Imperial College, London, S.W.7, Englandronney.usc.edu › AME513b › Lecture5 › Papers ›...

T H E E F F E C T O F P L A S M A C O N S T I T U T I O N O N L A S E R I G N I T I O N E N E R G I E S

RICHARD G. KINGDON AND FELIX J. WEINBERG Imperial College, London, S.W.7, England

The extent to which minimum ignition energies depend on the species which carry the plasma energy is investigated, with particular referenee to inhibitors and to the usual constituents of electrodes used in spark ignition. The fundamental limitations which apply to ignition by laser beams focused into gases are avoided by using minute targets in the form of wires or fibres of the order of 10-5 m. in diameter. The extent of the plasma can be varied by defocusing. Its energy is measured by a specially developed suction mieroca- lorimeter. It is found that, for Q-switched pulses of about 40 ns duration, minimum ignition energies are independent both of the carrier material constitution and of the amount of foreign substance in the plasma in accord with the underlying propagation-limited theory. For nmch longer durations however, of the order of 1 ms, plasma constitution plays the role expected from each additive. Thus inhibitors, for example, inhibit if the continuing energy input enables them to catch up with the expanding flame fronts.

Introduetion

A prel iminary invest igat ion i into the use of focused laser beams for min imum ignit ion energy studies revealed certain fundamenta l l imitat ions when the radiat ion is focused into a f lammable gas mixture. Ini t ial ly the gas is t ransparent and, unless the power flux exceeds the threshold for b reakdown (typically 101~ I0 n W.cm -2) it will remain so for the durat ion of the pulse and no energy will be absorbed. If the power flux is suff icient to generate an appreciable electron concentrat ion at the peak of the pulse, the p lasma front facing the incident beam absorbs incoming energy dur ing the t ime between the onset of breakdown and the end of the pulse. This leads to an absorpt ion threshold which has been shown to be a l ready in excess of the m i n i m u m ignit ion energy for mixtures which are not ei ther far from stoi- chiometr ic or at sub-a tmospher ic pressures. The use of minute targets at the focal spot obviates this diff icul ty. It offers not only a s impler al ternative to reduc ing the pulse duration much below 40 nanosec but makes available a range of other degrees of freedom. In part icular, the init ial volume of the plasma, its chemical const i tut ion, and the amount of the target material it carries can be changed

747

by varying the target 's posi t ion in relation to the focal spot, the mater ial of which it is made, and its size, respectively.

The main object of the present work was to investigate to what extent min imum ignit ion energies depend on the species carrying the p lasma energy. The range of interest includes inhibitors , e.g. sod ium chloride, and this is relevant also from the poin t of view of sheathed explosives for use in mines and f lammable atmospheres. At the other extreme, it might be thought that some spe c i e s - - e ve n ~oossibly just m o i s t u r e - - w o u l d act as promoters by generat ing free radicals active in chain propagation within the plasma. Moreover, if the ignit ion energy d id d e p e n d on p lasma consti- tut ion it would be impor tant to know the effect of inevitable contaminants such as copper or other electrode materials in electrical ignit ion. The results should therefore be relevant to a wide range of igni t ion phenomena.

Experimental System and Preliminary Investigations

The general layout of the experimental system is shown in Fig. l(a). A "Laser Associates 211 A" ruby laser was used with a dye ceil Q-switch, for most of the experiments. The

748 IGNITION, OPTICAL AND ELECTRICAL PROPERTIES

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FIG. 1. Apparatus. (a).Layout of system. A, ruby laser; B, pulse counter; C, beam energy calorimeter; D, amplifier; E, pen recorder; F, neutral density filters; G, iris; H, rotatable polaroid; I, target; J, ignition cell; K, sighting microscope; L, He-Ne laser; M, schlieren stop; N, high speed camera. (b) Suction microcalorimeter.

pulse reproduc ib i l i ty was improved by a factor of about 10 by us ing vanadyl ph tha locyan ine dye in n i t robenzene at twice the normal concentration, the laser be ing operated well above the threshold. It was fired at two minu te intervals and the first six shots were d iscarded. In this way a s tandard deviat ion of 4% of the

incident energy was obtained. This was measured by ref lect ing a constant fraction into a calorimeter. The pho tod iode moni tored the number of pulses. The beam width was l imi ted by an iris s ince it was found that it could not be focused to a spot less than 1 mm. in diameter wi thout a circular stop. The sheet

LASER IGNITION ENERGIES 749

of polaro id could be rotated so that the beam energy could be a t tenuated in a control led manner.

