Laminar Flame Speeds of Gasoline Surrogates Measured with the Flat

Jan 26, 2016 - Surrogates used in the current work are the primary reference fuels (PRFs, ... mixtures of toluene and PRFs), and the ethanol reference...
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Laminar Flame Speeds of Gasoline Surrogates Measured with the Flat Flame Method Y.-H. Liao*,† and W. L. Roberts‡ †

Department of Mechanical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia

‡

ABSTRACT: The adiabatic, laminar flame speeds of gasoline surrogates at atmospheric pressure over a range of equivalence ratios of Φ = 0.8−1.3 and unburned gas temperatures of 298−400 K are measured with the flat flame method, which produces a one-dimensional flat flame free of stretch. Surrogates used in the current work are the primary reference fuels (PRFs, mixtures of n-heptane and isooctane), the toluene reference fuels (TRFs, mixtures of toluene and PRFs), and the ethanol reference fuels (ERFs, mixtures of ethanol and PRFs). In general, there is good agreement between the present work and the literature data for single-component fuel and PRF mixtures. Surrogates of TRF mixtures are found to exhibit comparable flame speeds to a real gasoline, while there is discrepancy observed between isooctane and gasoline. Moreover, the laminar flame speeds of TRF mixtures with similar fractions of n-heptane are found to be insensitive to the quantity of toluene in the mixture. Mixtures of ERFs exhibit comparable flame speeds to those of TRFs with similar mole fractions of n-heptane and isooctane.

1. INTRODUCTION Laminar flame speed of a fuel/oxidizer mixture has been extensively studied in combustion research and widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism because it is indicative of the reactivity, diffusivity, and exothermicity of the given mixture.1−3 In many combustion applications, such as in engines and gas turbines, flame speeds play a significant role in their performance and emissions. Moreover, the autoignition of the mixture ahead of a propagating flame front in an engine, known as engine knock, is greatly associated with the speed of flame propagation.4 It has been shown that flame speeds are fundamental data in turbulent combustion modeling, which often assumes that combustion takes place in the so-called flamelet regime.5,6 In the flamelet regime, turbulent flames can be considered as an ensemble of laminar-like flame fronts, which are locally propagating through turbulent flow fields, resulting in the local flame experiencing flame stretch, a combination of hydrodynamic and curvature effects.5 This flame stretching increases the flame surface area, resulting in an increase in the turbulent flame speed. Thus, the accurate determination of laminar and turbulent flame speeds is essential in designing more efficient engines and developing more accurate computational models. The accurate chemical modeling unquestionably requires a correct representation of the fuel of interest. Gasoline is a very complex mixture of hundreds of hydrocarbons, primarily consisting of paraffins, naphthenes, and aromatics.7 In addition, there is a wide variation in composition between market gasoline fuels.7 It is therefore impossible and impractical to develop a detailed chemical model that can accurately predict and exhibit the combustion characteristics of gasoline. To meet the increasing demand in fuel efficiency and fuel flexibility and the stricter requirement of reduced combustion emissions, simplified surrogate fuels are often used to model the physical and chemical characteristics of practical fuels and to improve © XXXX American Chemical Society

the understanding of the reaction chemistry of combustion processes.7,8 Mixtures of n-heptane and isooctane are the primary reference fuels (PRFs) for octane ratings and are commonly used as convenient surrogates for fuels with variable octane ratings.7,9,10 Jerzembeck et al.3 showed that PRF87, consisting of 87% isooctane and 13% n-heptane by liquid volume, appeared to exhibit similar flame speeds to a commercial gasoline with an octane number (ON) of 90 over a pressure range from 10 to 25 bar. Although PRF mixtures are being widely used as gasoline surrogates, practical fuels are very different from PRFs in compositions and behave quite differently than PRFs in engines as design and operating conditions change.11 Pitz et al.7 suggested that any gasoline surrogate should contain n-heptane, isooctane, and toluene as a result of the fact that toluene is typically the most abundant aromatic in gasoline. Several studies10−13 have shown that the characteristics of a real gasoline at engine operating conditions can be satisfactorily predicted and successfully repeated using a toluene reference fuel (TRF), mixtures of n-heptane, isooctane, and toluene, with the same octane rating. Ethanol has gained interest as an alternative fuel primarily as a result of the increased concern of fossil fuel ability and environment issues arising from the release of anthropogenic carbon.14 The advantages of having a high ON [research octane number (RON) of 109 and motor octane number (MON) of 90]15 and being easily blended with hydrocarbon fuels make ethanol very attractive in practical combustion applications.14 Some studies have suggested that the addition of ethanol in gasoline leads to an increase in fuel efficiency and engine performance16,17 as a result of the presence of the oxygenated compound. The fuel, consisting of 85% ethanol with gasoline, is Received: June 26, 2015 Revised: January 26, 2016

