the optimum condition for ignition of gases by...

THE OPTIMUM C O N D I T I O N FOR IGNITION OF GASES BY COMPOSITE SPARKS

M. KONO, S. KUMAGAI AND T. SAKAI University of Tokyo, Japan

To determine the optimum ignition condition for sparks consisting of a capacitance spark followed by a dc- (glow) or ac-discharge (1 MHz), the effects of gap width, electrode configuration, mixture strength, spark duration, and energy distribution between the two components on the minimum ignition energy were investigated, using a quiescent propane-air mixture. The condition in question is conveniently characterized by the optimum spark duration for which the minimum ignition energy is lowest and the corresponding energy value. For a dc-discharge spark, the well-defined optimum spark duration varies from about 50 to 300 Ixsec and the minimum ignition energy for spark durations larger than the optimum increases in different modes, depending on the mixture strength and the quenching effect of spark electrodes. For an ac-discharge spark, the optimum condition for ignition is much the same as for a tic-discharge spark, but the minimum ignition energy and the spark duration are always proportional to each other above the optimum, and therefore the optimum spark duration is easily obtained up to about 5 msec. Flash-schlieren photographic observations of the initial behavior of the spark kernel confirmed that such differences in the mode of minimum ignition energy are related to electrostatic attraction by the negative electrode.

Introduction

The electric spark used current ly in pract ical ignit ion systems is the so-called long durat ion composi te spark which consists of capaci tance and inductance components . From the s tandpoint of electric discharge characteristics, one of them equals a capaci tance spark and the other, a glow or arc discharge of direct or al ternating current. Igni t ion by single- component sparks has been the subject of numerous experiments and theoretical treat- ments. For example, Lewis et al. I have a l ready es tabl ished the concept for ignit ion by capacitance sparks of very short duration. However , there have been few pub l i shed papers on the ignit ion phenomena associated with the composi te spark because these phenomena are too complex to he s tudied and the importance of both components has not been noticed by investigators. A signif icant contr ibut ion in this area was made by Kumagai et al.,Z who investigated the ignit ion mechanism of composi te sparks produced by igni t ion coils for internal combust ion engines and emphas ized the co-

operative funct ion of both components for ignition.

In the present study, using two kinds of wel l -def ined electric discharge instead of the second component of the practical composi te spark, the effects of gap width, electrode configuration, mixture strength, and energy dis- t r ibut ion between the two components on the min imum ignit ion energy have been investigated. The op t imum igni t ion condi t ion of such composi te sparks is convenient ly character ized by the spark durat ion for which the min imum ignit ion energy is lowest, and the lowest value itself.

Apparatus and Procedure

The ignit ion unit used for generat ing a composi te spark is composed of a capaci tance- spark circuit and dc- or ac-discharge supply systems. Block diagrams of the electrical cir- cuits are shown in Figs. 1 and 2. The capacitance-spark circuit, which is actuated by the start pulse as shown in Fig. 1, produces

757

758 IGNITION, OPTICAL AND ELECTRICAL PROPERTIES

ELECTRO- MAGNET

DO

STOP _L PULSE -- I

CAPACITANCE SPARK CIRCUIT

Rc DC

AC PULSE

SPARK OSCILLOSCOPE

F1c. 1. Ignition system for generating capacitance spark and following dc-discharge component.

capacitance sparks whose durat ion is about 0.5 Ixsec and whose energy is control led by changing the values of R c and C c which in- cludes about 4 p F stray capacitance inherent in the circuit. The energy of capacitance sparks obtained in this way is calculated from the breakdown voltage at the spark gap and the value of Co, and can be varied between 0.05 and 10 mJ.

The capaci tance spark acts as a tr igger for the dc discharge. The stop pulse, which is delayed by a known time interval after the start pulse, makes the thyratron T terminate the dc discharge across the spark gap. Simulta- neously, the vacuum switch is opened by the electromagnet which is operated by the current through the thyratron. The system is then reset.

The current and voltage traces of dc-discharge sparks on a dua l -beam osci l loscope are qui te rectangular and their durat ion and energy densi ty which are control led by the var iable resistance R i can be var ied independen t ly from 0 to 70 msec and from 3.4 to 100 J / s e c , respectively. The traces show that such a dc discharge is a glow discharge and that above 100 J / s e c it enters the transient range from glow to arc discharge, where the energy dens i ty is nei ther s teady nor reproducible . In order to produce the ac discharge, when a capacitance spark occurs across the spark gap, the tuned-push-pul l ampl i f ie r shown in Fig. 2 begins to be excited by a wel l -def ined rectangular pulse of a known duration. For the durat ion, therefore, the oscil lator output of 1

VARIABLE- OUTPUT OSC.

