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Fuel Processing Technology

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Research article

Experimental and kinetic modeling study of laminar flame characteristics ofhigher mixed alcohols

Qianqian Li⁎, Hu Liu, Yemiao Zhang, Zhiyu Yan, Fuquan Deng, Zuohua HuangState Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, People's Republic of China

A R T I C L E I N F O

Keywords:Higher mixed alcoholsMethanolLaminar flame speedMarkstein lengthKinetic modeling

A B S T R A C T

Mixtures of alcohols combining strengths of lower and higher alcohols show promising alternative properties asengine fuel. To aid the extensive application of the mixed alcohols, detailed combustion investigation is ne-cessary. In this study, laminar flame speeds and Markstein lengths were measured for simplified higher mixedalcohols (blends of methanol and n‑hexanol/n‑heptanol/n‑octanol) with spherically propagating flame at0.1MPa, elevated temperature of 433 K, and different mixing ratios. A comprehensive model was developed todescribe the high temperature chemistry of the mixed alcohols. The accuracy of the model was validated withpresent data. Results reveal that the three higher alcohols exhibited close laminar flame speeds and Marksteinlengths resulting from their similar thermal, diffusion, and chemical kinetic properties. With the increasingmixing ratio of the higher alcohols, the laminar flame speed decreased, especially for the rich mixtures. Themaximum laminar flame speed slightly shifted to the lean side. The Markstein length increased at extremely leanmixtures and decreased at extremely rich mixtures, with the intersection occurring at the equivalence ratiobetween 1.2 and 1.3. The laminar flame speed variation of the mixed alcohols was dominated by chemicalkinetics. Reaction pathway analyses indicated that the cracking process of each component in the mixed alcoholsremains as that in the pure alcohols. Such result demonstrated the chemical kinetic effect resulting from changesin fuel components.

1. Introduction

Alcohols are considered to be promising alternative fuels because oftheir good performance in decreasing harmful emissions and reducingthe need for traditional fossil fuels. Lower alcohols such as methanolhave been extensively studied because of their low production cost andsimple structure [1–3]. Lower alcohols are good octane enhancers, butthey have problems of cold start, low heating value, corrosion, andhygroscopicity [3–5]. These disadvantages limit their use in internalcombustion engines. Compared with lower alcohols, higher alcohols aremore soluble in traditional fuels, less corrosive, and have higher heatingvalue and lower latent heat [6–9]. Higher alcohols also show lowknocking resistance. Therefore, the higher mixed alcohols, namely themixtures of lower and higher alcohols, were proposed to leverage theirstrengths.

Higher mixed alcohols can be produced through biochemical pro-cesses from carbon resources, including coal, natural gas, biomass, etc.Mixed alcohols were initially studied as an octane enhancer to replacemetallic octane enhancers. In 1976, Chevron reported a mixed alcohol(a mixture of tert‑butanol, isopropanol, methanol, etc.) that eliminates

the use of metallic octane enhancers. Subsequently, mixed alcohols andother higher mixed alcohols were developed by several enterprises,including BP and Dow Chemical. These alcohols are produced in dif-ferent ways and contain several components which are mainly C1–C8primary alcohols. Lower alcohols especially methanol always takes thehighest proportion. Their production process is environmentallyfriendly. In addition, they are good octane enhancer and have higherheating value than lower alcohols do [10,11]. There was continualresearch focusing on economical production of mixed alcohols in recentyears [12]. Studies have been carried out to evaluate the performanceof mixed alcohols. Gautam et al. [10,11] investigated the effects ofblending C1–C5 mixed alcohols into gasoline on the characteristics ofcombustion and emission. Their results indicate that the addition ofalcohols accelerates flame propagation and generates cycle emissions ofCO and HC that are comparable to that of neat gasoline. Moreover,alcohol blended fuel shows excellent knock resistance, which improvesengine power output. Sathiyagnanam et al. [9] studied the engineperformance and emission characteristics of a diesel engine fueled by ablend of ethanol–hexanol–diesel and found that the soot emissions aresignificantly low but that the NOx emissions are slightly higher than

https://doi.org/10.1016/j.fuproc.2019.01.010Received 2 October 2018; Received in revised form 23 January 2019; Accepted 24 January 2019

⁎ Corresponding author at: State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China.E-mail address: qianqianli@mail.xjtu.edu.cn (Q. Li).

