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Corona-induced oxidation of reactive gaseous mixtures
C O M B U S T I O N A N D F L A M E 92:197-200 (1993)
197
BRIEF COMMUNICATION Corona-Induced Oxidation of Reactive Gaseous Mixtures G. O. THOMAS,* M. J. EDWARDS, and S. A. JONES Department of Physics, UniversityCollegeof Wales,Aberystwyth,Dyfed SY23 3BZ, UK
INTRODUCTION Bone, et al. [1] investigated the propagation of detonation waves through regions of intense electromagnetic field. They observed that, under certain conditions, these fields severely perturbed detonations in mixtures of carbon monoxide and oxygen. Waves propagating from a negative to a positive potential decelerated, and were occasionally quenched. Propagation in the reverse direction resulted in no change or a very slight velocity increase followed by a deceleration on leaving the field. There was no apparent effect when they used a dry methane-oxygen mixture. They postulated that these observations were due to the drift of CO + and electrons. Ion drift within the reaction zone would occur according to the polarity of the field, and the direction of detonation propagation. The consequence would be a modification of the spatial distribution of species and hence of the overall reaction process, Jaggers and yon Engel [2] observed a similar phenomenon with flames. They attributed increases in burning velocity to increased reaction rates, the result of electron collision with other species. Malinowski [3], working with transverse electric fields also cited electron drift as the cause of similar observations with hydrocarbon-air mixtures, During initial studies to investigate the possible use of field-induced ion drift to study detonation propagation an interesting effect has been observed, believed to be due to corona discharge,
*To whomall correspondenceshouldbe addressed, Copyright © 1993by The CombustionInstitute Published by ElsevierSciencePublishingCo.,Inc.
LONGITUDINAL ELECTRIC-FIELD INTERACTION WITH DETONATION
Experimental Details The experiments were performed in a 5-m-long 19-mm-internal-diameter steel tube connected to a 0.5-m-long perspex section of the same diameter, the latter fitted with two circular brass electrodes 25 cm apart. DC fields of up to 500 kW m -1 could be applied across the electrodes, using a series resistor to limit the discharge current. Detonation propagation was monitored using smoked foils. A detonation is not a truly planar one-dimensional wave [4] and the three-dimensional structure exhibited by a gas detonation is characterized by the average spacing between transverse perturbations. For subatmospheric pressures any change in spacing, and hence composition, is easily identified on a "smoked foil," the pattern left on a lightly sooted surface. Experimental Observations Two detonable mixtures were tested, stoichiometric acetylene-oxygen and carbon-mon: oxide-oxygen, both at initial pressures in the range 30-60 torr. However, no changes in propagation characteristics with applied electrical field were observed. This result was surprising, for Bertrand [5], in a similar experiment with acetylene-oxygen, noted a significant variation in transverse structure. The cell size increased markedly as the detonation approached the positive electrode, and this continued as the wave propagated between the electrodes. Beyond the negative electrode the 0010-2180/93/$5.00
198 cell size gradually recovered to its initial value, although, in some cases, explosive reinitiation occurred. This was confirmed by streak photography. Control tests with no applied field showed no such effect, The results of the two investigations therefore appear contradictory. However, on closer examination, a fundamental difference can be discerned. During the present studies great care was taken to ensure that there was no electrical breakdown prior to detonation. In Bertrand's apparatus, however, a bluish discharge was visible in the region of the positive electrode. This low-pressure electrical discharge, or corona, was facilitated by the presence of the smoked foil, which acted as an electrical conductor. It was therefore speculated that the discharge caused a partial oxidation of the mixture reflected by the modified propagation characteristics of the detonation.
