Sooting structure of methane counterflow diffusion flames with preheated reactants and dilution by products of combustion

Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, 1992/pp. 1049-1057

SOOTING S T R U C T U R E OF M E T H A N E C O U N T E R F L O W D I F F U S I O N FLAMES WITH P R E H E A T E D REACTANTS A N D D I L U T I O N BY P R O D U C T S OF C O M B U S T I O N C. ZHANG, A. ATREYA* AND K. LEE

Combustion~Heat Transfer Laboratory Department of Mechanical Engineering Michigan State University, East Lansing, MI 48824 USA This paper presents the results of an experimental investigation into the sooting structure of methane counterflow diffusion flames where the fuel and the oxidizer streams were preheated and/or diluted by products of combustion such as CO2 and HzO. Low strain rates were employed to spatially resolve the inner structure of these flames and detailed measurements of temperature, species, PAH, soot particle number density and volume fraction were made. Strain rate, methane concentration and fuel flow rate were held constant throughout this work. In these fuel-rich flames a three-color (blue-yellow-orange) structure was observed. Primary combustion reactions were found to occur in the blue zone and measurable size soot particles (>5 nm) were observed only in the orange zone. In the yellow zone, which lies between the blue and the orange zones, blue-green fluorescence was measured but soot particle scattering was found to be negligible. Thus, it seems that large PAHs responsible for fluorescence may also be responsible for yellow emission. Soot inception occurs at the diffuse interface between the yellow and the orange zones whose location depends on the local thermochemieal conditions. It was found that an increase in the reactants preheating temperature (while simultaneously reducing the Oz concentration to hold the temperature throughout the reaction zone at or below the original flame temperature) led to an early inception and increased soot volume fraction. However, addition of COz and HzO (while holding all other conditions constant) resulted in delayed soot nucleation and a significant reduction in the soot volume fraction. These two observations are consistently explained by the mechanism of OH interference with soot inception. An increase in the CO2 and/or HzO concentrations (brought about either by an increase in the Oz concentration or by direct addition) result in an increase in the OH concentration. This reduces the PAH and C2H2 concentrations and delays soot inception. Thus, thermochemical conditions that favor soot inception substantially increase the ultimate soot loading because they control the residence time available for surface growth.

Introduction Soot formation during combustion is an active area of research. Considerable progress regarding the fundamental understanding of sooting process in laminar flames has been reported. 1-5 A general picture that emerges from this previous work shows four distinct processes during soot formation: (i) Precursor formation: This involves a complex sequence of fuel pyrolysis reactions which eventually result in the formation of precursors (large PAHs) needed to form the first particles; (ii) Particle in*Current address: Dept. of Mech. Engineering and Applied Mechanics, The Univ. of Michigan, Ann Arbor, MI 48109.

ception: Here, chemical and physical growth of precursors result in the formation of numerous small (diameter - 2 n m ) primary particles; (iii) Particle growth: The primary soot particles formed during inception grow by surface reactions as well as by coagulation; and (iv) Particle oxidation: The soot particles are reduced in size due to attack by OH radicals and 02 molecules. Most of the above understanding was derived from detailed measurements in co-flowing Wolthard-Parker or axisymmetric burners. In this work, through similar detailed measurements of temperature, species, PAH, soot particle number density (N) and volume fraction (~b) in counterflow diffusion flames, we show where relative to the primary reaction zone these processes occur and how they are affected by fuel and oxidizer preheat and dilution by primary

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FIG. 1. Schematic of the experimental apparatus (C: chopper, COR: digital correlator; F: filter; GC: gas chromatograph; HV: high voltage supply; I: irises; L: lens; LA: lock-in amplifier; M: mirror; ND: neutral density filter; P: pin hole; PD: photodiode; PMT: photomultiplier tube; PI: polarizer; TC: thermocouple). combustion products (CO2 and H20). Since strain rate essentially controls the residence time, by employing low strain rates it has been possible to expand the flame and quantitatively examine its sooting structure. Counterflow diffusion flame was chosen because it provides a well-defined one-dimensional system convenient for both experiments and theoretical modeling.