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0378-3820/ © 2019 Elsevier B.V. All rights reserved.

T

those of pure diesel. In their study, the engine performed well withethanol in blends extended up to 45% due to the existence of hexanol.Mixed alcohols clearly show promising alternative properties over purealcohols. However, fundamental studies on mixed alcohols are limited,let alone higher mixed alcohols which contain C6 or much heavier al-cohols. Hence, a comprehensive study on the characteristics of mixedalcohols is necessary to support their wide-ranging applications.

In combustion research, laminar flame speed is a key parameter thatis indicative of the chemical and physical properties of combustion[13,14]. Laminar flame speed can be adopted to validate the accuracyof chemical kinetic models [15]. The laminar flame speeds and che-mical kinetics of pure alcohols have been studied by several re-searchers. Specifically, the laminar flame speeds of methanol weremeasured by Egolfopoulos et al. [16], Veloo et al. [17], Beeckmannet al. [18,19], Li et al. [20], Zhang et al. [21], Kun et al. [22], etc. Onthe basis of their data, the accuracy of the methanol models proposedby Li et al. [23] and Egolfopoulos et al. [16], as well as the other modelsof hydrocarbons containing a sub-model of methanol, were validated.Unlike methanol research, studies on higher alcohols were started later.Nevertheless, higher alcohols continue to attract increasing attention. Liet al. [24,25] measured the laminar flame speeds of six pentanol iso-mers at a wide temperature and pressure range and the data wereemployed to validate and improve the accuracy of previously proposedmodels. Nativel et al. [26] developed a comprehensive study on themeasurement of laminar flame speed and the chemical kinetics of n‑and isopentanol. With the data presented, they subsequently developeda model for pentanol combustion. Togbé et al. [27,28] studied theoxidation of n‑pentanol and n‑hexanol in a jet-stirred reactor and pro-posed corresponding oxidation models. To further evaluate the accu-racy of the models, they measured the laminar flame speeds of n‑pen-tanol and n‑hexanol in a combustion bomb and compared the resultswith those of the model simulations. Cai et al. [6] proposed a detailedmodel of n‑octanol covering low and high temperatures and validatedthe model accuracy against ignition delay times and stable speciesconcentration profiles. However, this model was not validated with thedata of laminar flame speeds. Sarathy et al. [29] proposed a model formixed alcohols composed of C1–C5 alcohols to illustrate the effects ofstructure differences on the laminar flame speeds of various alcohols. Atpresent, laminar flame studies focus on C1–C6 alcohols, leaving C7 andC8 alcohols, which comprise higher mixed alcohols, barely explored. Tothe best of our best knowledge, the laminar combustion characteristicsof higher mixed alcohols are rarely studied.

In the present study, laminar combustion characteristics were in-vestigated for a simple model of higher mixed alcohols (metha-nol–n‑hexanol/n‑heptanol/n‑octanol mixtures) at 0.1 MPa, elevatedtemperature of 433 K, wide equivalence ratio range of 0.8–1.5, andvarious mixing ratios. The laminar flame speeds of different mixed al-cohols were determined, and their variations with mixing ratios wereillustrated. A lumped model was constructed to describe the hightemperature chemistry of the mixed alcohols. The performance of thenew model was evaluated. Finally, a comprehensive analysis was car-ried out to explore the underlying behavior of laminar flame speedvariations and demonstrate the interactions between methanol andhigher alcohols.