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Figure 1. Schematic of the experiment setup.

commonly used in flexible-fuel vehicles in the United States and Europe. The present work is to measure the laminar flame speed of gasoline surrogates at an initial temperature, ranging from 298 to 400 K, using a flat flame burner. Components of surrogate fuels used in the current study are n-heptane, isooctane, toluene, and ethanol. Measurements performed at an initial temperature of 298 K are favorable for chemical model validation, while measurements at elevated temperatures are beneficial for practical combustion applications. The flame speed of surrogates, consisting of different blend concentrations and octane ratings, are compared to that of a real gasoline.

equivalence ratio. Condensation of fuel is ensured not to occur as a result of the fact that the partial pressure of the fuel vapor in the mixture is well below its saturation point, corresponding to the local gas temperature at any location in the flow system.9 Moreover, the fuel pyrolysis is considered to be negligible as a result of the highest temperature in the system being lower than 500 K and the maximum estimated flow residence time being shorter than 2 s.9 This arrangement of the fuel vaporization, similar to that in the study of Davis and Law,19 shows no flow rate oscillation and facilitates to produce a steady, consistent flame. Mixtures in the present work were prepared on the basis of liquid volume fraction. n-Heptane and toluene were from Sigma-Aldrich with a purity better than 99 and 99.8%, respectively; isooctane and ethanol were from Fisher Scientific with a purity better than 99.8 and 99.5%, respectively. One concern in pressurizing the liquid fuel is the possibility of gas dissolution into the liquid fuel, which could, in turn, deteriorate the fuel purity and affect the validity of the measurements. Helium rather than nitrogen was selected as the pressurization gas as a result of its low dissolubility, an order of magnitude lower than that of nitrogen.20 According to the work of Hesse et al.20 and van Lipzig et al.,21 the helium dissolution has a negligible impact on the fuel purity. Ambient air entrainment into the flame was considered to be negligible as a result of the introduction of the nitrogen shroud gas, which isolates the flame from the ambient air. The flow rate of nitrogen was varied such that the velocity of the shroud flow was at least twice of that of the reactant mixture. The burner temperature, varying from 298 to 400 K, was controlled by the combination of an electric heater wrapped around the burner and the coolant flowing through the porous plug. The inlet and outlet temperatures of the coolant were monitored by a pair of K-type thermocouples. The temperature of the reactant gas was also monitored by a K-type thermocouple, and the variation in the temperature was found to be less than 2 K.

2. EXPERIMENTAL SECTION The measurement of the laminar flame speeds in the current work was conducted with a McKenna burner at atmospheric pressure. The burner is made with a bronze porous plug and capable of producing a flat flame free of stretch,18 facilitating the flame speed determination. A schematic of the experimental setup is shown in Figure 1. The premixed mixture is prepared by adding the required amount of vaporized fuel to a metered quantity of air. The desired amount of liquid fuel is metered by pressurizing the fuel with helium through a Coriolis mass flow controller (Brooks Instrument, model QMBC-3), while the flow rate of air is controlled by calibrated mass flow controllers (MKS Instruments). The liquid fuel is atomized into fine droplets using a spray nozzle assisted by preheated air and is converted to the vapor phase inside a heated evaporation chamber. The temperature of the evaporation chamber is set at a temperature of 50 K higher than the boiling point of the fuel to ensure that all of the injected fuel completely vaporizes before leaving the chamber9 while minimizing the risk of pyrolyzing the fuel. The charge is then mixed with the required amount of heated air to achieve the desired B

