Development of hot nitrogen kernel, produced by a very fast spark discharge

Twenty-Second Symposium(International)on Combustion/The Combustion Institute, 1988/pp. 1651-1659

DEVELOPMENT

OF HOT

NITROGEN

KERNEL,

PRODUCED

BY A

VERY FAST SPARK DISCHARGE A. BORGHESE,* A. D'ALESSIO,** M. DIANA,** AND C. VENITOZZI*** *Istituto Motori, C.N.R., Napoli, Italy **Dipartimento di Ingegneria Chimica, Napoli, Italy ***Istituto di Ricerche sulla Combustione, C.N.R., Napoli, Italy

In this work, an experimental description has been given of some physical processes occurring and induced in atmospheric pressure nitrogen by very fast spark discharges. The time range investigated spans from the very first tens of nanoseconds (spark phase) up to the submillisecond scale. Experiments were based upon the application of both the laser-schlieren technique, yielding 2-D qualitative visualization, and the 1-D quantitative Rayleigh laser-light scattering, from which the gas mass density radial distributions were derived, for delays from the spark greater than 1 Ixs. Following the energy deposition stage, three main time domains have been identified, corresponding respectively to i) the fast expansion of the excited gas kernel and of the resuiting shock wave, (0.2-10 p~s) ii) the subsequent, ordered gas flow-field, involving recirculation of fresh gases into the spark gap (10-40 ixs) and, finally, iii) the transition of the coherent fluid dynamic structures toward chaotic, turbulent mixing of excited and cold gases (40-200 ixs). Next, from the depolarized light scattering data in the range 5-15 Ixs, some indications were drawn, about the recombination process of the atomic nitrogen, which has been shown to have lifetimes of the order of tens of microseconds, thus exhibiting interesting properties as an efficient energy reservoir.

Introduction The interest in operating spark ignited i.e. engines in a wider range of fuel-air ratios, with low exhaust emission levels, has triggered the employment of nonconventional ignition sources like fast electrical discharges and plasma jets, which exhibit attractive features for enhanced ignition.I'2 The evaluation of such devices inside real engines gave mixed results, because of the complex interplay between the operating characteristics of the ignition sources and the fluid dynamics of the air-fuel mixture inside the combustion chamber. 3'4 The physical interpretation of the observed effects is consequently rather confusing because it is not clear whether the mechanisms, controlling the fuel-air mixture ignition, are of thermal nature, due to steeper temperature gradients generated by a faster deposition of energy into the medium, or of chemical nature, as a consequence of the relaxation of radicals and vibrationally excited molecules involving the reacting species. Furthermore, it has also been pointed out that the ignition processes by spark discharges, as well as by plasma jets, may be controlled by gas convective effects produced as a consequence of the

ignition energy deposition process. 5'6 This status of the knowledge suggests that probably to look for a simple controlling mechanism or parameter is a too ambitious approach and the ignition problem has to be considered for what it is: namely a time-varying, spatially structured phenomenon in which external energy is sequentially absorbed and redistributed. Therefore the availability of diagnostics with adequate temporal and spatial resolution becomes a crucial point if the processes of ignition by whatsoever sources have to be understood and then properly applied in practical engines. Following this approach, this paper has the specific purpose of studying the deposition and transfer in nitrogen at atmospheric pressure, of electrical energy released by capacitive spark discharges with characteristic times of 50 nanoseconds.

The Electrical Excitation Source The experiments carried out in this work deal all with the excitation of inert gas (pure nitrogen at atmospheric pressure) by means of very fast spark discharges. The sparks were generated between two, 2 mm spaced, tungsten electrodes, arranged coax-

1651

1652

IGNITION/EXTINCTION

ially to a cylindrical (50 mm I.D.), stainless steel chamber, which was also provided with optical access through quartz windows; further details of the experimental apparatus have been reported elsewhere. 7 A spark pulser was purposely developed, in order to generate spark discharges as short as possible. Suitable choices of the circuit electrical parameters, as well as of the charging voltage Vo, allowed us to produce critically damped 50 ns-wide current pulses with a peak power of 0.9 MW after 10 ns and an energy amount of Es = 14 mJ, released to the spark channel] Figure 1 shows the normalized waveforms of the spark voltage (V), current (I), power (P) and energy (E) as functions of time, along with the corresponding absolute maximum values.

