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The effect of nonsteady electric fields on sooting flames
COMBUSTION A N D F L A M E 7 8 : 3 5 7 - 3 6 4 (1989)
357
The Effect of Nonsteady Electric Fields on Sooting Flames M. KONO* Department of Aeronautics, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, Japan
F. B. CARLETON, A. R. JONES, and F. J. WEINBERG Department of Chemical Engineering and Chemical Technology Imperial College, London, England
The effects of transient dc and 50-Hz ac fields acting on a sooting acetylene diffusion flame were compared with those of steady de fields. The object was to separate the velocities of charged soot particles from those of the ionic wind by making use of the difference in their relative evolution times. This has not been considered previously, largely beacuse much of the research was carried out on flat counterflow flames, but is here shown to be important for conventional flame shapes. Laser Doppler and scattering methods were used to determine particle velocities and sizes. Soot particles of the order of 10 t~m diameter were found to have mobilities of the order of 5 x 10 -6 m 2 sV- ~ and zero for particles exposed to a flux of positive and negative charge carriers, respectively. Major perturbations to flow due to ionic wind effects set in after a delay of the order of 10 ms. Similar results were obtained whether the transients followed the switching on of a dc field or occurred during the 50-Hz cycling of the ac field.
INTRODUCTION The major effects that dc fields exert on all phases of soot formation in flat flames have been exhaustively investigated [1--4]. Alternating radial fields at frequencies up to 400 Hz were applied to sooting turbulent diffusion flames as early as 1965 [5] whereas, much more recently, frequencies up to 10 7 Hz were applied to fiat counterflow diffusion flames and to premixed acetylene flames, revealing very interesting variations with the period of oscillation [6]. The use of the dependence of the various effects on the frequency of the ac field as a diagnostic tool has further been discussed in Ref. 7, based on the large differences between the mobilities of the charge carriers of interest: particles, ions, and electrons, which differ so greatly from one another that it is possible to define which band of frequencies will cause each to oscillate within certain limits. Academic visitor at Imperial College. Copyright © 1989 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York. NY 10010
One characteristic inertial time that has not been considered in any of this is that required for the establishment of an ionic (Chattock) wind. Yet this is of considerable importance, if only as a perturbing influence (at least in systems other than flat flames perpendicular to lines of field in which it is opposed by the gas flow). Thus the measurement of particle velocities in dc fields is always complicated by the contribution of this field-induced gas velocity. The main object of the present study of transients was to utilize the difference between the time of evolution of an ionic wind and the much shorter times of charge acquisition and drift velocity development by particles, to establish relative particle velocities with respect to the gas and hence absolute mobilities of fully grown soot particles. EXPE~MENTAL The burner system is shown in Fig. 1. An acetylene diffusion flame was stabilized on the
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l ..
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nozzle exit of a copper robe of internal and external diameters 6 and 7 ram, respectively. The fuel flow rate was 0.7 cm3/s. As shown in Fig. 1, the field was essentially along rather than across the flame, applied between the burner and a cylindrical copper electrode of 18 mm i.d. and 250 nun length, located above the nozzle tube. The distance between the lower end of the cylindrical electrode and the nozzle exit was 22 ram. Both dc and ac power sources were used, the latter being supplied from the 50 Hz mains. Two forms of dc electric field were studied, one continuous and the other transient. In the latter case, relevant parameters of the flame were measured immediately after the electric field was switched on. The switch for this purpose was equipped with an inductor and a capacitor to minimise the transient arc occurring when it was turned on.
