Laminar Flame Speeds and Flame Instabilities of Pentanol Isomer–Air

Article pubs.acs.org/EF

Laminar Flame Speeds and Flame Instabilities of Pentanol Isomer− Air Mixtures at Elevated Temperatures and Pressures Qianqian Li, Erjiang Hu,* Xinyi Zhang, Yu Cheng, and Zuohua Huang* State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China S Supporting Information *

ABSTRACT: Laminar flame speeds of three pentanol isomer (1-, 2-, and 3-pentanol)−air mixtures were measured at equivalence ratios of 0.6−1.8, initial pressures of 0.10−0.75 MPa, and initial temperatures of 393−473 K using the outwardly propagating spherical flame. A recently developed kinetic mechanism of 1-pentanol oxidation (Dagaut model) was used to simulate the laminar flame speeds of 1-pentanol−air mixtures under experimental conditions. A comparison between simulation and measurement shows that the simulation yields good agreement on the stoichiometric and fuel-rich side, but it gives lower values on the fuel-lean side. A kinetic modeling study was performed, and several rate constants of selected elemental reactions were modified on the basis of the sensitivity analysis. The modified model gives good prediction on the laminar flame speed under all experimental conditions. The modified model is also validated against the jet-stirred reactor (JSR) experimental data, and it exhibits good prediction for most species. 1-Pentanol gives the fastest laminar flame speed, followed by 3- and 2-pentanol. 2- and 3-pentanol have very close values considering the experimental uncertainty. With the increase of the pressure, the difference in the laminar flame speed among pentanol isomers is decreased. The flame instability of three pentanol isomers was also analyzed. 2- and 3-pentanol have similar instability behavior with a close density ratio, flame thickness, and Lewis number, while 1-pentanol shows slightly high instability behavior. In comparison to 2- and 3-pentanol, 1-pentanol has a smaller critical radius and Peclect number, and this suggests its high instability behavior. Table 1. Physical Parameters of Fuels1,9,10

1. INTRODUCTION With an increasing demand for environmental protection and energy savings, the research of alternative clean fuels has been attracting more and more attention. Researchers have conducted a large number of studies to research more efficient production of clean alternative fuels. Alcohols, as a representative of biofuels that can be derived from biomass, are attracting the attention from governments, industrial sectors, and scientists. Up to now, most of the studies on alcohols concentrate on low-carbon alcohols, such as methanol and ethanol. Some fundamental studies on fuel pyrolysis and oxidation kinetics, fuel ignition, and laminar flame speed as well as the operation of alcohol−diesel and alcohol−gasoline on engines have been conducted.1−4 With the development of production technology, interests in propanol, butanol, and pentanol have increased recently. Pentanol, with five carbons, exhibits even better combustion characteristics and high energy density. Studies by Nielsen et al.5 and Cann and Liao6 showed that pentanol and its isomers were very promising alternative fuels. In comparison to low-carbon alcohols, pentanol is a better non-hygroscopic, less corrosive fuel, with a close octane number to that of gasoline, higher heating value close to gasoline, higher vapor pressure to make it safer in storage and transportation, easier to mix with gasoline at various proportions, and improved cold-start performance in engines.7,8 The physical parameters of different fuels are compared in Table 1. Several kinds of methods have been developed to more efficiently produce pentanol and its isomers. Pentanol, as one kind of biofuel, can be produced through fractional distillation of fossil oil and fermentation. It can also be produced in the © 2013 American Chemical Society

ethanol oxygen content (mass %) energy volume density (kJ/cm3) cetane number octane number (R + M)/2 lower heating value (MJ/kg) enthalpy of vaporization (kJ/g) solubility in water at 20 °C (mL/100 H2O)

nbutanol

npentanol

gasoline 0 31.87

0.35 21.11

0.22 26.9

0.18 28.38

8 100

17 87

18.2

28.9

33.1

34.65

42.5

837.86

584.19

503.69

348.88

unlimited immisciblity

9.1

2.7

82−88

Escherichia coli and terpenoid pathway and the Fischer− Tsopsch process.6,11−13 Engineering-scale production of pentanol was also tested in the past several years, and further work is underway to search for cost-effective methods for the production of biopentanol. Up to now, only limited studies on fundamental combustion of pentanol were reported. Kinetic mechanisms of n-pentanol and isopentanol were developed by Dagaut and co-workers7,14 They extended the reaction mechanisms from previous scheme for the oxidation of lighter hydrocarbons or alcohols. Stable species concentration profiles were measured in a jet-stirred reactor (JSR) at 10 atm and various equivalence ratios, and the Received: November 22, 2012 Revised: December 26, 2012 Published: January 2, 2013 1141

