The influence of pressure and temperature on soot formation in premixed flames

Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 403-411

THE

INFLUENCE OF PRESSURE AND TEMPERATURE FORMATION IN PREMIXED FLAMES

ON

SOOT

H. BI~HM, D. HESSE, H. JANDER, B. L1]ERS, J. PIETSCHER, H. GG. WAGNER AND M. WEISS

Institut fiir Physikalische Chemic Universitiit GOttingen, W. Germany The formation of soot has been investigated in fiat C2H4-air and CrHr-air flames, with special reference to low flame temperatures. Measurements of the threshold for soot formation show that as well as the "'high temperature threshold" (slightly temperature dependent) there exists a low temperature threshold. For a given C / O ratio the mass of soot formed passes through a maximum between low and high temperature threshold at a temperature near 1600 to 1650 K. These soot yield curves are similar to those obtained in shock wave pyrolysis of benzene etc. by Graham, Frenklach et al. With increasing pressure the soot thresholds widen slightly. Their minima shift towards lower C / O ratio and the maxima of the soot yield curves rise. For the pressure dependence of the soot volume fraction finally formed, a p2 dependence has been found at constant flame temperature for flame temperatures above 1650 K and C / O ratios from 0.65 to 0.75. For benzene-air flames the pressure dependence is almost the same as for C~H4-air flames. The dependence of soot yield on the C / O ratio at constant flame temperature is a little weaker than the data obtained for constant flow velocity. Measurements of light absorption and light scattering along the flame axis show that coagulation and mass growth of the soot particles in the low temperature flames proceed in a similar way to that in flames at temperatures above 1600 K.

Introduction The formation of aerosols in combustion processes is of significant interest. It effects the operation and design of technical combustion devices and has implications for both effective operation and pollutant emission. Many experiments have been carried out in order to determine the effect of various operational parameters such as e.g.: fuel structure, mixture strength, temperature, pressure, flow conditions. This also includes the influence of additives on the formation and emission of carbon from burners, combustion chambers and piston engines.1 Pressures applied in technical combustion processes range from atmospheric to about 200 bar. In modern Diesel engines pressures of about 200 bar, in combustion chambers up to 30 bar are obtained while in general stationary burners operate close to atmospheric pressure. Combustiou temperature values around 3000 K can be reached, on the other hand in certain parts of diffusion flames, combustion can also take place at rather low temperatures. 2-8 Laboratory experiments, especially mechanistic studies on carbon formation, usually concentrate on conditions which are favourable for the measure403

ments. These sometimes differ from conditions in technical applications. Laboratory experiments in premixed flames and pyrolysis experiments under shock wave conditions were useful for the study of the mechanism of carbon formation. 9-u The application of premixed flames to the full range of technical conditions is, however, limited by problems of flame stability. 12"13 The formation of polyhedral flames is a serious problem when approaching high pressures. ~4't5 At low temperatures flames on a fiat flame burner tend to extinguish. For high temperatures the adiabatic flame temperature practically sets an upper boundary. Despite these problems, it seems worthwhile (and desirable) to extend the range of conditions for laboratory experiments on laminar and turbulent premixed flames towards high pressure and towards low temperature. In previous paper, 14 we reported the influence of pressure on the formation of carbon in premixed laminar flames. The results indicated that the effect of temperature and the influence of pressure on final soot volume fraction (fee) had not been completely separated. It is well-known, ~'13'16 that temperature has a very strong influence on f~= so that

404

COMBUSTION-GENERATED PARTICULATES: SOOT IN DIFFUSION FLAMES

an increase of temperature can drastically reduce f,~ in fiat laminar flames. Previous experiments have indicated that a pressure rise also increases fv~ more than proportional to pressure. 14.17 A first hint that an interrelation exists between temperature and pressure on soot formation had already been found by Bone and Townendt8 in their bomb explosion experiments. In this paper we will report: 1) experiments on the influence of temperature on soot formation in premixed flames mainly in C2H4-air as well as in CzH2- and C6Hr-air flames over an extended temperature range and 2) experiments on the influence of pressure on soot formation in C~H4-air flames of constant maximum temperature.