2. Experimental apparatus and data processing

2.1. Experimental apparatus

Experiments were carried out with the spherical propagating flamemethod. The apparatus is shown in Fig. 1. This apparatus is composedof a constant volume chamber, ignition system, heating system, opticalpath, and data acquisition system. The cylindrical chamber was made ofstainless steel. Two quartz glasses were mounted at the ends of thechamber, with the optical path provided in a circle measuring 80mm indiameter. The elevated temperature in the chamber was achieved with

a heating tape of 1500 kW around the chamber. The temperature wasmonitored by a K type thermocouple, and the pressure was detectedand displayed with a pressure transmitter. A high-speed camera(Phantom V611) recorded the flame propagation in 10,000/s. Thetarget fuels were all liquid, and thus, the initial temperature was set to433 K to ensure full evaporation. When the fuel evaporated, air com-posed of 79% N2 and 21% O2 was introduced into the chamber. Uni-form mixtures were achieved in 5min and then ignited by centrallylocated electrodes. At the same time, the high-speed camera traced theflame propagation process. The alcohols had purity levels of over99.5%, and the gases (O2 and N2) had purity levels higher than 99.95%.

2.2. Data processing

With the photos of the flame propagation, flame radius rf(t) wasmeasured, and the stretched flame propagation speed Sb was derived by

=S r ttb

d ( )df . To eliminate the stretch effect, we adopted the nonlinear

extrapolation method developed by Frankel and Sivashinsky [30,31] incalculating the unstretched flame propagation speed, which is ex-pressed as

= − ⋅S S S L r2/b b0

b0

b f (1)

where Lb is the Markstein length and Sb0 is the unstretched flamepropagation speed.

Fig. 2 shows the relationship between the stretched flame propa-gation speed and the stretch rate (κ) for different higher alcohols andthe n‑hexanol–methanol blends at the equivalence ratio of 1.2. Thisfigure also presents the implementation of the nonlinear extrapolationmethod. The unstretched flame propagation speed was obtained at thestretch rate equal to zero. The stretch rate was calculated as

=κ r r t(2/ )(d /d )f f (2)

The blends of hex25, hex 50, hex75, and hex100 indicated blendingratios of n‑hexanol in liquid volume of 25%, 50%, 75%, and puren‑hexanol, respectively. This rule was also applied to the blends ofmethanol and the else two higher alcohols in the following analyses.The three higher alcohols exhibited similar stretched flame propagationspeeds at the same stretch rate and smaller values than methanol. Forthe n‑hexanol–methanol blends, the stretched flame propagation speedshowed a decreasing trend with the increasing blending ratio ofn‑hexanol. With the unstretched flame propagation speed, the laminarflame speed was calculated on the basis of mass conservation across theflame front.

Several factors affect the measurement accuracy of laminar flamespeed [32–37]. Great efforts have been made to estimate and decreasethe uncertainty of laminar flame speed measurement with sphericallypropagating flame in recent years [33,34,38,39]. To reduce the effectsof ignition and confinement of the chamber wall, we used a flame ra-dius between 8 and 22 cm for data processing. The present data werederived at 0.1MPa, whereas the schlieren pictures with cellular struc-ture were not available. At extremely rich mixture conditions, cracksmay arise but affect the measurement accuracy little. Therefore, theeffects of flame instability were neglected in the current work. Flameradiation exists during flame propagation and retards the flame pro-pagation. The measured laminar flame speed was corrected by ac-counting for the radiation effect through the equation [35].

⎜ ⎟⎜ ⎟= + ⎛

⎝⎜

⎞

⎠⎟ ⎛

⎝⎞⎠

⎛⎝

⎞⎠

− −

S S SS

STT

PP

0.82u,RCFS0

u,Exp0

u,Exp0 u,Exp

0

0

1.14u

0 0

0.3

(3)

where Su, Exp0 and Su, RCFS

0 indicate the measured and radiation-corrected laminar flame speed, respectively; S0= 1 cm/s; T0= 298 K;and P0= 0.1MPa. The total experimental uncertainty was determinedbased on the theory of Moffat et al. [40]. Detailed description of thistheory has been provided in previous studies [25,41], and a brief in-troduction was given here for clarity. The total experimental

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uncertainty can be calculated with the equation as,

⎜ ⎟= + ⎛⎝

⎞⎠

−( )δ Bt S

MS SM S

0u

2 1,952

u0

0u

(4)

where BS 0uindicates the systematic uncertainty; tM−1, 95 is student's

multiplier withM-1 degrees of freedom and confidence interval of 95%;SSu0 is the standard deviation of M repeated experiments.

