Plasma assisted combustion: Effects of O3 on large scale turbulent combustion studied with laser diagnostics and Large Eddy Simulations

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Available online at www.sciencedirect.comProceedingsScienceDirectProceedings of the Combustion Institute xxx (2014) xxxxxxwww.elsevier.com/locate/prociof theCombustionInstitutePlasma assisted combustion: Effects of O3 on largescale turbulent combustion studied withlaser diagnostics and Large Eddy SimulationsA. Ehn a,, J.J. Zhu a, P. Petersson a, Z.S. Li a, M. Alden a, C. Fureby b,T. Hurtig b, N. Zettervall b, A. Larsson b, J. Larfeldt caDivision of Combustion Physics, Lund University, SwedenbDefence & Security Systems and Technology, Swedish Defence Research Agency FOI, SwedencSiemens Industrial Turbomachinery AB, Finspang, SwedenAbstractIn plasma-assisted combustion, electric energy is added to the flamewhere the electric energywill be trans-ferred to kinetic energy of the free electrons that, in turn, will modify the combustion chemical kinetics. Inorder to increase the understanding of this complex process, the influence of one of the products of the alteredchemical kinetics, ozone (O3), has been isolated and studied. This paper reports on studies using a low-swirlmethane (CH4) air flame at lean conditions with different concentrations of O3 enrichment. The experimentalflame diagnostics include Planar Laser Induced Fluorescence (PLIF) imaging of hydroxyl (OH) and form-aldehyde (CH2O). The experiments are also modeled using Large Eddy Simulations (LES) with a reactionmodel based on a skeletal CH4-air reaction mechanism combined with an O3 sub-mechanism to includethe presence of O3 in the flame. This reaction mechanism is based on fundamental considerations includingreactions between O3 and all other species involved. The experiments reveal an increase in CH2O in the low-swirl flame as small amounts of O3 is supplied to the CH4-air stream upstream of the flame. This increase iswell predicted by the LES computations and the relative radical concentration shift is in good agreement withexperimental data. Simulations also reveal that the O3 enrichment increase the laminar flame speed, su, with10% and the extinction strain-rate, rext, with20%, for 0.57% (by volume) O3. The increase in rext enablesthe O3 seeded flame to burn under more turbulent conditions than would be possible without O3 enrichment.Sensitivity analysis indicates that the increase in rext due to O3 enrichment is primarily due to the acceleratedchain-branching reactions H2 O() OHH, H2OO() OHOH and HO2 () OHO.Furthermore, the increase in CH2O observed in both experiments and simulations suggest a significantacceleration of the chain-propagation reaction CH3 O() CH2OH. 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Keywords: Plasma-assisted combustion; Ozone-assisted combustion; Large Eddy simulations; Laser-induced fluores-cence; Turbulent combustionhttp://dx.doi.org/10.1016/j.proci.2014.05.0921540-7489/ 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Corresponding author. Address: Division of Combustion Physics, Lund University, Box 118, S-221 00 Lund,Sweden. Fax: +46 46 222 45 42.E-mail address: andreas.ehn@forbrf.lth.se (A. Ehn).Please cite this article in press as: A. Ehn et al., Proc. Combust. Inst. (2014), http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.092mailto:andreas.ehn@forbrf.lth.sehttp://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.0922 A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxx1. Introduction and backgroundElectrical aspects of combustion have beenstudied for a long time as exemplified by the clas-sic work of Lawton & Weinberg, [1]. During thelast decade, the interest has increased significantlyin an electrical aspect of combustion, as inspiredby the world-wide concern of environmentallysustainable energy and transport systems, and ithas evolved into a separate research disciplinenamed Plasma Assisted Combustion (PAC). Thisdevelopment is surveyed in the special issue ofthe IEEE Transactions on Plasma Science, [2],and recent review articles, e.g., [3]. The main ideais to add a comparatively small amount of electricenergy to a flame to achieve a relatively largeeffect. The electric energy is transferred to freeelectrons that excite, dissociate, ionise and heatgas molecules. Apart from employing PAC inreforming of combustible gases or flue-gas treat-ment, the electric energy can be affixed directlyto the flame to increase the laminar flame speedand to stabilize the flame. Adding electric