4900r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4900–4907 : DOI:10.1021/ef900378sPublished on Web 08/27/2009

Measurements of Laminar Burning Velocities andMarkstein Lengths of n-Butanol-Air

Premixed Mixtures at Elevated Temperatures and Pressures

Xiaolei Gu, Zuohua Huang,* Qianqian Li, and Chenglong Tang

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic China

Received April 29, 2009. Revised Manuscript Received August 5, 2009

Measurements of laminar burning velocities and Markstein lengths of n-butanol-air premixed mixtureswas made over a wide range of equivalence ratios at initial temperatures of 413, 443, and 473 K and initialpressures of 0.1 and 0.25 MPa using the high-speed schlieren photography and outwardly propagatingflame. Effects of laminar flame thickness, thermal expansion ratio, and flame Lewis number on flamestability response were studied. Schlieren photos of flame propagation are recorded. The results show thatlaminar burning velocities of n-butanol-air premixed mixtures are increased with the increase of initialtemperature, and they decrease with the increase of initial pressure. Markstein lengths are decreased withthe increase of the equivalence ratio, and they decreasewith the increase of initial temperature, and all theseindicate that the flame instability is increased with the increase of equivalence ratio, the decrease of initialpressure, and the increase of initial temperature.

1. Introduction

The energy crisis and air pollution created an incentive tostudy and evaluate alcohols as alternative fuels or fuel additivesin spark-ignition engines. Ethanol is proposed as a replacementbecause it is biodegradable and less detrimental to groundwaterwith the positive impact of emissions reduction and octaneimprovement. Unfortunately, ethanol is classified as a solventand is fully miscible in water, preventing it from being trans-ported via pipeline like pure gasoline. n-Butanol has severaladvantages over ethanol as a transportation fuel. It has a higherenergy density than ethanol. The volumetric energy density ofn-butanol is only 9% lower than that of gasoline1 while ethanolhas 30% less energy per unit volume compared with gasoline,which has been shown to negatively impact vehicle fuel eco-nomy, especially at higher blend ratios. n-Butanol also has amuch lower vapor pressure than ethanol, reducing the evapora-tive emissions and chance of explosion. n-Butanol is much lesshygroscopic compared to ethanol, preventing it from watercontamination. n-Butanol has also shown to be less corrosiveto materials in automotive fuel systems and existing pipelinesthanethanol and is very close inoctane rating togasoline. In fact,n-butanol has physical properties similar to gasoline and can beused as a direct replacement for gasoline in spark-ignitionengines with few or no modifications.2 The fuel properties ofthe base fuels are summarized in Table 1.3

Rice et al.5 and Yacoub et al.6 have studied engine emissionsand knock characteristics of the butanol-gasoline blends.Liao at al.7 measured the laminar burning velocities for etha-nol-air mixtures experimentally over a wide range of equiva-lence ratios in a constant volume combustion bomb, and theeffect of flame stretch at the flame frontwas discussed.Alasfour8

experimentally investigated the availability analysis of a spark-ignition engine using butanol-gasoline blends. The resultsshowed that maximum power is greater when using the buta-nol-gasoline blend compared to that of pure gasoline. Dagautet al.9 have recently investigated the oxidation of blends ofn-butanol-gasoline surrogate mixtures (85-15 vol %) using ajet-stirred reactor (temperatures range from 770 to 1220 K at10 bar). Yang et al.10 studied combustion intermediates inpremixed, low-pressure (30 torr, 1 torr= 7.5 kPa), laminar,n-butanol-oxygen flames, for all four isomers, by usingphotoionization mass spectrometry. McEnally et al.11 studiedmethane-air flamesdopedwith four isomersofbutanol and twoisomersof butane.Themajor species, thedecompositionkineticsof the butanol dopants, and the emissions of toxic byproductswere measured. Oehlschlaeger et al.1 experimentally studied theautoignition of four isomers of butanol at high temperatures in ashock tube, and a kinetic mechanism for description of theirhigh-temperature oxidation was developed. Sarathy et al.12