Experimental Arrangement In order to extend the temperature range of the burned flame gases towards low temperatures it proved to be necessary to stabilize the flames by shielding them at all pressures. The central flat flame burners were surrounded by a flat shielding burner. Both burners were protected against convection of the surrounding air by a glass tube (about 20 cm height) with a grid on top. At elevated pressures a special burner housing was used. For the measurements at p = 1 bar, a cooled porous sinter plate burner (60 mm diameter) and a 56 mm diameter steel capillary burner19 were used; for the high pressure experiments, a silver capillary burner was used. This pressure burner system has been described in Ref. 14. The following changes have been made: the plate of the shielding burner is located 5 mm below the tubular burner. The shielding quartz tube used in Ref. 14 is replaced by a closely fitting ceramic tube with a quartz window of 5 × 5 cm. The diagnostic light beam passed through the vessel via nitrogen of helium purged metal pipes. The packing of the silver capillaries in the central burner had a rather strong influence on the temperature profile across the burner. This influenced the homogeneity of the soot concentration somewhat. For this reason, the absolute values off~ given here and in Ref. 14 differ slightly. Absorption measurements were performed at k = 488 nm but mostly at longer wavelength in the range from 650 to 810 nm, selected by narrow interference filters. For light scattering measurements an argon ion laser line of h = 488 nm was used. The depolarization ratios were determined for all flames along the flame axis. When the particle size exceeded the Rayleigh regime (d (( 0.1 k), Mie corrections were applied. Soot volume fraction, fv, particle volume, ~3, and particle number density, N,

were evaluated from scattering and absorption signals. The data evaluation proceeded as described in Refs. 20, 21, taking into consideration the influence of absorbers other than soot and molecular fluorescence. As in previous experiments21 the complex refraction index of Sarofim and Dalzell 2z was used (m = 1.57 - 0.560. For flames with lower temperatures and low soot volume fraction growth rates df~/dt, the final soot volume fraction fw has been determined as follows27: rates for formation fv along the flame have been plotted as a function of the local f~ values, The extrapolation of these curves to ft- = 0 are taken as fv~ values. This assumption should characterize the sooting tendency under the applied conditions (see discussion). The depolarization ratios and the wavelengths dependence of the absorption coefficients were used to correct the measured optical signals for the determination offv. Profiles below 1500 K were not evaluated. Test measurements in the burned gases were used to support the extrapolations towards the lower (C/O)cri t curves. The volume flow rates and temperatures were measured as in Refs. 14, 24. Liquid benzene was vaporized by passing air through a high pressure saturator. The high pressure vaporizer was constructed and tested. Temperatures TM given here are those 10 mm above the burner. For the variation of the flame temperature the total flow through the burner at constant fuel-air ratio was changed. This proved to be more convenient for a wide range of conditions than the variation of the inert gas.2~ For the gas analysis a quartz sampling probe was inserted along the flame axis. It was partly filled with quartz wool which acted as a soot filter. The flame gases were analyzed gaschromatographically (Hewlett-Packard). The polycyclic aromatic hydrocarbons were condensed (N2 liq. cooling) and analyzed with GCMS using two standards, z5

Experimental Results Soot Fotvnation Threshold for Different Temperatures: The critical C/O ratio #/ ((C/O)crit) for the onset of soot formation was visually determined as a function of input flow velocity and of pressure. The critical C/O ratio#/represents the mixture composition at which the flame just starts, respectively stops #q'he carbon to oxygen atom ratio C/O in the mixture is related to its equivalence ratio k = [C/O]/[C/O]s,o~ch with [C/O]s~o,ch = 1/3 for C2H4air, [C/O]s~olch = 0.4 for C2H2 and CsHr-air.

PRESSURE AND TEMPERATURE sooting, characterized by the yellow luminosity in the burned gases. It is well established by several authors x3,16,z6 and confirmed by these measurements that at elevated temperatures the threshold for soot formation in flat (non-polyhedral!) flames depends on temperature: With increasing temperatures the critical C/O ratios increases (Fig. 1). In addition the slope of the curves of the (C/O)~rit as a function of temperature is similar for the fuels tested in that range of temperature. When the measurements are extended towards lower temperatures by reducing the cold gas flow velocity, the curves start to deviate from a straight line and reach a minimum at (C/O)eri t = 0.55 for CzH4-air. For still lower flow velocities the threshold curve bends back towards higher (C/O)~nt values. In a lower flow velocity range the flames become rather instable but the presence of a shielding flame keeps them on the burner. The flow lines of the inner flames contract somewhat (see Fig. 2 in Ref. 13). Nevertheless, the yellow luminosity and its disappearance at still lower flow velocities can be clearly recognized. Different observers obtain similar results. (It should be noted that the same luminosity, assuming the soot particles at 1900 and at 1400 K are similar, corresponds to soot loadings which differ by a factor of about 40.) Below the critical C/O ratio the flames show the usual blue-green but no yellow luminosity in the burned gas. For C2H4-air flames the determination of the critical C/O ratios, based on the appearance of the yellow luminosity, were extended to C/O ratios larger than one at flow velocities near 1 cm/s. -