energydirectly to the flame is the most attractive use ofPAC with the potential proficiency to improveflame stability and to delay lean blow-out, thusallowing for leaner fuel mixtures, or for use of bio-fuels or other fuels for which the combustion sys-tem was not originally designed or optimized for.Deposition of electric energy in the flame zonehas greater potential than reforming due to short-lived radicals and excited states that may not havesufficient lifetimes to be convected from thereforming zone to the combustion zone. Manyspecies and excited states are generated whenadding electric energy to a flame, such as freeelectrons (e), ions (O2+, O2, CHO+, . . .), ozone(O3), singlet oxygen in the delta, O2(a1Dg ), andsigma, O2(b1Rg+), states, which are well-knownto influence the combustion, transport andthermal processes of laminar flames, [48]. Manyof the species and excited states mentionedabove already exist in a flame but the increase indensity of these elements and the increase in freeelectron energy result in new reaction pathwayswhereby electron-molecule interactions, such asO2 e () OO e, CH4 e () CH3H e and CO2 e () COO e are theforemost radical producers. Ozone combustionenhancement is examined experimentally andcomputationally [4,5], and both studies show anon-negligible increase in the laminar flame speed,su, with only a modest O3 enrichment. From the1D laminar flame results in [4,5], using detailedhydrocarbon-air reaction mechanisms, includingsub-mechanisms for O3, it is suggested that theO atoms contributed by O3 accelerate the chain-branching reactions in the flame preheat zone,which leads to an increase in su. In studies of sin-glet oxygen [68], it is found that the electronicallyPlease cite this article in press as: A. Ehn et al., Proc.j.proci.2014.05.092excited O2 molecules are more chemically active.This is manifested by acceleration of chain-branching reactions that form new reaction path-ways, leading to an increase in reactive atoms andradicals, therewith increasing su. The studies [48]focus on laminar low-power density flames,whereas the current study concerns high-powerdensity turbulent flames that are more relevantfor gas-turbine combustion.The concept of O3 enrichment is here examined,experimentally and computationally, in a highpower density turbulent swirl-stabilized flame tostudy if improved flame stabilization and fuel flex-ibility can be achieved for real-life combustionapplications. To accomplish this, O3 was addedseparately to the fuel-air mixture in a premixedlow-swirl burner, [9,10]. This is achievable sinceO3 is one of the more long-lived active species pro-duced by electrical discharges in air, [5], which hasa lifetime of hours at NTP, [11]. CombustionLarge Eddy Simulations (LES), using skeletalreaction mechanisms amended with O3-mecha-nisms and Planar Laser-Induced Fluorescence(PLIF) imaging of hydroxyl (OH) and formalde-hyde (CH2O) are combined to explain the differ-ences in flame characteristics on addition of O3.2. Experimental set-up and diagnosticsA low-swirl flame is a detached flame thatpropagates freely in a diverging turbulent flowfield. In low swirl flames there are no significantvortex breakdown and hence no significant oscil-latory precessing vortex core motion. The flowat the leading edge of the flame is free from recir-culation zones that carry hot products back to thereaction zone, thus reducing NOX-production.The flame is stabilized, not in a central recircula-tion zone typically existing in high swirl flames,but in a low speed zone in a central region down-stream of the swirler where there is no recirculat-ing flow. Due to this feature low swirl flames aresuitable for studying turbulence-flame interactionsin turbulent premixed flames, and for this reasonit is also suitable for examining PAC and idealizedaspects thereof such as O3 enrichment. Detailsabout the low-swirl burner used here have beenpreviously presented in [9,10] and referencestherein. A schematic of the gas supply system isshown in Fig. 1a. The burner is fed from two sep-arate channels; the first is for the methane (CH4),air (oxygen (O2) and nitrogen (N2)) mixturewhereas the second is for the O3 and O2 mixture.A schematic of the low-swirl burner is presentedin Fig. 1b. Mixing is performed in the bottom ofthe burner, where the gas mixture passes throughseveral perforated plates, each with 37 holes witha diameter of 3.6 mm, before entering the swirlerswhere the swirling flow motion is generated. Theannular swirler has an exit angle of 37 in relationCombust. Inst. (2014), http://dx.doi.org/10.1016/http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.092Fig. 1. (a) Schematic of gas-supply system used in the experiment. O2 is fed to O3 generators and the O3/O2 gas is mixedwith N2, CH4 and air through the inlet of the burner. (b) Semitransparent illustration of the low swirl burner nozzle.