Experimental Methodology The experimental apparatus is schematically shown in Fig. 1. Two streams of gases which can be electrically preheated impinge against each other at a stable stagnation plane, which lies approximately at the center of the burner gap. Upon ignition, a flat axisymmetric diffusion flame roughly 8 cm in diameter is established above the stagnation plane. All measurements are taken along the axial streamline. Co-flowing nitrogen is introduced along the outer edge of the burner to eliminate oxidizer entrainment and to extinguish the flame in the outer jacket. Methane, oxygen, nitrogen, helium and carbon dioxide used during these experiments were obtained from chemical purity gas cylinders and their flow rates were measured using calibrated critical

flow orifices. Water vapor was generated by passing a stream of inert gas (helium or nitrogen) through a distilled water saturater maintained at a specified temperature. As shown in Fig. 1, a beam of argon-ion laser operating at 514 nm is modulated by a mechanical chopper and then directed by a collimating lens to the center of the burner. This beam is used for classical light scattering and extinction measurements. A photomultiplier tube and a photodiode are used to detect the scattered and transmitted signals respectively. These signals are processed by a lockin amplifier interfaced with a microcomputer. The extinction coefficient was experimentally corrected for gas absorption by subtracting the extinction coefficient of a reference flame. This reference flame was carefully chosen by slightly reducing the fuel and the oxidizer concentrations such that soot scattering was reduced to less than 0.5% of the original flame. Emission of soot particles was not observed from this blue-yellow "scattering limit" flame. Laserinduced broadband fluorescence (LIF) measurement were also made by operating the laser at 488 nm and detecting the fluorescence intensity at 514 • 10 nm. This was taken proportional to the PAH concentration. 3 In the subsequent data reduction, the soot aerosol is assumed monodispersed with a complex refractive index of 1.57-0.56i. 6 Temperatures were measured by a 0.076 mm diameter Pt/Pt-10%Rh thermocouple. The thermocouple was coated with SiOz to prevent possible catalytic reactions on the platinum surface. It was traversed across the flame in the direction of decreasing temperature at a rate fast enough to avoid soot deposition and slow enough to obtain negligible transient corrections. For radiation corrections, separate experiments were performed to determine the emissivity of the SiO2 coating as a function of temperature. The maximum radiation correction was found to be 150 K. These temperature measurements were repeatable to within • K. The chemical species concentrations in the flame are obtained by an uneooled quartz microprobe and a gas chromatograph. A 70/.Lm sampling probe was used for most of the analysis except for the heavily sooting flame where a larger (90 /xm) probe was used. This probe was positioned radially along the streamlines to minimize the flow disturbance. Concentrations of stable gases (H2, COz, 02, N2, CH4, CO and H20) and light hydrocarbons (up to C6) were measured. In this paper, measurements for two sets of experiments are presented (see Table I for flame conditions). A sooty flame with no preheat (BA) was first chosen as the base flame. The effects of preheating the reactants and dilution by products of combustion were then studied by varying the chemical-composition and/or the preheat temperature. In the first set of experiments, the oxidizer

SOOTING STRUCTURE OF COUNTERFLOW DIFFUSION FLAMES

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16%Oz + 84%He (300 K) 16%O~ + 84%He (300 K) 16% ()2 + 84%11e (300 K) 16%O~ + 80.4%He-3.6 %HzO (30OK) 11%O2 + 89%N2 (900 K) 9.7%O2 + 90.3;%N2 (1200 K)

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side was preheated to temperatures of 900 K (BB) and 1200 K (BC) while the fuel side was preheated to 835 K and 900 K respectively. Lower preheat for the fuel side was used to avoid CH4 decomposition prior to entering the flame (confirmed by chemical measurements). Also, while helium was used as the inert diluent in the oxidizer stream for the BA flame (to eliminate buoyant instabilities at low strain rates) it was replaced" by N2 for BB and BC flames. The preheat made these flames stable for the same strain rate. Preheat also made the BB and BC flames extremely sooty and unsuitable for probe measurements. Ilence, oxygen concentration was reduced to reduce the maximum flame temperature. In the second set of experiments, inert diluents used in BA, BB and BC flames were partially replaced by CO2 and He() while holding the fuel and the oxidizer concentrations constant. The additive gas mixture and its amount was calculated to maintain the same adiabatic flame temperature and the same stagnation plane h)cation. The objective was to provide nearly identical thermal and strain rate fields while changing the chemical environment to enable quantitatively studying the effect of COz and H,20 bn the sooting structure. Fuel concentration (65%), fuel flow rate and strain rate (8 sec -l) were held constant throughout this work. Results To obtain the sooting structure of these flames, detailed measurements of temperature, species, PAll, 6 and N were made along the burner axis normal to the flame. To consistently compare the measurements for different flames, the distance