For C6H6-air flames the tendency of the (C/O)cnt T curve at low temperatures is similar to that

ltC/OJtrit' ' OSQ 0.75 ~

a Ref.26 A Ref.Z6 e_ Ref. t6 ~ !}presentStudy

C2H2

0.60 tL55 l

1400

i

1600

I

I

1800

I

T[KI

I

2000

i,

FIG. 1. Threshold of soot formation as a function of temperature at normal pressure for C2H4-air, CzH~-air and C~H~-air flames. Flame temperature TM at l0 mm height above the burner.

405

2 ~0-~ 5 Z

=0-8

J

1

1600

J

l

17~

l

~ I Kl

.

~eoo

FIG. 2. Temperature dependence off.~ for C 2 H 4flames at C/O = 0.72 (*), C2Hz-air at C/O = 0.70 (O) and CeH6-air at C/O ~ 0.74 (O). air

for CzH4-air. There exists a minimum (C/O)c,it which is slightly below C/O = 0.6. It can be expected that CzHz-air flames behave in a similar manner. The influence of pressure on the threshold curves has been investigated for pressures of up to 5 bar for C2H4-air flames. At the high temperature side of the curve the influence of pressure is very small. The low temperature branch of the (C/O)~it - T curve shifts a little towards lower temperatures and the minimum of the curve in Fig. 1 moves with increasing pressure towards slightly lower temperatures and towards lower (C/O)cri t values (about 0.52-0.53 at 5 bar). This tendency seems to continue towards higher pressures.

Influence of Temperature on the Soot Volume Fraction f~: In the high temperature region the final soot volume fractions fw decrease strongly with increasing temperature.l°.U.13,16 As shown in Fig. 2, the fv~ - T curves reach a maximum value towards lower temperatures and startto decrease. 28 These declining curves could not be measured to very low temperatures. One may expect, however, that they tend towards the curves of the critical C / O ratios at the soot threshold shown in Fig. 1. The absolute values fv= on these curves depend strongly on the C/O ratio. For a given fuel the positions of the maxima of these bell-shaped curves vary only slightly with temperature in the range around 1600 to 1650 K. A three dimensional representation of the data on f~= as a function of C/O and the flame temperature TM is given in Fig. 3 for C2H4-air flames. (TM means the flame temperature at 10 mm height above the burner.) It clearly shows the domain in

406

COMBUSTION-GENERATED PARTICULATES: SOOT IN DIFFUSION FLAMES

fv~

10-~

"-6

C2H4/oir

10 -6

,-7 ~ 10-7 ,-9

\

Z bor

~" I 1600

10-8 10-5

FIG. 3. f~.~ plotted as a function of the C / O ratio and the flame temperatures TM for C2H4-air flames at normal pressure. T M m e a n s flame temperature at 10 mm height above the burner. The broad solid line is the threshold for soot formation. which soot formation in premixed flames proceeds most favorably and where it is less efficient. In Fig. 2 there are also data for C2H2-air flames at C / O = 0.70 and for C6H~-air flames at C / O = 0.74. The shape of these curves is quite similar. Their maxima for the different fuels appear at different temperatures. The plots corresponding to Fig. 3 for C6He-air and C2H2-air flames are in principle similar to that for CeH4-air flames. The fact that the fv~ - T curves for the C2H2-air flames fit rather well into the curves of the other fuels supports the assumption that the low temperature threshold for soot formation in CzH2-air flames is close to that of the other two fuel-air flames (Fig. 1).