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2. EXPERIMENTAL AND NUMERICAL APPROACHES

proposed model could give a good prediction. Engine studies on alcohol fuels containing pentanol were also reported.9,15,16 To provide a greater understanding of the combustion behavior and to extend the application of pentanol, a further fundamental study on pentanol is necessary. Fuel property varies with its structure. Fuel isomers possess the same thermodynamic and transport properties but different chemical properties. Previous studies have been reported on the propanol and butanol isomers. Veloo and Egolfopoulos13 studied the laminar flame speed and extinction strain of propanol isomers. Their experimental results showed that npropanol demonstrated a higher laminar flame speed and extinction strain than those of isopropanol. Through sensitivity and reaction path analyses, they summarized two major contributors to this difference. One contributor is higher concentrations of propene in the isopropanol flame, which results in relatively unreactive allyl radicals. Another contributor is higher concentrations of formaldehyde reacting readily to form formyl radicals, which are overall reactivity enhancers. Veloo and Egolfopoulos17 and Gu et al.18 conducted the kinetic modeling of laminar flame speed for four butanol isomers. Both studies indicated that n-butanol gave the fastest flame speed, followed by isobutanol and sec-butanol, while tert-butanol gave the slowest flame speed. Moss et al.19 and Sarathy et al.20 developed the oxidation kinetics for the four butanol isomers. Dominant reaction pathways were provided, and simulations with mechanisms were compared to experimental data, including premixed laminar flame speed, ignition delay, and premixed flat flame species profiles. A reasonably good agreement was achieved. Pentanol has eight isomers because of the different positions of −OH and −CH3. Some physical properties of pentanol isomers, such as viscosity, density, and self-diffusion, have been studied in the past decade.1,21−23 Very recently, thermal decomposition of pentanol was reported.8 Temperature- and pressure-dependent Rate constants for the dominant reaction channels were computed, and the difference with butanol was analyzed. In this study, an experimental and kinetic study on laminar flame characteristics of the three pentanol isomer (1-, 2-, and 3pentanol)−air mixtures was conducted. As shown in Figure 1,

2.1. Experimental Approach. The experiment apparatus has been described in detail in refs 24 and 25. A brief description is given here. The experimental apparatus consists of four parts, including a heating system, a photography acquisition system, an ignition system, and a constant volume combustion bomb. A high-speed camera with a speed of 10 000 frames per second was used to record the flame images. The bomb was heated with the heating tape wrapped around the bomb. When the bomb is heated to the experimental temperature, the liquid fuel is injected into the bomb with a syringe. Oxygen and nitrogen are introduced according to their partial pressures. Ignition starts after keeping for 10 min. To ensure a full vaporization of liquid fuel, the partial pressure of liquid fuel in all mixtures is below 70% of its saturated vapor pressure. Experiment tests show that 10 min is sufficient for the vaporization of pentanol isomers and to form a homogeneous mixture. For each condition, experiments are repeated 3 times to ensure its repeatability. For a spherically expanding flame, the raw flame radius, rf(t), is derived directly from the photography. The flame propagation speed is derived by Sb = drf/dt. The flame stretch rate is calculated by κ = 2Sb/ rf. At the early stage of flame propagation, there exists a linear relationship between the flame propagation speed and stretch rate, S0b − Sb = Lbκ.26 Through linear regression, the unstretched flame propagation speed, S0b, and Markstein length, Lb, are obtained. The laminar flame speed is obtained through S0u = (ρbS0b)/ρu based on mass conservation across the flame front. ρu and ρb are the unburned and burned gas densities, respectively. The adiabatic temperature, Tad, and gas density are calculated through thermal equilibrium. The radii used to derive the laminar flame speed in this paper are in the range of 6− 25 mm to eliminate the effects of ignition energy and pressure rise.25,27,28 In addition, the radius is restricted by the occurrence of the cellular structure. All data of laminar flame speeds of pentanol isomers are provided in the Supporting Information of this paper. 2.2. Kinetic Model. Simulation on laminar flame speeds was performed using the Chemkin and Premix codes.29,30 The kinetic model (Dagaut model) used in this study was developed on the basis of the measured species concentration in JSR and the previously proposed C1−C4 alcohol oxidation mechanism.7 It contains 2099 reactions and 261 species. In addition, modification on the Dagaut model was made to obtain better predictions on the measured laminar burning velocities and in comparison to JSR experimental data.