The target fibres or wires were pos i t ioned in the test cell, their pos i t ion with respect to the beam being moni tored by a microscope. Examinat ion under a more powerful microscope al lowed the amount of solid matter vaporised from them to be assessed. The targets themselves were th in wires or quartz fibres subsequent ly coated wi th the material under test.

Long uniform quartz fibres were produced in two stages. In the first, a heated quartz rod was pul led mechan ica l ly in the usual manner. The fibre so p roduced was heated in an oxygen-enriched gas flame from which it was expelled in a mol ten form. After r is ing b y convection it was caught while s lowly descending. Uniform fibres several feet long and 8 ixm in diameter were produced in this way. The diameters of the targets ranged from 12 to about 50 ixm.

The basic premisses were establ ished in p re l iminary experiments. It was demonstra ted that in the presence of any target no p lasma energy threshold exists; i.e., if there are energies so small that they would be comple te ly absorbed or reflected by such targets, they must be negligible by compar ison with minimum ignit ion energies.

In the absence of the Q-switch, irregular sp ik ing occurs for about 1 ms. Using a mixture of 8.6% methane in a i r - - w h i c h corresponds to the min imum quench ing d i s t ance - - i t was shown that the laser beam energies required to produce ignit ion indeed differed greatly for copper and steel wire and for sodium chloride coated quartz fibres, all of about 50 ~.Llll d iameter. The value for copper was over twice that for iron, while the entire laser output (nomi- nal ly 30 J) was insuff ic ient to produce ignit ion in the presence of sod ium chloride.

The Suction Microcalorimeter

The calorimeter shown in Fig. l(a) is s imilar to that used in previous laser ignit ion experiments/~) and measures a constant fraction of the total energy in the inc ident beam. It would of course be possible, as in this previous investigation, to use a second such calorimeter beyond the test region and subtract the depart- ing beam energy from the incident one. How- ever the difference would not necessari ly yield the p lasma energy since reflection, scattering, heat absorpt ion by unvapor i sed parts of the target, etc., may play a part.

Since errors in t roduced by such effects

would be expected to vary with the target material, it was considered essential to measure the p lasma enthalpy direct ly. To this end a microcalor imeter had to be deve loped which aspirates the p lasma and measures its heat content, independen t of its composi t ion. This proved one of the most diff icult parts of the investigation and the calorimeter design passed through several i terations and extensive cal ibrat ion procedures. The final des ign which was used throughout the subsequent work is shown in Fig. l(b).

It works by recording the temperature rise of a minute quant i ty of copper "wool" to which the heat is t ransferred from the aspirated gas. To realise the scale, it is important to appreciate that the entrance aper ture is less than a mil l i - metre in diameter, the nest of "wool" is woven of 12 ~m diameter copper wire and the total mass is only about 8 mgm.

Developing a sui table cal ibrat ion source for total energies a round 1 mJ proved equal ly taxing. Devices ranging from CW lasers to small heated solid objects were considered as pr imary standards. The cal ibrat ion source final ly evolved consis ted of an electrically heated fi lament, weighing about 0.1 mgm and lagged near its points of contact with the current- carrying leads. The l agg ing - -cons i s t i ng of cement bonded silk f i b r e s - - w a s added as a result of temperature traverses along the fila- ment. Gas could be aspirated, either continu- ously or in puffs contro l led by a solenoid valve, over the central coi led por t ion of the f i lament which was about 1 mm in length.