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Energy & Fuels The adiabatic, laminar flame speed free from the effect of stretch, denoted S0L, was determined from the heat extraction through the coolant, a technique similar to that described in the study of Botha and Spalding.22 The flame was established with a flow rate that gives a velocity slightly higher (typically 5−7% higher) than the adiabatic flame speed, resulting in an unstable, lifted flame. The flow rate was then lowered until a flat, stable flame was stabilized above the surface of the burner. The heat loss from the hot flame to the cool porous plug leads to an increase in the coolant temperature. Through measurements, the entire burner was thermally insulated and the temperature was monitored to ensure that there was negligible heat loss to the ambient and the heat was entirely carried out by the coolant. The inlet and outlet coolant temperatures were recorded when the steady state was achieved at each flow rate adjustment. It has been shown that there is a region of linear variation between the unburned mixture velocity, Vu, and the heat extracted per milliliter of fuel consumed.22,23 The adiabatic, laminar flame speed is therefore obtained through the linear extrapolation to zero heat loss.22,24 One major challenge in the method of Botha and Spalding is the measurement of the heat loss through the coolant,25 which, in turn, affects the accuracy of the flame speed determination. In the work of Botha and Spalding,22 the burner mainly consisted of a burner tube, which, at the top, carried a porous disk made of bronze and a cooling water jacket. The cooling water jacket was wrapped around the tube and extended over the whole length of the burner tube. To form a good thermal contact between the disk and the cooling water jacket, the porous disk was soldered to the burner tube. The reactant mixture entered the burner tube and passed through the steel wool and finally through the porous disk. The capability of extracting the entire heat loss necessary for stabilizing the flame in this arrangement is questionable, mainly as a result of the indirect contact between the porous disk and the cooling water jacket, the accumulating heat in the porous disk, and the potential heat loss to the long burner tube inside, which was packed with steel wool. In the present study, the burner is a McKenna burner, which exhibits some distinct features from that used in the study of Botha and Spalding. First of all, the cooling water jacket in the McKenna burner is embedded and evenly distributed inside the porous disk and closely sits (≈9.5 mm, according to the burner manufacturer) beneath the top surface of the disk, facilitating prompter and more complete extraction of heat through the water jacket. Second, the mixture enters the burner through a plenum chamber and then directly passes through the porous disk. The heat, therefore, conducted away through the burner is minimized, and the heat absorbed by the porous disk is immediately brought out by the coolant. Finally, the McKenna burner is equipped with a shroud flow, in which nitrogen was used in the current work, as mentioned previously. The shroud gas is heated to the target temperature and isolates the burner from the ambient, avoiding possible heat loss to the ambient. During each run of the experiment, the temperatures of the plenum chamber, the shroud gas, and the outer wall of the shroud flow were monitored and set at the target temperature. The temperatures stayed nearly constant (the variation was found to be about 1 K) through each run of the experiment. The uncertainty of the flat flame method has been reported in details in refs 22, 24, and 25, and the primary sources of uncertainty are the accuracy of the flow controllers, the measurement of heat loss, and the validity of the extrapolation. The mass flow controllers used in the current work were regularly calibrated (repeatability is within 0.6%) with a gas flow calibrator (Mesa Laboratories, Inc., Definer Series), and the accuracy was found to be within ±1.0%. The gas flow calibrator is sent back to the manufacturer each year for recalibration to ensure that the desired accuracy is achieved. The maximum deviation in the flow rate generally occurs at the leanest and richest conditions, where mixtures have a relatively lower flow rate as a result of the low flame speed. Throughout the series of experiments, the flow rate of the coolant was kept constant to give a rise in the temperature of at least 15 K (in comparison to the maximum of 10 K in the study of Botha and Spalding), resulting in a fluctuation in the temperature reading of less than 0.5 K. Although a large temperature rise in the coolant (by lowering the flow rate of the coolant) facilitates the

measurement, it was found that the plenum chamber started heating as the flow rate was decreased. The maximum rise in the coolant temperature was controlled to be less than 20 K, resulting in an uncertainty range of 2.5−3.3% in the temperature measurement without heating the plenum chamber. As mentioned in the work of Botha and Spalding,22 the extrapolation is considerably accurate because the measurement data generally fall within a region of linear variation. An example of the flame speed extrapolation for the methane/air mixture at a stoichiometric ratio is shown in Figure 2a. In

Figure 2. (a) Flame speed of the methane/air mixture at a stoichiometric ratio as a function of the amount of heat loss. (b) Laminar flame speed of methane/air mixtures at 298 K and atmospheric pressure. general, the data points used for extrapolation fell within a range of 40−80% of the flame speed, as shown in Figure 2a. This range brought about a maximum of approximately 3% in the variation of the linearity, depending upon the quantity of data points. The validity of the proposed technique is shown in Figure 2b, in which there is generally good agreement between the present study and the literature data on the laminar flame speed of methane/air mixtures at 298 K. Overall, the uncertainty of the flame speed determination, when neglecting the little increment in the plenum temperature, was found to be between ±1.4 and 1.8 cm/s.