Influence of Pressure on f,~ for C2H4-Air Flames: In order to obtain the influence of pressure on fv~ at constant temperature flame profiles at a given C / O ratio and pressure were investigated for different flow velocities. A plot offt~ as a function of TM for different pressures P is shown in Fig. 4a for C / O = 0.72. For higher pressures the maxima of the f ~ - T curves shift towards lower temperatures. In the high temperature region the slope of the f ~ curves are similar for the pressures applied and also for the fuels used. Fig. 4b shows a log f w log P plot for temperatures of 1800, 1700 and 1600 K and a C / O ratio of 0.72. From the resulting straight lines in Fig. 4b an exponent m for the expression f ~ ~ P" of about 2 results for 1700 and 1800 K. This exponent m seems to increase slightly

i 1700

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I 1800

TN

i [ K]

fv ~ 1600 K

~- - ~

./"

-- 1700 K

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10-7

10 -8

1 I

, 0.1

2 I 0.3

~

3 J, 0.5

,

5 I 0.7

p [bor] log p

FIG. 4. a) Soot volume fractions of C2H4-air flames for C/O = 0.72 as a function of temperature T M for different pressures b) Plot of the log f~.~ as a function of pressure for different temperatures. with increasing temperature and also for temperatures below 1600 K. The same exponent m ~ 2 for the pressure dependence off,~= as for C / O = 0.72 is obtained for C / O ratios from 0.68 to 0.75 and temperatures at and above 1700 K in C2H4-air flames. Figure 4 shows clearly that even a slight decrease of the flame temperature TM with increasing pressure will produce higher exponents m for the pressure dependence o f f ~ and vice versa (see Refs. 14, 17). The dependence offw on the C / O ratio is rather strong. It rises approximately proportional to (C/O -- (C/O)crit) n with n ~ 3.5 - 4 for CzHa-air flames on the hot side 1'29 in Fig. 3. These data have been obtained for flames with constant flow velocities. The maximum flame temperature in sooting fiat flames usually decreases somewhat with increasing C / O ratio. This shifts the corresponding ft~= values (for high temperatures flames) upwards. For a constant temperature of 1700 K the exponent n decreases to

PRESSURE AND TEMPERATURE n ~ 2.5 - 3, at p = 1 bar, while at p = 5 bar the exponent n is about 2. For temperatures below 1650 K this dependence o f f ~ on C/O for different pressures becomes more complex as can be seen from Fig. 3. For pressures above atmospheric the '~{~.~ mountains" in Fig. 3 are higher and have steeper slopes in T direction.

Benzene Flames: For benzene-air flames the tendency to form polyhedral flames becomes more pronounced at higher pressures. Therefore, only a fairly small range of conditions could be covered for the experiments with stable fiat flames at elevated pressures with the silver capillary burner (C/O = 0.8; 1550 K --< T -< 1850 K; flow speed 2.5 to 5 cm/s. Below 2.5 cm/ s the flame flashed back). The dependence of f ~ on the C/O ratio is similar to that for C2H4-air flames. For a constant flow velocity (4.3 cm/s) the relation fv~ ~ (C/O - (C/ O ) c r i t ) n c a n be described with n ~ 3. For constant temperature T = 1730 K the exponent comes close to two (n ~ 2.2). In a plot of log f ~ against pressure the curve for benzene-air at 1730 K is nearly identical to the curve in Fig. 4b for CeH4-air at T = 1800 K (m ~ 2).

Influence of Temperature on the Gas Composition: The bell-shaped curves for soot loading of the burned gases might suggest that there are similar or related changes of the concentration of the hydrocarbons in the burned gases. Therefore, these burned gases have been analyzed as a function of temperature at 30 mm above the burner for CzH4air flames. Some typical results are given in Fig. 5. For the water gas components H2, CO, CO2 and H20 there is a small systematic change with temperature which does not reflect the appearance of solid carbon. (The measurements have shown that the water gas is equilibrated in the measured temperature region.) The concentrations of the main hydrocarbon components such as acetylene, polyacetylenes, methane, etc. decrease with increasing temperature. Their changes in concentrations are continuous with temperature and relatively small. For the temperature range from 1500 to 1700 K there is no indication that these concentrations follow the tendency of soot formation. In order to check the behaviour of polycyclic aromatic components (PCA) the concentrations of many of them have been analyzed for temperatures between 1300 and 1700 K under the conditions mentioned above. Results for some selected PCA are shown in Fig. 5. These PCA increase continuously from low temperatures up to about 1600 K (for C /

407

1300

10-4:

c/0.0.TZ

CzH4 kf.fv=IS-q

10- s

lw e

~1o"

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-.