(I MHz)

t RECTANGULAR,

PULSE

F]c. 2. Generator of ac-discharge component,

B+

SPARK o CAPACITANCE SPARK CIRCUIT

IGNITION OF GASES BY COMPOSITE SPARKS 759

MHz frequency is ampl i f i ed and a steady oscil latory discharge is p roduced across the spark gap whose durat ion and energy densi ty can be also varied independen t ly from 10 to 50 msec and 5 to 50 J / s e e , respectively. The energy densi ty in this case is determined by means of a ca/orimeter. 3

The spark gap is located at the center of a combust ion chamber, 40 mm both in diameter and in length, with glass windows for schl ieren-photographic observation. The spark electrodes are of a 0.3 mm diameter tungsten wire ending in a 30 ~ half angle cone. These electrodes were used in most eases, but in order to achieve a very high quenching effect for spark kernels, a 3.0 mm diameter steel rod t ipped with a 45~ angle cone was also used.

After the combust ion chamber has been fi l led with a lean propane-a i r mixture at room temperature and a tmospher ic pressure, spark discharges are repeated in the quiescent mixture 25 to 30 times, recording the number of ignit ions, and thus the percentage of ignit ion. The min imum igni t ion energy obtained under various experimental condi t ions is def ined at 50% ignit ion, the igni t ion energy be ing de-

f ined as the total energy of the capaci tance and other components .

Results and Discuss ion

DC-Discharge Spark Using the de-discharge spark, the effects of

gap width, electrode configurat ion, mixture strength, spark durat ion, and energy distr ibu- t ion between the two components on the minimum ignition energy were investigated.

Figure 3 shows the effect of spark durat ion on the min imum igni t ion energy for different mixture strengths wi th electrodes of 0.3-turn diameter. The gap wid th is about the same as the quenching distance, viz. 3.5, 3.0 and 2.5 mm for 3.0-, 3.2- and 3.5-% (vol.) propane, respectively. This quench ing dis tance is the quenching distance de termined for the lowest value of the min imum ignit ion energy. The min imum ignit ion energies for a spark dura- t ion of 0.5 ixsec were obta ined by using only capaci tance sparks; their values were found to be much the same as those reported pre- v ious ly ) For a spark durat ion of more than

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F~G. 3. Effect of spark duration on minimum ignition energy for gap width nearly equal to quenching distance.


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FIG. 4. Effect of spark duration on min imum ignit ion energy for gap width smaller than quenching distance.

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Fzc. 5. Effects of mixture strength and capacitance-component energy on min imum ignition energy. Gap width: 1.0 mm; electrode: 0.3 mm in diameter.


0.5 Ixsec, the ignit ion energy always inc luded the capaci tance-component energy of 0.07 mJ. With increasing spark durat ion the min imum igni t ion energy decreases appreciably , reach- ing its lowest value at a durat ion of about 50 p, sec and then increas ing in proport ion to the duration. The increase of the min imum igni t ion energy direct ly proport ional to the spark durat ion probab ly means that ignit ion has a l ready been es tabl ished. The op t imum spark durat ion for the min imum ignit ion energy (about 50 p~sec in this case) seems to be independen t of the mixture strength employed in such a narrow range.

F igure 4 shows the quench ing effect on the min imum ignit ion energy, this effect being s t rengthened through the use of smaller gap widths and thicker electrodes. As the gap width is decreased, the op t imum spark durat ion increases. As is shown in Fig. 5, the op t imum spark durat ion also increases with decreasing mixture strength. However , the op t imum spark durat ion above about 300 p.sec is diff icult to de termine clearly, as shown in the case of 0.5-mm gap width, because the min imum igni t ion energy increases more s lowly after the

op t imum point (in propor t ion to the square root of the spark durat ion on the average in this case) and before the opt imum point it hard ly varies over the narrow range of energy densi ty applied. The less- than-proport ional increase in the min imum ignit ion energy means that, in such a case, the spark energy released after the op t imum spark durat ion is largely available for igni t ion, in other words, ignit ion has not been established. Such situa- t ions, however, do not hold for the cases where the quenching effect of the electrodes is ex- t remely high a n d / o r the mixture strength is much leaner. In the former case, for example in Fig. 4, with 3-mm diameter electrodes the min imum ignition energy reaches the lowest value for a spark durat ion of about 70 p.sec and increases in propor t ion to the durat ion in the same way as in Fig. 3. In the latter case, for 2.8% propane and a 0.07 m J capacitance component as shown in Fig. 5, the min imum ignit ion energy lacks a clearly de- f ined opt imum spark durat ion, showing less dependence on spark durat ion than in the former case.