(c) Optical set-up with mirrors (M) and lenses (L) for performing PLIF imaging.A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxx 3to the vertical axis. The mass flow split betweenswirler and perforated plate is 1.5:1, and the flowwas kept at 42 m3/h, resulting in a flame power of30 kW. Four O3 generators (ICT-20-120 fromOzone Tech systems) convert a fraction of O2 intoO3. The maximum rate of O3 was 0.24 m3/h whenthe O3 generators were fed with 3.30 m3/h of O2.The total flow was kept constant to facilitate com-parative studies of flames without and with O3enrichment. Consequently, additional N2 was sup-plied to the flame due to the small volumetric lossas O3 is produced from O2.The flame was operated at two different O3seeding concentrations (Case 1 and Case 2), aswell as a reference case without O3 seeding (Case0) corresponding to the case studied in [9,10].Flow data for the three different cases are pre-sented in Table 1.Two different laser systems were employed toperform PLIF imaging of OH and CH2O of thelow-swirl burner flame. A frequency tripled Bril-liant B laser (Quantel) enabled CH2O imagingby 355 nm excitation with a pulse energy of180 mJ. An Nd:YAG-Dye system was used forPLIF imaging of OH. The [A2R+(t = 0) X2P(t00 = 0)] OH transition was excited using a laserwavelength of 308 nm, with a pulse energy of10 mJ. The optical setup is presented in Fig. 1c.Two quartz lenses, L1 and L2, were used to formlaser sheets of 50 mm height that were focusedat the center of the low-swirl burner. The focusinglens had a focal length of 500 mm, providingsharp PLIF images. Intensified CCD cameras(Princeton Instruments PIMAX II) equipped withquartz camera lenses (L3: B. Halle, f = 100 mm,f/2 and L4: UV-Nikkor f = 105 mm, f/4.5) capturedTable 1Experimental composition. S denotes the swirl number, / theCase S Re /0 0.50 20,000 0.651 0.50 20,000 0.662 0.50 20,000 0.68Please cite this article in press as: A. Ehn et al., Proc.j.proci.2014.05.092the OH and CH2O fluorescence. A GG385 Schottfilter was used to discriminate the laser light in theCH2O imaging whereas no filter was used for theOH imaging.The experimental PLIF data was filtered with amedian filter to emphasize the phenomenologicalchanges that occur when the O3 seeding concen-trations was varied. The data was corrected forbackground and in cases where measurementswere performed at different instants of time thedata has been normalized against reference mea-surements that were conducted during each mea-surement occasion. Grid images were routinelyacquired to ensure that the experimental datarepresent a spatial coordinate above the burnernozzle. The spatial resolution in the measurementsis less than 200 lm and can thus resolve typicalflame fronts.3. Large Eddy Simulation and Numerical MethodsThe LES model is based on a reacting flowmodel in which the mixture is assumed to be a lin-ear viscous fluid with Fourier heat conduction andFickian diffusion. The viscosity is computed usingSutherlands law whereas the thermal conductivityand species diffusivities are computed using theviscosity and constant Prandtl and speciesSchmidt numbers, respectively, [12]. The mixturethermal and caloric equations-of-state areobtained under the assumption that each speciesis a thermally perfect gas, with tabulated forma-tion enthalpies and specific heats. The combustionchemistry is based on Guldberg-Waages law ofmass-action involving the summation over allequivalence ratio and Re the Reynolds number.O2 (%) N2 (%) O3 (%) CH4 (%)19.7 74.0 0 6.419.5 74.0 0.11 6.418.8 74.2 0.57 6.4Combust. Inst. (2014), http://dx.doi.org/10.1016/http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.0924 A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxxparticipating reactions with reaction ratesobtained from Arrhenius rate laws.The combustion chemistry is here modeled bythe skeletal mechanisms of Smooke & Giovangigli(SG25), [13], and Sher & Refael (SR20), [14],respectively, together with the O3 mechanism ofWang et al., [5]. As a reference, the GRI-3.0 mech-anism, [15], is used together with the O3 sub-mechanism of Wang et al., [5]. SR20 includes 17elementary reactions and one global reaction toembody the C2 channel, and was developed fromthe detailed reaction