Soot Surface Growth and Oxidation in Laminar Unsaturated-Hydrocarbon/Air Diffusion Flames

A02-14223

AIAA 2002-1116 Soot Surface Growth and Oxidation in Laminar Unsaturated-Hydrocarbon/Air Diffusion Flames

A.M. EI-LeathyG.Md an . u FaetX . F , h The University of Michigan, Ann Arbor, Michigan 48109-2140

40th AIAA Aerospace Sciences Meeting & Exhibit 14-17 January 2002 / Reno V ,N

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AIAA 2002-1116 SOOT SURFACE GROWTH AND OXIDATION IN LAMINAR UNSATURATED- HYDROCARBON/AIR DIFFUSION FLAMES A.M. El-Leathy*, F. Xu1 and G.M. Faeth** Universite Th Michiganf yo Arborn An , , Michigan 48109-2140

ABSTRACT OH mechanism wit a hcollisio n efficiencf o y 0.10, with no significant effect of fuel type, in The structure and soot surface growth and good agreement wit classicae hth l premixed flame oxidation propertie f rouno s d laminar coflowing measurements of Neoh et al. (1980). Finally, t diffusioje n flames were studied experimentally. direct rates of soot surface oxidation by O2 were Test conditions involved ethylene-, propylene-, small compare sooo dt t surface oxidatio, OH y nb propane benzene-acetylene-fueled an - d diffusion based on estimated soot surface oxidation rates by flames burning in coflowing air at atmospheric O2 due to Nagle and Strickland-Constable (1960), pressure wit reactante hth t normasa l temperature. because soot surface oxidatio s completewa n d Measurements were limited to the axes of the neae flamth r e sheet where maximum soot flame d includean s d soot concentrations, soot concentrations were generally smaller than 1% by temperatures, soot structure, majo s speciega r s volume for present test conditions. concentrations, radical (H, OH, O) species Nomenclature concentrations s velocitiesga d e resultan ,Th . s show that fuel decomposition yields significant mas carbof so n oxidize molr dpe e acetylene concentration t fuel-rica s h conditions, of species i reacted (kg kgmol") that soot formation begins where significant dp - mean primary soot particle concentration botf so h acetylen H-radicad ean e lar diameter (m) present, and that soot formation ends when fs = soot volume fraction (-) acetylene concen-trations become small even ni [i] - molar concentration 01 species i (kgmo) lm «-3\ the presenc f significaneo t concentration- H f o s k = ^o^tzmann constan moleculeJ ( t " radical. Present measurements of soot surface growth rate diffusion i s n flames were consistent a masmolecul f o s f specieo eg (k i s with earlier measurement n boti s h acetylene- molecule") fueled diffusion flames and in ethylene- and Ri = terms of the HACA soot surface methane-fueled premixed flames, with results growth rate formulas (kg m" s") over this entire data base exhibiting encouraging S = soot surface area per unit volume agreement with existing Hydrogen- Abstraction/Carbon-Addition (HACA) soot t tim) e(s surface growth mechanisms. It also was found T - temperatur) e(K that present surface oxidation rate diffusion i s n u = streamwise velocity (ms~) flames could be described reasonably well by the V;i = mean molecular velocity of species Urns'). . _ wg = soot surface growth rate (kg m" s") W soot surface oxidation rate (kg 'Research Scholar, Department of Aerospace Engineering. ox ) ms Research Associate, Departmen Aerospacf o t e z = streamwise distance (m) Engineering Assistanw ;no t Professor, Departmenf o t = empiricaa; l (steric) factore th n si Mechanical, Materials and Aerospace Engineering, Central HACA soot surface growth rate Florida University, Orlando, Florida. formulas A.B. Modine Professor, Departmen Aerospacf o t e = collisior|i n efficienc r soofo y t surface Engineering; Fellow, AIAA. oxidatio specief no si sood an t s densitiega p,p m"g = ss (k ) Copyright © by G.M. Faeth. Published by the American = fuel-equivalenc(j) e ratio (-) Institut Aeronauticf eo Astronauticsd san , Inc., with Subscripts permission. CH = HACA soot surface growth mechanis Colkemf o Halld an t 11 1 American Institute of Aeronautics and Astronautics (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

F W= HAC A soot surface growth mech- existing Hydrogen-Abstraction/Carbon-Addition anism of Frenklach and cowork- (HACA) soot surface growth mechanisms of ers8-10 Frenklach and coworkers8"10 and Colket and = burne o r exit condition Hall,11 finding that the HACA soot growth mechanisms yielded excellent correlatione th f so INTRODUCTION measurements soot surface growth rates for quite e presencTh f a commosooo e s i t n featurf o e reasonable values of the steric factors that appear nonpremixed (diffusion) flames fueled with theoriese inth . hydrocarbons, affecting their structurd an e Xu and Faeth7 continued study of soot surface reaction mechanisms. As a result, soot processes growth by considering diffusion flame affect capabilitie r computationasfo l combustion environments thamore ar t e relevan practicao t t l due to the complexities of soot chemistry, public soot formation processes in flames. These health due to emissions of particulate soot, fire experiments involved acetylene/air laminar safety due to increased fire growth rates caused by coflowin t diffusiogje n flames usin fule gth l suite soot radiation, and combustor durability due to of measurements developed during the earlier undesirable heat loads caused by continuum studies of premixed flames, e.g., Refs. 4-6. These radiation from soot. Motivated by these results showed that H concentrations ranged from observations, the present investigation sought to near-equilibrium conditions to super-equilibrium extend past work on soot surface growth in ratios of 20-30 in the soot surface growth region laminar premixed and diffusion flames in this of the diffusion flames, that soot surface growth laboratory, see Refs. 1-7, seeking to gain a better rates in premixed flames and diffusion flames understanding of effects of fuel type on flame satisfy similar reaction rate expressions, and that structure mechanismth d an e f sooo s t surface soot surface growt hdiffusioe rateth n i s n flames growt d oxidatioan h f laminao n r coflowint gje were well represente HACe th y dAb mechanisms diffusion flames t atmospheria c pressure wite hth of Refs. 8-11, with steric factors essentially reactant normat sa l temperature. unchanged from observations in premixed flames. 1 3 Sunderland and coworkers " experimentally Soot surface oxidatio premixen i diffusiod dan n studie e structurth d d sooan et surface growth flame environments must also be understood if propertie f laminao s r hydrocarbon-air diffu-sion reliable predictions of soot emissions from flame y makinb s g direct measurement f sooo s t combustio ne madeb n processeI o t . e ar s residence times, soot concentrations, soot addition, soot formation and oxidation generally structure s ga e temperaturesth , d an , procee e samth t eda tim botn ei h premixed dan concentrations of stable gas species. It was found diffusion flame environments so that soot surface that soot surface growth only occurred when oxidation rates must be estimated reliably in order acetylene was present and that growth rates were to obtain reliable estimates of soot surface growth roughly first-order with respect to acetylene rates.1"7 Potential soot oxidant flamen si s include concentrations. Existing detailed soot surface 8 11 O2? CC>2, H2O, O and OH and numerous growth models, " however, could not be simplified treatments can be used to estimate soot evaluated using these results because radical surface oxidation rates when e onlth y species concentrations important for these concentrations of the stable species, O2, CO2 and mechanisms wer measuredt eno . H2O known.e ar , 12"17 Recent work, howevers ha , X t al.ue 4"6 extended Sunderlan coworkersd dan 1"3 concentrate n moro d e fundamental mechanisms by considering soot surface growth in premixed of soot surface oxidation, finding that soot surface flames. Experimental methods were similar to oxidation by O2, CO2, H2O and O were relatively Sunderland and coworkers1"3 except that unimportan premixen i t d flame environ-mentt sbu could be described by reaction with OH, see Neoh concentrations of radical species (H, OH and O) 18 20 were determined both computationalld an y and coworkers. " Subsequent studies of soot surface oxidation in diffusion flames due to Garo experimentally. It was found that concen-trations 21-2z 2 Z4 25 of H closely approximated equilibrium et al., , Puri et al. ' and Haudiquert et al., requirements in the relatively slowly evolving however, did not find OH collision efficiencies in good agreement wit e findinghth f Neoo sd han soot growth regions of premixed flames. The 18 20 new measurements were then used to evaluate the coworkers " in premixed flames. One