Figure 2 shows the optical system for measuring the velocity of soot particles. The principle of this method is based on interferometry of Dopplershifted fight applied to the plus and minus diffraction orders [8]. The laser beam of about 1 mm diameter traversed the central pan of the flame at right angles to the direction of soot particle movement. The focal length of the schlieren lens (L~) was 500 ram. The stop at the focus of the laser beam was provided with two slits (0.3 x 55 mm), 2.8 mm apart. Interference between the two beams was achieved by a biprism and a cylindrical lens (L2) as shown in Fig. 2 and the beat frequency was recorded using an EMI 9798B photomultiplier. The value of the beat frequency was determined from the display of a digital storage oscilloscope, or by using a frequency spectrum analyzer. The velocity of soot panicles is given to a sufficient approximation by kfb/(20), where 20 is approximately equal to the ratio of the distance between slits of the stop to the focal length of lens L1 andfb is the beat frequency. The object of the work was to measure the mobility of large soot panicles and it was found that this measuring method could be applied to soot particles of the order of 1 pm and larger. The size of soot particles was determined on the basis of Mie scattering theory [9]. The intensity of scattered light was measured at scattering angles of 1 and 3 degrees by means of the photomultiplier used also in the measurement of particle velocity. This combination of angles was chosen because their scattering ratio varies monotonically in the region of the panicle size relevant to the present experiment, d being less than 50 ~ x . The intensity of the scattered light as monitored on the oscilloscope fluctuated considerably with time, due to variations in soot particle size. However, the aim of the present work was to study the average properties of fully formed particles rather than those of the precursors and the large particles were selected to correspond to measuremerit of soot panicle velocity. In each case, the five largest signals occurring during 10 ms were used as data for calculating size from the intensity of scattered light. The particles were treated as being spherical, although it is likely that some aggregation into chains had already occurred in
ELECTRIC FIELDS AND SOOTING FLAMES
359
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C2H2 Fig. 2. Opticalsystemfor measurementof soot particlevelocity.Schliereninterferometryof Doppler-shiftedlight. the gas phase. The refractive index of soot was assumed to be 1.9 - 0.55P ° but it can be shown that for forward scattering the result is remarkably insensitive to the precise value. RESULTS AND DISCUSSION Throughout what follows, the dc field direction is designated according to the polarity of the upper electrode. In figures as well, plus and minus signs denote the polarity of that electrode. Because the flame plasma is essentially electrically continuous with the burner, being separated from it by only a small dead space, as compared with the much larger separation between the top of the flame and the cylindrical upper electrode, it follows that a "negative field" implies that soot particles are bombarded with an increased flux of positive ions, whereas a "positive field" exposes them to an increased electron flow. Clearly these practical flame systems do not share the simple geometry of fields lines orthogonal to flat flames [1, 31. In the absence of a field, the particle velocity at any height corresponds to the local gas flow velocity made up of natural and forced convection consequent upon the gas flow from the burner. In the presence of a d c field, two additional effects occur: the gas flow is modified by the superimposition of the ionic wind and, if the particle is charged, its own drift velocity in the field is added algebraically to the convective motion. The Ap-
pendix compares the response time of particle drift consequent upon its own charge acquisition with that of the development of the ionic wind. It is clear that the latter is several orders of magnitudes slower than the former. It is this major difference in response time that makes it possible to study particle drift velocities, and hence deduce mobilities, during the early stages of the application of transient fields. The subsequent changes in particle velocity also indicate the development in time of the evolution of the ionic wind. Continuous dc Electric Field
Figure 3 illustrates how the velocity of soot particles varies with the applied field at two heights above the burner port. The upper part shows the variation of current with applied field, which is independent of the polarity of the electrodes. The linear relationship indicates simple charge removal from an ionization-recombination equilibrium plasma, well below saturation conditions [2]. The particle velocities, however, show a strong polarity dependence. Considering, first of all, the case of zero applied potential when the soot particles act simply as flow tracers, we see a gas flow velocity of approximately 82 cm/s at 1.2 cm above the burner port, rising to approximately 127 cm/s over the next 8 mm downstream. Application of a positive field produces no great change, the velocity being
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altered by only 10%-20%. This implies both that the particles are largely uncharged and the ionic wind velocity is small. Figure 4 shows photographs of flame luminosity that indicates the extent of distortion by ionic wind effects. None of this is very surprising; it has been shown [1, 3] that the predisposition of soot particles to carry positive charge can be neutralized when they are subjected to a stream of electrons from the flame (or even reversed, when bombardment with negative charge carriers is large enough). There is, in fact evidence here of some residual charge effects: after long periods of running, small deposits of soot appear on the burner rim, indicating some positive charging, and both the lowest curve in Fig. 3 and the luminous photograph in Fig. 4 indicate some downward wind effect due to positive charge. In the case of the negative field, when electrons are rapidly removed by flow to the burner and a column of positive ions ascends the flame, large increases in particle velocity are observed. At 2 cm above the burner the velocity is almost trebled. Again, these must be due to both the ionic wind effect and the charge on the soot particles. The force that drives the ionic wind (j/k per unit volume, where j is the current density and k the
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ELECTRIC FIELDS AND SOOTING FLAMES
361
mobility of the relevant charge carrier) increases with the size of the charge carrier. It may be orders of magnitude greater for a positive molecular ion as compared with an electron and again for a charged soot particle as compared with a molecular ion. The appearance of the flame in this case (Fig. 4) is characteristic of an increased convective flow. The rapid acquisition of positive charge by soot particles from an early stage in their growth has clearly been demonstrated for fields of this polarity [2]. Figures 5 and 6a show the size of large soot particles as a function of applied field and distance downstream from the burner. In the case of a negative field, appreciable reduction in size occurs as a result of decreased residence times. In the case of the opposite polarity, the changes are very small. This again accords with previous work [1, 2]. Even the small increase at intermediate potentials in Fig. 5, if that is significant, has its counterpart in the observed growth of large particles when the field is used to prolong the residence time in the pyrolysis zone--see Ref. 1. However, the small difference between absence of field and the positive case suggests that the
particles carry very little charge under these circumstances. Unlike in a counterflow system, where deposition of particles on the burner in opposition to the metered gas flow has been used to deduce mobility [1 I], no simple method of separating the drift of charged particles from the ionic wind effect suggests itself here, in the steady state.