CORONA DISCHARGE IN REACTIVE MIXTURES Previous Studies of Corona in Reactive Mixtures The effects of applied electric fields on the combustion process have been reviewed by Bradley [6]. For example, Bradley and Gupta [7] attempted to improve combustion in burners by heating (by field-induced collisions between chemi-ionised species). Ali, et al. [8], on the other hand, investigated the direct influence of corona-induced ionisation on the combustion chemistry. They were primarily concerned with CO and NO production in the after burning gases in a jet-stirred reactor. A similar influence of corona on chemical reaction was studied by Bradley and Nasser [9], in this case, on the stability of premixed burner flames. They observed a significant shift in the blowoff conditions with corona but noted that, for an unlit burner, direct ignition of the gas only occurred when a spark discharge was formed, Virtually all previous studies have been concerned with the influence of corona during combustion. Only one previous study indicates a possible effect on the unreacted mixture, During a study on flame propagation, Inomata
G . O . THOMAS ET AL. et al. [10] noted a decrease in flame velocity in the vicinity of a continuous electrical discharge, with the velocity change proportional to the duration of the discharge prior to ignition (of the order of tens of seconds). They introduced the possibility that not only did the discharge influence flame propagation by the production and diffusion of active species within the reaction zone, but that it could also cause partial oxidation of the mixture prior to flame front arrival. Present Studies To investigate whether corona discharge could lead to such partial oxidation, a series of mass spectrometric experiments have been performed to monitor the gas composition as a function of time. These were performed in a 50-mm-diameter 370 mm long Pyrex vessel. A voltage of the order 20-25 kV was applied across the vessel and resistors of between 18.6 and 100 MI~ limited the discharge current to 140-1000 mA. The voltage across the electrodes was of the order of 1 kV with gas pressures in the range 30-75 tort. Stoichiometric ethylene, methane, and acetylene mixtures with oxygen were tested with 5% argon added in an attempt to provide a reference composition. In all cases, after application of corona, a depletion of the initial reactants was observed with a corresponding increase in carbon dioxide concentration. Typical mass spectrometer outputs obtained before and after the application of the corona to a stoichiometric acetylene and oxygen mixture at 60 torr are given in Fig. 1. This shows a depletion of acetylene and an increase in carbon dioxide after 6 min. There was some variation in the time taken to totally oxidize the various fuels, but all were of the order of many minutes. Propagating flames were only obtained if transition to a spark occurred. As the mass spectrometer studies had indicated that oxidation reactions were induced by corona, and, as visible radiation could be observed, tests were then attempted to detect radiation characteristic of the oxidation process. Using a monochromator and a photomultiplier, spectra were obtained, but, unfortunately, they were dominated by nitrogen emis-
CORONA-INDUCED OXIDATION OF MIXTURES
199
iC2H2 26
0,02++
16
N2 28
0 2 32
Fig. 1. Mass spectra (a) before and (b) after application of a corona discharge to oxy-acetylene at an initial pressure of 60 ToE.
sion bands, even when using nitrogen-free gases (nominally 6 ppm N2). Emission corresponding "to OH was identified, but the discharge intensity levels observed were too small to permit any quantitative measurements. In all cases the light output was in the form of highfrequency pulses, in the range 10-20 kHz. DISCUSSION
.