Optical Set-Up and Techniques Two complementary optical diagnostic tools, namely the laser-schlieren photography and laser Rayleigh light scattering technique have been applied; the former provided a two-dimensional (2-D) qualitative description of the perturbed gas volume, whereas the latter was applied in order to extract I-D, quantitative informations on radial distributions of gas density. In both techniques, the light source was a Q-switched, frequency doubled, NdYAG laser.

NORMALIZED ELECTRICRL I~FIVEFORMS

,8-

.8-

.4-

The "schlieren" set-up, not shown here, is nearly conventional, except the use of a "point-shaped" light stop, instead of the more familiar "knife-edge," in order to preserve the uniform detection of differently oriented gradients. Figure 2 shows the optical set-up for the light scattering experiments, where the laser beam is focused in the region between the electrodes by means of a low power lens (L), along an optical axis at 90 degrees to the spark channel; as a result, the laser beam defines a line-shaped scattering volume, passing through the center of the spark. The scattered light is then collected to form a one-dimensional image onto an optical diode-array detector (OMA, P.A.R. Co., Mod. 1420); the detection branch consists of two coupled, high aperture collecting optics (Canon F.D., 50 mm f/1.2 objectives), arranged along an axis at 90 degrees from both the spark axis and the laser beam, the overall magnification ratio being 1:1; the adoption of double, cascade imaging, coupled to a linear spatial filter (slit), arises from the need to cancel out the intense stray light, due to the presence of the electrodes, very close to the laser beam. For these experiments, an accurate control of the time delays between the spark event and the laser pulse is needed, especially if temporal probing of the phenomenon within the first microseconds is required. Accordingly, a timing procedure has been set up as follows (see also Fig. 2): a master trigger pulse, under software control, fires the pumping flash lamps of the laser; next, a delay tl is generated which separates the master trigger from the firing of the spark trigger; the spark itself, which occurs at a time affected by a statistical time jitter +At, triggers a second delay generator (tz), whose output opens the laser Q-switch; in this way, the spark-to-laser delay t2 turns out to be unaffected by the spark jitter, whereas the overall master trigger-laser pulse delay (tl + tz +- At), kept at about 200 ixs, does not suffer particularly from such uncertainty. The ultimate accuracy of this timing is within few nanoseconds, thus allowing very high overall time-resolution.

c ,

,F

•

s

o,

A

Y-,.r

-I

I"e ,.~

i b 26 36 46 s6 ~6 76 o~ do i~o NFINOSECONI]S

FIG. 1. Normalized waveforms of the electrical excitation as functions of time. Also shown are the peak absolute values of the corresponding quantities. I = current pulse (experimental); V = gap voltage; P = power delivered to the gas; E = energy deposition law.

[ T~

FLASN LAMPS

FIG. 2. Experimental set-up for laser light scattering experiments. P = half-wave plate; L = focusing lens; C = current probe; A = polarization analyzer; S = adjustable slit; Oi,O2 = objectives; IF = interference filter k = 532 nm.

HOT NITROGEN KERNEL Evaluation of Gas Density from the

Scattering Data The data from the scattering experiments consist of a series of scattered 1-D "images," recorded at definite time delays from the spark; every image is made up of 700 pixels, each corresponding to a radial position R from the spark axis, located at R = 0. The acquisition procedure is as follows: first, the test chamber is evacuated and a series of pulsed images is summed up under vacuum conditions and stored; thus, a "noise" image Io(R) is recorded which is due to the residual scattering from solid surfaces, mainly the electrodes, and to spurious reflections. Next, the chamber is filled with nitrogen at 1 arm pressure and an analogous image It(R) of unperturbed or "cold" conditions, is recorded taking into account the overall spatial responsivity of the detector, The Io(R) and I¢(R) images are, of course, time-independent and are acquired "una tantum'" for normalization purposes. Finally, the spark pulser is enabled and, for a given spark-to-laser delay t, a signal I~(R,t) is recorded, either in a "single-shot" mode or, if high signal/noise ratios are needed, in the "averaging" mode; in the latter case, the overall spatial resolution turns out to be reduced, due mainly to the finite random wandering of the spark axis. The image processing procedure consists in the evaluation of the cleaned and normalized image, given by: I(R,t) = [I,(R,t) -