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Transient
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Figure 7 illustrates the velocity transients observed during the first 25 ms after switching on the dc field. The polarity difference bears out previous observations; in the positive field case all the results are largely independent of the applied potential, suggesting that the particle essentially follows the general gas velocity and, therefore, carries little charge. For large and moderate negative fields, however, there is an initial surge in particle velocity at times too short for the ionic wind to become established (see Appendix). The growth during the first 5 ms, or so, is thought to be due to progressive charge acquisition because any changes in particle size appear far too small to account for such an effect. This is further illustrated in Fig. 6b, which shows the variation in particle size induced during the positive and negative half cycles of ac (50 Hz) fields. The rate of charge acquisition is, of course, itself affected by the field strength. After about 10 ms, the velocity of all soot particles rapidly decreases, irrespective of the polarity of the upper electrode. This is thought to be due to ionic wind effects. Bearing in mind that at sufficiently long times these curves must eventually coincide with their steady dc case values, as portrayed in Fig. 3, it is clear that the details of ionic wind and its development in time are complex. It follows from the theory (e.g. Ref. 2) that gas flows accelerate towards both electrodes, gas being entrained around the flame to make its acceleration possible, in incompressible flow. These primary flow modifications in turn affect the shape of the flame and the position of the pyrolysis and ionization regions, resulting in further readjustments of the flow pattern. Under certain conditions, oscillations are set up. These
362
M. KONO et al. I
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The object of this exercise was not to analyze the ionic wind effects, which are clearly too complex and geometry-dependent, but to confirm experimentally the calculation in the Appendix and establish the period during which charged particle drift can be measured independently of them. Comparison with the negative field case in Fig. 7 indicated that mobilities may be calculated from the results at short times. For this case, the velocity of the soot particles relative to the gas velocity was obtained by subtracting the velocity in the absence of an electric field from that in its presence, during the early stages. These relative velocities are plotted in Fig. 8 as a function of the mean electric field. Gas velocities in the absence of an applied field were obtained from Fig. 7 at 12 and 20 mm from the burner port. The corresponding mobilities were 3.9 x 10 -6 and 5.9 x 10 -6 m: s- ~V - I for the two heights respectively. In the positive field case the mobility, along with the charge on the particle, was regarded as close to zero or, at least, too small to measure.
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ELECTRIC FIELDS AND SOOTING FLAMES
363
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versus negative electric field strength. The gradient of lines shows mobilityof soot particles. burner port. Here again, during the positive half cycle, the velocity of the particles tends to approach, in time, that of the gas, whereas during the negative half cycle, the additional velocity of the soot particles alone can be determined. The corresponding relative velocity is shown as the solid symbols in Fig. 8, with the electric field strength averaged over the half cycle, i.e., the peak to peak value multiplied by 0.637. It will be apparent from Fig. 8 that the mobility obtained in this way is in fair agreement with that deduced from the transient dc electric field. The agreement may be improved further if the reduction in particle size shown in Fig. 6b is taken into account. CONCLUSIONS A study of the transient behavior of soot particles, velocities, sizes, and flame luminosity leads to a number of interesting conclusions. Comparison of the events immediately following the switching on of a dc field with one in the steady state, and also with various phases during an ac field of 50 Hz frequency, shows that ionic wind effects are complicated but can be separated in time from the drift of the charged particles in a field. Among the results are that large soot particles of the order of 10 pm diameter attain mobilities of the order of 5 x 10 -6 m 2 s -~ V -~ when subjected to a flux of positive ions, but acquire very little charge (effectively zero mobility) in the opposite electric field.