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normal to the flame was normalized according to the following reasoning. The diffusion flame establishes itself above the stagnation plane at a location where the diffusion of fuel against the incoming oxidizer stream creates stoiehiometric conditions. As the strain rate is increased, the flow veh)city which becomes zero at the stagnation plane is increased. Thus, for the same diffusion veh)city (or difft,sion coefficient) the stoichiometric plane which marks the flame location is moved closer to the stagnation plane. Clearly, the appropriate length scale is the square root of the appropriate diffusion coefficient divided by the strain rate. This is proportional to the distance between the flame location (Zt, marked by the measnrcxt flame temperature) and the stagnation plane (Z~, obtained from TiCI4 particle track photography). Since the appropriate diffusion coefficient for different thermochelnical environments is nut well knowu and varies with the addition of different inerts, the measured (Zt - Z~) is used as the length scale. This is listed in Table I. Hence, all the results presented here are plotted against the normalized distance perpendicular to the flame where the stagnation plane is the origin (Z,, = (Z - Z~,)/(Zt - Z,)). Temperature

a n d Velocity:

Fig. 2 shuws the measured temperature and calculated velocity profiles for the flames listed in Table I. These velocity profiles were numerically calculated by specifying the measured boundary conditions and by using the measvred temperature and species profiles. Properties of the muhi-component mixture were obtained as a function of temperature from the NASA code. 7 The continuity and

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momentum equations employed along with the ap~ propriate boundary conditions are well-known. These calculated velocity profiles were checked by using the measured location of the stagnation plane. They matched within the experimental error of ---0.2 mm. Also included in Figure 2 are the computed residence times measured from the flame location (Z, = 1). The temperature and velocity profiles were used to calculate the heat release rates from the energy equation. 8 These heat release rates show a maximum at Z, = 1 but they become negative between 0.4 < Z,, < 0.8. This endothermic effect increased with soot loading and was the highest for the BC flame. Previously, it was believed that this endothermicity is due to methane decomposition. 9 However, we find that it corresponds better to the hydrogen production rate because the CH4 destruction rate peak does not coincide with the minimum in the heat release rate curves. Thus, it seems that the hydrogen abstraction reactions responsible for soot precursor formation v2 may be net endothermie. It should be emphasized that measurement errors are amplified while determining rates of measured quantities. However, while the magnitude of the reaction or heat release rates may have

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Chemical Species: Fig, 3 shows the measured concentrations of stable gases, several intermediate hydrocarbons and large PAHs for the BA flame. Similar measurements were made for all other flames. The overall accuracy was -+5% for stable gases (determined from the summation of species mole fractions). Among the stable gases, H20 was the least repeatable and had the largest error. Intermediate hydrocarbons were much lower in concentration and their peak values were roughly at the same Z, location which changed with the flame conditions. The concentrations of these hydrocarbons increased considerably with increase in soot loading. They were the highest for the BC flame. The chemical reaction rates, Ri were calculated from the measured species con-

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Zn FIG. 4. Calculated production/destruction rates of stable gases, intermediate hydrocarbons and large PAHs for the BA flame (Top: Stable gases; Bottom: Intermediate hydrocarbons). Stable g a s e s : Hydrocarbons: H20 • 1.5 * * C~,H~, ~ ' - - * CO • 1.5 H LIF (in arbitrary) CH4 A - - A C2H~ ~ - - A O~ ~ C2H4 H2 (3----0 C~H~ • 5 CO• centrations using the species conservation equation. s The overall production and destruction rate profiles so obtained are presented in Fig. 4 for the BA flame. It is seen that primary combustion reactions occur in the narrow blue flame zone, (Z, = 0.9-1.1) where H20 and CO2 are produced and 02 is consumed. Here, H2 and CO are oxidized to CO• and H20. The peak destruction rate of methane, however, has shifted to the fuel side in a region spatially corresponding to the appearance of intermediate hydrocarbons and PAH species. Also, the destruction rate of PAH around Z,, = 0.5 seems to coincide with the nucleation of the soot particles (Fig. 4, BA fame).