Corona discharge is one of many electrical effects observed in gases on application of intense electric fields [11]. Corona differs from arcs or sparks in that the field strength across the discharge is high with correspondingly lower current. Sparks normally exhibit higher currents with lower voltage drops. Also, unlike glow discharge, which is associated with low pressures ( < 1 torr), corona can occur over a wide range of gas pressures. Corona arises when electrons are accelerated to energies which are high enough to ionise neutral molecules. This causes a further increase in the electron concentration and leads to an avalanche process. Once the threshold for corona is reached the discharge is composed of many short pulses, with frequencies of up to 100 kHz, commonly known as Trichel pulses, Usually corona are studied using a point to plane configuration and this can give rise to
positive and negative corona, depending on the polarity of the point electrode. The nature of ionic reactions in corona discharges in air have been reviewed by Shahin [12] and Cross [13]. In positive corona, the main ion has been calculated to be 02 +. N2 + ions are also created but they react rapidly to give NO + and 02 +. In negative corona, the predominant species at the anode were CO 3- and O3-. Complex water-based ions are also observed. In the present reactive systems, it is postulated that radicals are formed in the corona (ozone production could be particularly important) and that these react to give products similar to those from a propagating combustion wave. In a corona, however, the bulk of the gaseous mixture is never heated to the autoignition temperature, despite the presence of very-high-temperature ions and electrons. The rate of production of ions must therefore act as a limiting process for any endothermic reactions. The pulsed nature of the discharge would be a contributory factor in this respect. The phenomenon would not be dissimilar that of an "atomic flame," where a preionized oxidant is mixed with a fuel in a flowing reactor. An example of such a study of atomic flames is given by Gaydon and Wolfhard [14], although they conclude that the flames in their study were probably "fairly hot," presumably
200 referring to more complete thermal equilibrium. Corona-induced oxidation could therefore provide a possible explanation of the behaviour observed by Bone et al. [1]. Detonation failure was related to the polarity of the applied field and could be related to positive or negative corona. Dried mixtures of carbon monoxide and oxygen exhibit larger cell sizes and are closer to the limits of propagation [4], but it was the moist mixtures that were affected by the application of a high-voltage field. Water vapor, however, has been observed to affect the onset of corona discharges [11] and also the nature of the ions formed [15].
CONCLUSIONS
The present results would appear to show fairly conclusively that corona dicharge can lead to slow, non-self-sustaining oxidation reactions in flammable mixtures. This is inferred from mass spectrometric studies of fuel depletion and the pulsed nature of the light output. Such a mechanism provides one explanation for the effects observed when detonation are quenched as they propagate in regions with large electric fields: a depletion of the initial reactants prior to detonation initiation. The phenomena could have a potential beneficial applications as a method of limiting fuel concentration. The authors acknowledge the insight into this phenomenon gained during a visit by G O T to the
G. O. THOMAS ET AL. laboratory o f Prof. 's Lee and Knystautus at McGiU University.
REFERENCES 1. Bone, W. A., Frazer, R. P., and Wheeler, W. H., Philos. Trans.A 235:29 (1935). 2. Jaggers, H. C., and yon Engel, A., Combust. Flame 16:275 (1971). 3. Ma~inowsld, M. A-E., Z Chim. Phys. 21:469 (1921). 4. Edwards,D. H., Twelfth Symposium (International) on Combustion,The Combustion Institute, Pittsburgh,
1969, pp. 819-828. 5. Bertrand, P., Meeh II Lab. Report, Dept. Mech. Eng., McGill Univ. (1983). 6. Bradley, D., in Advanced Combustion Methods (F. Weinberg, Ed.), Academic, New York, 1986. 7. Bradley, D., and Gupta, M. L., Combust. Flame 40:47 (1981). 8. Ali, M., Bradley, D., and Gupta, M. L., Proceed/rigsof
the Seventh International Conference on Gas Discharges and their Applications, 1982. 9. Bradley, D., and Nasser, S. H., Combust, Flame
55:53-58 (1984). 10. Inomata,T., Okazaki, S., Moriwaki, T., and Suzuki, M., Combust. Flame 50:361 (1983). 11. Loeb, L. B., Fundamental Processes of Electrical Discharges in Gases, Chapman Hall, London, 1939. 12. Shahin, M. M., in Chemical Reactions in Electrical Discharges (B. D. Blaustein, Ed,) (Advances in Chemistry Series 80), American Chemical Society, Wash-
ington, D.C., 1969. 13. Cross, J. A., J. Phys. D. Appl. Phys. 10:1003-1009
(1977).
14. Gaydon, A. G., and Wolfhard, H. G., Proc./Z Soc. A 213:366 (1952). 15. Gardiner, P. S., and Craggs, J. D., J. Phys. D.: Appi. Phys. 19:1007-1017 (1977).