3. RESULTS AND DISCUSSION 3.1. n-Heptane/Air Mixtures. The adiabatic, laminar flame speeds of n-heptane/air mixtures, along with the data measured with different techniques in the literature,10,19,21,26 at an initial temperature of 298 K are shown in Figure 3. The data from the work of Davis and Law19 and Huang et al.26 were measured in the counterflow twin-flame configuration with linear extrapC

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by Davis and Law19 for stoichiometric and rich mixtures. However, a discrepancy appears for lean mixtures. Similar experimental setups with the heat flux method were employed by van Lipzig et al.21 and Sileghem et al.,10 but the measurement performed by van Lipzig et al. consistently exhibits higher values than that by Sileghem et al. The maximum accessible equivalence ratio, as mentioned previously, is limited by the onset of the cellular flame. The maximum flame speed is found to occur at an equivalence ratio of Φ = 1.1 with a value of 35.1 ± 1.4 cm/s, as shown in Figure 4. In most practical hardware, the combustion occurs at elevated temperatures and isooctane can be considered as the simplest surrogate as a result of its high octane rating. The effect of the initial mixture temperature on the adiabatic, laminar flame speed of the isooctane/air mixture is shown in Figure 5. Good agreement is found between the present work

Figure 3. Adiabatic, laminar flame speeds of n-heptane at 298 K and atmospheric pressure.

olation to zero stretch, and those in the work of van Lipzig et al.21 and Sileghem et al.10 were obtained in a flat flame burner with the heat flux method. The principle of the latter technique is similar to that employed in the current study, while the heat flux method has a slightly better accuracy than the flat flame method as a result of the fact that the adiabatic, laminar flame speed measured with the heat flux method is found by interpolation25 rather than extrapolation. In general, there is good agreement among measurements with different techniques, particularly for those by Huang et al., van Lipzig et al., and the present work. The peak value of the adiabatic, laminar flame speed in the current study is found to be 39.8 ± 1.4 cm/s at an equivalence ratio of Φ = 1.1. As a result of the flame instability limit associated with the flat flame burner,23,27,28 the richest and leanest accessible equivalence ratios in the current study are Φ = 1.3 and 0.8, respectively. Although the high vapor pressure of n-heptane allows for the preparation of richer mixtures, flames with an equivalence ratio of Φ > 1.3 appear to have a cellular flame structure, similar to that reported by van Lipzig et al.21 3.2. Isooctane/Air Mixtures. The adiabatic, laminar flame speeds of isooctane/air mixtures at an initial temperature of 298 K are shown in Figure 4. Similar to n-heptane, the excellent agreement is also found for isooctane among measurements by Huang et al.,26 van Lipzig et al.,21 and the present work. For both n-heptane and isooctane, the values of the flame speed obtained in the present work are generally consistent with those

Figure 5. Adiabatic, laminar flame speeds of isooctane at various temperatures (298−400 K) and atmospheric pressure.

and that of Dirrenberger et al.29 for all initial temperatures investigated. The flame speed obtained in the present work compares well to that by Kumar et al.9 at 298 K. However, an inconsistency obviously arises at elevated temperatures, with the data by Kumar et al. being generally higher than those in the present work. 3.3. Toluene/Air Mixtures. The adiabatic, laminar flame speed of toluene/air mixtures at an initial temperature of 298 K is shown in Figure 6. In contrast to n-heptane and isooctane fuels, there is little consistency in the flame speed for toluene among the various measurements. The inconsistency even exists between measurements carried out with similar techniques. Davis and Law19 and Hirasawa et al.30 both employed the counterflow twin-flame configuration, and the discrepancy between these two studies is between 2 and 3 cm/ s. The discrepancy probably results from the extrapolation method, either linear or nonlinear, which generally results in the linear technique being 2 cm/s higher than nonlinear.2 The data by Davis and Law were obtained using a linear extrapolation, while those by Hirasawa et al. were obtained using a nonlinear extrapolation. The flame speeds in both studies of Sileghem et al.10 and Dirrenberger et al.29 were determined via the heat flux method, and the difference between their results is found to be approximately 2 cm/s. The values in the present work generally fall between the minimum (the work by Hirasawa et al.) and the maximum (the work by Dirrenberger et al.) among the measurements in the literature.

Figure 4. Adiabatic, laminar flame speeds of isooctane at 298 K and atmospheric pressure. D

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Figure 6. Adiabatic, laminar flame speeds of toluene at 298 K and atmospheric pressure.