explanatio r thesfo n e discrepancie s thai e s th t nitrogen reactant mixture flow for the premixed optical scatterin d extinctioan g n measurements diametem m 0 flame6 r A coannula. r outer port that were use infeo dt r soot structure properties was used for the air coflow of the diffusion durin diffusioe gth n flame studies were basen do e nitrogeth r fo nflame e d cofloth an s f o w models that hav t beeno e n very successfur fo l premixed calibration flame. The air coflow of the representin e opticath g l propertie f sooo sn i t diffusion flames serve o t eliminatd e natural flame environments Neoe se , t al.Koylhe d 1an 9 u convection instabilities whereas the nitrogen and Faeth.26 Supporting this e vieus wthas e i th t coflow of the premixed flame served to eliminate of similar methods during early studies of soot e annulath r diffusion flam r thifo es fuel-rich surface growt fuel-ricn hi h premixed flames also flam orden i e improvo t r accurace H eth e th f yo prove problematicale b o dt discussioe th e se , f no concentration calibration. this point in Xu et al.4'5 and references cited therein. Whateve e sourcprob-leme th th r f o e , Burner gas flow rates were measured with however, these differences between observations rotameters. Mixtures were allowed to mix within of soot surface oxidation rates in premixed and feed lines that were at least 1000 diameters long diffusion flames obviously mus resolvede tb . to ensure that they were uniform. The flames burne n rooi dr witmai h room disturbances Based on this status, the present investigation controlled by surrounding them with several sought to better understand the structure and soot layers of screens and a plastic enclosure. The surface growth properties of laminar jet diffusion burner coul movee db d horizontall verticalld yan y flames fueled by hydrocarbons other than with linear positioners in order to accom-modate acetylene that wer t directleno y involvee th n di rigidly-mounted optical instrumentation. HACA soot surface growth mechanism, e.g., ethylene, propylene, propane and benzene. In Instrumentation particular, benzene was considered in order to Present measurements were similar to Xu and stud behavioe yth fuelf ro s more closely relateo dt Faeth7 and included the following properties Polycyclic Aromatic Hydro-carbons (PAH) than measured along the axes of the flames: soot e otheth r fuels that were studied becausH PA e volume fractions, flame tem-peratures, soot surface growth mechanisms have been concentration f majoo s r stabl s speciesga e , soot considered to be a potential alternative to HACA 27 structure s velocitiega , d concentrationan s f o s soot surface growth mechanisms Howarde se , some radical species (H, OH and O). and Richte d Howard.an re presenTh 28 t measurements also provided information about Soot volume fractions were measurey b d soot surface oxidation rates whic s useho wa dt deconvoluting laser extinction measure-ments at gain a better understanding of soot surface 632.8 nm for chord-like paths through the flames. oxidatio flamn ni e environment improvo t d san e Data was reduced using the refractive indices of correction measuref so d soot surface growth rates Dalzel d Sarofiman l , simila o past r1 1t work" ; 1 H for effects of soot surface oxidation compared to these values have recently been confirmey b d earlier wor n thii k s laboratory, e.g., Refs. 4-7. Krishnan et al.32 The experimental uncer-tainties The following description of the research is (95% confidence) of soot volume fractions are relatively brief, XuEl-Leathyd 2an 9 30 shoule db estimate smallee b o dt r than 10 soor %fo t volume consulte r morfo d e detail d completan s e fractions greater than 0.02 ppm, increasing tabulations of data. inversely proportiona sooe th t o volumt l e fraction for smaller values. EXPERIMENTAL METHODS Soot and gas temperatures are essentially the 7 Experimental methods were unchanged from Xu same; thus soot (gas) temperatures were and Faeth,7 wit e samth h e burner arrangement measured by deconvoluting spectral radia-tion used to observe soot processes in laminar intensitie chord-likr sfo e paths throug flamee hth s diffusion flames and to calibrate the H and computing temperatures at several concentration measurements using a laminar wavelength pairs (550/700, 550/750, 550/830, premixed flame. The burner had a 34.8 mm 600/700, 600/750, 600/83 d 650/75an 0 0 nm). diameter inner port for the fuel stream flow of the Temperature differences between the average and diffusio e methane-oxygenth nr fo flame d an s - any of the line pairs were less than 50-100 K and