*Corresponding author. Tel.: þ0086 29 82665075. Fax: þ0086 2982668789. E-mail address: zhhuang@mail.xjtu.edu.cn.(1) Moss, J. T.; Berkowitz, A. M.; Oehlschlaeger, M. A. An experi-

mental and kinetic modeling Study of the oxidation of the four isomersof butanol. J. Phys. Chem. A 2008, 112 (43), 10843–10855.(2) ButylFuel LLC. www.butanol.com (accessed October 2008).(3) Wallner, T.; Miers, S. A. A comparison of ethanol and butanol as

oxygenates using a direct-injection, spark-ignition engine. ASME J.Eng. Gas Turbines Powers 2009, 131, 032802-1–032802-9.(4) Hara, T.; Tanoue, K. Laminar flame speed of ethanol, n-heptane,

Iso-octane air mixtures; JSAE Paper No. 20068518, Society of AutomotiveEngineers of Japan: Tokyo, 2006.(5) Rice, R. W.; Sanyal, A. K.; Elrod, A. C.; Bata, R. M. J. Eng. Gas

Turbines Power 1991, 113, 377–381.(6) Yacoub, Y.; Bata, R.; Gautam,M. Proc. Inst. Mech. E Part A: J.

Power Energy 1998, 212, 363–379.

(7) Liao, S. Y.; Jiang, D. M.; Huang, Z. H.; Zeng, K.; Cheng, Q.Determination of the laminar burning velocities for mixtures of ethanoland air at elevated temperatures. Appl. Therm. Eng. 2007, 27 (2), 374–380.

(8) Alasfour, F. N. Butanol- A single-cylinder engine study: avail-ability analysis. Appl. Therm. Eng. 1997, 17 (6), 537–549.

(9) Dagaut, P.; Togb�e, C. Oxidation kinetics of butanol-gasolinesurrogate mixtures in a jet-stirred reactor: Experimental and modelingstudy. Fuel 2008, 87, 3313–3321.

(10) Yang, B.; Osswald, P.; Li, Y.; Wang, J.; Wei, L.; Tian, Z.; Qi, F.;Kohse-Hoinghaus, K. Identification of combustion intermediates inisomeric fuel-rich premixed butanol-oxygen flames at low pressure.Combust. Flame 2007, 148 (4), 198–209.

(11) McEnally, C. S.; Pfefferle, L. D. Fuel decomposition and hydro-carbon growth processes for oxygenated hydrocarbons: butyl alcohols.Proc. Combust. Inst. 2005, 30 (1), 1363–1370.

(12) Sarathy, S. M.; Thomson, M. J.; Togb�e, C.; Dagaut, P.; Halter,F.;Mounaim-Rousselle, C. An experimental and kineticmodeling studyof n-butanol combustion. Combust. Flame 2009, 156 (4), 852–864.

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presented new experimental data for n-butanol in three experi-mental configurations. Species concentration profiles and lami-nar flame speed were obtained for n-butanol in the aforemen-tioned experimental configurations. The improved detailed che-mical kinetic mechanism (878 reactions involving 118 species)has been derived, and the proposed mechanism showed goodqualitative agreement with the experimental data. However,there were few reports on the laminar burning velocity ofn-butanol-air at elevated initial temperature and pressure.

The objective of the present paper is to measure laminarburning velocities of n-butanol-air over wide range ofequivalence ratio at elevated initial temperature and pressure.Markstein lengths and unstretched laminar burning velocitiesof n-butanol-air premixed mixtures at various equivalenceratios are obtained.Flame instability is also analyzedbasedonthe experimental data and flame schlieren photos.

2. Experimental Setup and Procedures

Adetailed descriptionof the experimental apparatus usedfor this study is given in the work of Zhang et al.13 For clarity, themain features of the equipment and the procedures of themeasurements are described here. Figure 1 shows the experimen-tal arrangement. It includes a constant volume combustionchamber and the systems for heating, ignition, data acquisition,and high-speed schlieren photography. The combustion chamberis a cylinder typewith an inner diameter of 180mmandvolumeof5.5 L as shown in Figure 2. The centrally located electrodes areused to ignite the combustible mixture. The pressure transmitter,thermocouple, pressure transducer, liquid fuel injection valve,and inlet and outlet valves are mounted on the chamber body.Two quartz windows of 80mmdiameter are mounted on the twosides of the vessel. A high-speed digital camera (HG-100K)operating at 10000 frames per second recorded the flame pro-gression during the combustion. The partial pressures of eachcomponent are regulated by a mercury manometer when theinitial pressure of themixtures in the vessel is less than or equal to0.1 MPa, and the partial pressures are regulated by the pressuretransmitter at initial pressures greater than 0.1 MPa. The entirevessel was heated by a 2.4 kW heating-tape wrapped outside thechamber body. The thermocouple measures the initial tempera-ture of mixtures in the vessel with an accuracy of 1 K. The initial