Figure 5 shows a typical effect of the energy

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FIG. 6. Effect of electrode configuration on minimum ignition energy. Gap width: 1.0 mm; mixture strength: 3.0% (vo].) propane.

762 IGNITION, OPTICAL AND

of the capaci tance component on the m i n i m u m ignit ion energy. The op t imum spark dura t ion decreases as the energy of the capaci tance component increases, and for a more p ro longed spark, an increase in the energy of the capacitance component causes an increase in the min imum igni t ion energy larger than the capacitance componen t increment. This is easily understood, for example, for 3.0 % propane by subtract ing 2.80 mJ from the m i n i m u m ignition energy for the 2.80 mJ capaci tance component and compar ing the rest wi th that for the 0.07 mJ capaci tance component . This situation holds for a larger gap width, namely a smaller quench ing effect. However, for an extremely large quench ing effect, an energy increase in the capaci tance component is found to produce a s l ight decrease in the m i n i m u m ignition energy.

Figure 6 shows the experimental results in relation to e lectrode configuration. As can be seen, the m i n i m u m ignit ion energy obta ined by using a 3 mm diameter electrode for the negative and that of 0.3 mm diameter for the posit ive is about five times the value ob ta ined by using 0.3 mm diameter electrodes for bo th

ELECTRICAL PROPERTIES

the posit ive and negative, whereas the value for the reverse polar i ty is only about twofold. Such a marked effect of the negative e lect rode on the min imum igni t ion energy, in contrast with the results ob ta ined by Lewis eta]. 1 us ing a capacitance spark of very short durat ion, suggests that especia l ly for a long spark duration the spark kernel p roduced by the glow discharge is d rawn to the negative electrode by an electrostatic attraction, which may affect the mode of the min imum ignit ion energy versus spark durat ion.

It is also very interest ing that the above results are qual i ta t ively compat ible wi th the experimental facts reported previously relat ive to the op t imum spark durat ion 4,~ and the mini - mum ignit ion energy, which can be regarded as directly propor t ional to the spark dura t ion 6 or to its square root. 4

AC-Discharge Spark

In order to avoid the effect of an electrostat ic force, a 1 MHz f requency ac discharge spark was subst i tuted for the dc-discharge spark. Figure 7 shows the effect of spark dura t ion

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FIG. 7, Comparison of minimum ignition energy obtained with dc- and ae-discharge sparks. Gap width: 1.0 mm; electrode: 0.3 mm in diameter.


on the minimum ignition energy for different mixture strengths, together with that obtained by using the dc-discharge spark. The minimum ignition energies of both cases are found to be nearly equal near the optimum spark duration, but after the optimum point there exists a pronounced discrepancy between them. The minimum ignition energy in the case of the ac-discharge spark increases approximately in proportion to the spark duration. This suggests that in the case of the dc discharge, the increase of the minimum ignition energy nearly proportional to the square root of the spark duration is due to the electrostatic force. The optimum spark duration varies with the mixture strength and also with the gap width and electrode configuration, but its maximum value of about 5 msec is obtained near the lower limit of flammability, the corresponding minimum ignition energy being about 200 mJ.

Observation of Spark Kernels

In order to clarify factors governing the optimum ignition condition of composite

sparks, microphotographic observations on the behavior of spark kernels were made by a conventional flash-schlieren method. Figure 8 shows serial photographs of incipient flames produced with de- (a) and ac-discharge sparks (b) at 50% ignition, where time is given in microseconds from initiation of the capacitance component. Since there exists a small fluctuation in the spark kernel size at a given time, especially for dc discharge (Fig. 9), each of these photographs, which represents a sepa- rate trial for ignition, is of averaged size. In Fig. 8 (a), a considerable volume of the spark kernel is attracted around the negative electrode. Furthermore, the spark kernel divides clearly into two portions; one is produced by the capacitance component and the other by the dc discharge. The spark kernel due to the capacitance component fades out as time elapses and therefore seems not to contribute to the formation of a self-propagating flame, These are general features of spark-kernel behavior for a dc-discharge spark of longer- than-optimum duration. For an ac-discharge spark, as shown in Fig. 8 (b), such features

- - +

(a)

TIME .508 698 1200 2000 3250 / J . sec o

(b)

TIME 122 280 655 1340 3170 /.L s e c

FIG. 8. Schlieren photographs of spark kernels produced with dc- (a) and ac-discharge sparks (b) with identical duration of 1.2 msec. Gap width: 1.0 ram; electrode: 0.3 mm in diameter; mixture strength: 3.0% (vol.) propane; minimum ignition energy: 6.50 mJ for (a) and 8.34 mJ for (b).


are not observed and its spark kernel develops with rather a smooth surface around the spark path.