Io(R)]/[Ic(R)

-

Io(R)]

(1)

which is unity where the gas is unperturbed. The experimental data reported here refer to an optical arrangement, where the incident laser beam is polarized linearly and normally to the horizontal scattering plane; the scattered radiation, collected at 90 degrees from the incident beam can be analysed in the vertical or horizontal plane and it is referred to as Ivv(R,t) or IHv(R,t) signal, respectively. In order to extract total gas density and composition informations from the data I(R,t), some relations must be recalled, giving the expression for the Rayleigh scattered signals:s

Iw

= klL

E

(NiCi)= Z

i

i

Irtv = klL E (NiC/pv,i)

Iw,i(R,t)

(2.1)

(2.2)

i

PV,MIX = IHV/Ivv

1653

energy, J: k = proportionality factor, taking into account the collection efficiency and the size of the detector element. In the case of pure nitrogen, dealt with here and introducing the scattering efficiencies Q~. and Q,v, Eqs. (2.1)-(2.3) become: QVV,A+M = Ivv/klL = (NMCM + NACA)

(3.1)

QHV,A+M = I H v / k I L

= (NMCMPv,M + NACAPv,A) PV,A+M =

(3.2)

(3.3)

QnV,A+M/Qvv.A+M

where subscripts A,M and A + M refer to atoms, molecules and a mixture of them, respectively; computed and experimental values of the molecular CM and atomic CA cross-sections from the corresponding mean polarizabilities, are reported in Table I; in the considered case of a mixture of molecular and atomic nitrogen, the quantity Qvv can be expressed as: Qvv = NM" CM + NA" CA = CM/O,M • (IXMNM + P.A(2CA/CM)NA)

(4)

where IXM and P,A = p,M/2 are the molecular and the atomic weight of nitrogen, respectively. From Table I, the term 2CA/CM is 0.77 and it follows that the Qvv quantity is proportional to the total mass density, everywhere molecular or weakly dissociated nitrogen is present, with a maximum relative error of -23% in the limiting case of purely atomic nitrogen. However, solving any two of the Eqs. (3.1)-(3.3) with respect to NM and NA allows one to evaluate rigorously the total mass density as well as the gas composition in any case.

Schlieren Visualization An overall description of the temporal evolution of the excited gas is given by the laser schlieren TABLE I Shows the values of polarizabilities ~r, the elastic Rayleigh cross-sections C and the depolarization ratios Pv = Qnv/Qvv for molecular and atoinic nitrogen, as inferred from quoted references.

(2.3) 0", c m 3

where Ni = number concentration of the i-th species, cm-3; C i = Rayleigh scattering cross-section of the i-th species, cm 2sr- 1; Pv,~ = depolarization ratio of the i-th species; Iu = incident laser light

N2

1.76 × 10 ~4~

N

1.10

x

C, cm2sr t

6.1 × 10 2~.,

10 ~4" 2.35 × I0 ~

Px

1.0 × 10 ~ 0 (S state)

1654

IGNITION/EXTINCTION

photographs of Fig. 3 where selected pictures representative of the process are given in the time interval from 0.2 Ixs to 200 p,s. The first 10 Ixs phase (A) puts in evidence the kernel enlargement from an initial "bean" shaped structure to a prolate spheroidal one. In the same time scale a shock wave front develops, leaves the kernel with an ellipsoidal shape and finally evolves toward a spherical geometry after 10 Ixs. The central part (B) of Fig. 3 documents the kernel growth up to 40 txs: the initial spheroid is enlarged on the equatorial plane, passes through a sort of four leaved structure and finally assumes a toroidal shape. The pictures in the part (C) of the figure document the final transition form 40 I~s to 200 IXS from a relatively ordered structure to much more complex ones which are significantly enlarged in the equatorial plane. The schlieren technique can just furnish the external shape of the kernel, in a semiquantitative way, but a more detailed and quantitative characterization of the internal core can be gained by the linearly imaged polarized and depolarized light scattering measurements.

tl

m

.