Soot Volume Fraction r and Number Density N: Figs. 5 and 6 show @ and N profiles along with PAH and C2Hz profiles. The location where the first measurable soot particle appears (particles larger than 5 nm; herein defined as the inception location) changes considerably with the introduction of CO.z

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and HzO (Fig. 5) and preheating of the reactants (Fig. 6); Z, at inception changes from 0.3 to 0.85. The corresponding peak values of ~b, LIF and C2H-2 decrease as the inception location shifts away from the flame front. The C2Hz peak also shows a similar shift toward the stagnation plane. Thus, the optically measured shift in the inception location is consistent with the chemical measurements. Figure 2 shows that the temperature at the inception location varies from 1400 K to 1750 K and the residence time varies from 10 to 50 ms. Thus, it is not possible to conclude that inception occurs at a specified temperature or residence time. The reasons for the shift in the inception location will be discussed later. However once inception occurs, the newly formed soot nuclei are driven by convection and thermophoresis away from the hot reaction zone into the cooler growth zone (O -< Z, <- 0.6). Here, surface growth occurs readily and ~b increase rapidly. Both coagulation and surface growth processes contribute to the increase in the soot particle size

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Zn FIG. 6. Effect of preheating (BA, BB and BC) on soot characteristics: volume fraction, number density and growth species (C2Hz and large PAH). Top: Bottom: [] N(BA) C) C2H2(BA) [] N(BC) (3 CzH2(BB) 9 N(BB) 9 C2H2(BC) O ~b(BA) [] LIF(BA) ~b(BC) [] LIF(BB) 9 ~b(BB) 9 LIF(BC) from 5 to 100 nm. The coagulation rate becomes less significant as temperature and N decrease toward the stagnation plane. Thus, for Zn < 0.3, the surface growth becomes the dominant mechanism and results in a considerable increase in the particle size. It is interesting to note that ~b for the flames in Figs. 5 and 6 change by an order of magnitude even though the fuel concentrations and the fuel flow rates are identical. Also note that the temperature for the BC flame is lower than the WAO flame throughout the sooting zone, yet ~b is 10 times larger. The reason for this difference is the delay in inception resulting in reduced particle residence time in the growth region. The surface growth rate also decreases because of decrease in soot surface area and CzHz concentration, but this may not be as significant. Discussion

Sooting Structure: The observed structure of these flames is schematically shown in Fig. 7. Essentially a three-color

flame was observed which extends from Zn = 0 to Z n -- 1.1. While traveling along the central stream line from the oxidizer side to the fuel side, first a light blue zone is encountered in which the peak flame temperature (-1900 K) and primary combustion reactions occur. Next, we encounter a bright yellow zone where the temperature is between 1500 K and 1800 K. Between the blue and the yellow zones there exists a fairly thin dark zone whose origin is unclear to the authors. In the yellow zone, scattering by soot particles was found to be several orders of magnitude smaller than at the stagnation plane. However, the measured extinction coefficient used to determine ~b and the fluorescence signal used to estimate the PAH concentration were not negligible in the yellow zone. It is likely that both extinction and yellow emission are caused by large PAH molecules and other intermediate hydrocarbons that are considered as precursors to soot formation. 1-3 Hence, it may be reasonable to conclude that soot precursor are formed in the yellow zone but their concentration is not high enough for nucleation to occur and form measurable size soot particles 4 (>5 nm). This is probably because of the oxidative attack by OH radicals on precursor species. The thickness of the yellow zone is approximately equal to the distance between the location of the sharp rise in N and the location where ~b becomes zero. Between the yellow zone and the stagnation plane, a dark orange zone exists. Here the temperature is between 1200 K and 1600 K. Soot inception occurs at the diffuse interface between the yellow and the orange zones. Soot particulate scattering and ~b in the orange zone increase until we reach the stagnation plane. This three-color sooting structure is similar to previous 1~ observations in co-flow diffusion flames. However, the interpretations are different. In the previous work 1~ the existence of soot particles in both the yellow and the orange zones was a priori assumed because soot scattering was not measured. To explain the color difference, it was suggested that