In present study, BS 0udepends on the measuring equipment adopted.

The flame radius was derived with an image processing program andthe measurement uncertainty of flame radius is 2.5%. The uncertaintyof temperature are 2 K, thus the relative temperature error is 0.5%. Theoverall pressure measurement accuracy was 1–2%. The liquid fuelswere injected into the chamber with the microliter syringe and thepartial pressure of each component was monitored with the pressuretransmitter. Therefore, the uncertainty of equivalence ratio is caused by

Fig. 1. Experimental apparatus.

Fig. 2. Stretched flame propagation speed versus stretch rate for different alcohols and n‑hexanol–methanol blends at 0.1MPa and 433 K.

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the measurement errors of both pressure gauge and syringes. The un-certainty of equivalence ratio is estimated to be 2–3%. Based on theanalysis of different factors above, BS 0

uwas determined to be

1.3–3 cm·s−1. For each condition, the experiments were repeated forthree times. The standard error, caused by the extrapolation methodand the repetition, were derived by taking the nonlinear fitting and datascatter into account. Finally, the total uncertainty of laminar flamespeed measurement is determined as 1.5–3.9 cm/s.

3. Model development and simulation method

A comprehensive high temperature model, referred to as the MAmodel, was developed for present mixed alcohols (methanol, n‑hexanol,

n‑heptanol, and n‑octanol) on the basis of the n‑octanol model proposedby Cai et al. [6]. The base model of methanol was replaced by a cor-responding part from the IM model proposed by Li et al. [20] for me-thanol–isooctane blends because the IM model performs well in pre-dicting the laminar flame of methanol. The submodels of n‑hexanol andn‑heptanol were supplemented. The submodel of n‑hexanol was con-structed by Togbé et al. [27] and was adopted in the current work. Asthe oxidation chemistry of n‑heptanol has not been studied yet, a sub-model of n‑heptanol was constructed in this work.

The oxidation model of n‑heptanol was formulated by referring tothe oxidation model of n‑octanol. Similar to n‑octanol, n‑heptanol ownsa functional group of hydroxyl and a long alkyl chain. It was labeled asnC7H15OH in the chemical kinetic files of the MA model for consistency

Fig. 3. Laminar flame speeds of cyclohexane and n‑hexanol in the present study and the literatures at 0.1MPa.

Fig. 4. Laminar flame speeds of mixed alcohols at 0.1MPa and 433 K.

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with n‑hexanol and n‑octanol. The intermediates were named followingthe rules for n‑octanol. The high temperature submodel of n‑heptanolwas developed by providing different reaction classes, which have beenextensively discussed for n‑butanol, n‑pentanol, and n‑octanol.

H abstraction reaction always plays a dominant role in the initialreaction process. Here, n‑heptanol mainly reacts with simple radicals(H, O, OH, CH3, etc.) to produce heptoxy and hydroxyheptyl radicals.According to the previous research [42,43], the CeH bond energies atthe α and β carbon sites are lower than those at other carbon sites due tothe influence of the hydroxyl group. Thus, H abstraction reactions fromthe two sites are identified as alcohol specific. H abstraction reactions atthe other carbon sites are treated as alkyl like because they are con-sidered to be affected minimally by the hydroxyl group. The reaction

rates in the current study were determined by analogies with n‑butanol,n‑octanol, and alkanes [6,43,44]. Similarly, the unimolecular decom-position reactions occurred near the hydroxyl group used the reactionrates of n‑octanol [6]; the other reactions are given the rate constants asthe analogy reactions of n‑heptane [45]. The fuel radical decomposi-tion, isomerization reactions, subsequent reactions of enols, and otherimportant high temperature reaction classes were included in the MAmodel. The rate constants were derived by following the rate rules forn‑octanol, which mostly adopted the analogy reactions of alkanes andalcohols [6,43,45,46]. The thermal and transport parameters of theintermediate species involved in the submodel of n‑heptanol were cal-culated with the RMG program [47]. The MA model consisted of 403species and 2374 reactions. The data files of this model have been

Fig. 5. Markstein lengths of mixed alcohols at 0.1MPa and 433 K.