~f... //

v

*

-

10-s

~S ~ t

10"1

~, 10-? 104

10-g lO-tC

i0~o

Thre~old of SoatFormatmn 1300

TM [xl

FIG. 5. Carbon density in the burned gases of C~H4-air flames burning at normal pressure as a function of temperature. Shown are: the total carbon content present as (CO + CO2) and the hydrocarbons C2H2, CH,, the concentrations of the PCA: acenaphthylene, naphthalene, anthracene, fluorene (measured 30 mm above the burner), the soot volume fraction f,~ and the formal maximum fv-growth rate ks.f~ in [cm3/cm3- s] and the threshold for soot formation. O = 0.72) and then drop towards higher temperatures. The maximum concentration of the PCA is at a lower temperature than the f ~ maximum. This agrees with measurements of fluorescence in the burned gases of premixed21 and diffusion flames. 7 The drop of PCA concentrations towards low temperature is less steep than that of soot. There is no large change of their concentrations at the threshold of soot formation which is indicated in Fig. 5. At 1600 K the total carbon density in the PCA measured is about 3.6 10-s g/cm 3. The carbon content of the PCA not measured here contribute only very little to the total carbon content of the PCA so that the total PCA in the burned gases contain only a small fraction of the carbon present in C2H2.

Discussion

A Comparison With Shock Waves Pyrolysis: The experiments reported here have been performed in order to establish the phenomenology of soot formation towards low temperatures. They show that pressure and especially temperature strongly

408

COMBUSTION-GENERATED PARTICULATES: SOOT IN DIFFUSION FLAMES

influence the formation of soot in premixed flames. In addition they show that there exists an upper and a lower temperature limit for the formation of soot in these premixed flames. Within these upper and lower thresholds of soot formation the total amount of carbonaceous aerosol found in the fiat flames passes through a maximum at final flame temperatures around 1650 K (see Fig. 5). At that maximum the carbon atom density present as soot and hydrocarbons in the burned gases is [C]t = 2.1017 C atoms/cm a for atmospheric pressure and C/O = 0.72, The soot contains [C]t = 4" 1016 C atoms/cm a and about 1015 C atoms/cm 3 are stored in PCA. For the same carbon atom density [C]t shock tube pyrolysis experiments of benzene and several other hydrocarbons show a similar bell-shaped curve for the amount of soot formed after a given time l°'H as the fiat flame experiments (Figs. 2, 4 and 5) presented here. Although the appearance of these ft~ - T curves is similar for both sets of experiments, they are at least partly due to different chemical effects. On the high temperature side of the f~= T curves both sets of experiments, shock tube pyrolysis and fiat flames, evaluate f ~ , the amount of soot finally formed. They can, therefore, be compared. At the low temperature side most of the shock tube pyrolysis resultsao are strongly influenced by the induction period of soot formation (fv after 1 or 2 Its!). Contrary to that the fiat flame results refer to much longer reaction times than the shock tube data. Therefore, the low temperature branch of the fv~ - T curves for flames is shifted towards lower temperature compared to the shock tube pyrolysis data. 9-11 If the shock tube data could be extended to longer reaction times, their low temperature branch would also be shifted towards lower temperatures. It is, therefore, common for both types of experiments that there exists a field of experimental conditions which is especially favourable for the formation of soot.

Low Temperature Soot: One could argue that near the low temperature soot threshold (Fig. 1) the visible yellow luminosity might not be due to carbonaceous aerosol but to some other emitter. The fact that solid material can be collected from the flame on hot or cooled probes does not mean that this material is present in the burned gases of the flame, The following observations can be quoted: 1) the concentration of PCA especially for those which are responsible for fluorescence varies continuously from below to above the "soot threshold" (see Fig. 5) 2) the depolarization ratio QHv/Qw and the differ-

ance (Qw - Qr~v) tend towards the particle limit with increasing pressure. The depolarization ratio for measurements near the soot threshold at lower temperatures tends towards the particle limit with increased distance from the burner. Measurements of Qvo(X), Qnv (k) and of the "polarized fluorescence" at wavelength different from h show that a part of the Qt,~ signal comes from particles. 43 We, therefore, think that the yellow luminosity stems from carbonaceous aerosol. Its optical properties may, due to a higher hydrogen content, be different from those of the soot particles usually obtained at higher temperatures. 16 The appearance of the material taken from these low temperature flames in different ways fits to the description of soot obtained from hot C2H2-air low pressure flames under certain conditions, al A more detailed investigation of that material and its properties is necessary and under way.