Received22 January 1992; revised 6 September 1992
197
BRIEF COMMUNICATION Corona-Induced Oxidation of Reactive Gaseous Mixtures G. O. THOMAS,* M. J. EDWARDS, and S. A. JONES Department of Physics, UniversityCollegeof Wales,Aberystwyth,Dyfed SY23 3BZ, UK
INTRODUCTION Bone, et al. [1] investigated the propagation of detonation waves through regions of intense electromagnetic field. They observed that, under certain conditions, these fields severely perturbed detonations in mixtures of carbon monoxide and oxygen. Waves propagating from a negative to a positive potential decelerated, and were occasionally quenched. Propagation in the reverse direction resulted in no change or a very slight velocity increase followed by a deceleration on leaving the field. There was no apparent effect when they used a dry methane-oxygen mixture. They postulated that these observations were due to the drift of CO + and electrons. Ion drift within the reaction zone would occur according to the polarity of the field, and the direction of detonation propagation. The consequence would be a modification of the spatial distribution of species and hence of the overall reaction process, Jaggers and yon Engel [2] observed a similar phenomenon with flames. They attributed increases in burning velocity to increased reaction rates, the result of electron collision with other species. Malinowski [3], working with transverse electric fields also cited electron drift as the cause of similar observations with hydrocarbon-air mixtures, During initial studies to investigate the possible use of field-induced ion drift to study detonation propagation an interesting effect has been observed, believed to be due to corona discharge,
*To whomall correspondenceshouldbe addressed, Copyright © 1993by The CombustionInstitute Published by ElsevierSciencePublishingCo.,Inc.
LONGITUDINAL ELECTRIC-FIELD INTERACTION WITH DETONATION
Experimental Details The experiments were performed in a 5-m-long 19-mm-internal-diameter steel tube connected to a 0.5-m-long perspex section of the same diameter, the latter fitted with two circular brass electrodes 25 cm apart. DC fields of up to 500 kW m -1 could be applied across the electrodes, using a series resistor to limit the discharge current. Detonation propagation was monitored using smoked foils. A detonation is not a truly planar one-dimensional wave [4] and the three-dimensional structure exhibited by a gas detonation is characterized by the average spacing between transverse perturbations. For subatmospheric pressures any change in spacing, and hence composition, is easily identified on a "smoked foil," the pattern left on a lightly sooted surface. Experimental Observations Two detonable mixtures were tested, stoichiometric acetylene-oxygen and carbon-mon: oxide-oxygen, both at initial pressures in the range 30-60 torr. However, no changes in propagation characteristics with applied electrical field were observed. This result was surprising, for Bertrand [5], in a similar experiment with acetylene-oxygen, noted a significant variation in transverse structure. The cell size increased markedly as the detonation approached the positive electrode, and this continued as the wave propagated between the electrodes. Beyond the negative electrode the 0010-2180/93/$5.00
198 cell size gradually recovered to its initial value, although, in some cases, explosive reinitiation occurred. This was confirmed by streak photography. Control tests with no applied field showed no such effect, The results of the two investigations therefore appear contradictory. However, on closer examination, a fundamental difference can be discerned. During the present studies great care was taken to ensure that there was no electrical breakdown prior to detonation. In Bertrand's apparatus, however, a bluish discharge was visible in the region of the positive electrode. This low-pressure electrical discharge, or corona, was facilitated by the presence of the smoked foil, which acted as an electrical conductor. It was therefore speculated that the discharge caused a partial oxidation of the mixture reflected by the modified propagation characteristics of the detonation.