Figure 9 shows the development of the maximum spark kernel diameter perpendicular to the spark path, which is determined from the schlieren photographs, and compares the eases of ignition and extinction, using the two kinds of sparks corresponding to Fig. 8. For the dc-diseharge spark, the difference in the kernel diameter between ignition and extinction occurs after about 1.2 msee, whereas that for the at-discharge spark seems to occur after about 650 txsec for the identical spark duration. This time of about 650 Ixsec is in order-of- magnitude agreement with the optimum spark duration, as shown in Fig. 7. The period until a change in the kernel development occurs may be the time required for ignition to be established. In other words, subsequently the spark discharge and electrodes can no longer affect the ignition process, because the flame front of the kernel is already far away from them.

As regards the spark energy released after the optimum point, these facts confirm that it is largely available for ignition when the minimum ignition energy increases rather gra-

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Fro. 9, Diameter of spark kernels produced with dc- and ac-discharge sparks vs. time from initiation of capacitance component. Other parameters are the same as in Fig. 8.

dually, e.g., in proportion to the square root of the spark duration, whereas it is hardly available when the rate of increase is directly proportional. Therefore, for the ac-discharge spark, as shown in Fig. 8 (b), the net useful energy for ignition is only about 4.52 mJ (= 8.34 mJ • 650/1200), which is considerably smaller than the minimum ignition energy of 6.50 mJ for the dc-discharge spark. Such supe- riority of ac-diseharge sparks with respect to the ignition energy is well understood by considering the heat loss from the spark kernel to the negative electrode, as suggested from Fig. 8 (a).

Figure 10 shows serial schlieren photographs of incipient flames, which grow to self-propagating flames at 50% ignition produced by composite sparks with capacitance components of 0.07 mJ (a) and 2.80 mJ (b). In Fig. 10 (a), the spark kernel of the dc discharge fits entirely into the aperture of the torus formed by that of the capacitance component. By contrast, in Fig. 10 (b), the following kernel is sucked into the initial torus together with the surrounding unburned mixture forming a wedge-shaped groove, and fin- ally the spark kernel develops in a torus as a whole. From this, it is apparent that an increase in the minimum ignition energy with increasing energy distribution of the capacitance component is closely related to the spark kernel behavior and leads to increased heat loss from the spark kernel to the unburned mixture. 7,s Figure 10 also suggests that with increasing capacitance-component energy the spark kernel begins to move away from the electrodes at an earlier stage of the ignition process, and that the heat loss from the spark kernel to the electrodes decreases. This situation certainly corresponds to the fact that, for an extremely high quenching effect, an increase in the capacitance-component energy results in a slight decrease in the minimum ignition energy.

Conclusions

The optimum ignition condition of composite sparks which consist of a capacitance spark and a dc- (glow) or ac-discharge (1 MHz) was investigated, using lean propane-air mixtures.

In the case of de-discharge sparks the optimum condition for ignition is concluded to be as follows:

1) The well-defined optimum spark duration varies from about 50 to 300 Ixsec, according to mixture strength and the quenching action of the spark electrodes.

IGNITION OF GASES BY COMPOSITE SPARKS

+

765

(a)

TIME 47 88 195 425 1950 /zsec

- - - t -

(b)

TIME 47 88 195 425 1950 / ~ s e c

FIG. 10. Schlieren photographs of spark kernels produced with dc-discharge spark. Minimum ignition energy (including capacitance component): 3.10 (0.07) and 5.20 (2.80) mJ for (a) and (b), respectively; spark duration: 80 p~sec. Other parameters are the same as in Fig. 8.

2) Except in the case of an extremely high quenching effect, the op t imum spark durat ion decreases and the cor responding lowest value of the min imum igni t ion energy increases, wi th increasing energy of a capaci tance component of about 10% at least of the total spark energy. 3) The min imum igni t ion energy obtained as a funct ion of the spark durat ion increases after the op t imum point in propor t ion to the spark durat ion for an extremely high or low quenching effect.

Conclusions for ac-discharge sparks are the fol lowing:

1) The op t imum igni t ing condi t ion is much the same as for dc-discharge sparks. 2) The min imum igni t ion energy and the longer- than-opt imum spark durat ion are always in propor t ion to each other. Therefore, the op t imum spark dura t ion is easily obta ined up to about 5 msec.