experimental uncertainties (95% confidence) of data base from the present diffusion flames was these measurements were less. thaK 0 n5 extende o includdt e earlieth e r acetylene-fueled diffusion flame results of Xu and Faeth7 in order Concentrations of major gas species (N2, H2O, to more completely assess effects of fuel type on H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, diffusion flame structur sood ean t surface growth CsHg, C6H6 and neon, (the last being a tracer gas and oxidation rate properties. Test conditionr sfo use o estimatt d e effect f radiao s l diffusiof o n e diffusioalth l n flames considered durine th g lithium-containing species that were use fino dt d present study are summarized in Table 1. The H concentrations) were measure sampliny b d g diffusion flames considered included acetylene- and analysis by gas chromatography. nitrogen, ethylene, propylene-nitrogen, propane Experimental uncertain-ties (95% confidence) of and benzene-acetylene-nitrogen fuel streams these measure-ment mainl e calibratioo st ar e ydu n burning in coflowing air at atmospheric pressure uncertaintie estimatee ar d lese b san s o dthat % n5 e witreactantth h t a normas l temperature. speciel foal r s concentrations reported here. Nitrogen dilutio fuee th l f nstreao s user mwa d fo Soot structure measurements were limiteo t d some of the flames in order to keep maximum primary soot particle diameters carried out by soot volume fractions smaller than 2 ppm so that thermophoretic samplin d analysian g s using problems of large soot concentrations on the Transmission Electron Microscopy (TEM), measurements coul avoidede db . Stoi-chiometric following Dobbins and Megaridis, similar to flame lengths (the vertical heighpositioe th f o t n earlier work in this laboratory.26 Primary particle along the axis where the local fuel-equivalence diameters were nearly monodisperse at given s ratistoichiometricwa o ) were 54-13, mm 0 positions in each flame (standard deviations were whereas luminous flame lengths (du yelloo t e w less tha nexperimentae 10%th d )an l uncertainties light emitted from heated soot particles) were 80- (95% confidence) of soot primary particle 103 mm. All the flames were well attached, diameter measurements are estimated to be less stationary and laminar over the region where than 10%. measurements were made. Streamwise gas velocities were measured using RESULTS AND DISCUSSION laser velocimetry (LV) with experi-mental Soot Structure uncertainties (95% confidence) less than 5%. photograpM ATE f sooo h t particles alone gth Measurement concentrationH f o s s were carried ethylene/aie axith f so r flame (Flam , obtainee4) d ouy deconvoluteb t d absorption followine th g near the end of the soot formation region where Li/LiOH atomic absorption techniqu f Neoo e h 18 20 the soot volume fraction is a maximum, is and coworkers. " Correctio e radiath r lfo n illustrated in Fig. 1. Soot particle images at other diffusio f o LiOn H s seefounwa d d from locations and for other flames are qualitatively the measurements of the concentrations of neon seed, same and were generally similar to TEM images assuming that the diffusivities of LiOH and neon of soot particles obtained in other premixed and were similarconcentratioH e Th . n measurements diffusion flames that have been studied, see Refs. were calibrated using a premixed flame as 1-7 and references cited therein. In general, soot discussed by Xu and Faeth6 and similar to Neoh 18 20 particles consis roughlf o t y spherical primary soot and coworkers. " Measurements with different particles that have nearly constant diameters at a seeding levels showed that effects of the LiOH particular flame condition, collected into seed on flame properties were negligible, similar 6 aggregates that are known to be mass fractal to past work. ' Experimental uncertainties (95% objects.26'32 The mean number of primary soot confi-dence) of the H concentration particles per aggregate increased with increasing measurements are estimated to be smaller than distance from the burner exit, similar to the 30%. measurement thif so s propert Sunderlano t e ydu d 1 Test Conditions et al. The properties of the laminar premixed flame Flame Structure used to calibrate the H concentration 6 Measurement s (sootga f )o s temperatures, measurements are reported by Xu and Faeth. The streamwise gas velocities, soot volume fractions,

primary soot particle diameters, concentrationf so soot particles forme de soo lateth t n i formatior n majo s speciega r concentrationd an s f radicao s l region tha smallee shorteo t ar t e du rr periodf o s species (H, OH and O) are plotted as a function of soot surface growth. This behavior is aided by the distance alon e flamth g n ea r axifo s the fact that rates of soot surface growth remain acetylene/air flame (Flame 1), the ethylene/air relatively rapid near soot inception conditions, as flame (Flame 4) and a benzene-acetylene/air note Tesner,y db 34'35 which allow relativele sth y flame (Flame 8) in Figs. 2-4. Corresponding few primary particles present ther groo et w quite residence times, foun integratiny db velocite gth y large. measurementse th f o p to e indicate ar e , th t a d plots residence Th . e time relative sfirsar e th t o et Significant level soof so t formation (evidencey db position where detectable soot volume fractions the appearance of measurable soot were threth er e observe fo 10-2= z m t 0m (a d concentrations), were generally asso-ciated with flames). The stoichiometric ((|)=1), or flame sheet the first streamwise locations where detectable condition for the three flames occurs at z = 54, 81 concentration f boto s h acetylen werH d e an e and 130 mm, with the last lying beyond the region observed, similar to the observations of Figs. 2-4. tha s illustratei t n Figi d. 4 .Luminou s flame Soot formation (as indicated by soot surface lengths coincide wit righe hth t hand boundarief so growth) became small again where maximum l e threfigureal th r e fo sflames , although soot soot volume fractions were reached; which concentrations near these boundaries wero to e occurree presencth n i f drelativelo e y large small to be accurately measured and reported concentrations of H but where acetylene using present methods. concentrations became smaller than roughly 1% by volume. This behavior was also similar to Gas (soot) temperatures along the flame axes in other soot-containing lamina t diffusiorje n flames Figs. 2-4 reach a maximum well before the that have been studied in this laboratory, see Refs. maximum soot concentration and flame sheet 1-3 and 7. An interesting feature of conditions condition reachede ar s . This behavio s similai r r where soot formation again became small in to other soot-containing lamina t diffusioje r n flames involving benzene as a portion of the fuel, flames that have been studied in this laborator s thay wa e tFig , se thi4 . s even s definitelwa t y and is caused by continuum radiation heat losses correlated with concentrations of acetylene from soot.1"3'7 becoming small, whereas concentrations of benzene remained relatively constan n thii t s Gas velocities alon flame gth e axe Figsn i s4 2- . region. increase from burner exit values smaller than 0.1 m/ valueo t s highese excesn i sth m/ 2 t f a so s t The concentrations of major stable gas species in positions that were measured. These velocity Figs. 2-4 are similar to concentrations of major increases were caused by effects of buoyancy due stabl species ega othen si r laminar flames burning relativele toth y small flame densitie burned san r in air, involving acetylene and other hydrocarbons exit velocities of the test flames. s fuels a Refse se , originae . th 1-3 f I .t l no fue s i l acetylene, decompositio e originath f o nl fuel Similar to earlier observations of soot-containing yields significant concentration f acetyleno s e laminar jet diffusion flames in this laboratory, relatively near the burner exit. As a result, Refs. 1-3, and 7, primary soot particle diameters concentration f acetyleno s e within flames fueled reach maximum values relatively early in the soot by hydrocarbons other than acetylene are formation region. This behavior is caused by generally quite simila acetyleno rt e concentrations accelerating primary soot particle nucleation rates within acety-lene/air flames, compare Figs. 2-4. with increasing streamwise distance in the initial Thus, concentrations of the hydrocarbon fuels portions of the soot formation region, which is decreas d C>an 2e concentrations increase with cause y b progressiveld y increasinH g increasing distance withi e sooth n t formation concentrations with increasing streamwise region, whereas intermediate hydrocarbon— s distance, see Xu and Faeth.7 This behavior causes CH4, C2H2 (when not the fuel), C2HU (when not the relatively few primary soot particles formed the fuel) and C2H6 — reach a maximum near soot inception conditions, that become large somewhat befor e locatioth e n where soot due to long periods of soot surface growth, to be concentrations reach a maximum. The final supersede muce th hy d b large r numbe primarf ro y combustion products, CC>d HbOan 2 , increase

