand see Fig. 7. In the experiment shown in Fig. 7, although power consumption was approximately 2.7 kw i.e., 5.0 kv and 540 ma, and the gas flow rate in the GAT was approximately 1 l/s these parameters correspond to the average temperature of the exit gas equal to 2090 K, the quartz reactor wall remained relatively cold as manifested by the fact that one could touch the quartz tube wall of operating plasma reactor with the naked hand. The arc in the present system had remarkable stability at high flow rates and over a wide range of power consumption.

5 Gliding arc in reverse vortex flow Rev. Sci. Instrum. 76, FIG. 7. Demonstration of almost-perfect thermal insulation of GAT. B. Operation with spiral electrode In the second case utilizing a spiral electrode, the arc ignited at the smallest gap between the spiral and flat circular disk electrode see Fig. 8. The arc was then forced by the strong vortex flow to move down along the spiral electrode and elongate see T1, T2, and T3 taken at three consecutive times in Fig. 8 to demonstrate the process of elongation of the arc. Its behavior was similar to that of the conventional 2D flat gliding arc, but in a 3D cylindrical volume, an ideal geometry for high volume plasma processes. The gliding arc would finally extinguish when the cooling of the arc could no longer be compensated by the power supply. As soon as the arc extinguished, a new arc was ignited at the top where the distance between the two electrodes was small, thus starting a new cycle of the gliding arc in tornado. In the case utilizing a circular ring electrode near the end of the spiral electrode see Fig. 8 and 10 in Fig. 3, which was smaller than the spiral electrode diameter, the gliding arc had sufficient power to reach the far end of the spiral electrode and remained there. Thus, a stable GAT was maintained, similar to the case of the circular ring electrode. V. DISCUSSION Gliding arcs require gas velocities large enough to move the discharge along diverging electrodes, but some applications such as plasma catalytic fuel conversions require a relatively large residence time for a high degree of completion of the chemical process. In the conventional 2D flat plasma geometry wherein a large amount of gas passes around the arc, the average specific power delivered remains low in the case of nonequilibrium regime, and the residence time in the reactor is relatively small. These are the major drawbacks of the flat GA. In order to overcome these shortcomings, the present work introduced a gliding arc discharge in a circular geometry using a reverse vortex flow. Additional advantages of the GAT are near-perfect thermal insulation and its practical utility in most plasma processes. The electric field in transitional plasmas is relatively strong, and both translational and electron temperatures are strongly coupled. 9 Since the decrease of the temperature in the translational plasma results in the increase of the electric field and electron temperature, the plasma becomes more nonequilibrium. 6 The present GAT works in regimes where extinction and reignition do not take place regularly; these regimes can be described as more or less constant, strictly as nonequilibrium, transitional or thermal, depending first of all, on the current and the intensity of the flow. According to the electric circuit arrangement of the 2D flat gliding arc shown in Fig. 1, the maximum current in the arc J max and the current at maximum power dissipation J W are as follows: J max = V/R, J W = V/2R. According to the thermal GA theory, 2 J W has a minimal current at the maximum power dissipation, which is the current just before the extinction of the arc. Current below this value of J W is possible only for a nonequilibrium gliding arc. 10 The electric circuit arrangement for the present GAT was essentially the same as the one shown in Fig. 1, with the power supply having dc output voltage V, current J, and internal resistance R. Figure 9 provides a plot from an oscilloscope recording of the current signal over time for the present gliding arc. In this experiment, the voltage applied on the system was 10 kv, and the resistance was 28.5 k. Thus, according to Ohm s law, the maximum current and J W in the present GAT become, respectively, J max = 350 ma, J W = 175 ma. In the present GAT experiments it was observed that the current remained relatively constant and the value of current varied in a range of ma. The current that corresponded to the maximum current during the present GAT cycle was not observed in the present study; this value should have been obtained at a short distance ignition regime FIG. 8. a Photograph of gliding arc with spiral electrode at normal exposition time of 0.2 s; b photographs of gliding arc with shot exposition time s, at time T1, T2, and T3, illustrating elongation sequence. c Schematic of GAT reactor showing elongation, the positions 1 and 2 correspond to T1 and T2 in b.

6 Kalra et al. Rev. Sci. Instrum. 76, FIG. 9. Current vs time for GAT. Point A: current J =0; point B: J=175 ma, thermal arc limit. T is the time period of gliding arc in operation. if periodic extinction and reignition were present. The lowest current observed 150 ma, see Fig. 9 in the present GA experiment was less than J W =175 ma. Thus, it was concluded that the GAT uses an overshooting regime, which was also observed in the case of the 2D flat low-current gliding arc. 10 The average current J avg was about 200 ma, which was close to the current J W corresponding to the maximum power dissipation on the arc. As the distance between the electrodes was increased further, a much flatter curve was obtained as shown in Fig. 10. The reason for this is that when the electrodes were further apart, the shortest arc was already relatively long with no possibility of further elongation. Thus the gliding arc could be stabilized in strictly nonequilibrium transitional regimes. In these regimes a relatively high specific energy of approximately 1.0 kw h/m 3 with power of 3.5 kw and flow rate of 1 l/s, corresponding to an average air temperature of about 2500 K, could be added with the reactor wall temperature remaining as low as 60 C. The GA in tornado remained stable at a very low energy input of about 0.1 kw h/m 3 corresponding to average air temperatures of 650 K with gas velocities inside the reactor reaching as high as 60 m/s. The period of a GAT shown in Fig. 9 was about 4 ms not very regular because there was no special place for arc extinction, and its corresponding frequency was 250 Hz. The frequency of the present GAT cycle involving elongation, extinction and reignition varied according to the airflow rate. As the flow rate of air varied from 0.5 to 2.5 l/s in the present study, the gliding arc frequency varied from 100 to 500 Hz. Additionally as the arc discharge rotated inside the cylinder, minor fluctuations were observed in the current, whose frequency was about 1000 Hz see Fig. 9. Time for each minor fluctuation cycle approximately corresponded to the time taken by the gas flow to travel one circumference inside the reactor. Hence, the fluctuations of the current signal could be attributed to some minor asymmetric characteristics in the reactor design and operation, i.e., finite inlet jets, electrode alignment, etc. The present GAT existed in a manner such that the nonequilibrium transitional gliding arc was extremely elongated between two electrodes with a constant distance. An overshooting regime of the arc was observed with the current value less than the minimum current predicted by the theory of thermal arc. The overshooting regime observed in the present atmospheric pressure GAT confirmed that the discharge was in nonequilibrium conditions. In contrast to conventional thermal and nonthermal discharges, the transitional GAT discharges were able to provide key advantages of both thermal and nonthermal discharges. The benefits of the present GAT, such as a high level of nonequilibrium and a large residence time, make the GAT very attractive for numerous industrial applications including fuel conversion, car- FIG. 10. Current vs time for GAT. Point A: current J =0. When separation of electrodes was increased to the maximum distance, the variations in the current were small.

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