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Fig. 9. Sootparticle velocityin ac electric field (50 Hz) for 20 mm downstreamfrom nozzle exit. The method developed may lend itself to more detailed studies, for example of the evolution in time of charge acquisition and growth of soot particles, in flames that are not restricted to the somewhat artificial academic geometries studied previously.
REFERENCES I. Place, E. R., and Weinberg, F. J., Proc. R. Soc,. A 289:192 (1965). 2. Lawton, J., and Wemberg, F. J., ElectricalAspects of Combustion, Clarendon, Oxford, 1969. 3. Mayo, P. J., and Weinberg, F. J., Proc. R. Soc., A 319:351 (1970). 4. Wagner, H. Gg., Seventeenth Symposium (InternationaO on Combustion, The Combustion Institute,
Pittsburgh 1979, p. 3. 5. Mitchell, J. E., and Wright, F. J. Combust. Flame 13:413 (1%9). 6. Kono, M., Iinuma, K., and Kumagai, S., Eighteenth
364
7.
8. 9.
10.
11.
M. KONO et al. Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p 1167. Weinberg, F. J., Electrical Intervention in the Sooting of Flames, NATO Workshop on Sooting Combustion Systems and Its Toxic Properties, NATO, 1981, p. 1. Schwar, M. J. R., Thong, K. C., and Weinberg, F. J., J. Phys. D 3:1962 (1970). Bayvel, L. P., and Jones, A. R., Electromagnetic Scattering and Its Applications, Applied Science Publishers, London, 1981. Lee, S. C., and Tien, C. L., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 1159. Hardesty, D. R., and Weinberg, F. J., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 907.
of 10 #m diameter, the response time, t p , is calculated as 1.2 x 10 -9 S. The calculation procedure is based on Ref. 6. The ionic wind is caused by ions in the flame region whose number density is many orders of magnitude less than that of neutral molecules. In evaluating the effect, in a transient electric field, of the ionic wind on flame shape or gas velocity in the flame region, the collisions between ions and neutral molecules must therefore be considered as the predominant rate-controlling process. On simple kinetic theory, the collision response time, tc can be expressed as tc = z / ( 1 . 4 r o 2 c N ) ,
APPENDIX The equation of motion of a charged soot particle in quiescent gas may be expressed as mdoldt + olB = Eq,
(1)
where t is time, E the electric field strength, B the constant in the viscous drag expression, and m, v, and q the mass, velocity, and charge, respectively, of the particles. Integration of Eq. 1 with v -- l, initially, leads to v = E q B { 1 - exp[ - t / ( m B ) ] } .
(2)
The term m B in this equation corresponds to a response time. In the region where the time exceeds the value of m B , the soot particle moves with approximately constant velocity. For large particles, of the size discussed in this work, e.g.,
(3)
where z, o, c, and N are the ratio of number density of neutral molecules to that of ions ( = 5 × 10 7 [2]), collision diameter, mean molecular velocity, and number density of molecules, respectively. For air at 1500 K and 1 atm, approximate values ofo, c, and N a r e 4 × 10- l0 m, 1 × 10 3 m/s, and 5 × 10 ~ m -3, respectively. The value of tc that emerges from this calculation is 14 ms. A rapid experimental test was carried out to verify the above rough estimate. High-speed cine photography was used to record the onset of distortion on the luminous region when a transient dc electric field was applied. The cine sequence confirms that no change occurs in the flame shape prior to 10 ms after the application of the electric field. Received 3 August, 1988: revised 7 November 1988
357
The Effect of Nonsteady Electric Fields on Sooting Flames M. KONO* Department of Aeronautics, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, Japan
F. B. CARLETON, A. R. JONES, and F. J. WEINBERG Department of Chemical Engineering and Chemical Technology Imperial College, London, England
The effects of transient dc and 50-Hz ac fields acting on a sooting acetylene diffusion flame were compared with those of steady de fields. The object was to separate the velocities of charged soot particles from those of the ionic wind by making use of the difference in their relative evolution times. This has not been considered previously, largely beacuse much of the research was carried out on flat counterflow flames, but is here shown to be important for conventional flame shapes. Laser Doppler and scattering methods were used to determine particle velocities and sizes. Soot particles of the order of 10 t~m diameter were found to have mobilities of the order of 5 x 10 -6 m 2 sV- ~ and zero for particles exposed to a flux of positive and negative charge carriers, respectively. Major perturbations to flow due to ionic wind effects set in after a delay of the order of 10 ms. Similar results were obtained whether the transients followed the switching on of a dc field or occurred during the 50-Hz cycling of the ac field.