Fig. 6. Comparison among laminar flame speeds and Markstein lengths of C6–C8 primary alcohols at 0.1MPa and 433 K.

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provided as the Supplementary materials.To verify the model accuracy, the simulated laminar flame speeds of

the mixed alcohols were compared with the experimental data. Thesimulations were carried out using CHEMKIN Pro software [48,49]. Theinitial temperature was set at 433 K, and the initial pressure was

0.1 MPa. The calculations had 1000 maximum points and 400 adaptivepoints, and the adaptive grid control was finally decreased to 0.03 forthe gradient and curvature solutions. The Soret diffusion effect andmixture-averaged transport were employed in all simulations.

4. Results and discussion

4.1. Uncertainty of experimental apparatus

Before performing the laminar flame speed measurement on themixed alcohols, the uncertainty of the experimental apparatus wasevaluated by comparing the laminar flame speeds of cyclohexane andn‑hexanol with the published data [27,50–52], as shown in Fig. 3. Theerror bars are denoted for the present data in the figure. Cyclohexanehas been extensively studied with a large set of laminar flame speeddata published at different initial conditions. As seen in Fig. 3a, presentdata agreed well with the literature data at all equivalence ratio rangefor cyclohexane at 400 K. Among the three higher alcohols involved inpresent study, only the laminar flame of n‑hexanol has been studied byTogbé et al. [27] at 423 K. In current work, comparison was performedbetween the data of Togbé et al. and the present data, as shown inFig. 3b. The present data were slightly higher than the data of Togbéet al. due to the 10 K temperature difference. To account for the 10 Ktemperature effect, the data of Togbé et al. was adjusted by employingthe method given by Wu et al. [53]. Results demonstrate that the dif-ference between the present data and the T-adjusted data was withinexperimental uncertainty. Therefore, the experimental apparatus wasconcluded to yield satisfactory experimental data and can be adopted toin further measurements.

4.2. Laminar flame speed

Fig. 4 plots the laminar flame speeds of the mixed alcohols at0.1 MPa and 433 K. The error bars were indicated to show the experi-mental uncertainty. Methanol exhibited faster laminar flame speed thanthe higher alcohols did, as similarly found by existing studies on me-thanol and n‑butanol/n‑pentanol [17,18,54]. With the increasingmixing ratio of the higher alcohols, the laminar flame speed decreased,especially for the rich mixtures. The maximum laminar flame speedslightly shifted to the lean side for the pure higher alcohols had theirpeak values around the equivalence ratio of 1.1.

The Markstein length acquired along with the extrapolation of theunstretched flame propagation speed is also given here to illustrate theflame front instability property and response of laminar flame speed toflame stretch. Fig. 5 plots the Markstein lengths of the different mixedalcohols at 0.1MPa and 433 K. The random uncertainties calculatedwith the repeated experiments are also displayed. With the increasingmixing ratio of the higher alcohols, the Markstein length tended to

Fig. 7. Laminar flame speeds and Markstein lengths of three mixing alcohols at 0.1MPa and 433 K.

Fig. 8. Adiabatic temperature of higher alcohols at 0.1MPa and 433 K.

Fig. 9. Experimental results of higher alcohols and simulations of differentmodels at 0.1MPa and 433 K.(Solid lines: simulations of MA model; Dash lines: simulations of the modelsproposed in the literature).

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(a) n-hexanol

(b) n-heptanol

(c) n-octanol(caption on next page)

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increase at lean mixtures and decrease at extremely rich mixtures. Thecritical values were observed between 1.2 and 1.3. This behavior de-monstrated that the addition of higher alcohol tended to stabilize theflame front at lean mixtures and destabilize the flame front at richmixtures.

The laminar flame characteristics of the higher alcohols werecomparatively investigated. Fig. 6 shows the laminar flame speeds andMarkstein lengths of the three higher alcohols at 0.1 MPa and 433 K.

The error bars were not indicated for clarity. The three alcohols yieldedclose laminar flame speeds and Markstein lengths, and this behavior issimilar to the heavy straight-chain alkanes studied by Kelley et al. [55].Serving as heavy hydrocarbons, the Markstein lengths of the higheralcohols monotonically decreased with the increase of the equivalenceratio due to the inherent non-equal diffusion of heat and mass.