Figure 7. Adiabatic, laminar flame speeds of PRF/air and TRF/air mixtures at 298 K and atmospheric pressure.

Good agreement is found between the present work and the work by Sileghem et al. over a range of equivalence ratios of Φ = 0.9−1.2. The peak value of the flame speed occurring at Φ = 1.1 in the present work is 35.5 ± 1.4 cm/s. 3.4. TRF Mixtures. Although mixtures of n-heptane and isooctane, i.e., PRFs, are often used as a surrogate, TRFs are considered as a more appropriate surrogate than PRFs as a result of the presence of toluene in real gasolines, particularly with increasing ON7 and its similar combustion characteristics to gasoline in practical applications.11,31 In the present work, mixtures of TRF were prepared in two groups. The first group consists of a constant percentage of toluene (5%), while the volume fractions of n-heptane and isooctane are varied such that the RON ranges from 86 to 96. The second group allows for the percentage of toluene to vary, ranging from 5 to 41%, with the RON varying from 84 to 95. Details of the mixture compositions investigated are given in Table 1.

regardless of the blend concentration and the ON. The negligible difference between PRFs and TRFs is probably due to the combination of the small quantity of toluene and the similar composition of n-heptane and isooctane in both mixtures. The flame speeds of PRF mixtures in the present work are in excellent agreement with those by Huang et al.26 The adiabatic, laminar flame speeds of surrogates A and B, whose composition is listed in Table 1, at a range of initial temperatures of 338−400 K are shown in Figure 8. Both

Table 1. Compositions of Fuel Mixtures by Liquid Volume

PRF85 PRF90 PRF95 TRF86 TRF91 TRF93 TRF94 TRF96 surrogate surrogate surrogate surrogate surrogate ERF90.4 ERF90.8 ERF91.2

A B C D E

n-heptane (%)

isooctane (%)

15 10 5 15 10 10 10 5 17.6 17 21.9 26 17 10 10 10

85 90 95 80 85 80 75 90 77.4 69 58.1 33 42 85 80 75

toluene (%)

ethanol (%)

RON32,33

5 10 15

85 90 95 86 91 93 94 96 84 87 85 87 95 90.4 90.8 91.2

5 5 10 15 5 5 14 20 41 41

Figure 8. Adiabatic, laminar flame speeds of surrogates A and B at various temperatures (338−400 K) and atmospheric pressure.

surrogates have a comparable volume fraction of n-heptane, but surrogate B consists of more toluene (14%) than surrogate A (5%). The RONs estimated on the basis of the second-order model in ref 32 for surrogates A and B are 84 and 87, respectively. Over the initial temperatures investigated, both surrogates generally exhibit comparable flame speeds, as shown in Figure 8. Moreover, the flame speeds of both surrogates compare well to isooctane at 338 K over a range of equivalence ratios of Φ = 0.9−1.2. However, there are differences observed at elevated temperatures, and the difference is seen to increase with an increasing temperature. Surrogate B has been studied in a shock tube with pressures and temperatures similar to those in homogeneous charge compression ignition (HCCI) engines, and strong agreement in the ignition delay time was found between a real gasoline with RON = 87 and surrogate B.12 Additionally, Andrae et al.11 employed a semi-detailed mechanism to predict the laminar

The adiabatic, laminar flame speeds of PRF and TRF mixtures with a RON of 85−96 at 298 K are shown in Figure 7. The flame speeds of PRF and TRF are generally found to lie between n-heptane and isooctane but are generally more comparable to those of isooctane. Similar to PRF, TRF mixtures in the current study exhibit comparable flame speeds, E

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Energy & Fuels flame speed of surrogate B, and the prediction compared well to the measurements of the laminar flame speed of a real gasoline performed by Zhao et al.13 The gasoline used by Zhao et al. is CR-87, which has an ON of 87. CR-87 contains more than 100 components, and the relative concentration of any single component is less than 2%.11 The comparison of the flame speeds of isooctane, surrogates A and B, and CR-87 at 358 K is shown in Figure 9. The flame

Figure 10. Comparison of the adiabatic, laminar flame speeds of gasoline surrogate fuels at 358 K.

compensated for by increasing the toluene fraction in the mixture, such as surrogates B and D in the current study. 3.5. Ethanol Reference Fuel (ERF) Mixtures. As mentioned previously, ethanol has an advantage of increasing the knock resistance in engines as a result of its inherently high ON, making it an attractive alternative to gasoline. The adiabatic laminar flame speeds of ERFs, along with TRFs, at 298 K are shown in Figure 11. For both reference fuels with a