SOOTING STRUCTURE OF COUNTERFLOW DIFFUSION FLAMES orange corresponds to radiant emission from soot particles whose temperatures are equilibrated with those of the gas, while in the yellow region the soot particles are hotter than the surrounding gas. The present work suggests that the yellow emission is not due to soot particles because the measured scattering is very small. Instead, it is due to large hydrocarbon molecules produced prior to soot nucleation. While the species responsible for yellow emission are not known, similar hypothesis have been suggested previously, 14

Effect of C02 and HeO on Soot Formation: Evidence of reduction in ~b as a result of carbon dioxide or water vapor addition to the flame is presented in Fig. 5. A substantial decrease in ~b is observed with addition of CO2 and HzO. It further appears that water displays more effectiveness in inhibiting soot formation, because the result of addition of 3.6% H20 (WAF and WAO flames) to the fuel or the oxidizer streams are comparable to the result of adding 21% CO2 (MA flame). Here, 14%He was used in the fuel stream of the MA flame (to obtain the same average molecular weight) while N2 was used for WAF and WAO flames. Considering that helium is the most mobile inert and yields the least amount of soot when added to the fuel stream, 11 the soot yield for the MA flame would be even higher if He was not used. This further supports our conclusion that H20 is more effective than COz. Since there is no oxygen on the fuel side of the flame, the reduction in soot formation due to addition of CO2 and H20 can occur either through homogeneous gas phase reactions of the soot precursors with the OH radicals or, to a lesser extent, through heterogeneous reactions on the particle surface. The possible chemical mechanisms for production of OH from CO2 and H20 are: 12 H20 + H = OH + H2, CO2 + H = OH + CO. Thus, addition of water vapor or carbon dioxide to the flames boosts the OH concentration. This excess OH lowers the soot precursor concentration. As a result, the soot inception zone is shifted toward the stagnation plane leaving little time for surface growth. It is also evident from Fig. 5 that the thickness of the yellow zone increases with increase in the local concentrations of CO2 and H20. These results are consistent with the result of D u e t al.t3 where addition of CO2 was found to delay soot inception. It is important to note that the inlet CH4 and O2 concentrations, flow rates and flame temperatures were maintained constant (to within ---30 K; _+25 K was the measurement repeatability) while admitting CO2 or H20. Thus, the effect on soot formation can only be attributed to a chemical mechanism, specifically to oxidation of gas phase soot

1055

precursors and incipient particles by the OH radical. The recent work of Smyth et. a112 provides the reason for why water vapor is more effective in inhibiting soot formation than COz. Through absolute concentration measurements of OH radicals they found that the reaction H20 + H ~ OH + H2 is 14 times faster than the reaction COz + H ~ CO + OH. Thus a lot more OH is formed by H20 than by COz, A comparison of WAO and WAF flames also shows that diluents are more effective in reducing ~b when they are added to the oxidizer side. This is simply because the flame lies on the air side of the stagnation plane resulting in a higher concentration of CO2 and H20 in the yellow-orange zone when they are added to the air side.