B

.

.

.

.

.

.

i

. . . . . . . . . . . . . . . . .

The Shock Phase Figure 4 reports the radial profiles, in the equatorial plane, of the normalized scattering coefficients, in the time range 1-10 p,s. Unfortunately, time delays shorter than 1 Izs were not allowed in the light scattering experiments, due to the unavoidable disruptive absorption of the laser light by the still ionized channel. Furthermore, at t = 1 2 IXS, some scattered light peaks have been observed around R = 0, in "single-shot" mode, which are heavily leveled down by averaging and have been attributed to Thomson scattering from the last electrons in the spark channel; presently, no attempt has been made to characterize quantitatively these effects. The central deep well defines the central core of the kernel, which does not enlarge so much in th~ radial direction, and the external peaks indicate the shock front, traveling outward. The region between the kernel core and the shock front, characterized by a flex in the density profiles, contains a gas annulus heated up by the shock passage more than by direct energy transfer from the c o r e .

The data in Fig. 4, refer to "averaged" signals over several shots; this leads to higher S/N ratios but also to some smoothing of the sharp gradients, particularly at the shock fronts. Additionally, the data are affected by the blurring of the diode array detector, which limits the accuracy of quantitative estimates of mass density p and temperature T in the axial region, unless proper deconvolutions are performed. 13 Bearing in mind these limitations, the evalua-

NORMRLIZED GRS DENSITY DISTRI~pUTIONS

FIG. 3. Schlieren photographs of the spark-induced density perturbations, at different time delays from the spark. A) the shock phase (0.2-10 ~s) B) the recirculation phase (14-40 Ixs) C) the turbulent phase (80-200 ps) The spark luminous channel is shown superimposed.

RRDIRL

COORDINQTE (mm)

FIG. 4. Radial patterns of gas mass density, normalized to the unperturbed value, at different time delays from the spark (t = 1-10 txs)

HOT NITROGEN KERNEL tions of axial temperature and compositions have been carried out from the experimental Qvv'S (Fig. 4) and the corresponding Qrtv's (not reported) as follows. From Eqs. (3.1)-(3.3)the molar fractions of nitrogen atoms X, and molecules XM are readily found to be: XA = (1 -i- [1

-

PV,A+M/DV,M)

--

(1 --

CA/CM)(PV,A+M/PV,M)]

1655

MOLRR FRRCTION

XR

OF N-RTOMS

t

-

7

Hs

.~.% %. =" o.

XM = 1 -- X, On the other hand, the gas (translational) temperature T can also be derived assuming constant pressure p = 1 Arm for t > 5 Ixs, from the law of perfect gases, in the form p/p = KT/(Ix), where p = tXAN, + I-I,MNM, is the mass density, (~t) = I~AXa + IXMXMis the molecular weight of the mixture and K is the Boltzmann constant. Some remarks are needed about the sensitivity of the depolarized scatteri_n~g technique, involving signals as small as 10-3-10 -~ times the corresponding Qvv's and becoming indeed unpractical as mostly atomic gases are probed. However, the S/N ratio can be raised to acceptable values if a number large enough of laser shots (several hundreds in our conditions) are summed up. As an example, in Fig. 5 the radial pattern of XA at a 7 Ixs delay from the spark is shown, along with the corresponding Qvv, also reported for easy reference; an almost full dissociation of nitrogen resuits and the axial temperature, evaluated as shown above, amounts to roughly 2500 K. These findings, compared to the equilibrium composition of nitrogen, x4 prove that the excitation conditions at this time are very far from local thermodynamic equilibrium (L.T.E.). Analogous indications were found until 15 ItS, although the resulting S/N ratio proved to be more critical, as this time was approached. The large dissociation and the corresponding relatively low temperature found experimentally can be justified in the frame of the "active nitrogen" properties 15 and particularly of the three-body recombination process of nitrogen, 16 from where lifetimes of atomic nitrogen, evaluated in the conditions of the channel axis, of about 10-20 p,s are predicted. Furthermore, the negative temperature coefficient of such a kinetic process determines low recombination rates inside the hot channel, whereas higher rates should be expected on the "wall". On the other hand, further experiments, not reported here, v and carried out adding traces of hydrocarbon to the nitrogen, showed a strong emission from CN radicals, produced somehow by the interaction of inner excited species with outer cold gas, on the same space and time scales. Both the depolarization and CN emission mea-