INTRODUCTION The major effects that dc fields exert on all phases of soot formation in flat flames have been exhaustively investigated [1--4]. Alternating radial fields at frequencies up to 400 Hz were applied to sooting turbulent diffusion flames as early as 1965 [5] whereas, much more recently, frequencies up to 10 7 Hz were applied to fiat counterflow diffusion flames and to premixed acetylene flames, revealing very interesting variations with the period of oscillation [6]. The use of the dependence of the various effects on the frequency of the ac field as a diagnostic tool has further been discussed in Ref. 7, based on the large differences between the mobilities of the charge carriers of interest: particles, ions, and electrons, which differ so greatly from one another that it is possible to define which band of frequencies will cause each to oscillate within certain limits. Academic visitor at Imperial College. Copyright © 1989 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York. NY 10010
One characteristic inertial time that has not been considered in any of this is that required for the establishment of an ionic (Chattock) wind. Yet this is of considerable importance, if only as a perturbing influence (at least in systems other than flat flames perpendicular to lines of field in which it is opposed by the gas flow). Thus the measurement of particle velocities in dc fields is always complicated by the contribution of this field-induced gas velocity. The main object of the present study of transients was to utilize the difference between the time of evolution of an ionic wind and the much shorter times of charge acquisition and drift velocity development by particles, to establish relative particle velocities with respect to the gas and hence absolute mobilities of fully grown soot particles. EXPE~MENTAL The burner system is shown in Fig. 1. An acetylene diffusion flame was stabilized on the
O0 ! 0-2180/89/$03.50
358
M. KONO et al.
l ..
:::
"
[I
g
( 6mm
I, Diam.
Acetylene Fig. 1. Burner system.
nozzle exit of a copper robe of internal and external diameters 6 and 7 ram, respectively. The fuel flow rate was 0.7 cm3/s. As shown in Fig. 1, the field was essentially along rather than across the flame, applied between the burner and a cylindrical copper electrode of 18 mm i.d. and 250 nun length, located above the nozzle tube. The distance between the lower end of the cylindrical electrode and the nozzle exit was 22 ram. Both dc and ac power sources were used, the latter being supplied from the 50 Hz mains. Two forms of dc electric field were studied, one continuous and the other transient. In the latter case, relevant parameters of the flame were measured immediately after the electric field was switched on. The switch for this purpose was equipped with an inductor and a capacitor to minimise the transient arc occurring when it was turned on.
Figure 2 shows the optical system for measuring the velocity of soot particles. The principle of this method is based on interferometry of Dopplershifted fight applied to the plus and minus diffraction orders [8]. The laser beam of about 1 mm diameter traversed the central pan of the flame at right angles to the direction of soot particle movement. The focal length of the schlieren lens (L~) was 500 ram. The stop at the focus of the laser beam was provided with two slits (0.3 x 55 mm), 2.8 mm apart. Interference between the two beams was achieved by a biprism and a cylindrical lens (L2) as shown in Fig. 2 and the beat frequency was recorded using an EMI 9798B photomultiplier. The value of the beat frequency was determined from the display of a digital storage oscilloscope, or by using a frequency spectrum analyzer. The velocity of soot panicles is given to a sufficient approximation by kfb/(20), where 20 is approximately equal to the ratio of the distance between slits of the stop to the focal length of lens L1 andfb is the beat frequency. The object of the work was to measure the mobility of large soot panicles and it was found that this measuring method could be applied to soot particles of the order of 1 pm and larger. The size of soot particles was determined on the basis of Mie scattering theory [9]. The intensity of scattered light was measured at scattering angles of 1 and 3 degrees by means of the photomultiplier used also in the measurement of particle velocity. This combination of angles was chosen because their scattering ratio varies monotonically in the region of the panicle size relevant to the present experiment, d being less than 50 ~ x . The intensity of the scattered light as monitored on the oscilloscope fluctuated considerably with time, due to variations in soot particle size. However, the aim of the present work was to study the average properties of fully formed particles rather than those of the precursors and the large particles were selected to correspond to measuremerit of soot panicle velocity. In each case, the five largest signals occurring during 10 ms were used as data for calculating size from the intensity of scattered light. The particles were treated as being spherical, although it is likely that some aggregation into chains had already occurred in
ELECTRIC FIELDS AND SOOTING FLAMES
359
L
Loser 25mr/
He-Ne
L2
q.