Fig. 7 shows the comparison among the laminar flame speeds ofthree mixing alcohols of hex50, hep50 and oct50. Obviously, threemixing alcohols had close laminar flame speeds as well as Marksteinlengths, demonstrating that the addition of three higher alcohols ex-hibited similar effects on the variation of laminar flame characteristics.

4.3. Analysis of laminar flame speed variation

4.3.1. Comparison among higher alcoholsLaminar flame speed is a parameter reflecting the combined con-

tributions of thermal, diffusion, and chemical kinetics. As concluded byLaw et al. [13], laminar flame speed is promoted by high temperature,strong diffusion, and fast chemical reaction rates. The contribution oftemperature is always characterized by adiabatic temperature, Tad,which can be calculated on the basis of thermal equilibrium theory withthe CHEMKIN software. The diffusion effect can be characterized by theMarkstein length [53].

As mentioned above, the three higher alcohols showed similarMarkstein lengths, thereby demonstrating similar diffusion. Therefore,we simply need to identify the fundamental effects of thermal andchemical kinetics. Fig. 8 shows the adiabatic temperatures of the threehigher alcohols at 0.1 MPa and 433 K. The three alcohols presentedalmost the same Tad with the peak value obtained around 1.1. This

Fig. 10. Reaction pathways of three higher alcohols at 1.2, 433 K, and 0.1MPa.

Fig. 11. Concentrations of key intermediates in the three higher alcohol flames at 0.1MPa, 433 K, and 1.2.

Fig. 12. Adiabatic temperatures of n‑octanol-methanol blends at 0.1MPa and433 K.

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result suggested that the thermal effects of the three alcohols con-tributed to the laminar flame speeds approximately.

Identifying the contribution of chemical kinetics calls for an accu-rate model. The performance of the MA model was thus tested. Thelaminar flame speeds of the three higher alcohols were simulated withthe MA model, and the results were compared with the experimentaldata, as shown in Fig. 9. This figure also plots simulation results ofprevious models. The model of n‑hexanol developed by Togbé et al.[27] yielded relatively low predictions, especially around the stoi-chiometric ratio. The n‑octanol model developed by Cai et al. [6]yielded satisfactory predictions at the lean mixture but overpredictedthe experimental data at stoichiometric to rich mixture conditions. TheMA model yielded a reasonable agreement with the experimental dataat the equivalence ratio range for the three alcohol flames. In addition,the simulations of the MA model were close for the three alcohols andthus accurately captured the differences among the alcohols.

To learn the detailed chemical kinetics of the higher alcohols, weperformed reaction pathway analyses of the three higher alcohols(Fig. 10) at 0.1 MPa, 433 K, and equivalence ratio of 1.2. It is revealedthat the cracking process of the three alcohols followed a similar rule.The fuels were primarily consumed through H abstraction reactions bybreaking the CeH bond, producing various alcohol radicals. These ra-dicals then underwent decomposition and isomerization reactions,generating small species (e.g., olefins, enols, and alkyls) and evensmaller radicals (e.g., CH2OH, CH2O, CH3, C2H3, C2H4, and C2H5). Asexpected, these species finally cracked into smaller molecule species,which dominated the flame propagation.

The concentrations of the key intermediates of CH3, HCO, C2H3, andC2H4 in three alcohol flames at 0.1 MPa, 433 K, and equivalence ratio of1.2 are shown in Fig. 11a and b. It is seen that the differences in the

concentrations of these radicals are quite small. These radicals can befurther consumed through chain terminating or branching reactions,contributing the consumption or accumulation of the reactive radicalpool. As seen in Fig. 11c, it is observed that the concentrations of H, OHand O are almost the same in three alcohol flames, which finally resultsin close laminar flame speed.