Effect of Preheating on Soot Formation: In diffusion flames, it is widely accepted that higher flame temperature leads to increased soot production provided the residence time and the fuel concentration are held constant.2 In this work, the fuel flow rate, the fuel concentration and the strain rates were held constant and the flame temperature was controlled by preheating both the fuel and the oxidizer streams. Composition measurements were made to ensure that fuel pyrolysis did not occur prior to entering the flame zone. As expected, preheat resulted in an increase in the flame temperature and made a non-sooty blue flame very sooty. To further investigate the effect of preheat, oxygen concentration was reduced as the preheating temperature was increased in order to hold the temperature throughout the reaction zone at or below the original flame temperature [temperature profiles for three such flames (BA, BB and BC) are shown in Fig. 2]. It is interesting to note that the actual measured temperatures were the highest for the BA flame and the lowest for the BB flame throughout the critical soot formation zone. Yet, the maximum ~b, and the maximum concentrations of PAH (LIF) and C2H2 (X) are changed roughly by a factor of 3 [Fig. 6] with: [qS, LIF, X]aA < [~b, LIF, X]BB < [~b, LIF, X]Bc This phenomena was unexpected. However, it can be explained based on the chemical effect discussed above. As a consequence of reduced oxygen concentration, the concentrations of COz, H20, O and OH decreases in the yellow zone. Thus, the oxidation of soot precursors and incipient particles by O and/or OH also decreases. This results in early inception increasing the time available for soot growth. Hence, for the same fuel concentration and flow rate and even with a slightly diminished temperature field, reduced oxidation leaves more hydrocarbon species available for soot growth. In the

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limit of zero oxygen, if the temperature field can be externally maintained, the amount of soot formed will depend only on the residence time or the strain rate. However, this limit could not be experimentally achieved.

Conclusions Several conclusions regarding to the sooting structure of counterflow diffusion flames can be derived from this study: (i) In a fuel-rich counterflow diffusion flame, a three-color structure was observed: an orange zone in which a substantial amount of soot was detected, a yellow zone in which negligible scattering from soot particles was observed and a light thin blue zone in which the peak flame temperature and primary reactions occur. (ii) Soot inception occurs at the diffuse interface between the yellow and the orange zones and its location depends on the thermochemical environment. It was found that an increase in the reactants preheating temperature (while simultaneously reducing the oxygen concentration to hold the temperature throughout the reaction zone at or below the original flame temperature) led to an early inception and increased th for the same fuel flow rates, fuel concentrations and strain rates. However, addition of CO2 and H20 (while holding all other conditions constant) resulted in delayed soot nucleation and significant reduction in th. These two observations are consistently explained by the mechanism of OH interference with soot inception. An increase in the CO2 and/or H20 concentrations (brought about either by an increase in the Oz concentration or by direct addition) result in an increase in the OH concentration. This reduces the PAH and C2H2 concentrations and delays soot inception. (iii) Thermochemical conditions that favor soot nucleation substantially increase the ultimate soot loading because they control the residence time available for surface growth. These conditions become favorable with (a) decrease in the concentration of combustion products in the reaction zone; (b) increase in fuel concentration; and (c) increase in flame temperature.

Acknowledgements This work was supported by the Gas Research Institute under contract number GRI 5087-260-1481 and the technical direction of Drs J. A. Kezerle and T. R. Roose and by National Science Foundation under contract number NSF CBT-8552654. The second author would also like to thank Professor G. F. Carrier and Drs F. E. Fendell and K. C. Smyth for their interests in this study and several helpful discussions. REFERENCES 1. HAYNES, B. S. AND WAGNER, a . G.: Prog. Energy & Comb. Sci. 7, 229 (1981). 2. GLASSMAN, I.: Twenty-Second Symposium (International) on Combustion, p. 295. The Combustion Institute (1988). 3. SMrrH, K. C., MILLER, J. H., DORVMAN,R. C., MALLARD, W. G. AND SANTORO,R. J.: Combust. & Flame 62, 157 (1985). 4. HARalS, S. J. AND WEINEa, A. M.: Twentieth Symposium (International) on Combustion, p. 969, The Combustion Institute (1984). 5. HARRIS, S. J. AND WE1NER, A. M.: Ann. Rev. Phys. Chem. 36, 31 (1985). 6. DALZELL, W. M. AND SAROFIM, A. F.: J. Heat Transfer 91, 100 (1969). 7. GORDON, S. AND MCBRIDE, B. J.: NASA SP-273, (1970). 8. SMOOKE, M. D., SESHADRI, K. AND PURl, I. K.: Combust. & Flame 73, 45 (1988). 9. Tsujl, H.: Prog. Energy Comb. Sci. 8, 93 (1982). 10. SAITO, K., WILLIAMS, F. A. AND GORDON, A. S.: Combust. Sci. Tech. 51, 285 (1987). 11. AXELBAUM,R. L., LAW, C. K. AND FLOWER, W. L.: Twenty-Second Symposium (International) on Combustion, The Combustion Institute, (1988). 12. SMrrH, K. C., TJOSSEM, P. J. H., HAMINS, A. AND MILLER, J. H.: Combust. & Flame 79, 366 (1990). 13. Do, D. X., AXELBAUM,R. L. AND Law, C. K.: Twenty-Third Symposium (International) on Combustion, The Combustion Institute, (1990). 14. GAYDON,A. G. AND WOLFHARD, H. G.: FlamesTheir Structure, Radiation and Temperature, p. 196, John Wiley & Sons, New York (1979).