temperature is adjusted by a thermoregulator. When the mixturereaches the designated initial temperature, the power is switchedoff. The required liquid fuel is injected into the chamber by themicroliter syringes corresponding to the given initial temperature,initial pressure, and equivalence ratio. Bomb dry air is suppliedinto the chamber through the inlet/outlet valve. A wait of 8 to10 min is allowed before the ignition starts.

For the spherically expanding flame, the stretched flamevelocity, Sn, reflecting the flame propagation speed, is derivedfrom the flame radius versus time,16,17

Sn ¼ dru

dtð2Þ

Where, ru is the radius of flame and t is the elapsed time fromspark ignition, thusSn canbedirectly obtained fromflamephotosin schlieren photograph.

The flame stretch rate, R, represents the expanding rate offlame front area. In a quiescent mixture, it is defined as

R ¼ dðln AÞdt

¼ 1

A

dA

dtð3Þ

Where, A is the area of any infinitesimal element of the flamesurface. For the spherically outwardly expanding flame front, theflame stretch rate can be simplified as

R ¼ 1

A

dA

dt¼ 2

ru

dru

dt¼ 2

ruSn ð4Þ

In respect to the early stage of flame expansion, there exists alinear relationship between the flame speeds and the flame stretchrates;18 that is,

Sl -Sn ¼ LbR ð5ÞWhere, Sl is the unstretched flame propagation speed, which canbe obtained as the intercept value atR=0, in the plot of Sn versusR. The burned gasMarkstein lengthLb is the negative value of theslope of the Sn-R curve. The diffusional-thermal instability offlame front is dependent upon the Markstein length. Withpositive values of Lb, which correspond to Lewis numbers largerthan unity, the flame speed is decreased with the increase of flamestretch rate. If any kind of protuberance appears at the flamefront (stretch increasing), the flame speed at the flame protrudingposition will be suppressed, and this makes the flame stable. Incontrast to this, negative values of Lb correspond to Lewisnumbers smaller than unity, indicating that the flame speed isincreased with the increase of the flame stretch rate; in this case, ifany kind of protuberance appears at the flame front, the flamespeed at flame protruding position will be increased, and thisdecreases the flame front stability.15,19

The characteristics of the igniter can influence the measuredvalue of burning velocity. Previous study showed that the flamespeeds were independent of ignition energywhen the flame radiusis larger than 5 mm.14 Together with the consideration of theisobaric combustion, the data reduction is limited within theflame radius ranging from 5 to 25 mm (the pressure rise rate isless 1.0%).

Table 1. Typical Properties of Base Fuels3

gasoline ethanol n-butanol

chemical formula C4-C12 C2H5OH C4H9OHcomposition (C, H, O) (mass%) 86, 14, 0 52, 13, 35 65, 13.5,

21.5lower heating value (MJ/kg) 42.7 26.8 33.1density (kg/m3) 715-765 790 810octane number (R þ M)/2 90 100 87boiling temperature (�C) 25-215 78 118latent heat of vaporization (kJ/kg) 380-500 904 716self-ignition temperature (�C) ∼300 420 343stoichiometric air/fuel ratio 14.7 9.0 11.2laminar flame speed (cm/s)a,b,4 ∼33 ∼39mixture calorific value (MJ/kg)b 3.75 3.85 3.82ignition limits in air (vol %) 0.6 3.5 1.4solubility in water at 20 �C(mL/100 mL H2O)

<0.1 fully miscible 7.7

a T = 325 K and p = 100 KPa. b Stoichiometric mixture, standardtemperature and pressure (STP).

