(c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. 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 For permission to copy or republish, contact the copyright owner named on the first page. For AlAA-held copy- right, write to AIAA, Permissions Department, 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191-4344. (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. 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 mech- anis 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 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. 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.