RRDIRL

COORDINRTE (mm)

FIG. 5. Radial pattern of the N-atoms molar fraction XA at 7 txs delay from the spark. The corresponding radial profile of the normalized gas mass density distribution is also plotted by dots, for reference.

surements suggest the existence of a highly reactive hot channel, where non-thermal energy is stored (9.76 eV/molecule), which is released later on through recombination processes. An Analysis of the Shock Phase Exploiting the data of Figs. 3A and 4, a quantitative analysis has been carried out correlating the shock front position R, along the equatorial plane, with time; the following analytical expression:

R2= Rzo+ klt+c2t 2 with R o = 0 . 7 6 m m kl = 1.87mZ/s

c=350m/s

(5)

gave the best fit with the experimental results, with a very high correlation factor. The Ro value is very close to that evaluated from the schlieren data at 200 ns (see Fig. 3.A) and the coefficient c comparing in the quadratic term turns out to be exactly the sound speed in pure nitrogen at room conditions. A physical meaning can be attributed also to the linear term; the strong shock theory/7 predicts that, under the hypothesis of cylindrical geometry and instantaneous deposition of energy per unit length Es, the shock moves from the origin according to the expression R2 = b(Es/pl)l/2t

(6)

IGNITION/EXTINCTION

1656

where b is a coel~cient near to one and Pl is the initial density of the medium. In the present case, the Eq,(6) reproduces the linear term in Eq.(5) if a value for b of about 0.8 is assumed. This agreement should however be faced with the actual geometry, which undergoes a transition from an essentially cylindrical to a spherical one, at longer times. Equation (5) can be extrapolated at t = 100 ns to obtain the boundary conditions of the kernel, soon after the electrical energy deposition phase when the shock travels with a velocity of -1080 m/s. Then the relevant physical quantities at the shock front can be computed from the Rankine-Hugoniot relations and the perfect gas law, given by:

Pz/P~ =

(2M2~/- ~/ + 1)/(~/ + 1)

(7)

P2/Pl =

[MZ(~/+ 1)]/[M2(~/ - 1) + 2]

(8)

T2/TI = (P2" Pl)//(Pl " P2)

uz/c

= (1 - pl/p2)" M; ul = 0

(9) (10)

where p, p and T are pressure, density and temperature, respectively; subscripts 1 and 2 show the values ahead of the shock wave and immediately behind it; u and c represent the flow velocity and the speed of the sound; M and 3' = 1.4 are the Mach number, R/c, and the ratio of the specific heats of the nitrogen. At initial conditions (t = 100 ns), the shock front is characterized by a P2 = 1.1 MPa, p2 = 4.0"pl and T2 = 2.70" T1 = 800 K. A further characterization of the initial kernel can be carried out, solv-

NORMBLIZEDI GRSDENSITY DISTRIBUTIONSRTII, 15,2B,25,38.48~us

FIG. 6. Radial patterns, as in Fig. 4, for t = 1040 p,s.

ing the conservation equations for the inner part of the kernel, using the above data as boundary conditions; assuming similarity expressions for the radial profiles of the density, velocity and pressure and the existence of L.T.E., values along the axis of temperature around 34000 K and a pressure of about 0.4 MPa have been estimated is with an electron density in the range 1017-10 is cm -3 This result is consistent with the previous determination of electron density through spectroscopic and interferometric measurements. 7,19 Equation (5) predicts also a very. fast decay of the Mach number and thus the expansion velocity of the channel u2 drops from an initial high value, close to that of the shock, to a very low one, and the pressure becomes almost atmospheric after 1-2 Ixs, as can be derived from Eqs. (7)-(10), thus confirming the assumption made above for the evaluation of the axial temperature.