(13) Zhang, Zh. Y.; Huang, Z. H.; Wang, X. H.; Xiang, J.; Wang,X. B.; Miao, H. Y. Measurements of laminar burning velocities andMarkstein lengths for methanol-air-nitrogen mixtures at elevatedpressures and temperatures. Combust. Flame 2008, 155, 358–368.(14) Huang, Z. H.; Wang, Q.; Yu, J. R; Zhang, Y.; Zeng, K.; Miao,

H. Y.; Jiang, D. M. Measurement of laminar burning velocity ofdimethyl ether-air premixed mixtures. Fuel 2007, 86 (15), 2360–2366.

(15) Qin, X.; Ju, Y. G. Measurements of burning velocities ofdimethyl ether and air premixed flames at elevated pressures. Proc.Combust. Inst. 2005, 30 (1), 233–240.

(16) Bradley, D.; Gaskell, P. H.; Gu, X. J. Burning velocities,Markstein lengths, and flame quenching for spherical methane-air flames:a computational study. Combust. Flame 1996, 104 (1-2), 176–198.

(17) Lamoureux, N.; Chaumeix, N. D.; Paillard, C. E. Laminar flamevelocity determination for H2-air-He-CO2 mixtures using the sphe-rical bomb method. Exp. Therm. Fluid Sci. 2003, 27 (4), 385–393.

(18) Gu,X. J.;Haq,M.Z.; Lawes,M.Burning velocity andMarksteinlengths of methane-air mixtures. Combust. Flame 2000, 121 (1-2),41–58.

(19) Liao, S. Y.; Jiang,D.M.;Gao, J.;Huang, Z.H.Measurements ofMarkstein numbers and laminar burning velocities for natural gas-airmixtures. Energy Fuel 2004, 18 (2), 316–326.

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When the observation is limited to the initial part of the flameexpansion, where the pressure varies little, a simple relationshiplinking the flame propagation velocity Sl, to unstretched laminarburning velocity ul, is given as follows,

ul ¼ FbSl=Fu ð6ÞWhere, Fb and Fu are the densities of the burned gas and theunburned gas respectively.The termFu canbeobtained accordingto the initial state, and Fb is determined from thermal equilibriumcalculation.

Due to the finite thickness, there are two possible definitionsfor the stretched laminar burning velocity depending on whetherthe burning velocity is defined at the unburned gas side or at theburned gas side. These two burning velocities are the stretchedlaminar burning velocity un, which is the burning velocity relatedto the entrainment of the unburned gas, and the stretched massburning velocity unr, which is the burning velocity related to theproduction of the burned gas, proposed by Bradley16 and calcu-lated by

un ¼ S SnFbFu

" #ð7Þ

unr ¼ FbFb -Fu

ðun -SnÞ ð8Þ

where S is a function that depends on the flame radius and thedensity ratio. It accounts for the effect of the flame thickness on

the mean density of the burned gases. The expression of S in thestudy used the formula given by Bradley et al.,16,20

S ¼ 1þ1:2δlru

FuFb

!2:224

35-0:15

δlru

FuFb

!2:224

352

ð9Þ

Here δl is laminar flame thickness given by δl=ν/ul, where ν is thekinematic viscosity of unburnedmixture and ul is the unstretchedlaminar burning velocity of the flame.

A dimensionless Markstein number,Ma, can be obtained,

Ma ¼ Lb=δl ð10ÞThe stretch rate, R, can be nondimensionalized by characteri-

stic flow time to obtain the Karlovitz number, Ka, as follows,

Ka ¼ R=ðSl=δlÞ ð11ÞFrom eqs 5, 10, and 11, the following relationship is derived,

Sn

Sl¼ 1-MaKa ð12Þ

Figure 1. Experimental arrangement.

Figure 2. Schematic representation of the combustion vessel.

(20) Bradley, D.; Hicks, P. A.; Lawes, M.; et al. The Measurement oflaminar burning velocities and Markstein numbers for iso-octane-airand iso-octane-n-heptane-air mixtures at elevated temperatures andpressures in an explosion bomb. Combust. Flame 1998, 115 (1-2),126–144.

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It is found that the decrease in the burning velocity of stretchedflame becomes relatively smaller with decreasingMa at the sameKarlovitz number.