throughou e sooth t t formation region, reaching e locas velocityath tga l e sooth ; t densits i y broad maxima near the flame sheet (<|)=1). constant; and the surface area available for soot Intermediate combustion products, associated surface growth and oxidation is equivalent to with water-gas equilibrium, e.g., CO and H2, are constant diameter spherical primary soot particles present in relatively large concentrations that mee pointa t Refse a t Se . . justificatio1-r 7fo n throughou e sooth t t formation region, reaching of these assumptions. broad maxima somewhat upstrea e flamth f meo sheet. Finally, nitrogen concentrations remain The first soot formation property of importance is relatively uniform for the flame regions illustrated the number of primary particles per unit volume, in Figs. 2-4. found fro e measuremth d soot volume fractions and primary soot particle diameters, as follows:1 Concentrations of H, OH and O all increase monotonically as the flame sheet is approached. np = (1) Near-equilibrium concentra-tions were observed near the onset of soot formation, but H and OH The experimental uncertainties (95% confidence) concentrations eventually become 10-20 times of np are estimated to be less than 32% for fs > 0.1 larger than equilibrium concentrations neae th r ppm, increasing inversely proportionar o fo ft sl flame sheet. Concentration exhibiO f o s t even smaller value e soo Th f ftso . surface arer ape larger superequilibrium ratios; neverthelessO , unit volum same gives ei th ey n b measurements , concentrations generall e 100-100ar y 0 times as follows:1 smalle concentrationH O rd thaan nH s throughout most of the region where soot is present. S - 6fs/dp (2) The experimental uncertainties (95% confidence) The region where soot is present in Figs. 2-4 estimatee ar o fS lesfr e s b >fo s o dt 0.tha % 1n16 contains significant concentrations of species ppm, increasing inversely proportionar o fo ft l potentially responsibl r soofo e t oxidation, e.g., s 12 20 smaller value f fo s. Definin e sooth g t surface O2, CO2 ,OH. d H2an O O particular,n 'I , Os 2i growt rate h f increasth rateo s ea f sooeo t mass invariably present at concentrations on the order per unit surfac e soo th ared timetd an aan , of 0.1% (by volume) throughout most of the soot surface oxidatio ratne f decreasth rateo s ea f eo surface growth regio f laminao n r coflowint gje 1 soot mass per unit surface area and time, diffusion flames. "' Similar concentrations of O2 conservatio f sooo n t mass alon a gstreamlin e e alsar o observe e fuel-ricth n i d h regionf o s 36 under the previous assumptions gives the soot opposed-jet diffusion flames; therefore, this surface growt oxidatiod han n rates follows:s a , 1 effect does not appear to be due to O2 leakage throug e bas th f hcoflowino e t diffusioje g n w = -w = (p/S)d(p f /p)/dt flames. Thus, soot formatio d oxidatioan n n g ox s s (3) proceed at the same time, with soot formation where present measurements of species dominatin maximue procesth e gth o t p msu soot concentrations and temperatures yield the gas volume fraction conditio d sooan nt oxidation density, assumin n idea ga s mixture th ga l d an e dominating the process thereafter. minus sig s inserteni thao s d t positiva w s oi x e number temporae Th . s l wa derivativ3 . Eq n i e Soot Surface Reaction Rate Properties found from three-point least-squaree sth fitf o s argument of the derivative, similar to past work. d Paspresenan t t measurement f o laminas r The soot density in Eq. 3 was taken to be equal to premixed and diffusion flames were used to study 1850 kg/m3, similar to past work.1"7 Finally, soot surface growt d oxidatioan h n similao t r consideratio f o soon t surface oxidatios wa n earlier studies base laminan do r flames.1"7 Major limite o t earld y soot oxidation (soot mass assumptions were identical to the earlier work, as consumption less than 70%) where problemf o s follows: soot surface growth, rather than soot soot aggregate breakup, the development of nucleation, dominates soot mass production; soot primary soot particle porosity d internaan , l surface oxidation dominates soot oxidation; oxidation of primary soot particles, do not yet effect f o diffusios n (Brownian motiond an ) occur, see Neoh et al.20 Estimated experimental thermophoresi soon so t motio smalle nar thao s , t uncertainties (95% confidence) of soot surface soot particles convect alon flamee axee th g th f so s 6 American Institute of Aeronautics and Astronautics (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