CORONA DISCHARGE IN REACTIVE MIXTURES Previous Studies of Corona in Reactive Mixtures The effects of applied electric fields on the combustion process have been reviewed by Bradley [6]. For example, Bradley and Gupta [7] attempted to improve combustion in burners by heating (by field-induced collisions between chemi-ionised species). Ali, et al. [8], on the other hand, investigated the direct influence of corona-induced ionisation on the combustion chemistry. They were primarily concerned with CO and NO production in the after burning gases in a jet-stirred reactor. A similar influence of corona on chemical reaction was studied by Bradley and Nasser [9], in this case, on the stability of premixed burner flames. They observed a significant shift in the blowoff conditions with corona but noted that, for an unlit burner, direct ignition of the gas only occurred when a spark discharge was formed, Virtually all previous studies have been concerned with the influence of corona during combustion. Only one previous study indicates a possible effect on the unreacted mixture, During a study on flame propagation, Inomata
G . O . THOMAS ET AL. et al. [10] noted a decrease in flame velocity in the vicinity of a continuous electrical discharge, with the velocity change proportional to the duration of the discharge prior to ignition (of the order of tens of seconds). They introduced the possibility that not only did the discharge influence flame propagation by the production and diffusion of active species within the reaction zone, but that it could also cause partial oxidation of the mixture prior to flame front arrival. Present Studies To investigate whether corona discharge could lead to such partial oxidation, a series of mass spectrometric experiments have been performed to monitor the gas composition as a function of time. These were performed in a 50-mm-diameter 370 mm long Pyrex vessel. A voltage of the order 20-25 kV was applied across the vessel and resistors of between 18.6 and 100 MI~ limited the discharge current to 140-1000 mA. The voltage across the electrodes was of the order of 1 kV with gas pressures in the range 30-75 tort. Stoichiometric ethylene, methane, and acetylene mixtures with oxygen were tested with 5% argon added in an attempt to provide a reference composition. In all cases, after application of corona, a depletion of the initial reactants was observed with a corresponding increase in carbon dioxide concentration. Typical mass spectrometer outputs obtained before and after the application of the corona to a stoichiometric acetylene and oxygen mixture at 60 torr are given in Fig. 1. This shows a depletion of acetylene and an increase in carbon dioxide after 6 min. There was some variation in the time taken to totally oxidize the various fuels, but all were of the order of many minutes. Propagating flames were only obtained if transition to a spark occurred. As the mass spectrometer studies had indicated that oxidation reactions were induced by corona, and, as visible radiation could be observed, tests were then attempted to detect radiation characteristic of the oxidation process. Using a monochromator and a photomultiplier, spectra were obtained, but, unfortunately, they were dominated by nitrogen emis-
CORONA-INDUCED OXIDATION OF MIXTURES
199
iC2H2 26
0,02++
16
N2 28
0 2 32
Fig. 1. Mass spectra (a) before and (b) after application of a corona discharge to oxy-acetylene at an initial pressure of 60 ToE.
sion bands, even when using nitrogen-free gases (nominally 6 ppm N2). Emission corresponding "to OH was identified, but the discharge intensity levels observed were too small to permit any quantitative measurements. In all cases the light output was in the form of highfrequency pulses, in the range 10-20 kHz. DISCUSSION
.