Influence of Pressure on Final Soot Volume Fraction: The experiments reported above for C2H4-air flames show that at constant temperature TM and for a given C/O ratio the f ~ values at higher temperature increase approximately proportional to p2 while the total carbon density rises as P. The concentration Pc of hydrocarbons such as CzH2 in the burned gases depends little on temperature (see Fig. 5) and it rises nearly proportional with increasing C/O ratio. 29 It is to be expected that the ratio of hydrocarbons to (CO + CO2) in the burned gases does not z2 vary strongly with pressure for pressures up to a few bar. The contribution of C2H2 (and that of polyacetylenes) to the surface growth per unit area F of the soot particles in C2H4-air flames is nearly proportional to the C2H2 concentration. 27'29 It should therefore be proportional to pressure. This means that the effective particle area F, the size of which is determined early in the soot forming process (in the later phase of combustion F remains nearly constant), aa should be proportional to pressure in order to obtain a p2 dependence of f~®. One might expect that the increase of f ~ with increasing C / O ratio and its increase with pressure for a given C//O ratio are related to pc in a similar manner. For the benzene-air flames both dependences of fv~ on P and C / O are indeed found to be similar at constant final flame temperature. In C2H4-air flames burning at constant flame temperature (high temperature side in Fig. 4a) the exponents for the dependence o f f ~ on P and C / O ratio differ somewhat probably due to processes in the very early phase of soot formation where the effective F values are determined. Nevertheless the

409

PRESSURE AND TEMPERATURE observations show that the quantitative relations for the influence of pressure and C/O ratio on f ~ in benzene- and ethylene-air flames are quite similar, expecially at elevated pressures.

Soot Formation Rates Towards Low Temperatures: The kinetics of soot formation in the "low temperature range" cannot be discussed here. A few remarks concerning coagulation and surface growth shall, however, be added. The main part of soot mass growth can often be described 19'33 by L = kf(fw - f~)~. For the low temperature flames the ft,~ values are approached at large distances above the burner. This complicates the evaluation of ky. In the range of temperatures where the kf could be determined they fitted however well into the Arrhenius plot of log ky - 1/T given in Refs. 27, 33. At 1550 K the value of ky found is about 15 s-l corresponding to a half value time of active soot growth of already 70 ms. A quantity which is another characteristic measure for the growth of soot11 and which can easily be obtained from experiments is the maximum growth rate (formally (df/dt)m~ = kf'fv~ the steepest slope of the fv profile). It is plotted in Fig. 5 as a function of temperature. This maximum growth rate curve looks very similar to what has been obtained in shock pyrolysis measurements.9'11 At the high temperature side the shape of this curve is strongly influenced by kf which is a measure for the time of active growth of soot (f~), independent of the model applied, f ~ decreases with increasing temperature because the time for the active particle mass growth decreases. 31'35 On the low temperature side the curve rises strongly with increasing temperature and the influence of processes determining kf is much less pronounced. Therefore, the "'maximum growth rate" is more closely related to the real rate of the soot volume fraction growth in that temperature range. This should be helpful for a quantitative interpretation of the soot growth rate.

Particles Coagulation: The temperature dependence of the coagulation rate constant k¢o~ ( - T 1/2) is much weaker than that of k£. One, therefore, expects at low temperatures that f,~ continuous growing while the "final particle ~The optical properties of low temperature soot may differ from that in flames with high temperature. As long as the complex refraction index of the particles does not vary along the flame axis, kf can be obtained as usual from absorption measurements without knowing the complex refractive index.

number" is already reached; at higher temperature the opposite should happen. This is in agreement with other observations27 and the results obtained here. Coagulation rate constants kcoagat normal and reduced pressures are usually found to be somewhat above the theoretical value for spheres with log normal size distribution. 1'z°'23'33-4° For low temperature flames there is a tendency for a decrease of k~oag below the theoretical value which could be due to the higher mean hydrogen content of the particles. A similar, but much larger effect has been observed for flames seeded with certain metals. 24 Generally speaking the "cool sooting flames" show some properties which are different from those of the "hot flames." There are similarities to the pyrolysis of aromatic compounds.l°'2° In order to close the gap with experiments as those of Anderson et al.41 and Homann~2 from the particles side, it seems worthwhile to look more closely to specific properties of these "cool flame soot particles."