3. RESULTS AND DISCUSSION 3.1. Laminar Flame Speed of 1-Pentanol and Kinetic Analysis. Figure 2 shows the variation of the flame radius versus time for the stoichiometric 1-pentanol−air mixture at different initial temperatures and pressures. The linear relationship between flame radius and time is presented. An increasing temperature or a decreasing pressure will increase the flame propagation speed. Figure 3 shows the stretched flame propagation speeds versus stretch rate at different initial temperatures and pressures for the stoichiometric 1-pentanol−air mixtures. A linear relationship between the stretched flame propagation speed and stretch rate is presented. The stretch rate decreases with the increase of the flame radius. Flame propagation speeds up as the flame develops. When the radius tends toward infinity, the stretch rate reaches zero. The unstretched flame propagation speed is derived through extrapolation of the stretched flame propagation speed versus stretch rate to the zero stretch point. Figure 4 gives both measured and simulated laminar flame speeds of 1-pentanol−air mixtures versus equivalence ratio at three initial temperatures and four initial pressures. A reasonable prediction is presented at the stochiometric and rich mixtures. Simulation underpredicts the laminar flame speed

Figure 1. Chemical structures of 1-, 2-, and 3-pentanol isomers.

these three pentanol isomers have a similar structure with the straight carbon chain and one hydroxyl attached. The only difference is the position of hydroxyl attached to the carbon. Experiments were performed at equivalence ratios of 0.6−1.8, three initial temperatures (393, 433, and 473 K), and four initial pressures (0.1, 0.25, 0.5, and 0.75 MPa), using an outwardly propagating spherical flame and high-speed schlieren photography. First, laminar flame speeds of 1-pentanol at various temperatures and pressures were measured. The kinetic model was modified on the basis of the recently developed mechanism by Togbé et al.7 Second, a comparison on the laminar flame speed of three pentanol isomers was made. Finally, the flame instability of three pentanol isomers was analyzed. 1142

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Figure 2. Flame radius versus time.

Figure 4. Laminar flame speeds versus equivalence ratio. Symbols, measurement; lines, calculation.

Figure 5 gives the relatioship between the adiabatic temperature, Tad, and equivalence ratio. The adiabatic temperature is relatively sensitive to temperature rather than pressure. This suggests that temperature contribution to the laminar flame speed is stronger than pressure. Sensitivity analysis in

Figure 3. Stretched flame propagation speed versus stretch rate.

at the lean mixture side but represents the tendency well. Laminar flame speeds reach their peak values at an equivalence ratio of 1.1. A comparison suggests that further improvement on the kinetic model is required to make it more accurately predict at lean mixture. The decreasing trend in the laminar flame speed becomes weakening as the pressure increases.

Figure 5. Adibatic temperature versus equivalence ratio. 1143

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Figure 6 shows that main-chain branching reactions and termination reactions are sensitive to temperature and pressure.