Some of the intr icacies of the cal ibrat ion will become apparent from Fig. 2. Figure 2(a) shows the effect of changing the rate of energy release. It will be seen that even when the t ime of releasing 1 mJ is as long as 0.5 s., the response differs from that for the .highest powers by only about 30%, so that the difference between the shortest times of del ivery from a f i lament and those from the laser pulse would be expected to be negligible. Calibra- t ion for various aspirat ion r a t e s - -F ig . 2 ( b ) - - was necessary to ensure complete p lasma cap- ture. As expected, it p roduced curves showing the reading to be independen t of flow rate (until very high values are reached) once a certain threshold is exceeded. This occurred at about 4 e.c. s -1. The need to look at varying separat ions between source and probe arose when it was found that the probe picked up some radiation direct from the plasma. This contr ibut ion could be subtracted s imply by deduc t ing the reading at zero aspiration rate, provided the probe month was far enough away for the hot gas not to reach it in the

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absence of suction. F igure 2(e) illustrates the l ineari ty of the cal ibra t ion relat ionship, under the op t imum condi t ions establ ished, which makes it relat ively easy to measure the energy content of any of the p lasmas used. When flame propagat ion follows ignit ion, the p lasma energy can be measured in an inert a tmosphere, separately, t h o u g h - - a s d iscussed b e l o w - - o x i - d izable metals can give different results in the presence of oxygen.

Results

Use of the suction calorimeter revealed at once the important d i f ference between the incident beam energies and those contained in the plasma. Thus even though the beam energies for igni t ion in the ease of iron and copper targets were very different, as men- t ioned above, the p lasma energies were very s i m i l a r - - o f the order of 2 mJ. The reason is that the iron plasma releases heat by react ing with the oxygen of air and, while this energy is p icked up by the suct ion calorimeter, it means that much less incident beam energy is required for ignit ion. This was conf i rmed by repeat ing the exper iment for the same incident beam energy in an atmosphere of ni t rogen which y ie lded a greatly reduced plasma ene rgy - - a s registered by the suction calorimeter.

In order to approach the ideal of instanta- neous point igni t ion as closely as possible , the laser was next pass ive ly Q-switched and the target size d e c r e a s e d - - g i v i n g a pulse dura- t ion of approximate ly 20 ns half-width and an initial target d iameter of approximate ly 12 p.m. Figure 3 shows that min imum igni t ion energies obtained in this manner for mixtures conta ining between 6 and 8.6% CH 4 in air do not differ greatly (curve 5) from those obta ined in spark igni t ion experiments. T M

Curve 3, which is for that circuit inductance which yields lowest igni t ion energy, was drawn 3 so that no igni t ion ever occurred be low it, whi le curves 2 and 4 represent 1% and 80% ignit ion probabi l i ty . 2,4 In contradis t inct ion 1 to condit ions at large quench ing distances, that is at reduced pressures or close to the l imits of f lammabi l i ty , there is here no evidence that laser ignit ion energies are apprec iab ly smaller in consequence of the absence of losses to massive electrodes.

These measurements were carried out us ing a single strand of 12 Ixna copper wire across a focal spot of approx. 80 Ixm diameter. The next step was to vary the amount of material in the p lasma and to vary the target material.

The former was achieved s imply b y p lac ing several strands or fibres side by side in the focal spot, the latter by us ing different wires and quartz fibres and by coating wi th sodium chloride. In order to vary the quenching distance, which might affect the result , these measurements were carr ied out for very lean (6%) and near s toiehiometr ic (8.6%) meth- a n e / a i r mixtures. The two ignit ion energies were close to 1.5 and 0.5 mJ in the two cases. There is no point in d iscuss ing these measurements or results in more detai l since no matter what variable was imposed, the min imum ignit ion energy in this part icular system proved to be totally independent of plasma constitution both as regards different sub- stances and different amounts of the same substance within the plasma.