/ L1
~. •~
i
Blprlsm
Photonlultlpller
StOP with double s i l t s
C2H2 Fig. 2. Opticalsystemfor measurementof soot particlevelocity.Schliereninterferometryof Doppler-shiftedlight. the gas phase. The refractive index of soot was assumed to be 1.9 - 0.55P ° but it can be shown that for forward scattering the result is remarkably insensitive to the precise value. RESULTS AND DISCUSSION Throughout what follows, the dc field direction is designated according to the polarity of the upper electrode. In figures as well, plus and minus signs denote the polarity of that electrode. Because the flame plasma is essentially electrically continuous with the burner, being separated from it by only a small dead space, as compared with the much larger separation between the top of the flame and the cylindrical upper electrode, it follows that a "negative field" implies that soot particles are bombarded with an increased flux of positive ions, whereas a "positive field" exposes them to an increased electron flow. Clearly these practical flame systems do not share the simple geometry of fields lines orthogonal to flat flames [1, 31. In the absence of a field, the particle velocity at any height corresponds to the local gas flow velocity made up of natural and forced convection consequent upon the gas flow from the burner. In the presence of a d c field, two additional effects occur: the gas flow is modified by the superimposition of the ionic wind and, if the particle is charged, its own drift velocity in the field is added algebraically to the convective motion. The Ap-
pendix compares the response time of particle drift consequent upon its own charge acquisition with that of the development of the ionic wind. It is clear that the latter is several orders of magnitudes slower than the former. It is this major difference in response time that makes it possible to study particle drift velocities, and hence deduce mobilities, during the early stages of the application of transient fields. The subsequent changes in particle velocity also indicate the development in time of the evolution of the ionic wind. Continuous dc Electric Field
Figure 3 illustrates how the velocity of soot particles varies with the applied field at two heights above the burner port. The upper part shows the variation of current with applied field, which is independent of the polarity of the electrodes. The linear relationship indicates simple charge removal from an ionization-recombination equilibrium plasma, well below saturation conditions [2]. The particle velocities, however, show a strong polarity dependence. Considering, first of all, the case of zero applied potential when the soot particles act simply as flow tracers, we see a gas flow velocity of approximately 82 cm/s at 1.2 cm above the burner port, rising to approximately 127 cm/s over the next 8 mm downstream. Application of a positive field produces no great change, the velocity being
360
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300tl00 200 100
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0.5 1.0 1,5 2,0 Appl led Voltoge kV
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Fig. 3. Velocityof soot particles and current versus applied voltage for continuousdc electric field.
altered by only 10%-20%. This implies both that the particles are largely uncharged and the ionic wind velocity is small. Figure 4 shows photographs of flame luminosity that indicates the extent of distortion by ionic wind effects. None of this is very surprising; it has been shown [1, 3] that the predisposition of soot particles to carry positive charge can be neutralized when they are subjected to a stream of electrons from the flame (or even reversed, when bombardment with negative charge carriers is large enough). There is, in fact evidence here of some residual charge effects: after long periods of running, small deposits of soot appear on the burner rim, indicating some positive charging, and both the lowest curve in Fig. 3 and the luminous photograph in Fig. 4 indicate some downward wind effect due to positive charge. In the case of the negative field, when electrons are rapidly removed by flow to the burner and a column of positive ions ascends the flame, large increases in particle velocity are observed. At 2 cm above the burner the velocity is almost trebled. Again, these must be due to both the ionic wind effect and the charge on the soot particles. The force that drives the ionic wind (j/k per unit volume, where j is the current density and k the
+
T
A
E E
t~J
l + No electric f i e l d
2 kV dc
2 kV dc Fig. 4. Photographs of flames.