The Early Convective Phase The schlieren pictures of Fig. 3 showed that between 10 and 40 I~S the external shape of the kernel changed from an ellipsoidal to a toroidal one. Correspondingly, from Fig. 6 a central peak of density appears at the center of the kernel between 15 and 20 Ixs, its intensity and width increase with the time and at 40 Ixs the density at the center equals that of the unperturbed gas. The external part of the kernel, still occupied by low density gases, shows a relatively low expansion in the radial direction. The density profile at 40 Ixs clearly indicates that, at that time, the central part of the kernel is occupied by room temperature gas. Hence a convective process should take place which brings fresh gas to the center of the kernel from the surroundings. Since the periphery of the kernel remains at high temperature in this time interval, the fresh gas must come in from the electrodes region and move along their axis. Since the time scale of the effect (tens of p~s) is much shorter than that for the molecular heat and mass transfer, the gaussian shape of the central density peaks is not to be attributed to diffusion, but should be interpreted as an indication of the statistical variance of the inlet of the fresh gas jets, as also confirmed by "single-shot" recordings of radial scattering patterns. Considering that the onset of the peak takes place at 15 p,s, an average velocity of the fresh gas jet around 100 m/s is estimated. Some physical mechanisms can be invoked in order to justify the observed effects. Firstly, it is worth noting that no such effects can take place, in the cases of "point-source" and "infinite line-source" geometries, because of symmetry considerations; namely they should be connected somehow to the

HOT NITROGEN KERNEL finite height, roughly cylindrical shape of the kernel in the first microseconds, with strong gradients of curvature and related velocity, localized in the annular zones around the electrodes (see Fig. 3.A). According to fluid dynamic fundamentals, z° an axisymmetric rotational flow field is then forced, which consists of two annular vortices, each surrounding one electrode tip, and which brings the gas along curved trajectories firstly toward the spark axis and then axially toward the gap center. On the other hand, a magneto-hydrodynamical effect can also take place, zl,5 originated from the radial constriction of the plasma channel close to the electrode tips, which causes as well two counter propagating, axial gas jets. It seems hard to assess which of the two mechanisms can prevail in our conditions, however they, ff both present, reinforce each other to determine the observed effects. Moreover, a comparison between analogous observationszz'za'z4'zs shows that, for discharges of nearly the same energy, the shorter the spark pulse, the larger the spatial extent and the faster the development of the toroidal structures. Finally, it should be pointed out that the described inlet gas flow acts as a shield around the electrodes, which inhibits their contact with the hot gases, so that heat conduction or otherwise perturbing effects are expected to be minimized.

1657

NORMRLIZED DISTRIBUTIONS AT

4o~

RRDIRL

GAS DENSITY ~,?B, 108. 140, 180jus

VCOORDINRTE

(mm)

FIG. 7. Radial patterns, as in Fig. 4, for t = 40180 I~s.

complex interactions which can take place between the kinetics and the fluid dynamics in this kind of turbulent mixing.

Discussion and Conclusions

The Turbulent Relaxation Phase Figure 7 documents the subsequent behaviour of the radial density distribution in the time range 40200 Ixs, recorded, as in Fig. 6, summing up several laser shots, thereby evidencing mainly the reproducible features of the spatial structures involved, whereas the corresponding pictures in Fig. 3.C refer to "'single-shot" exposures. In these data, it is apparent the enlargement of the outer shell of the relaxing gas kernel, as well as of the size of the central "cold" core, while the axial size of the perturbation does not vary significantly. This time phase is characterized, however, by an increasing statistical dispersion, due to the progressive destruction of coherent structures, and by turbulent mixing of excited and cold, recirculating gases. Furthermore, in Fig. 3.C, some bright, confined zones appear, even at t = 200 I~s, which correspond to locally high gradients, whereas in Fig. 7, the density patterns show very low "averaged" gradients; this can lead to the idea that the transition from coherent structures (t -< 40 Ixs) to chaotic, turbulent mixing proceeds so as to preserve locally confined, hot gas pockets. At the moment, no further reasonable interpretation can be drawn about the chemical composition of such hot pockets, although the matter lends itself to further speculations about the