mechanism, [16]. SG25includes 10 reversible and 15 irreversible reactionsand is applicable to both premixed and nonpre-mixed flames, [13]. Figure 2 compares predictionsof the adiabatic flame temperature, Tad, su, igni-tion delay time, sign, and extinction strain-rate,rext, between the SG25, SR20 and GRI-3.0 mech-anisms without O3, and with the amount of O3present in Cases 1 and 2, according to Table 1.Predictions from the chemical kinetics codeCantera and the LES code are found to be virtu-ally identical, supporting that the LES code pre-dicts the correct laminar flame behavior. TheSG25 mechanism shows a better overall agree-ment with the GRI-3.0 mechanism than theSR20 mechanism, particularly for higher seedingconcentrations of O3, and is therefore used inthe following LES computations. The chemicalkinetics effects of the two levels of O3 enrichmentstudied are shown in Fig. 2a and b: The O3 seed-ing YO3 0:019 and 0:0098 does not influenceTad, but increase su with approximately 2% inCase 1 and 12% in Case 2 compared to Case 0,which is in agreement with the data in [5,17].Moreover, with O3 seeding sign decreases margin-ally whereas rext increases with approximately 5%in Case 1 and 20% in Case 2. The increase in rexttheoretically enables an O3 seeded flame to with-stand higher levels of turbulence, larger velocityFig. 2. Results from the SR20 and the SG25 mechanisms are cmechanism for different O3 mass fractions, YO3 . Results of adiaare displayed as dashed and solid lines, respectively, in (a) for dand extinction strain rate, rext, represented by dashed and sofractions, YO3 . Legend: ( ) GRI-3.0, ( ) SR20 and ( )Please cite this article in press as: A. Ehn et al., Proc.j.proci.2014.05.092gradients, and therefore to increase the marginto lean blow-off.The LES model employ implicitly filteredmass, momentum, energy and species equations,[12], in which the subgrid stress tensor and fluxvectors are closed by the mixed model, [18]. Thefiltered reaction rate terms are modeled with thePartially Stirred Reactor (PaSR) model, [1920],which is a multi-scale model based on the observa-tion, [21], that combustion mainly takes place infine-structure regions distributed in a surroundingof low chemical activity. In the PaSR model, thefiltered reaction rates are taken as a weightedaverage of the fine-structure and surroundingsreaction rates using the reacting volume fraction,c*, as the weighting function. Subgrid mass andenergy equations are solved in all LES cells forthe fine-structure and surroundings temperatureand concentrations using the fine-structures resi-dence time, s*. Here, s* and c* are modeled fromthe subgrid, Kolmogorov, and chemical time-scales as described in [20]. The LES-PaSR modelhas been extensively used and validated for manycases, e.g. [1920,2224].The LES-PaSR model equations are solvedusing a semi-implicit finite volume code basedon the OpenFOAM C++ library, [25]. High-ordermonotonicity preserving reconstruction of theconvective fluxes and central differencing of theinner derivatives of the diffusive fluxes, [26], arecombined with Crank-Nicholson time-integrationto provide a second order accurate scheme. Thechemical source terms in the species transportequations are evaluated explicitly with tempera-ture and species concentrations from the previoustime-step. The code uses a compressible Pressure-based Implicit Splitting of Operators (PISO), [27],technique to manage the pressure-velocity-densitycoupling. Stability is enforced using compactstencils and by enforcing conservation of kineticompared to simulated results obtained with the GRI-3.0batic flame temperature, Tad, and laminar flame speed, su,ifferent equivalence ratios, /. (b) Ignition delay time, sign,lid lines, respectively, are shown for different O3 massSG25.Combust. Inst. (2014), http://dx.doi.org/10.1016/http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.092A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxx 5energy with a fixed time step and a Courant num-ber 6 A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxxproducing O and OH, respectively, from which Oreact rapidly with CH4, producing additional OH.The subsequent reaction of OH with CH4 and fuelfragments, such as CH2O, provided chemicalheat-release at lower temperatures to enhancethe laminar flame speed. The O radicals contrib-uted by O3 in the preheat zone accelerate thechain-branching reactions H2O + O,OH + OH,H2 + O,OH + H and H + O2,OH + O, henceincreasing the laminar flame speed, su, as clearlyobserved in Fig. 2a. These reactions are facilitatedby the new