REFERENCES 1. HAYNES, B. S. AND WAGNER, H. GG.: Progr.

Energy Combust. Sci. 7, 229 (1981). 2. JOST, W.: Explosionen und Verbrennungsvorg~inge in Gasen, Springer, 1938. 3. LEWIS, B. AND V. ELBE, G.: Combustion Flames and Explosions of Gases, Academic Press, 1961. 4. STREHLOW, R. A.: Combustion Fundamentals, Mac Graw Hill Inc., 1984. 5. GLASSMAN, I.: Combustion, Academic Press, 1977. 6. GUNTHER, R.: Verbrennung und Feuerungen, Springer-Verlag, 1974. Schug, K. P., Mannheimer-Timnat, Y. Yaccarino, P., Glassman, I.: Comb. Sci. Tech. 22, 235 (1980) (see Comb. Symp. for further ref.); Kern, J., Spengler, G.: Erd61, Kohle, Erdgas, Petrochemie 23, 813 (1970). 7. KENT, J. H., BASTIN, S. J.: Comb. Flame 56, 29 (1984); Kent, J. H., and Wagner, H. GG.: Comb. Sci. and Tech. 41, 245 (1984); Gomez, A. Littmann, M. G., Glassman, I.: Comb. Flame 70, 225 (1987); Haynes, B. S. and Wagner, H. GG.: Ber. Bunsenges. Phys. Chem. 84, 499 (1980). 8. SANTORO,R. J., SEMERJIAN, H. G., DOBBINS, R. A.: Comb. Flame 51, 203 (1983); Santoro, R. J., Semerjian, H. G.: Twentieth Symposium (International) on Combustion, p. 937, The Combustion Institute, 1985. 9. GECK, C. C.: Diplom Thesis, GiSttingen, 1975; Buckendahl, W., Diplom Thesis, G6ttingen, 1970. 10. GRAHAM,S. C.: Proc. Roy. Soc. A377, 119 (1981) (there further ref.).

410

C O M B U S T I O N - G E N E R A T E D PARTICULATES: SOOT IN D I F F U S I O N FLAMES

11. FRENKLACH, M.: Shock Tube Study of the Fuel S t r u c t u r e Effects on t h e C h e m i c a l Kinetics Mechanisms Responsible for Soot Formation, NASA CR 174661 (1983); F r e n k l a c h , M.: T w e n t y - F i r s t S y m p o s i u m ( I n t e r n a t i o n a l ) on Combustion, p. 1067, The Combustion Institute, 1988; Wang, T. S., Matula, R. A., Farmer, R. C.: Eighteenth Symposium (International) on Combustion, p. 1149, The Combustion Institute, 1981. 12. MARKSTEIN, G. M.: Nonsteady Flame Propagation, Pergamon Press, 1964; Krischer, B., Z.f. Elektrochem. 60, 1017 (1956). 13. FLOSSDORF, J. AND WAGNER, H. GG.: Z. Phys. Chem. NF 54, 113 (1967). 14. M)iTZING, H. AND WAGNER, H. GG.: Twenty-First Symposium (International) on Combustion, p. 1047, The Combustion Institute, 1988. 15. JOST, W.: Z. Phys. Chem. 193, 332 (1944); Miller, I. M.: NASA T. P. 1318, 1978; Maahs, H. G. and Miller, 1. M.: NASA T. P. 1673, 1980. 16. MILHKAN, R. C. : J. Phys. Chem. 66, 794 (1962); Millikan, R. C. and Foss, W. I.: Comb. Flame 6, 210 (1962). 17. MCFARLANE, J. J., HOLDERNESS, F. H., WHITCttER, F. S. E.: Comb. F l a m e 8, 215 (1964). 18. BONE, W. A. AND TOWNEND, D. T. A.: Flame and Combustion in Cases, Longmans Green and Co. Ltd, 1927. 19. BAUMGhRTNER,L., Dissertation, G6ttingen, 1983. 20. D'ALESS10, A., DI LORENZO, A., BERETrA, F., VENITOZZI, C.: Fourteenth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1973; Beretta, F., Borghese, A., D'Alessio, A., Venitozzi, C.: Laser Measurement Methods in Combustion Urbino, Italy 7-9. Sept. 1977 (see also Combustion Symposium). 21. HAYNES, B. S., JANDER, H., WAGNER, H. GG.: S e v e n t e e n t h Symposium (International) on Combustion, p. 1365, The Combustion Institute, 1979; Haynes, B. S. and Wagner, H. GG.: Ber. Bunsenges. Phys. Chem. 84, 499 (1980). 22. DALZELL, W. H., SAROFIM, A. F.: Trans. ASME, J. Heat Transl. 91, 100 (1969). 23. GAYDON,A. G., WOLFHARD, H. G.: Flames, their Structure, Radiation and Temperature, Chapman & Hall, 1979. 24. HAYNES, B. S., JANDER, H., MATZING, M., WAGNER, n . GG.: Nineteenth Symposium on Combustion, p. 1379, The Combustion Institute, 1983; Haynes, B. S., Jander, H., Miitzing, M., Wagner, H. Gg.: Comb. Flame 40, 101 (1981). 25. TOMPK1NS, E. E. AND LONG, R.: Twelfth Symposium (International) on Combustion, p. 625,