For " ins tantaneous" (i.e. 20 ns half widths) ini t iat ion by focused laser beam we therefore have here an independen t verif icat ion of the theory of point igni t ion: the cri terion is propagat ion- l imi ted and the decisive stage is reached when the inc ip ient flame kernel grows to a critical radius�9 The propagat ing reaction wave evident ly attains this quench ing dis tance before any of the p lasma material reaches it.

The next step was to establish how this conclus ion would be affected by changes in the p lasma d imens ions and the durat ion of the energy deposi t ion. The extent of the p lasma could be varied readi ly b y placing targets in a sl ightly defoeused area of the beam. Figure 4 shows results for two composit ions, Al- though the d imensions in both sets of the results are well wi th in the respective quenching distances, which are approx. 4 and 2 mm in the two cases, it is a much more appreciable fraction of it in the case of the 8.6% m i x t u r e - - for which a quite measurable increase is ob- served. Nevertheless every one of these results is once again independen t of the target materml or amount of it in the plasma.

As al ready indica ted in the section on pre- l iminary experiments, this si tuation of the igni t ion wave ou t s t r ipp ing and "forget t ing the const i tu t ion" of its parent p lasma does not app ly once the energy is deposi ted over an appreciable time. The essential dif ference between Q-switched and non-Q-swi tched pulses was investigated us ing twin 40 ns pulses at about 0.5 ms intervals.

This kind of laser ou tput can be p roduced s imply by changing the dye concentrat ion, though it does not a l low independent variat ion of the energy of the second pulse wi th respect to the first. Because of this and other l imita- t ions arising from the swi tching method, these results d id little to advance our unders tanding


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of the process. They may be summarised by saying that the two plasma blobs acted as quite separate igniters. Although the total energy depends on the total mass of target vaporised, it is the energy of each individual pulse that

determines whether or not ignition will take place. Of course, if the target is completely vaporised by the first pulse, subsequent pulses have no effect.

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produces many such spikes spaced over a per iod of about 1 ms. The reason why this durat ion of energy depos i t ion is long enough for the p lasma const i tu t ion to affect igni t ion energies was revealed by high speed laser schl ieren cin6 pho tography of expanding f ron t s - - see Fig. 5. It t ranspires that if energy continues to be fed to the target then the expanding plasma is able to catch up with the flame. The last frames of Fig. 5(c), which compare with an igni t ion in the absence of sod ium chloride in (b) and the non-igni t ion in (a), show how the rap id ly expanding c loud eventual ly becomes turbulen t as it overtakes the flame front, b reak ing through it locally and ext inguishing i t - - i f it contains inhib i t ing material.

In accordance with that picture it is necessary to provide suff ic ient target material for this sequence to go to complet ion. A 12 ~xm copper target used at the focus of a non Q-switched beam will vaporise complete ly at very low beam energies; thereafter no ignitions can be obtained even wi th high beam energies in the 8.6% mixtures. A 50 p.m diameter wire, on the other hand, al lows ignit ion to occur at an input of 2.1 +_ 0.2 mJ. Sodium chloride coated targets of the same d imensions will not produce i gn i t i on - -o r rather sustained flame p ropaga t i on - - even at full laser power, as al- ready ment ioned in an earl ier section.

Conclusions

becoming turbulent and breaking through the front. This can result in suppress ion or promo- t ion of propagat ion, depend ing on the consti- tut ion of the plasma. Thus, in the case of an inhibi tor , the leading edge of an extended pulse is responsible for ignit ion, the trai l ing edge for extinction; Q-swi tched pulses do not a l low time for the latter.

The conclusion is for tunate both in that it comple te ly vindicates the propagat ion- l imi ted theory of poin t igni t ion and validates previous experimental spark-based measurements , showing them to be independen t of plasma const i tut ion and hence electrode materials, so long as the discharge dura t ion is short enough. At the same time it offers a practical means of suppress ing igni t ion by use of inhibi tors inc luded in the igni t ion source (e.g. a sheathed explosive) by s imply extending the durat ion of energy supply. By the same token it suggests that promoters could be used s imilar ly to improve ignit ion eff ic iency in pract ical sys- tems by increasing the react ion rate on reach- ing the flame front under condi t ions when the flame might otherwise not have cont inued to propagate.