pathways from O3 to O2 and O in thereaction path diagram in Fig. 3d. Based on a sen-sitivity analysis, the increased extinction strain-rate, rext, observed in Fig. 2b, due to the O3enhancement, is primarily due to the acceleratedchain-branching reactions but also due to theaccelerated chain-propagation reaction CH3 +O)CH2O + H as evidenced by the increased levelof CH2O observed both in the laminar flame sim-ulations and in the low-swirl burner experimentsand LES predictions. As a consequence of theincrease in su and rext, the O3 enriched low-swirlburner flame can withstand a more turbulent envi-ronment, i.e. a turbulent flow with higher localvelocity gradients, than the corresponding flamewithout O3.Figure 4a and b compares time-averages andrms-fluctuations of the axial velocity, vx, andtemperature, T, across the flame at differentheights above the burner, h, at h/D = 0, 0.2, 0.5,0.6, 0.8, 1.0, 1.2 and 1.6, between LES predictionsof Cases 0, 1, 2 and experimental data for Case 0,[9,33,34]. Considering first the reference Case 0 wegenerally find good agreement between experi-mental data and LES predictions with a fewexceptions: At h/D = 0.2 the predicted hvxi distri-bution shows a double-peaked hvxi-profile, as aresult of the perforated plate-swirler configurationin the burner, Fig. 1b, that however is not fullyresolved in the experimental data, and at h/D = 0.6 hvxi is slightly overestimated in theFig. 4. Cross-sectional comparison of (a) time averaged axial vtemperature hTi and rms temperature fluctuations, Trms, at h/Dof Case 0, ( ) LES of Case 1, ( ) LES of Case 2, and (+) eCase 0.Please cite this article in press as: A. Ehn et al., Proc.j.proci.2014.05.092annular swirling jet expanding from the nozzle.The axial rms velocity fluctuations, vxrms, are rea-sonably well predicted. Regarding the tempera-ture and temperature rms fluctuations, hTi andTrms, respectively, hTi is slightly underestimatedbetween h/D = 0.5 and 0.8, suggesting that thelift-off height of the premixed cup-shaped flameis slightly overpredicted, and that the flame isslightly too wide at h/D = 1.6. The LES predic-tions of Trms show more radial variations thanthe experiments, but show a reasonable agreementwith the experimental data. The influence of O3enhancement, represented by Cases 1 and 2, seemsnot to significantly influence neither hvxi nor vxrms,whereas hTi and Trms are more influenced by theadditional O3. More specifically, hTi increasesomewhat with increasing O3 enrichment, particu-larly for h/D > 1.0. As a result of the differences inhvxi and hTi, Trms increase in the premixed cup-shaped flame between h/D = 0.5 and 0.6. Theseobservations are indicative of the more intensecombustion reactions taking place in the premixedflame caused by the O3 enrichment as indicated inFig. 3.Time-averaged OH and CH2O distributionsfrom the experimental PLIF data and LES predic-tions are compared in Figs. 5b and 5c, respec-tively. The windows used for comparison areshown by the box in Fig. 5a, and the correspond-ing PLIF and LES images are mirrored at the cen-terline. Albeit the time-averaged OH and CH2Odistributions from PLIF and LES agree reason-ably well with each other, the LES predictionsresult in a more pronounced W-shaped flamecup, with a higher centerline lift-off. Instanta-neous PLIF and LES distributions reveal theexistence of V-, multiple V-, and W-shaped flamecups but occurring in different proportions, result-ing in differently shaped flame cups. From theLES results of Cases 0, 1 and 2, and the resultsin [32], the W-shaped flame is statistically morefrequent. This flame shape is influenced byelocity, hvxi, and rms axial velocity fluctuations, vxrms, (b)= 0, 0.2, 0.5, 0.6, 0.8, 1.0, 1.2 and 1.6. Legend: ( ) LESxperimental data, using PIV, [9], and CARS, [33,34], forCombust. Inst. (2014), http://dx.doi.org/10.1016/http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.092Fig. 5. (a) Three-dimensional T (hot) and CH2O (gray-scale) distribution from LES for Case 2. The dashed boxdesignates the image area where LES results and PLIF data, from Case 2, are compared in (b) and (c). Time-averagedLES and PLIF images of OH and CH2O are shown in (b) and (c), respectively, together with radially averaged profiles.A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxx 7large-scale vortices (due to Kelvin-Helmholtzinstabilities) in the burner shear-layers, and withhigher su and rext, caused by the O3 seeding, thiseffect is more pronounced.The LES predictions reveal super-equilibriumOH levels at the outer edges of the flame not soapparent