Figure 9. Comparison of the adiabatic, laminar flame speeds for surrogates A and B and a real gasoline over a range of equivalence ratios.

speeds of both surrogates are found to be comparable to those of CR-87 over a range of equivalence ratios of Φ = 0.9−1.2. The flame speed of isooctane is generally lower than those of both surrogates and CR-87. Although the difference between isooctane and others is small, the value of the difference may increase at higher initial temperatures, as seen in Figure 8. Similar results were reported in the study by Stanglmaier et al.,34 where the flame speeds of gasoline were found to be considerably higher than those of isooctane, particularly at higher temperatures and pressures. The overall results may suggest that mixtures of TRFs could serve as better surrogates to a real gasoline than a single fuel component, particularly at more realistic initial temperatures. In the present work, the volume fraction of toluene in a surrogate was varied from 5 to a maximum of 41%. The maximum toluene fraction was chosen on the basis of the work by Pitz et al.,7 who reported that a real gasoline could contain up to 45% toluene. Once the toluene fraction was chosen, the faction of n-heptane and isooctane was varied such that RON had a value between 84 and 95. The comparison of the adiabatic, laminar flame speeds of surrogates at 358 K is shown in Figure 10. In general, the results are comparable, regardless of the octane rating, for different surrogates, while surrogate D, containing the largest fraction of n-heptane, is consistently higher than others. Good agreement for surrogates A, B, and E suggests that the laminar flame speed of surrogates, containing comparable volume fractions of n-heptane, is relatively insensitive to the quantity of toluene in the surrogate. A comparison between surrogates with a similar fraction of toluene, i.e., surrogates D and E, may suggest that an increase in the volume fraction of n-heptane could lead to an increase in the flame speed, although the increase is small, compared to the measurement uncertainty in the present work. The more nheptane a mixture contains, the higher the flame speed and the lower the RON. However, the decrease in the RON can be

Figure 11. Comparison of the adiabatic, laminar flame speeds for TRF/air and ERF/air mixtures at 298 K and atmospheric pressure.

similar percentage of toluene and ethanol, each mixture contains similar compositions of n-heptane and isooctane. In the current study, the maximum concentration of the ethanol blend in any mixture is below 20%; the RON of ERF mixtures is therefore estimated on the basis of the linear model, as shown by Foong et al.33 The flame speeds between TRFs and ERFs are generally found to be comparable, regardless of the blend concentration. However, ERF mixtures with 15% ethanol consistently exhibit higher flame speeds than the other fuels. The adiabatic, laminar flame speed of pure ethanol is found to have a peak value of approximately 42 ± 1.5 cm/s at Φ = 1.1,35 significantly higher than that of toluene, around 35.5 cm/s, as shown in Figure 6. Although pure toluene has a slightly higher RON of 12032 compared to RON of 109 for pure ethanol,33 the high flame speed of ethanol could be used to promote a higher flame speed of the mixture while not sacrificing the octane rating, as shown in Figure 11. F

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4. CONCLUSION The current work uses the flat flame method, which produces a one-dimensional fat flame free of stretch, to measure the adiabatic laminar flame speeds of gasoline surrogates at atmospheric pressure over a wide range of equivalence ratios and initial temperatures. The accuracy of the present work is approximately between ±1.4 and 1.8 cm/s. The laminar flame speeds of n-heptane, isooctane, and toluene are found to be consistent with the literature data measured with different techniques, validating the present technique. For TRF mixtures with 5% toluene, the flame speeds are comparable to those of PRF mixtures at 298 K. In comparison to isooctane, the flame speeds of the toluene reference fuels are found to be more comparable to those of a real gasoline, suggesting that the toluene reference fuels may serve as better surrogates to gasoline than a single-component fuel. This is especially true as the initial temperature increases. The actual quantity of toluene has a relatively minor impact on the flame speeds of TRF mixtures with similar fractions of n-heptane. The ERFs exhibit comparable flame speeds to the TRFs with similar compositions of n-heptane and isooctane at 298 K.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +886-3-5712121, ext. 55119. Fax: +886-35720634. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Research and Development Center, Saudi Aramco Fuel Technology, under the FUELCOM Program.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b01433 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b01433 Energy Fuels XXXX, XXX, XXX−XXX