4.3.2. Effects of the addition of higher alcohols to methanolAs the three higher alcohols exhibited similar laminar flame beha-

viors, the blend of n‑octanol and methanol was chosen as a re-presentative of the mixed alcohols to be analyzed. Fig. 5 shows thatwith the increasing mixing ratio of the higher alcohols, the Marksteinlengths of the mixed alcohols exhibited opposite behaviors at extremelylean and rich mixtures; such result differed from the monotonic varia-tion of laminar flame speed. Therefore, the diffusion effect was ex-cluded as the dominant factor governing the laminar flame speed var-iation of the mixed alcohols. Fig. 12 shows the adiabatic temperature ofthe n‑octanol-methanol blends at 0.1MPa and 433 K. Tad increasedconsiderably with the increasing blending ratio of n‑octanol for theincreasing value of C/H. A high Tad benefits the reduction of activationenergy and the acceleration of reaction rates and finally promotes flamepropagation. Nevertheless, the behavior of Tad in the current workdiffered from that of the laminar flame speed, thereby suggesting thatthermal effect did not contribute to the laminar flame speed variation.The analyses revealed that chemical kinetics should play an importantrole in the laminar flame speed variation of mixed alcohols.

The performance of the MA model in predicting the mixed alcoholswas evaluated first. Fig. 13 shows the comparison between the simu-lations of the MA model and the experimental data for the blends ofmethanol and the three higher alcohols. The MA model yielded a

Fig. 13. MA model performance in predicting the experimental results of the mixed alcohols at 0.1MPa and 433 K.(Symbols: experimental data; Lines: simulations of MA model).

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satisfactory agreement with the data for different blends over theequivalence ratio range and different mixing ratios. Specifically, withthe addition of higher alcohols, the simulation results increased slightlyat the lean mixtures and significantly increased at the rich mixtures.Besides, the maximum laminar flame speed shifted to the lean side.

Fig. 14 shows the reaction pathways of n‑octanol and methanol forthe n‑octanol–methanol blends at different mixing ratios (oct25, oct50,and oct75). The results showed that the cracking of n‑octanol in theblends was almost the same as that of pure n‑octanol. For methanol, itwas mainly consumed through H abstraction, thus generating twospecies of CH2OH and CH3O. However, the two species shift to CH2Oand subsequently to HCO. Most HCO was consumed via HCO+M⇔H+CO+M, which producing H radicals and promoted flame propa-gation. Therefore, the laminar flame speed difference between me-thanol and the higher alcohols was primarily caused by the differentproductions of smaller molecules in the flames. We should note thatwith an increasing amount of n‑octanol, the branching ratio of eachreaction path for methanol and n‑octanol varied only slightly. This re-sult indicated that the chemical disturbance of each fuel in the blendsmay be ignored. However, the ratio of methanol increases, thus pro-moting the production of H radicals and consequently acceleratingflame propagation. In sum, the effect of chemical kinetics inherentlyresults from changes in fuel components.

Fig. 15 shows the normalized flow rate sensitivity analysis for the

laminar flame speeds of n‑octanol-methanol blends at three equivalenceratios demonstrating the lean, stoichiometric and rich conditions, re-spectively. It is obviously seen that laminar flame speeds at differentinitial conditions were mainly sensitive to small molecule reactions,which emphasizes the characteristics of high temperature chemistry forthe hydrocarbon fuels. Most sensitive reactions were same, showingthese reactions play dominant role in the flame propagation. The mostimportant chain branching reaction of H+O2⇔OH+O had thehighest sensitivity coefficient and its coefficient showed increasingtendency with the addition of higher alcohols. However, most of thesensitive reactions had the reducing sensitivity coefficients especiallymethanol specific reactions due to the decreasing amount of methanolin the mixtures. The competitions among these reactions finally lead tothe change of whole reactivity.