growth ratee lesar ss than 30%; estimated net soot surface growth rates, considering effects experimental uncertainties of soot surface of the OH and 02 soot surface oxidation oxidation rate comparablee sar . mechanisms t sooe resultne th ,tr fo surfacs e growth rates considering the p2> CO2 and H2O Soot Surface Growth Rates soot surface oxidation mechanism, were limited o conditiont s where estimated soot oxidation Gross soot surface growte th l h al rater fo s rates never exceeded half the gross soot surface premixe diffusiod dan n flames considered during growt ha resul s ratef thiA o t s. conservative the present investigation were correcter fo d approach t soone te surfacth , e growth rates found effect f sooo s t surface oxidation because soot using the O2 and OH approach were essentially surface growth and oxidation proceed at the same e samth thoss ea d an e 2 founCO d, usin02 e gth time, as noted in connection with Figs. 2-4. The H2O approach. correction for soot surface oxidation, however, was not done in the same way as past work, e.g., Soot surface growth rates were interpreted using Refs. 1-7; instead, present findings concerning HACe th A soot growth mechanism Frenklacf so h soot surface oxidatio soot-containinn ni g flames coworkers,d an Colked an Hall8 d "° an t orde1n i 1 r were use estimato dt e soot surface oxidatior nfo to maintain consistency with past evaluations of all the flames. The details of the new results these mechanisms based on similar measurements concerning soot surface oxidation wile b l and due to the success of these approaches for presented subsequently maie th , n feature thesf so e correlating measurements of soot surface growth resultfollowss a e sar : soot surface oxidatios nwa n premixei d ethylene/ai d methane/oxygean r n dominated by the OH oxidation mechanism of flames4'5 and in acetylene/air diffusion flames.7 Neoh et al. 9 with nearly the same collisio l ncasesal t soon I ne , t surface growth rates were efficiency, after allowing for direct soot surface expressed, as follows: oxidatio C>y nb 2 mechanise baseth n do mNaglf o e 12 and Strickland-Constable (which was later w = (4) confirmed by Park and Appleton13). It was found that effect correctef o s d soot surface growth rates where i = FW or CH denotes reaction parameters in this way were small, except when soot surface e HACth for A soot surface growth rate growth rates themselves became small toward the mechanisms of Frenklach and coworkers,8"10 and end of the soot formation region. Thus, in order Colket and Hall11 which were found from the to be conservative about potential effects of soot measurements. The details of these mechanisms, surface oxidation, determinatio f soono t surface the formulas for the RI, and the reaction-rate growth rates, corrected for effects of soot surface t al.e 4u e founparameterb X n n di Re ca Ith r fo s oxidation, were limite o t conditiond s where e parametersTh , empiricae ocar i l steric factorn o s estimated soot surface oxidation rates never order of unity, with (XFW specified to be a function exceeded hal grose fth s soot growth rates, similar of temperature,8"10 and OCCH specified to be a to past work.11- n7 constant.11 Present measurements of soot surface growt hfirsa s t A approximatio premixee th l al d r dan nfo rates also were baselined with earlier resultn i s diffusion flames considered during this study, the laminar premixe d diffusioan d n flamey b s Ri are proportional to the product [H][C2H2]. correcting gross soot surface growth rates for Thus, values of wg/[C2H2] measured for all the effect f sooo s t surface oxidatio e samth en i n studies were plotted as a function of [H] to manne s earliea r r studie f sooo s t surface growth provid direcea maie t th tes nf o tfeature e th f o s in this laboratory.1"7 In this case, rates of soot HACA soot surface growth mechanisms without surface oxidation were estimate followss da : soot the intrusion of uncertainties due to the numerous surface oxidation by O and H was ignored, soot empirical parameters of the detailed HACA surface oxidatio resulte bases 6th y 2nb wa n do s mechanisms of Refs. 8-11. A plot of this type, of Nagle and Strickland-Constable,12 and soot includin l al soog t surface growth rate surface oxidation by HiO and HaO were measurements except the present measurements estimated following Libby and Blake14'15 and involving diffusion flames fueled with Johnstone et al.,16 which gave results similar to hydrocarbons other than acetylene provides i , n di Bradle t al.ye 17 Simila preseno t r t estimatef o s

Xu and Faeth.7 Adding present measurements and alone, there clearly is no difference between changin correctioe gth soor nfo t surface oxidation estimate wf so g HACe baseth n dAo mechanisms does not materially affect the correlation between as the hydrocarbon fuel type is varied. This wg and the product [CalHytH] and supports a includes flames with and without benzene first-order relationship between wg and the present, even though benzen s mori e e closely concentrations of acetylene and H. Notably, this tha H e relateotheth nPA ro t dfuel s that were behavior is also completely consistent with considered and might be thought to potentially present observations that significant soot enhance channel f sooo s t surface growte th y hb formation (growth s i firs) t observee th t a d PAH mechanism compared to the HACA location where significant concentration f botso h mechanism, see Howard.27 The behavior using acetylene and H are observed, that soot surface the HACA mechanism of Frenklach and growth ends in the presence of significant coworkers e El-Leathy,8'1se 0 s essentiall3i 0 e yth concentration wheH f o sn acetylene disappears, same. Naturally, these finding e alsar so f sooo and t dformatioen thae th t s observei n d consistent wite onse th hf sooo t t formation when acetylene disappears even though concen- occurring when [H] first appears in the presence trations of benzene are still significant (see Fig. of significant acetylene concentratione th n o s 4). burne e soor th sid tf eo formatio n e regioth d nan f sooo td formatioen n occurring when acetylene A more direct evaluatio e HACth f o An subsequently disappears in the presence of mechanism soof so t surface growt obtaines hi y db significan benzend an H t e concentratione th n so plotting wg directl a functio s a y f W ocpo n RF w flame shee soote sidth tf eformatioo n region. (after correlating OCFW a afunctios f o n temperature) for the Frenklach and coworkers8"10 Similar to earlier findings for soot surface growth mechanism a functio s a e f d RCth o n an r ,H fo rates,4'5'7 the HACA soot surface growth rate Colke Halld an t 11 mechanism. These resultr sfo mechanisms of Frenklach and coworkers8"10 and the Frenklach and coworkers n 81 " mechanism are Colket and Hall11 continue to be encouraging and discussed by El-Leathy;30 the corresponding eventuallthey yma y provid basie e th f reliabl so e results for the Colket and Hall11 mechanism are methods to estimate soot surface growth rates in illustrated in Fig. 5. In this figure, available flame environments fueled with hydrocarbons. measurement r botfo s h premixe d diffusioan d n Uncertainties remain, however, about effects of flames (after correctin l measurementgal soor sfo t pressure, about behavio t highea r r temperatures surface oxidatio e n presenth base n o td than considered thus far, and about effects of measurements) are illustrated along with best-fi fuelss a soon tH o , t PA surface growth rates. correlation for all the flames. The corresponding steri cs experimenta it facto d an r l uncertainties Soot Surface Oxidation Rates (95% confidence wit0 1. uncertaintn h a s i ) f yo 0.2. First of all, it is encouraging that the steric Present measurements of soot surface oxidation f o ordefacto f o magnituds i r r e unitys a , rates were correcte r effectfo d f sooo s t surface expected. Colkee th Hall d f o an t1 11e 1Nextus e th , growth based on the Colket and Hall11 soot HACA mechanism improves the correlation of surface growth mechanism, correlate s jusa d t soot surface growth rates solely in terms of the described. No condition is considered in the approximate correlation based on only the leading following, however, where the correction for soot HACterme th f so A mechanisms, appearinu X n gi surface growt mors hwa e than hal grose fth s soot and Faeth.7 The correlation for the diffusion surface oxidation rate. flamee th l s flameessentiallal i s d an s samee yth . Correlated values of soot surface growth rates for Similar to Neoh et al.,19 present soot surface the premixed flames are slightly lower than for oxidation rates (correcte soor dfo t surface growth) the diffusion flames (possibly because estimates were converted into collision efficiencies based of H concentrations are based on the assumption on kinetic theory estimates of the collision rates of local equilibrium of H, which is somewhat of a given gas species with the surface of primary larger tha measurementse nth , were e th use r dfo soot particles. Thus, the collision efficiency for a premixed flames) but the differences are potential oxidizing species is given by the comparabl preseno et t experimental uncertainties. following expression:2 Considering the results for the diffusion flames 8 American Institut Aeronauticf eo Astronauticd san s (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