TheLewis number is estimated through the ratio of themixturethermal diffusivity to the mass diffusivity of the limiting reactantrelative to the inert which is usually present in abundance. TheLewis number can be defined as

Le ¼ λ

FuCpDABð13Þ

Where λ is the thermal conductivity of unburnedmixture, F is thedensity of unburned mixture, Cp is the specific heat of theunburned mixture, and DAB is the binary mass diffusion coeffi-cient of n-butanol into nitrogen (φ < 1.0) and oxygen intonitrogen (φ> 1.0). All thermodynamic and transport propertieswere evaluated by CHEMKIN. Table 2 shows the properties ofn-butanol-air mixtures at an initial temperature of 413 K in thisstudy.

In this study, repeated experiments were conducted at leastthree times for one point and the averaged data are used in theanalysis. This can ensure repeatability of the results within theexperimental uncertainty (95%confidence level). The accuracyofthe thermocouple is 1K, and the variation in initial temperature is428( 3K.Thus, the relative error in initial temperature is 0.70%.The initial pressure is measured by amercurymanometer, and itsaccuracy is 1 mm Hg; thus, the variation in initial pressure is 0.1MPa ( 133.28 Pa, and the relative error in the initial pressure is0.133%. The mixture was prepared according to the partialpressures of the constituents. The accuracy of the microlitersyringes was 10 μL. The resolution of the digital flame photo-graphs taken is 352 pixel � 352 pixel. A Photoshop processingprocedure is used tomeasure the diameter of the flame. The ratioof picture scale and real scale is (5.93 ( 0.01):80 mm. So, therelative error of the measured radius has a maximum value of1.35% at small radius (ru=5 mm) and the minimum value of0.27% at large radius (ru=25 mm). The experimental standarderrors in the measurements are determined based on the methodof Kline and McClintock.21 The maximum standard errors are16.6% for Markstein length, 4.5% for stretched flame propaga-tion speed, 5.5% for unstretched flame propagation speed, 7.3%for stretched laminar burning velocity, 7.8% for stretched massburning velocity, and 8.6% for unstretched laminar burningvelocity.

3. Results and Discussion

3.1. Flame Morphology. Figure 3 shows the photos of theexpanding spherical flame at various equivalence ratios andinitial pressures. The instantaneous radii of the flame areindicated at the top of photos and flame thickness, thermalexpansion ratio, and Lewis number are tabulated at the rightside of the photos. The flame front remains a basicallysmooth surface at an initial pressure of 0.1 MPa. With theincrease of initial pressure, the flame surface stays smooth atearly stage, and then, some cracks appear at a large radius. Inthe absence of body forces, laminar premixed flames aresubjected to two modes of cellular instability; the hydro-dynamic instability and the diffusional-thermal instability.

Hydrodynamic instability is caused by the density disparityacross the flame, represented by the density ratio of burnedgas to unburned mixture, σ, while diffusional-thermalinstability occurs as a result of the nonequidiffusive proper-ties of the reactive mixture, and such instability may notpresent for flames that are nearly equidiffusive. An appro-priate mixture parameter indicating the extent and influenceof nonequidiffusion is the global Lewis number, Le, whichcan be estimated through the ratio of the mixture thermaldiffusivity to the mass diffusivity of the limiting reactantrelative to the inert which is usually present in abundance.Le < 1 and Le > 1 indicate the unstable and the stableflames, respectively.22,23 The photos also show the effects ofthe equivalence ratio on the propensity to destabilize. Theflame becomes progressively more unstable as the equiva-lence ratio increases. For Le > 1 of a positive stretchedflame, heat loss exceeds the mass gain and the flametemperature (Tb) is less than the adiabatic flame temperature(Tad), indicating that the flame is less intense. For Le < 1,the burning is more intense and can lead to flame instabilitydue to a nonequidiffusive effect. The results also show thatthe flame thickness and the Lewis number are decreased withthe increase of the equivalence ratio, indicating the decreaseof the flame stability due to the combined effects of ther-mal-diffusional instability and hydrodynamic instability.Flame thickness decreases significantly with the increase ofinitial pressure, but the Le number shows a little variation,and this indicates that flame instability is mainly due to theinfluence of hydrodynamic instability.