Following the energy deposition phase, three main time domains have been identified, correspondin~ respectively to i) the fast expansion of the excited gas kernel and of the resulting shock wave, ii) the subsequent, ordered gas flow-field, involving recirculation of fresh gases into the spark gap and, finally, iii) the transition of the coherent fluid dynamic structures toward chaotic, turbulent mixing of excited and cold gases. Next, from the depolarized light scattering data in the range 1-15 I~S, some indications were derived, about the recombination process of the atomic nitrogen, which has been shown to have lifetimes of the order of tens of microseconds, thus exhibiting interesting properties as an efficient energy reservoir. On the same time scale, a fast inlet of fresh gas into the hot kernel takes place, leading to a convective interaction with the residing excited gas. The effect should be due both to the intensity of the spark, responsible of the expansion of the hot kernel and to the finite axial size and the peculiar shape of the spark itself which, under this respect, cannot be described at all in terms of a "'point" source, nor as an infinite line. The description of the last temporal phase (t > 40 p~s), ahnost purely qualitative, evidenced the large

1658

IGNITION/EXTINCTION

spatial spreading of the spark excited gas as well as the chaotic features of its evolution. A quantitative analysis of the dynamics of the shock wave allows identifying the thermodynamic boundary conditions of the spark kernel at the end of the electrical energy release. This was used to build a model, descriptive of the initial kernel composition, which has been validated by independently achieved, spectroscopic results. An increasing complexity should be expected, if the dependence of such physical processes upon the features of the spark excitation, namely its duration and intensity, is accounted for, as well as if a reactive gas mixture is considered. Furthermore, in our experimental conditions, the time seales of the nitrogen recombination and of the establishment of fresh gas recirculation into the excited kernel turn out to be comparable (tens of microseconds); this leads to the interesting finding that the energy transfer from the excited species to cold gas is not limited to the outer shell of the hot channel ("surface" interaction), but ocvurs also inside the hot kernel, due to the forced convective flow, thereby suggesting the idea that some sort of "volume" interaction takes place additionally. The connection of such conclusions with the ignition studies is quite evident and related to the possibility of explaining some enhanced ignition capabilities of very fast discharges, taking into account simultaneously kinetic and fluid dynamic processes; on the other hand, the question arises about whether or not the long-lived nitrogen excitation still occurs in gas mixtures containing also oxygen as well as hydrocarbons; this should be the object of further investigations, which are actually running. REFERENCES 1. MALY, R., VOGEL, M.: Seventeenth Symposimn (International) on Combustion, p. 821, The Combustion Institute, 1979. 2. WEINBERG,F. J., HOM, K., OPPENHEIM, A. K., TEICHMAN, R.: Nature, Vol. 272, p. 341, March 1978. 3. ANDERSON, R. W., AslK, J. R.: SAE Paper No. 850076, 1985. 4. EDWARDS, C. F., OPPENHEIM, A. K., DALE, I. D.: SAE Paper No. 830479, 1983. 5. CHOMIAK, J.: Seventeenth Symposium (International) on Combustion, p. 255, The Combustion Institute, 1979. 6. OM1N, J. E., VINCE, I, M., WEINBERG, F. J.; F. J.: Eighteenth Symposium (International) on Combustion, p. 1755, The Combustion Institute, 1981.