To further evaluate the role of small molecules, Fig. 16 shows themole fractions of the key intermediates in the flames of the n‑octa-nol–methanol blends at different blending ratios. With the increasingmixing ratio of n‑octanol, the mole fraction of CH3 dramatically in-creased, whereas that of HCO decreased. CH3 was easily consumedthrough the chain terminating reaction, CH3+H(+M)⇔ CH4(+M),whereas HCO produced H radicals through the chain branching reac-tion, HCO+M⇔H+CO+M. Thus, H concentration dropped con-siderably. It needs to be noted that the mole fractions of C2H4 and C2H3

also greatly increased. The two species are very reactive; they

(a) n-octanol

(b) methanolFig. 14. Reaction pathways of n‑octanol and methanol for n‑octanol-methanol blends at 1.2 and different blending ratios (Black: oct25; Red: oct50; Blue: oct75). (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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participate in not only chain terminating reactions, such as C2H3+H(+M)⇔ C2H4(+M), C2H4+H⇔ C2H3+H2, but also chain branchingreactions, such as C2H3+O2⇔ CH2CHO+O, C2H3+O2⇔CH2O+HCO, etc. Therefore, the whole reaction rate depends on thecompetition between the production and consumption of reactive ra-dicals, especially H and OH. To further identify the roles of H and OH inflame propagation, we plotted the maximum concentrations of H+OHfor the blends of methanol and n‑octanol (Fig. 17). The laminar flamespeed shown in Fig. 17 directly denotes the correlation between radicalconcentration and laminar flame speed. H+OH concentration slightlyincreased at the lean mixtures and considerably increased at rich mix-tures. This behavior is similar to the laminar flame speed variation.

5. Conclusion

A comprehensive study was conducted to investigate simplifiedhigher mixed alcohols (blends of methanol and n‑hexanol/n‑heptanol/n‑octanol) by employing the spherically propagating flame at 0.1MPa,433 K, and various mixing ratios. The laminar flame speeds andMarkstein lengths of the mixed alcohols were measured. A detailedmodel was proposed to describe the high temperature chemistry of themixed alcohols. Model performance was tested against the present ex-perimental data. Detailed analyses regarding the effects of thermal,diffusion, and chemical kinetics were carried out to illustrate the dif-ferences among the higher alcohols and the laminar flame speed

Fig. 15. Sensitivity analysis of laminar flame speed for n‑octanol-methanol blends at three equivalence ratios.

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variation with increasing blending ratios of methanol. The main con-clusions can be summarized as follows.

1) The flames of the three higher alcohols yielded close laminar flamespeeds and Markstein lengths at the equivalence ratio range becauseof the alcohols' similar diffusion, thermal, and chemical kineticproperties.

2) With the increasing mixing ratio of higher alcohols, laminar flamespeed slightly decreased at lean mixtures and considerably

decreased at rich mixtures. The maximum laminar flame speedshifted to the lean side. The adiabatic temperature increased, andMarkstein length showed opposite behaviors at extremely rich andlean mixtures. Hence, the thermal and diffusion effects did not ex-plain the laminar flame speed variations of the mixed alcohols.Chemical kinetics should be considered as the main driving factorbehind such variations.

3) A comprehensive high temperature model was developed for themixed alcohols of methanol and C6–C8 primary alcohols. Thismodel accurately captures the rule of the equivalence ratio shift forthe laminar flame speeds of mixed alcohols, and it yields satisfactorypredictions at various mixing ratios.

4) Reaction pathway analyses revealed that the addition of higher al-cohols to methanol exerted little effect on the reaction path andbranching ratios. The effect of chemical kinetics was inherentlycaused by changes in fuel composition, which were ultimatelycharacterized by the concentration variations of small reactive ra-dicals. Specifically, the maximum concentrations of H+OH ex-hibited exactly the same behavior as the laminar flame speeds.

Acknowledgments

This work is supported by the National Natural Science Foundationof China (Grant No. 51776163, 51406159 and 91441203), the ChinaPostdoctoral Science Foundation (2018M633507), the ShaanxiProvince Postdoctoral Science Foundation, and State Key Laboratory ofEngines, Tianjin University (K2016-02).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://

Fig. 16. Concentrations of key intermediates in the mixed alcohol flames at 433 K, 0.1MPa, and equivalence ratio of 1.2.

Fig. 17. Relations between laminar flame speed and maximum concentrationsof H+OH for n‑octanol–methanol blends at 433 K and 0.1MPa.(Symbols: experimental laminar flame speed; Solid lines: simulations of MAmodel; Dash lines: maximum concentrations of H+OH).

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doi.org/10.1016/j.fuproc.2019.01.010.

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