COMMENTS Mary Julia Wornat, Sandia National Laboratories, USA. The measurement of PAH levels by fluorescence at 514 nm after excitation at 488 mn is very misleading. Many PAH (and most of not very

high ring number) do not absorb appreciably at 488 nm, and if they do, they do not necessarily fluoresce at 514 Am. Even compounds that fluoresce at 514 nm do not have uniform response. The flu-

SOOTING STRUCTURE OF COUNTERFLOW o r e s c e n c e m e a s u r e m e n t at 514 n m is thus not indicative of total P A H p r e s e n c e or a m o u n t . I s u g g e s t a m e a n s of b r o a d b a n d ultraviolet absorption.

Author's Reply. T h a n k you for t h e c o m m e n t . W h i l e we were aware that o u t of several different P A H species, a c e n a p t h a l y e n e is t h e d o m i n a n t fluorescing species with a fluorescence m a x i m a at 510 n m for excitation at 488 nm,~ this m e a s u r e m e n t was a s s u m e d to be r e p r e s e n t a t i v e of t h e concentration distribution of all PAHs. In light of y o u r c o m m e n t , this a s s u m p t i o n , t h o u g h c o m m o n l y m a d e , is not q u i t e accurate. T h u s , o u r P A H data should not be v i e w e d as r e p r e s e n t a t i v e of all P A H species. However, it is i m p o r t a n t to point o u t that this does not alter t h e conclusions of t h e paper. T h a n k you also for y o u r suggestions for future m e a s u r e m e n t s .

REFERENCE 1. COE, D. S., HAYNES, B. S. AND STEINFELD, J. I.: C o m b u s t . F l a m e 43, 211 (1981).

E. Brizuela, ARL, Australia. W h y p r e h e a t a n d

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s i m u l t a n e o u s l y reduce oxygen? This makes it difficult to separate the reasons for soot increase.

Author's Reply. It is i n d e e d logical to simply preh e a t t h e reactants and hold all o t h e r conditions c o n s t a n t to s t u d y the t e m p e r a t u r e effect. In tZaet, this is h o w we initially p l a n n e d o u r experiments. H o w e v e r , experimentally we found that if we start with a sooty flame at 300 K (no preheat) then t h e 900 K a n d 1200 K p r e h e a t flames are too sooty for p r o b e m e a s u r e m e n t s , whereas, if we start with a less sooty 1200 K flame (to enable p r o b e m e a s u r e m e n t s ) t h e u n - p r e h e a t e d flame was blue and nonsooty. T h u s , oxygen concentration was varied to reduce the flame temperature of the preheated flames. It was also one of our objectives to s t u d y soot formation in t h e absence of soot oxidation by s e q u e n tially increasing t h e oxidizer side p r e h e a t and red u c i n g t h e oxygen concentration to zero. However, this limit could not be experimentally achieved. E x p e r i m e n t s with sequential p r e h e a t i n g while holding all o t h e r conditions c o n s t a n t w e r e also cond u c t e d . However, this data could not be included in t h e p a p e r d u e to length limitations. F u r t h e r more, t h e conclusions of this data are rather obvious. W e choose to include t h e data p r e s e n t e d in t h e p a p e r b e c a u s e the results were m o r e interesting. W e were puzzled by t h e question: w h y are t h e flames at lower flame t e m p e r a t u r e m o r e sooty? W e tried to a n s w e r this question in our paper.