Acknowledgment

We are indebted to the Health and Safety Execu- tive who supported this work through the Safety in Mines Research Establishment and provided a bursary for one of us (R.G.K.)

The quest ion of the effect of p lasma consti- tut ion on min imum igni t ion energies thus appears to be independen t of the p lasma volume (at least for small volumes) but dependent on its duration. The init ial laser pulse, whether it is on its own or fo l lowed by many others, is responsible for the expand ing ignit ion front. The behaviour of this front is independent of the const i tut ion of the plasma, which is left behind. However , so long as the c loud remains dense enough, it continues to absorb energy for the durat ion of the incoming beam. This enables it to expand rapidly, eventual ly

REFERENCES

1. WEINBERG, F. J. AND WILSON, J. R., Proc. Roy. Soc. A 321, 41 (1971).

2. LEWIS, B. AND WON ELBE, G., Combustion, Flames and Explosions of Gases, Academic Pres*s, New York (1961).

3. TORIYAMA, Y. AND SARTOV, S., J. Inst. Elect. Eng., Japan, 62, 427 (1942).

4. SAYERS, J. F., TEWARI, G. P., AND WILSON, J. R., Gas Council Research'Communication, G.C. 171 (1970).

COMMENTS

J. M. Singer, Bureau of Mines, USA. Others have noted that the shape of the curves of minimum ignition energy versus gas-air concentrations depend on the type of ignition source. Are the minimum

ignition curves (especially the location of the trough-minimum concentration) changed when the ignition source is the Q-switched or non-Q-switched laser beam instead of an electric spark?

756 IGNITION, OPTICAL AND

Authors" Reply. I presume that Dr. Singer's ques- tion refers only to the work with targets; in the absence of targets (our reference 1) results for mixtures of large quenching distances are appreciably different. Non Q-switched pulses are also not relevant, as has been shown, since they do not yield correct minimum igni t ion energies. As regards Q- switched pulses, our results seem to follow those of workers using different ignition sources fairly closely, independent of the target material (see Fig. 3 of our paper). There must be some doubt about the min imum of the curve since we did not proceed beyond stoichiometric mixtures, to avoid any risk of clogging the suction microcalorimeter with soot particles.

D. R. Ballal, Cranfield Inst. of Tech., England. One observation of the authors of particular interest is that the beam energies required to produce ignit ion using copper wire is twice that for iron wire of same diameter. In our min imum ignition studies in fact the opposite is found to be true, i.e.

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T b = Boiling temperature of electrode material.

One can explain this difference however as follows. In our observation of von Engel 's rather l imited

and approximate data, we found that energy released in an arc discharge varies as

ER o~ T ~ (1)

Also measurements of are temperature T , (from which conduction heat loss to the electrodes E L can be calculated) in the vicinity of the electrodes show that

ELECTRICAL PROPERTIES

(T o - To) o~ E L oc T~ .75 To

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The ratio E L /Et~ represents the extra heat that must be supplied for ignition. Assuming that only Eq. (1) is valid for author 's case (since there are no electrodes to cause heat loss)

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However this implies (for are discharge) some form of dependence of m in imum ignition energy on plasma energy--whereas the authors have found none! Would the authors like to comment on this?

REFERENCES

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

Authors" Reply1. I do not think that much impor- tance should be attached to the beam energies. Different materials and plasmas are liable to absorb different proportions of them. That is why we considered it essential to measure the plasma energy itself. The constancy of the latter seems as incontro- vertible as it is theoretically pleasing. It may he that the difference between our experiments is due to the iron vapour ignit ing in air only under the conditions of a laser-induced plasma (see comparison with the nitrogen atmosphere). Since this heat release is also picked up by our suction calorimeter, and since the available plasma energy so measured is the relevant quantity, further attempts to reconcile the results would seem somewhat intractable.

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