50 HZ 5
kVp_a
ELECTRIC FIELDS AND SOOTING FLAMES
361
mobility of the relevant charge carrier) increases with the size of the charge carrier. It may be orders of magnitude greater for a positive molecular ion as compared with an electron and again for a charged soot particle as compared with a molecular ion. The appearance of the flame in this case (Fig. 4) is characteristic of an increased convective flow. The rapid acquisition of positive charge by soot particles from an early stage in their growth has clearly been demonstrated for fields of this polarity [2]. Figures 5 and 6a show the size of large soot particles as a function of applied field and distance downstream from the burner. In the case of a negative field, appreciable reduction in size occurs as a result of decreased residence times. In the case of the opposite polarity, the changes are very small. This again accords with previous work [1, 2]. Even the small increase at intermediate potentials in Fig. 5, if that is significant, has its counterpart in the observed growth of large particles when the field is used to prolong the residence time in the pyrolysis zone--see Ref. 1. However, the small difference between absence of field and the positive case suggests that the
particles carry very little charge under these circumstances. Unlike in a counterflow system, where deposition of particles on the burner in opposition to the metered gas flow has been used to deduce mobility [1 I], no simple method of separating the drift of charged particles from the ionic wind effect suggests itself here, in the steady state.
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Fig. 5. Soot particle size versus applied voltage.
Transient
dc F i e l d s
Figure 7 illustrates the velocity transients observed during the first 25 ms after switching on the dc field. The polarity difference bears out previous observations; in the positive field case all the results are largely independent of the applied potential, suggesting that the particle essentially follows the general gas velocity and, therefore, carries little charge. For large and moderate negative fields, however, there is an initial surge in particle velocity at times too short for the ionic wind to become established (see Appendix). The growth during the first 5 ms, or so, is thought to be due to progressive charge acquisition because any changes in particle size appear far too small to account for such an effect. This is further illustrated in Fig. 6b, which shows the variation in particle size induced during the positive and negative half cycles of ac (50 Hz) fields. The rate of charge acquisition is, of course, itself affected by the field strength. After about 10 ms, the velocity of all soot particles rapidly decreases, irrespective of the polarity of the upper electrode. This is thought to be due to ionic wind effects. Bearing in mind that at sufficiently long times these curves must eventually coincide with their steady dc case values, as portrayed in Fig. 3, it is clear that the details of ionic wind and its development in time are complex. It follows from the theory (e.g. Ref. 2) that gas flows accelerate towards both electrodes, gas being entrained around the flame to make its acceleration possible, in incompressible flow. These primary flow modifications in turn affect the shape of the flame and the position of the pyrolysis and ionization regions, resulting in further readjustments of the flow pattern. Under certain conditions, oscillations are set up. These
362
M. KONO et al. I
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Dlstance from nozzle extt mm Fig. 6. Soot panicle sizeve~usappliedvol~ge.(a)dc; ~)ac of SOHz. effects can be monitored, to some extent, by following temporal changes in flame luminosity obtained by means of a phototransistor of maximum response at 800 nm wavelength. This shows transient effects and confirms that the first rapid changes occur after about 10 ms.
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The object of this exercise was not to analyze the ionic wind effects, which are clearly too complex and geometry-dependent, but to confirm experimentally the calculation in the Appendix and establish the period during which charged particle drift can be measured independently of them. Comparison with the negative field case in Fig. 7 indicated that mobilities may be calculated from the results at short times. For this case, the velocity of the soot particles relative to the gas velocity was obtained by subtracting the velocity in the absence of an electric field from that in its presence, during the early stages. These relative velocities are plotted in Fig. 8 as a function of the mean electric field. Gas velocities in the absence of an applied field were obtained from Fig. 7 at 12 and 20 mm from the burner port. The corresponding mobilities were 3.9 x 10 -6 and 5.9 x 10 -6 m: s- ~V - I for the two heights respectively. In the positive field case the mobility, along with the charge on the particle, was regarded as close to zero or, at least, too small to measure.