7. BORGHESE, A., D'ALESSIO, A., RUSSO, G., VENITOZZI, C.: International Symposium on Diagnostics and Modeling of Combustion in Reciprocating Engines, p. 77, Tokyo, 1985. 8. KERKER, M.: "'The Scattering of Light," Academic Press, New York, 1969. 9. BRIDGE, N. J., BUCKINGHAM,A. D.: Proe. Roy. Sou., A295, p. 334, 1966. 10. RUDDER, R. R., BACH, D. R.: Journ. Opt. Sou., Vol. 58, No. 9, p. 1260, Sept. 1968. 11. NESBET, R. R.: Phys. Rev. A 16, Vol. 16, No. 1, July 1977. 12. HUDDLESTONE,R. H.. LEONARD, S. L.: "'Plasma diagnostic techniques," P. 440, Academic Press, New York, 1965. 13. LEWIS, R. W., TEETS, R. E.~ SELL, I. A., SEDER, T. A.: Appl. Opt., Vol. 26, No. 17, P. 3695, Sept. 1987. 14. CAPITELLI, M., FICOCELLI, E., MOLINARI, E.: "Equilibrium compositions of thermodynamic properties of mixed plasmas," Bari, Adriatica Editrice, 1969. 15. WRIGHT, A. N., WINKLER, C. A.: "'Active Nitrogen," Academic Press, 1968. 16. APPLETON,J. P., STEINBERG,M., LIGUOBNIK,D. J., Journ. Chem, Physics, Vol. 48, p. 599, 1868; Vol. 49, p. 2468, 1969. 17, LANDAU, L., LIFCHITZ, E.: "'Meeanique des Fluides,'" p.493, Editions MIR, Moscou, 1971. 18. BORGHESE, A., DIANA, M.: Proe. IX Int. Conference on Gas Discharges and their Applications, Venice, Sept. 1988. 19, ALDEN, M., GRAESTROM, P., HERTZ, H. M., HOLMSTEDT, G. S., HOGBERG,T., RUSSREaG, G., SVANBERG, S., International Symposium on Diagnostics and Modeling of Combustion in Reciprocating Engines, p. 85, Tokyo, 1985, 20. PRANDTL, L., TIETJENS, O. G.: "'Fundamentals of Hydro- and Aeromechanies," p.189, Dover Ed., New York, 1957. 21. FINKELNBURG,W., MAEGKER, H.: "Encyclopedia of Physics," Vol. XXII, p. 396, SpringerVerlag, 1956. 22, OLSEN, H. L., GAYrtART,E. L., EDMONSON, E. B.: Fourth Symposium (International) on Combustion, p. 144, Williams and Wilkins, 1953. 23. KONO, M., HATORI, K.: Proc. Eighth International Conference on "Gas Discharges and their Applications," p. 500, Oxford 1985, Leeds University Press. 24. "ZIECLER, G. F. W., WAGNER, E. P., MALY, R.: Twentieth Symposium (International) on Combustion, p. 1817, The Combustion Institute, 1985. 25. ElM, M. T., ANDERSON, R. W., ARPACI, V. S.: Comb. Flame 69 303, (1987).

HOT NITROGEN KERNEL

1659

COMMENTS J. Lee, McGiU Univ., Canada. It might be a cleaner experiment if you could put flanges on your electrodes to eliminate the complex time dimensional flows around the spark channel. Are you going to do this? Also, when you use combustible gas machines the spark channel is highly unstable shortly 'after the termination of the current pulse. Hence it may be difficult to do the detailed study of the kernel in that case. Author's Reply. Being interested to the interactions of the spark-excited gas and the associated fluid dynamic effects, it is quite appropriate to investigate the influence of the electrode geometry as well. This is, at the moment, just work to be done. However, as to your suggestion of employing flanged electrodes, we tried to check the effect of their presence, but no reliable experimental data could be obtained, due to the dramatic increase of stray-light introduced by the flanges, which were very close to the scattering volume. The consideration about the growing complexity of the phenomena involved by the addition of combustion effects is significantly connected to the methodological approach outlined explicitly in the work, which suggests to proceed from well-assessed excitation conditions of inert gases toward more complex ones, due to the oxidation processes.

Nevertheless, the very short duration of the sparks dealt with here, helps one to keep still separate on the time scale the excitation phase and the subsequent growing combustion, in the ease of reactive mixtures.

R. Maly, Daimler Benz AG, Fed. Rep. of Germany. Applying the gas law it would be possible to calculate temperature profiles from your data, at least for the "cold" wings. Did you calculate temperature and what were the results?

Author's Reply. A correct and meaningful evaluation of temperature data from the scattering experiments would require the consideration of "'single-shot" patterns, which take into account the presence of the confined, hot gas pockets, whereas the radial patterns presented here result from averaging over several shots. However, within the limits of considering averaged values, it is easy to evaluate, for instance from Fig. 7, that the local maxima of temperature should range between -1000 K at 40 p,s and - 6 0 0 K at 180 I~s, assuming atmospheric pressure and molecular nitrogen; these values turn out to much lower than those inferred from interferometric measurements carried out on very similar sparks (see Ref. 1 in the text).