in the PLIF data. The radially averagedOH profiles from the PLIF data are virtuallyunaffected by the O3 seeding, whereas the radiallyaveraged LES results show some difference in OHprofiles, particularly that the flame in Case 2 isshifted towards the burner due to the abovemen-tioned increase in su and sT due to the O3 seeding.Concerning CH2O we find good agreementbetween the PLIF data and the LES results bothwith respect to distribution and level. The radiallyaveraged CH2O profiles from the LES results andthe PLIF measurements show an increase inCH2O, with increasing O3 seeding, and that theLES predictions results in a small broadening ofthe CH2O profile with increasing O3 seeding.From Fig. 5a we also observe that the CH2O dis-tribution correlates with the strongest change inflame curvature.The observation that OH is virtually constantand CH2O increases under O3 enrichment can beunderstood by analyzing the reaction mechanismsummarized in Fig. 3d. The breakdown of O3 intoO2 and O, through reactions with O2, OH, H2Oand H, result in an abundance of O whichprimarily reacts with CH3 to produce CH2O andH, principally responsible for increasing su. ThePlease cite this article in press as: A. Ehn et al., Proc.j.proci.2014.05.092abundance of O also influences the chain-branch-ing reactions involving H and OH, but since OH iscommon to many elementary reactions, the over-all effect of O on OH concentration is diminished,and therefore no significant direct effect on OH isobserved.The increased strain rate achieved with O3seeding, Fig. 2b, creates possibilities to obtain amore stable flame with increasing the turbulentRe number. The quenching limit however isalmost unaffected in this configuration since thatis largely controlled by su which increase withO3 for this low equivalence ratio is only a fewpercentages.5. Concluding RemarksPlanar Laser-Induced Fluorescence (PLIF)imaging of hydroxyl (OH) and formaldehyde(CH2O), in combination with numerical simula-tions by Large Eddy Simulations (LES) using askeletal CH4-air reaction mechanism combinedwith an O3 sub-mechanism are employed toincrease the understanding of the convolutedprocess that occurs when electric energy is sup-plied to a turbulent, large scale flame. Further,this investigation provides a benchmark for futureinvestigations, where the influence of one of theever-present species of electrical discharges,ozone, O3, has been investigated in isolation usinga combination of experiments and numericalCombust. Inst. (2014), http://dx.doi.org/10.1016/http://dx.doi.org/10.1016/j.proci.2014.05.092http://dx.doi.org/10.1016/j.proci.2014.05.0928 A. Ehn et al. / Proceedings of the Combustion Institute xxx (2014) xxxxxxsimulations. The LES model employed is well val-idated and can be used to examine similar effectsin many other combustors, thus also providinginformation about effects related to other featuressuch as fuel, turbulence, mixing imperfections, etc.This current work reports on studies of a low-swirl methane (CH4) air flame at lean conditionswith different concentrations of O3 enrichment.Experiments and LES are in good agreement,revealing an increase in CH2O as small amountsof O3 is supplied to the CH4-air stream upstreamof the flame. The LES results also show that theO3 enrichment increases the laminar flame speedwith about 10% and the extinction strain-rate,rext, with 20%, for 0.57% O3. The increase in rextenables the O3 enriched flame to burn under moreturbulent conditions than would be possiblewithout O3 seeding. Sensitivity analysis indicatesthat the increase in rext due to O3 enrichmentprimarily results from the accelerated chain-branching reactions H2 + O,OH + H, H + O2,OH + O and H2O + O,OH + OH. In addi-tion, the increase in CH2O observed in bothexperiments and simulations suggest a notableacceleration of the chain-propagation reactionCH3 + O,CH2O + H.AcknowledgementsThis work was financed by the Swedish EnergyAgency via the EFFECT Project. Jon Tegner isacknowledged for assistance with evaluating thereaction mechanisms.References[1] J. Lawton, F.J. Weinberg, Electrical Aspects ofCombustion, Clarendon Press, 1969.[2] I.B. Mateev, L.A. Rosocha, Special issue onplasma-assisted combustion, in: IEEE Transactionson Plasma Science, 2006, 34.[3] A. Starikovskiy, N. Aleksandrov, Plasma-assistedignition and combustion, Prog. Energy Combust. Sci.39 (2013) 61.[4] T. Ombrello, S.H. Won, Y. Ju, S. Williams, Flamepropagation enhancement by plasma excitation ofoxygen. Part I: effects of O3, Combust. 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