Microphotographic observat ion of the behavior of incipient flames showed that:

1) For dc-discharge sparks of long durat ion, the spark kernel is drawn toward the negative electrode by an electrostat ic attraction. This may delay ignit ion and lead to a relat ively gradual increase in the min imum ignit ion energy with increasing spark durat ion after the opt imum point, e 2) For ac-discharge sparks whose durat ion is longer than the op t imum, the energy just re- qui red for ignit ion is assumed to be equal to the min imum ignit ion energy in the vicini ty of the op t imum spark durat ion.

REFERENCES

1. LEWIS, B. AND VON ELBE, G.: Combustion, Flames and Explosions of Gases, pp. 323-346, Academic Press, 1961.

2. KUMAGAI, S., SAKAI, T. AND KIMURA, I.: J. Fac. Engng. Univ. Tokyo, 24, 10 (1953).

3. KUMAGAI, S., SAKAI, T. AND YASUGAHI~, N.; Com- bustion Science and Technology, 6, 223 (1972).

4. SWETT, C. C. ja.: NACA RM E9E17 (1949). 5. BALLAL, D. R. AND LEFEBWE, A. H.: Combustion


and Flame, 24, 99 (1975). 6. KRAVCHENKO, V. S.,ERYGIN) A. T, AND YAKOVLEV,

V. A.: FGV (Combustion, Explosion and Shock Waves), 9, 603 (1973).

7. KONO, M. AND KUMAGAI, S.: Science of Machine (Japan), 26, 649 (1974).

8. KoNo, M., KUMAGAI, S. AND SAKM, T.: Combust ion and Flame, 27, 85 (1976).

COMMENTS

D. R. Ballal, Cranfield Institute of Technology, England. 1. Did you look at the current and voltage traces of the composite spark? Was the current value fluctuating between glow and arc discharge condition in your sparks?

2. We studied 1 opt imum spark durations in flowing mixtures using rectangular are discharge. We found that about 60 p.sec was the correct figure. However this op t imum duration (electrode gaps set at quenching distance)

1) decreases wi th increase in velocity 2) changes with pressure 3) changes with equivalence ratio (that is flame

temperature)- -a result different from authors ' work.

Moreover our results were supported by the heat loss equations, specially (3) above. I wonder if you have done any such analysis in your case?

3. Have you tried repetative spark discharge igni- t ion?

REFERENCES

1. BALLAL D. R. AND LEFEBVRE A. H.: Combust ion and Flame 24, 99 (1975),

Authors" Reply. 1. As described in our paper, the traces show that the de discharge used is clearly glow if its energy density is less than about 100 J / s e c . Above this value, the discharge current and voltage fluctuate between glow and arc condit ion even in our sparks.

2. With respect to the effect of opt imum spark duration on mixture strengths, such dependences as shown in your paper, where arc discharges are used, are not observed. This may be due to mixture strength of a considerably narrower range employed in our experiment, or the difference between arc and glow discharges. We can also find that your experimental results show weak dependence of optimum spark durations on mixture strengths in a range leaner than the stoichiometric ratio.

We are now performing experiments with f lowing systems. For a pressure effect, we are much interest- ed in the ignition phenomena at a pressure higher than the atmospheric.

3. We investigated the ignition by two successive sparks with reference to the frequency effect of capacitance spark and its results will be publ i shed in near future (by M. Kono, S. Kumagai and T. Sakai: Combust ion and Flame, in press).

Bernard T. Wolfson, AF Office of Scientific Re- search, USA. Your data apparently is associated only with static or nonf lowing reactive gaseous mixtures. Have you done similar experiments with f lowing systems? If so, what are the trends in ignition energy and extinction in these flowing systems and the differences in values between flowing and nonflow- ing systems?

Authors" Reply. We are now performing experiments with f lowing systems. According to the results already obtained, however, definite conclusions have not been derived yet, because many parameters interfere with each other. One can f ind some trends, which were obtained with particular ignit ion apparatus, in references cited.

E. K. Dabora, University of Connecticut, USA. Have you correlated your experimental results with rates of energy (or power) rather than just total energy? If not, would it be possible with your system to measure the spark energy as a function of t ime?

Authors" Reply1. Yes. The rate of energy is a significant but not predominant factor. Total energies, though not comprehensive, are capable of describing the results rather than rates of energy.

The rates of energy in our system are quite rectangular except for the capacitance component, so the released energy increases as a linear funct ion of time.

the optimum condition for ignition of gases by...

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