ii = 4w /(Ci[i] (5) that allowin direcr gfo t oxidatio Oy nb generall y ox 2 has a small effect on the collision efficiencies in where Q is the mass of carbon removed from the diffusion flames illustrated in Fig. 7. In addition, surface per mole of species i reacting at the ther s significani e t scatter (roughl a yfacto f o r s phasga e th surfaceconcentratio s i ] [i , i f o n e collisioth 100 f o ) n efficiencie n diffusioi s n adjacen surfacee th o d t t an , flames and a comparable scatter (roughly a factor

o fe collisio th 100 f o ) n efficiencie f Neoo s t he

9 j = (8kT/(7cmi)) 1/2 (6) al. in1 premixed flames. These findings, clearly t suppordono t Hmajoa 2s Oa r direct contributor is the (Boltzmann) equilibrium mean molecular to soot surface oxidatio flamesn i , either alonr eo velocit specief yo . sValuei r|f sio were measured in parallel with soot oxidatio y b On2 . for potential soot surface oxidation for i = O2, Consideratio f soono t surface oxidatio COy nb 2 . COOH 2 , d H2an OO , and O yielded similar conclusions, see El- Leathy.30 The collision efficiencies of O2 for soot surface oxidatio plottee n ar functio a s da heighf no t above Finally e collisioth , n efficiencie r soofo t H O f so the burne Fign ri . .Result6 s showfigure th n eno surface oxidation are plotted as a function of include the range of values observed by Neoh et height abov e burnee samth eth Fign en i i r , .8 19 al. in premixed flames, the values measured by manner as the results for H2O in Fig. 7. With Xu and Faeth7 and the present investigation in perhaps one exception (at an extreme condition diffusion flames, and values estimated from th1e0 where experimental uncertaintie e relativelar s y prediction f Naglo s d Strickland-Constablan e e large), direct O2 surface oxidatiot f soono no s i t for conditions in the diffusion flames considered very important for these conditions, as before. On presene Faethd th an d u an bt7 yX investigation . the other hand, similar to the observations of The Nagle and Strickland-Constable12 approach Neo t al.,he 19 collisioe nth efficiencier fo H O r sfo has exhib-ited effective capabilities to predict diffusion flames exhibit relatively small degrees soot surface oxidation by O2, see Park and of scatten i e r ar (roughl d an ) a yfacto3 f o r Appleton13 and references cited therein, and there excellent agreement with the results of Neoh et 19 are significant concentration soor f Oso 2fo t paths al. for premixed flames. In particular, the mean along the axes of the diffusion flames, see Figs. 2- collision efficiency of OH for soot surface 4. Thus, the fact that the Nagle and Strickland- oxidation in the diffusion flames is 0.10 with a 12 Constable estimatee Oth 2 f o collisios n standard deviatio f 0.0no 7 whic excellenn i s hi t efficiency are 10-100 times smaller than the agreement wit value hth f 0.1eo 3 foun Neoy db h measurements in diffusion flames, strongly et al.r soo1fo 9 t surface oxidatio n premixei n d suggests that some other species is mainly flames using the same treatment of soot structure responsibl r soofo e t surface oxidatioe th n i n as the present investigation. Finally, this diffusion flames. Other evidence that t no O s i 2 agreemen s achievewa t d ove relativela r y broad the main direct oxidizing species for flame range of flame conditions for the combined environment s providei se largth ey b dscatte r results in premixed and diffusion flames as (nearl a yfacto f o e 100rcollisioth f )o n follows: temperature f 1570-187o s , oxygeK 0 n efficiencie e diffusioth r fo s n flames, combined mole fractions of IxlO"5 - 1.2xlO"2, and levels of wit evee hth n larger scatter (more tha nfactoa f ro soot mass consumption less than 70% at collisioe 100th f o ) n efficiencie f Neoo s t al.he 19 atmospheric pressure for flames fueled with a for premixed flames. variet f fuelso y . Although these resulte ar s helpful, however, the properties of the final stage The collision efficiencies of H2O for soot surface of oxidation, where internal oxidation of primary oxidatio plottee n ar functio a s da heighf no t above soot particles becomes a factor, effects of pressure the burne Fign ri . .Result7 s showfigure th n eno on soot oxidation, and possibly effects of fuel include the range of values reported by Neoh et type on soot oxidation for hydrocarbons other al.19 for premixed flames, and values from the than those considered here l merial , t additional diffusion flames both considerin d ignorinan g g futurestude th n yi . the contributio f oxidatioo n y Ob n2 (the latter estimated usin e Nagl th d gStrickland an e - Constable12 correlation). Firs allf evidens i o t t ,i t