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PRESSURE AND TEMPERATURE

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COMMENTS T. Parker, Physical Sciences, Inc., USA. You rep o r t e d an increase in volume fraction with increased pressure. Is the increase in soot volume fraction totally due to an increase in pressure or is it partially due to increasing fuel concentration with increasing pressure in your burner. Author's Reply. At otherwise similar conditions (constant fuel mole fraction, etc.) the carbon concentration increases proportional to pressure. The final soot volume fraction for CeH4-air flames and the benzene flames reported here for pressures up to 10 bar increases proportional to the second, resp. third power of pressure. There is, therefore, an influence of pressure on soot formation which exceeds the influence of the growing initial carbon density. It should, however, be considered in connection with the dependence off,~ on [ C / O (C/O)o,,,].

P, E. Sojka, Purdue Univ., USA. It has been suggested that critical equivalence ratio data can depend on burner configuration. In a similar manner, would you expect the temperature at which the minimum critical equivalence ratio occurs to vary between burner types? Author's Reply. The critical (C/O)c,t ratios reported here have been obtained with different cold gas flow velocities on water cooled bronze porous plate burners and gold plated porous plate burners as well as on silver and steel capillary burners. No pronounced differences in the (C/O)cn, could be found on the high temperature side which could not be attributed to the temperature profiles across the burner. On the low temperature side of the curves, the situation is more critical. There, it is important to have stable shielding flames which stabilize the central flame at which the measurements are performed.

K. H. Homann, T.H. Darmstadt, Fed. Rep. of Germany. Can you explain the fact that the sum of the acetylenic hydrocarbons is constant with temperature while all the PAH decrease after going through a maximum? From a thermodynamic point of view one would expect an increase of the acetylenic species.

Author's Reply. You refer to a special case mentioned in the paper. For C/O ratios lower than 0.72 (in C2H4-air flames) the concentration of acetylenic hydrocarbons taken at a height of 30 mm above the burner starts to decrease at temperatures T ~ 1650 K. For C / O = 0.5 it decreases steeply such that at T = 1700 the acetylene concentration is below Xcanz < 10 s due to oxidation.

J. c. Hermanson, United Technologies Research Center, USA. The threshold values of soot formation in this work were determined by visual observation. Could you comment on the accuracy of the visual method for determining the critical C / O ratio, and how those results would compare with those that might be obtained from extrapolation of laser absorption data or by photometric measurements?

Author's Reply. The determination of the (C/O)c,t is a visual method and the accuracy of threshold of soot formation depends to some extent on the sensitivity of the observer's eye. A comparison with absorption and scattering measurements has been performed at atmospheric pressure. It requires a rather long extrapolation of the optical measurements to approach the critical (C/O) ratio in a range where the optical properties vary strongly with fuel concentration. This means that the optical measurements (including depolarization!) have to be performed with extreme care. For the system where the comparison has been performed the agreement between visual and laser measurements was about 2% at the high temperature branch and about 5-8% at the low temperature branch.