3.2. Flame Propagation Speed and Markstein Length.

Figure 4 shows flame radius versus the time for the stoichio-metric n-butanol-air flames. After the spark is obtained,local ignition occurs that propagates to the whole homo-geneous mixture as a spherically growing flame. The flameradius varies quasi-linearly with elapsed time when flameradius is larger than 6 mm. With an increase of the equiva-lence ratio, the flame propagation speed is increased.

Figure 5 gives the relationship of flame propagation speedversus flame radius. Flame propagation speed is increasedwith the increase of equivalence ratios from φ=0.8 to 1.3.And, flame propagation speed reaches the peak value at theequivalence ratio of 1.3. Flame propagation speed is in-creased quickly with the mixture from lean to rich.

Figure 6 plots theSn/Sl as a function ofKa for the specifiedequivalence ratios. A fitting line can be correlated betweenKa and Sn/Sl. In addition, Sn/Sl is found to depend on theequivalence ratio. That is, Sn/Sl is decreased with decreasingequivalence ratio (increasing Le, as seen in Figure 5) at thesame Ka. It should be noted that the slopes of these linescorrespond to the Markstein number, Ma. The slopes ofSn/Sl-Ka curves all take the negative values, and thisindicates the positive values of Markstein number and thestabilized flame fronts. However, the absolute values of theslopes of Sn/Sl-Ka curves are decreased with the increase ofequivalence ratio, and this indicates that the burned gasMarkstein number is decreased with the increase of theequivalence ratio and the flame becomes destabilized.The Lewis number varies very little at different initialpressures in Figure 3, and it may be that it gives no indication

Table 2. Properties of n-Butanol-Air Mixtures at Initial Tempera-

tures of 413 K

φ 0.8 0.9 1.0 1.1 1.2 1.3 1.4

DAB 10-5(m23 s-1) 1.65 1.65 3.69 3.69 3.69 3.69

λ [W/(m 3K)] 0.03346 0.03338 0.333 0.3323 0.3316 0.3309 0.3302

Le 2.14 2.11 0.92 0.91 0.90 0.89

(21) Kline, S. J.; McClintock, F. A. Describing uncertainties in singlesample experiments. Mech. Eng. 1953, 75, 3–8.

(22) Jomaas, G.; Law, C. K.; Bechtold, J. K. On transition tocellularity in expanding spherical flames. J. FluidMech. 2007, 583, 1–26.

(23) Law, C. K.; Sung, C. J. Structure, aerodynamics, and geometryof premixed flamelets. Prog. Energy Combust. Sci. 2000, 26 (4-6),459–505.

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of the pressure effect on flame instabilities. However, theMarkstein number is a global flame instability parameter.It may include the influence of Lewis number, thermal

expansion ratio, flame thickness, and Zeldovich number.So, maybe it is reasonable that the Markstein number ispositive and the Lewis number is less than 1.24

Figure 7 gives the burned gas Markstein length andunstretched flame propagation speed versus the equivalenceratio at different initial temperatures. The Markstein lengthis decreased with the increase of the equivalence ratio andinitial temperatures, indicating the increase of flame frontinstability with the increase of the equivalence ratio andinitial temperature. This is consistent to the schlieren photosin the study. The unstretched flame propagation speed isincreased with the increase of initial temperature. The peakvalue of unstretched flame propagation speed will move to aslightly rich mixture with the increase of initial temperature.Gas temperature has been recognized as a significant influ-encing parameter on the burning velocity as it influencesspecies equilibrium and the diffusion of heat/radicals into theflame front.

Figure 8 shows the laminar flame thickness (δl) anddensity ratio of unburned gas to burned gas (σ) versusequivalence ratio at different initial temperatures. The lowestvalue of δ will appear at equivalence ratio of 1.3. Flamethickness is decreased with the increase of equivalence ratio,and density ratio is increasedwith the increase of equivalence

Figure 3. Schlieren pictures of n-butanol-air mixtures at Tu = 473 K and various equivalence ratios and initial pressures. (a) Pu = 0.1 MPa,(b) Pu = 0.25 MPa. Flame thickness, thermal expansion ratio, and Lewis number are tabulated.

Figure 4. Flame radius versus time.

Figure 5. Flame propagation speed versus flame radius.

Figure 6. Sn/Sl versus Karlovitz number.