1.o
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acFields Figure 9 shows soot particle velocities at various times during the application of an ac electric field of mains frequency (50 Hz) 20 mm above the
ELECTRIC FIELDS AND SOOTING FLAMES
363
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versus negative electric field strength. The gradient of lines shows mobilityof soot particles. burner port. Here again, during the positive half cycle, the velocity of the particles tends to approach, in time, that of the gas, whereas during the negative half cycle, the additional velocity of the soot particles alone can be determined. The corresponding relative velocity is shown as the solid symbols in Fig. 8, with the electric field strength averaged over the half cycle, i.e., the peak to peak value multiplied by 0.637. It will be apparent from Fig. 8 that the mobility obtained in this way is in fair agreement with that deduced from the transient dc electric field. The agreement may be improved further if the reduction in particle size shown in Fig. 6b is taken into account. CONCLUSIONS A study of the transient behavior of soot particles, velocities, sizes, and flame luminosity leads to a number of interesting conclusions. Comparison of the events immediately following the switching on of a dc field with one in the steady state, and also with various phases during an ac field of 50 Hz frequency, shows that ionic wind effects are complicated but can be separated in time from the drift of the charged particles in a field. Among the results are that large soot particles of the order of 10 pm diameter attain mobilities of the order of 5 x 10 -6 m 2 s -~ V -~ when subjected to a flux of positive ions, but acquire very little charge (effectively zero mobility) in the opposite electric field.
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Fig. 9. Sootparticle velocityin ac electric field (50 Hz) for 20 mm downstreamfrom nozzle exit. The method developed may lend itself to more detailed studies, for example of the evolution in time of charge acquisition and growth of soot particles, in flames that are not restricted to the somewhat artificial academic geometries studied previously.
REFERENCES I. Place, E. R., and Weinberg, F. J., Proc. R. Soc,. A 289:192 (1965). 2. Lawton, J., and Wemberg, F. J., ElectricalAspects of Combustion, Clarendon, Oxford, 1969. 3. Mayo, P. J., and Weinberg, F. J., Proc. R. Soc., A 319:351 (1970). 4. Wagner, H. Gg., Seventeenth Symposium (InternationaO on Combustion, The Combustion Institute,
Pittsburgh 1979, p. 3. 5. Mitchell, J. E., and Wright, F. J. Combust. Flame 13:413 (1%9). 6. Kono, M., Iinuma, K., and Kumagai, S., Eighteenth
364
7.
8. 9.
10.
11.
M. KONO et al. Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p 1167. Weinberg, F. J., Electrical Intervention in the Sooting of Flames, NATO Workshop on Sooting Combustion Systems and Its Toxic Properties, NATO, 1981, p. 1. Schwar, M. J. R., Thong, K. C., and Weinberg, F. J., J. Phys. D 3:1962 (1970). Bayvel, L. P., and Jones, A. R., Electromagnetic Scattering and Its Applications, Applied Science Publishers, London, 1981. Lee, S. C., and Tien, C. L., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 1159. Hardesty, D. R., and Weinberg, F. J., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 907.
of 10 #m diameter, the response time, t p , is calculated as 1.2 x 10 -9 S. The calculation procedure is based on Ref. 6. The ionic wind is caused by ions in the flame region whose number density is many orders of magnitude less than that of neutral molecules. In evaluating the effect, in a transient electric field, of the ionic wind on flame shape or gas velocity in the flame region, the collisions between ions and neutral molecules must therefore be considered as the predominant rate-controlling process. On simple kinetic theory, the collision response time, tc can be expressed as tc = z / ( 1 . 4 r o 2 c N ) ,
APPENDIX The equation of motion of a charged soot particle in quiescent gas may be expressed as mdoldt + olB = Eq,
(1)
where t is time, E the electric field strength, B the constant in the viscous drag expression, and m, v, and q the mass, velocity, and charge, respectively, of the particles. Integration of Eq. 1 with v -- l, initially, leads to v = E q B { 1 - exp[ - t / ( m B ) ] } .
(2)
The term m B in this equation corresponds to a response time. In the region where the time exceeds the value of m B , the soot particle moves with approximately constant velocity. For large particles, of the size discussed in this work, e.g.,
(3)
where z, o, c, and N are the ratio of number density of neutral molecules to that of ions ( = 5 × 10 7 [2]), collision diameter, mean molecular velocity, and number density of molecules, respectively. For air at 1500 K and 1 atm, approximate values ofo, c, and N a r e 4 × 10- l0 m, 1 × 10 3 m/s, and 5 × 10 ~ m -3, respectively. The value of tc that emerges from this calculation is 14 ms. A rapid experimental test was carried out to verify the above rough estimate. High-speed cine photography was used to record the onset of distortion on the luminous region when a transient dc electric field was applied. The cine sequence confirms that no change occurs in the flame shape prior to 10 ms after the application of the electric field. Received 3 August, 1988: revised 7 November 1988