CONCLUSIONS foun y Neob d t al.e hr measurement fo 19 n i s premixed flame t a atmospheris c pressurt a e Flame structure and soot surface growth and similar O2 concentrations. The correction of oxidation rates were studied for coflowing present soot surface oxidation rates for oxidation lamina t je diffusior n flames. Present by O2 based on the results of Nagle and experimental measurements were supplemented Strickland-Constable12 was small (on average less earliee byth Faetrd findingthaan o e s hu tth X f so than 10%) compared to soot surface oxidation by complete data base involved acetylene, ethylene presenr fo , H tO conditions. propylene, propan d benzene-acetylene/aian e r flames burning at atmospheric pressure with the 4. Current findings concerning soot surface reactants at normal temperature as summarized in oxidation rates in diffusion flames were used to Table 1. This information about soot surface correct gross measurements of soot surface growth rates in diffusion flames was also growth rate r simultaneoufo s s effect f sooo s t supplemented by earlier measurements in surface oxidation. This approach involved premixed ethylene/air and methane/oxygen estimating soot surface oxidation rates by O2 t al.e 4u '5 X whic o flamet e h du alss o involved e resultbaseth f n Naglo o ds d Stricklandan e - flames at atmospheric pressure with the reactant ss proposea ConstableH O Neoy db y b t he d 1an 2 at normal temperature. The major conclusions of al. usint 1 9presenbu e gth collisioH O t n efficiency. the study are as follows: Corrected soot surface growth rates were only considered in cases where estimated soot surface 1. In all flames that were considered, the original oxidation rates were less than halgrose th f s soot fuel decomposes relatively early durin e soogth t surface growt l hflame al rate r fo ss considered formation process, yielding species generally here approace same , th th whic s ea s hwa used associated with soot surface growth and during earlier studies when other mechanisms of oxidation, , amone.g.OH ,d g C2Ha an others H , . soot surface oxidation were considered.4'5'7 Due The yields of these species are affected by flame to this relatively conservative approach for type (premixe r diffusiodo n flame), fuel species correcting soot surface growth rate effectr sfo f o s and flame operating conditions (e.g., nitrogen soot surface oxidation, however, present concentratio e fueth l n i nstream) , however, estimates of net soot surface growth rates were subsequent reaction of these species during essentially unchanged from earlier results, e.g., processes of soot surface growth and oxidation Refs. 4,5 and 7. are largely dependent on local flame conditions and are not materially affected by either the fuel It shoul recallee db resulte d th tha l sal t considered type or whether these processes are occurring in here involved flames at atmos-pheric pressure, premixed or diffusion flame environments. fueled wit a hlimite d numbe f hydrocarbonso r , and with the reactants initially at normal 2. Soot surface growth rates in laminar diffusion temperature; application thesf so e result otheo st r and premixed flames, for various fuel types in flame conditions should be approached with both types of flames, agree within experimental caution. uncertainties at comparable local conditions and coul correlatee db d reasonabl yHACe welth y Alb ACKNOWLEDGMENTS soot surface growth mechanism Frenklacf so d han coworkers Colked 8an "Hall,d 1 an t 11 with steric This researc s sponsorewa h NASy db A Grants factors in both these mechanisms having values NCC3-661, NAG3-1878, NAG3-2048 and on the order of unity, as expected. NAG3-2404 under the technical management of D. L. Urban and Z.-G. Yuan of the NASA Glenn 3. Soot surface oxidatio ne lamina rateth n i s r Research Center. diffusion flames, for various fuel types, could be correlate y assuminb d a gconstan t collision REFERENCES r soofo efficienc t H surfacO r fo ye oxidatiof no 0.10 with a standard deviation of 0.07. This ^underland, P.B., Koylii, U.O. Faethd an , , G.M., finding is in good agreement with the OH "Soot Formation in Weakly Buoyant Acetylene- collision efficienc r soofo y t surface oxidatiof no Fueled Laminar Jet Diffusion Flames Burning in 0.13 for assumed similar soot structure properties 10 American Institut Aeronauticf eo Astronauticd san s (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

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"Soot Oxidation in Flames," Paniculate Carbon Frenklach9 , M.d Wangan , , H., Soot Formationn i (D.C. Siegla and B.W. Smith, ed.), Plenum Press, Combustion (H. Bockhorn, ed.), Springer-Verlag York,w Ne , 1980 . 261-277,pp . Berlin, 1994, pp. 165-192. 20Neoh, K.G., Howard, J.B.Sarofimd an , , A.F., 10Kazakov, A.,Wang, H., and Frenklach, M., "Effec f Oxidatioo t Physicae th n no l Structurf eo "Detailed Modelin f o Soog t Formation i n Soot," Proceedings of the Combustion Institute, Laminar Premixed Ethylene Flame Pressura t sa e Vol.20, 1984 . 951-957pp , . Bar,0 1 f "o Combustion Flame,d an Vol. 100, Nos. 1/2, 1995 . 111-120,pp . 21Garo, A., Lahaye, J., and Prado, G., "Mechanisms of Formation and Destruction of HColket, M.B. Halld an , , R.J., "Successed an s Soot Particles in a Laminar Methane-Air Uncertainties in Modelling Soot Formation in Diffusion Flame," Proceedings of the Combustion Laminar Premixed Flames," Soot Formationn i Institute, Vol. 21, 1986, pp. 1023-1031.

22Garo, A., Prado, G., and Lahaye, J., "Chemical 32Krishnan, S.S., Lin, K.-C., Wu, J.-S. Faethd an , , Aspects of Soot Particles Oxidation in a Laminar G.M., "Optical Properties in the Visible of Methane-Air Diffusion Flame," Combustiond an Overfire Soot in Large Buoyant Turbulent Flame, Vol. 79, Nos. 3 and 4, 1990, pp. 226-233. Diffusion Flames," Journal f Heato Transfer, Vol. 122, No. 3, 2000, pp. 517-524. 23Puri, R., Santoro, R.J., and Smyth, K.C., "The Oxidation of Soot and Carbon Monoxide in 33Dobbins, R.A., and Megaridis, C.M. Hydrocarbon Diffusion Flames," Combustion "Morphology of Flame-Generated Soot as and Flame, Vol. 97, No. 2, 1994, pp. 125-144 Determine y b Thermophoretid c Sampling," (1994); also, Ibid., Erratum, Combustiond an Langmuir, Vol. , 1987 2 . 254-259 . pp 3, ,No . Flame, Vol. 102, No. 1/2, 1995, pp. 226-228. 34Tesner, P.A., "Formation of Dispersed Carbon 24Puri, R., Santoro, R.J., and Smyth, K.C., "The by Thermal Decompositio f Hydrocarbons,o n " Oxidatio f Sooo d nCarbo an t n Monoxidn i e Proceedings of the Combustion Institute, Vol. 7, Hydrocarbon Diffusion Flames," Combustiond an 1958 . 546-556pp , . Flame, Vol. 102, Nos. 1/2, 1995 226-228. pp , . 35Tesner, P.A., "Dispersed Carbon Formation by 25Hardiquert, M., Cessou, A., Stepowski, D., and Acetylene Self-Combustion," Proceedingse th f o Coppalle d Sooan t , H ConcentratioA."O , n Combustion Institute, Vol. 8, 1960, pp. 627-633. Measurement a High-Temperatur n i s e Laminar Diffusion Flame," Combustion and Flame, Vol. 36Lin, K.-C., and Faeth, G.M. "Structure of 111, No. 4, 1997, pp. 338-349. Laminar Permanently-Blue Opposed-Jet Ethylene-Fueled Diffusion Flames," Combustion 26K6ylti, U.O., and Faeth, G.M., "Structure of and Flame, Vol. 115, 1998,pp.468-480. Overfire Soo n Buoyani t t Turbulent Diffusion Flame t Lona s g Residence Times," Combustion Table 1. Summary of Laminar Jet Diffusion and Flame, , 1992 2 . 140-156 . pp , No Vol, 89 .. Flamesa