(24) Halter, F.; Foucher, F.; Landry, L.; Mounaim-Rousselle, C.Effect of dilution by nitrogen and/or carbon dioxide on methane andiso-octane air flames. Combust. Sci. Technol. 2009, 181, 813–827.

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ratio at equivalence ratios less than 1.3. The density ratio isdecreased with the increase of initial temperature. As shownin Figure 6, the burned gas Markstein length is decreasedwith the increase of the temperature.

Figure 9 gives the burned gas Markstein length andunstretched flame propagation speed versus equivalenceratio at different initial pressures. Markstein length andunstretched flame propagation speed are decreased withthe increase of initial pressure. Flame thickness is decreasedwith the increase of initial pressure. All these indicate that theflame instability will be increased with the increase of initialpressure. This is also consistent to the schlieren photos in thestudy.

Figure 10 gives the laminar flame thickness and densityratio versus equivalence ratio at different initial pressures.The results clearly show that the laminar flame thickness isremarkably decreased with the increase of initial pressure.However, the density ratio shows a little variation to equiva-lence ratio. This reflects the increase of the hydrodynamicinstability with the increase of initial pressure.

3.3. Laminar Burning Velocity. Figure 11 shows thestretched laminar burning velocity (un) and the stretchedmass burning velocity (unr) versus stretch rate at differentequivalence ratios and initial pressures. The difference bet-ween un and unr is clearly demonstrated. Here, un denotes therate of mixture entrainment, which usually increases as thestretch rate increases. In contrast, unr is the burning velocityrelated to the production of burned gas, which usuallydecreases as the stretch rate is increased. The differencebetween the stretched laminar velocity and the stretchedmass burning velocity (un- unr) is increasedwith the increaseof stretch rate, and this is due to the influence of flame

Figure 7. Lb and Sl versus equivalence ratio at different initialtemperatures. Figure 8. δl and σ versus equivalence ratio at different initial

temperatures.

Figure 9. Lb and Sl versus equivalence ratio at different initialpressures.

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thickness on the burning velocities. A large value of (un- unr)is presented at a small radius (corresponding to high stretchrate), where the flame thickness is the same order as the flameradius. As the definition indicated, a high value of stretchrate corresponds to a small flame radius; thus, the influenceby flame thickness becomes large. When stretch rate reacheszero, the flame radius becomes infinity, and the influence offlame thickness can be negligible, thus un and unr will get thesame value, that is, both un and unr will reach the unstretchedlaminar burning velocity ul. For a stoichiometricmixture, thevalue of (un - unr) at an initial pressure of 0.10MPa is largerthan that at an initial pressure of 0.25MPa, and this indicatesthat the flame thickness has a small influence on the burningvelocity as initial pressure is increased.

Figure 12 shows the unstretched laminar burning velocityversus equivalence ratio at different initial temperatures andpressures. The unstretched laminar burning velocity is de-creased with the increase of initial pressure, and it is in-creased with the increase of initial temperature. Themaximum value of unstretched laminar burning velocity ispresented at an equivalence ratio of 1.3 at different initialtemperatures and pressures.

4. Conclusions

Measurements of laminar burning velocities of n-buta-nol-air premixed mixtures were studied using the sphericallyexpanding flame at different initial pressures and tempera-tures over awide range of equivalence ratios. Themain resultsare summarized as follows.

1. Flame propagation speeds and laminar burning velo-cities decrease with the increase of initial pressure. The

Figure 10.δl andσversus equivalence ratioatdifferent initial pressures. Figure 12.Unstretched laminar burning velocity versus equivalenceratio.

Figure 11. Stretched laminar burning velocity versus stretch rate:(un) solid point; (unr) hollow point.

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maximum value of laminar burning velocity is presentedat an equivalence ratio of 1.3 for n-butanol-air premixedflames. The laminar burning velocity is increased with anincrease of initial temperature.2. The Markstein length is decreased with the increase ofinitial pressure. The flame thickness is decreased remark-ably with the increase of initial pressure. These indicate

that the thermal-diffusive instability of the flame front isincreased with the increase of initial pressures.3. Flame instability is increased with the increase of initialpressure and the increase of initial temperature.

Acknowledgment. The study is supported by the NationalNatural Science Foundation of China (50636040, 50821604).

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