Corona discharge is one of many electrical effects observed in gases on application of intense electric fields [11]. Corona differs from arcs or sparks in that the field strength across the discharge is high with correspondingly lower current. Sparks normally exhibit higher currents with lower voltage drops. Also, unlike glow discharge, which is associated with low pressures ( < 1 torr), corona can occur over a wide range of gas pressures. Corona arises when electrons are accelerated to energies which are high enough to ionise neutral molecules. This causes a further increase in the electron concentration and leads to an avalanche process. Once the threshold for corona is reached the discharge is composed of many short pulses, with frequencies of up to 100 kHz, commonly known as Trichel pulses, Usually corona are studied using a point to plane configuration and this can give rise to
positive and negative corona, depending on the polarity of the point electrode. The nature of ionic reactions in corona discharges in air have been reviewed by Shahin [12] and Cross [13]. In positive corona, the main ion has been calculated to be 02 +. N2 + ions are also created but they react rapidly to give NO + and 02 +. In negative corona, the predominant species at the anode were CO 3- and O3-. Complex water-based ions are also observed. In the present reactive systems, it is postulated that radicals are formed in the corona (ozone production could be particularly important) and that these react to give products similar to those from a propagating combustion wave. In a corona, however, the bulk of the gaseous mixture is never heated to the autoignition temperature, despite the presence of very-high-temperature ions and electrons. The rate of production of ions must therefore act as a limiting process for any endothermic reactions. The pulsed nature of the discharge would be a contributory factor in this respect. The phenomenon would not be dissimilar that of an "atomic flame," where a preionized oxidant is mixed with a fuel in a flowing reactor. An example of such a study of atomic flames is given by Gaydon and Wolfhard [14], although they conclude that the flames in their study were probably "fairly hot," presumably
200 referring to more complete thermal equilibrium. Corona-induced oxidation could therefore provide a possible explanation of the behaviour observed by Bone et al. [1]. Detonation failure was related to the polarity of the applied field and could be related to positive or negative corona. Dried mixtures of carbon monoxide and oxygen exhibit larger cell sizes and are closer to the limits of propagation [4], but it was the moist mixtures that were affected by the application of a high-voltage field. Water vapor, however, has been observed to affect the onset of corona discharges [11] and also the nature of the ions formed [15].
CONCLUSIONS
The present results would appear to show fairly conclusively that corona dicharge can lead to slow, non-self-sustaining oxidation reactions in flammable mixtures. This is inferred from mass spectrometric studies of fuel depletion and the pulsed nature of the light output. Such a mechanism provides one explanation for the effects observed when detonation are quenched as they propagate in regions with large electric fields: a depletion of the initial reactants prior to detonation initiation. The phenomena could have a potential beneficial applications as a method of limiting fuel concentration. The authors acknowledge the insight into this phenomenon gained during a visit by G O T to the
G. O. THOMAS ET AL. laboratory o f Prof. 's Lee and Knystautus at McGiU University.
REFERENCES 1. Bone, W. A., Frazer, R. P., and Wheeler, W. H., Philos. Trans.A 235:29 (1935). 2. Jaggers, H. C., and yon Engel, A., Combust. Flame 16:275 (1971). 3. Ma~inowsld, M. A-E., Z Chim. Phys. 21:469 (1921). 4. Edwards,D. H., Twelfth Symposium (International) on Combustion,The Combustion Institute, Pittsburgh,
1969, pp. 819-828. 5. Bertrand, P., Meeh II Lab. Report, Dept. Mech. Eng., McGill Univ. (1983). 6. Bradley, D., in Advanced Combustion Methods (F. Weinberg, Ed.), Academic, New York, 1986. 7. Bradley, D., and Gupta, M. L., Combust. Flame 40:47 (1981). 8. Ali, M., Bradley, D., and Gupta, M. L., Proceed/rigsof
the Seventh International Conference on Gas Discharges and their Applications, 1982. 9. Bradley, D., and Nasser, S. H., Combust, Flame
55:53-58 (1984). 10. Inomata,T., Okazaki, S., Moriwaki, T., and Suzuki, M., Combust. Flame 50:361 (1983). 11. Loeb, L. B., Fundamental Processes of Electrical Discharges in Gases, Chapman Hall, London, 1939. 12. Shahin, M. M., in Chemical Reactions in Electrical Discharges (B. D. Blaustein, Ed,) (Advances in Chemistry Series 80), American Chemical Society, Wash-
ington, D.C., 1969. 13. Cross, J. A., J. Phys. D. Appl. Phys. 10:1003-1009
(1977).
14. Gaydon, A. G., and Wolfhard, H. G., Proc./Z Soc. A 213:366 (1952). 15. Gardiner, P. S., and Craggs, J. D., J. Phys. D.: Appi. Phys. 19:1007-1017 (1977).
Received22 January 1992; revised 6 September 1992