27 C C C Howard, J.B., "Carbon Additio Oxidatiod nan n FLAME HC HC C2H2 N2 Ls Lv Reaction n i Heterogeneous s Combustiod an n TYPE (%) (%) (%) (mm) (mm) Soot Formation," Proceedings of the Combustion lb — — 16.9 83.1 54 82 Institute, Vol. 23, 1990, pp. 1107-1127. 2b — — 15.1 84.9 55 80 28 Richter, H. and Howard, J.B.,"Formation of 3b — ... 17.5 82.5 66 103 Polycyclic Aromatic Hydrocarbons and Their ... Growth to Soot — A Review of Chemical 4 C2H4 100.0 — 81 100 Reaction Pathways," Progress in Energy and 5 C H 18.8 — 81.2 74 100 Combustion Science, Vol, Nos26 . . 4-6, 2000. pp , 3 6 ...... 565-608. 6 C3H8 100.0 79 100 7 CeH 0.7 14.1 85.2 95 90 29 Xu, SootF. , Growth n Laminari Premixed 6 Flames, Ph.D. Thesis e UniversitTh , f o y 8 C6H6 1.4 10.2 88.4 130 90 Michigan, 1999. 9 C6H6 2.3 5.6 92.1 110 90 30El-Leathy, A.M., Effects of Fuel Type on Soot aRound lamina t diffusiorje n flame fueD I l sm witm 5 h3 Formation and Oxidation in Laminar Diffusion coflowinD I m m r port ai g0 6 . pord Reactanan t d an t Flames, Ph.D. Thesis, Mechanical Powerambient pressures and temperatures of 98 ± 1 kPa and Department, Helwan University, Cairo, Egyptn i , 294 ± 2 K. Reactant purities: C2H2, 99.5%; C2H4, press. 99.0%; C3H6, 99.5%; C3H8, 99.5%; C6H6, 99.5%; N2, 99.9%; laboratory air coflow having a 240 K dewpoint. 31 Dalzell, W.H.d Sarofiman , , A.F., "Optical Faethd bFroan u m7,X rest from present investigation. Constants of Soot and Their Application to Heat cPercent by volume of fuel port flow. Flux Calculations," Journal of Heat Transfer, , 1969 1 . 100-104 . pp , No Vol , 91 .. 12 American Institut Aeronauticf eo Astronauticd san s (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

ELAPSED TIME (ms) 0 20 30 40 45 50 55 60 65 2000

0 10 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 Fig. 1 Typical TEM photograph of soot HEIGHT ABOVE BURNER (mm) aggregate maximue th t sa m soot volume fraction location e ethylene-fiielealon th e axi f th go s d laminar jet diffusion flame burning in air at Fig. 3 Measured soot and flame properties along atmospheric pressure with the reactants at normal th ethylene/aie th axi f o s r lamina t diffusiorje n temperature (Flame 4). flam atmospherit ea c pressure (Flam. e4)

ELAPSED TIME (ms) ELAPSED TIME (ms) 0 9 5 8 0 8 5 7 6060 5 0 57 0 0 8 5 7 0 304 0 7 0 5 5056 0 56 2000 2000 *1750

10 20 30 40 50 60 0 79 0 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 HEIGHT ABOVE BURNER (mm) HEIGHT ABOVE BURNER (mm) Fig. 2 Measured soot and flame properties along Fig. 4 Measured soot and flame properties along the axis of an acetylene-nitrogen/air laminar jet th ea benzene-acetylene-nitrogen/ai axif o s r diffusion flame at atmospheric pressure (Flame laminar jet diffusion flame at atmospheric 1). From Xu and Faeth.7 pressure (Flame 8).

10" SYMBOL OXIDATION MECHANISM DIFFUSION FLAMES

SOLID H2O • C2H2 FLAMES OPEN A HYDROCARBON FLAMES

0 C2H2-BENZENE FLAMES . O UJ O i O) O 3CO 10- O XU ET. AL O PREMIXED CH4/O2 (1997) O

PREMIXED C2H4/AIR (1998) A XU & FAETH

DIFFUSION C2H2/AIR EL-LEATHL A . YET DIFFUSION HC/AIR (2001) DIFFUSIOUSION C?H?/CoHB/AIR(2001) 0 ,-6 10'= 10 40 50 60 70 80 90 HEIGHT ABOVE BURNER (mm) Fig. 5 Soot surface growth rates (corrected for Fig. 7 Soot surface oxidation collision soot surface oxidation e HACn termi th ) f o As efficiencies assuming soot burnou attaco t e kdu t mechanis f Colkemo r laminad Halfo an t l r by H2O as a function of height above the burner. flame t atmospherisa c pressure. Measurementf so Found fro measuremente mth Neof so n i t alhe . ethylene/air premixed flame st al.e fro;u mX premixed flames, estimated from the correlation measurement f o methane/oxyges n premixed for attack by C>2 of Nagle and Strickland- flame t se al. ;frou X mmeasurement f o s Constable for the conditions of the present acetylene-iritrogen/air diffusion flames from Xu diffusion flames, and found from the present and Faethmeasurementd an ; f ethylene/airo s , measurements in diffusion flames. Note that the propylene-nitrogen/air, propane/air and acetylene- dat presenae basth r efo t diffusion flames includes benzene-nitrogen/air diffusion flames from the the measurements of Xu and Faeth as well as the present investigation. present investigation.

SOLID PRESENT MEASUREMENT + C H, FLAMES 2 • C H FLAMES ' OPEN NAGLE AND STRICKLAND- 2 2 CONSTABLE CORRELATION HYDROCARBOA N FLAMES 0C2H2-BENZENE FLAMES

0 8 0 7 0 6 0 5 0 8 0 7 0 6 0 5 90 HEIGHT ABOVE BURNER (mm) HEIGHT ABOVE BURNER (mm) 6 FigSoo. t surface oxidation collision Fig. 8 Soot surface oxidation collision efficiencies assuming soot burnout due to attack efficiencies assuming soot burnou attaco t e kdu t b yfunctioa 6s 2a f heigho n t abov burnere eth . by OH as a function of height above the burner. Found fro measuremente mth n i Neof . so al t he Found from the measurements of Neoh et al. in premixed flames, estimated from the correlation remixed flames, estimated from the correlation for attack by C>2 of Nagle and Strickland- or attac y f 6Naglb o k2d Stricklandan e - e Constablconditionth e presenr th to f eo st Constable conditionth e r presenth fo f eo st diffusion flames, and found from the present diffusion flames, and found from the present measurements in diffusion flames. Note that the measurements in diffusion flames. Note that the data base for the present diffusion flames includes dat presenae basth r efo t diffusion flames includes the measurements of Xu and Faeth as well as the measuremente th Faetd e welan s th a hu s a lX f so present investigation. present investigation. 14 American Institut Aeronauticf eo Astronauticd san s