Competition between chain branching reactions and chain termination reactions determines the laminar flame speed variation tendency. Experimental results implicate that the effect of chain branching reactions increases with the increase of the temperature and pressure. On the basis of the sensitivity analysis from the effect of the equivalence ratio, as shown in Figure 6a, a modification is made on the Dagaut model. Several reaction rates are updated in values by referencing the latest literature. The modified rate constants of the targeted reactions are listed in Table 2. Simulations on laminar flame speeds were conducted using the modified model. Figure 7 gives the comparison between the

Figure 7. Laminar flame speed versus equivalence ratio. Symbols, measurement; lines, calculation with the modified model.

simulated and measured laminar flame speeds. It can be seen that the modified model can accurately predict the laminar flame speeds at all equivalence ratios, temperatures, and pressures. To validate the modified model for 1-pentanol combustion besides laminar flame speed, the modified model was also validated against the JSR data by Togbé et al.7 A comparison between modified model and Dagaut model predictions on species concentration was also made. Figure 8 gives the

Figure 6. Sensitivity analysis.

Table 2. Comparison of Reaction Rates between the Dagaut and Modified Models Dagaut model A

H + O2 + M = HO2 + M

(1)31

C2H 2 + O = HCCO + H

(2)32

C2H3 + O2 = CH 2CHO + O

(3)33

n

new model Ea

8.00 × 1017 −0.8 0 H2O, 16.25; CO, 1.875; CO2, 3.75; CH4, 16.25; C2H6, 16.25; and H2, 2.5 4.62 × 104

2.6

656

3.53 × 1017

−1.36

5580

1144

A

n

Ea

4.65 × 1012 0.44 0.00 × 100 20 LOW, 6.366 × 10 , −1.72, and 5.248 × 102; TROE, 0.5, 1 × 10−30, and 1 × 1030; H2, 2.0; H2O, 14; O2, 0.78; CO, 1.9; CO2, 3.8; AR, 0.67; and HE, 0.8 1.63 × 107 2 1900 5.50 × 1014

−0.611

5.26 × 103

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Figure 8. Experimental and computed species concentration profiles at ϕ = 1.0, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line, modified model; dashed line, Dagaut model.

MPa. 1-Pentanol gives a higher laminar flame speed than those of 3- and 2-pentanol. This is consistent with the previous study that n-alcohol gives the highest laminar flame speed among all isomers.34 2- and 3-pentanol give almost the same laminar flame speeds. To further analyze laminar flame behaviors of three pentanol isomers, extended experiments were carried out at an initial temperature of 433 K and four initial pressures, as shown in Figure 12. Consistent with the behavior in Figure 11, 1pentanol gives the highest laminar flame speed among three pentanol isomers at the elevated pressures. At a pressure of 0.1 MPa, the difference between 1- and 2- and 3-pentanol is increased when the temperature changes from 393 to 433 K. At elevated pressures, the difference among three pentanol isomers is decreased. An increasing temperature increases the laminar flame speed, while an increasing pressure decreases the laminar flame speed. The adiabatic flame temperature, Tad, is a very important parameter for the laminar flame. It has a positive relationship with the laminar flame speed. Figure 13 gives the adiabatic

comparison between model predictions and experimental data for the stoichiometric 1-pentanol−air mixture at p = 10 atm and τ = 0.7 s. Predictions on species, such as O2, CO, CH2O, pentanol, and n-pentanol, with the modified model are slightly improved in comparison to those with the Dagaut model. Predictions on other species, such as C2H4, CH4, and H2, with the modified model are greatly improved. Simulations on species concentration profiles for the lean and rich 1-pentanol− air mixtures were also performed, as shown in Figures 9 and 10, respectively. For the lean mixture (ϕ = 0.5), both Dagaut and modified models can accurately predict the CH4 concentration. The modified model improves the prediction on C2H4. Prediction on H2 is largely improved with the modified model. For the rich mixture (ϕ = 2.0), both modified and Dagaut models give the approximate prediction on species concentrations of CH4 and H2, while the modified model improves the prediction on the species concentration of C2H4. 3.2. Laminar Flame Speeds of Three Pentanol Isomers. Figure 11 shows laminar flame speeds versus equivalence ratio of three pentanol isomers at 393 K and 0.1 1145

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Figure 11. Laminar flame speeds versus equivalence ratio for three pentanol isomers.

flame temperatures for three pentanol isomers. A small difference in the adiabatic flame temperature is presented. An enlargened graph shows that the adiabatic flame temperature of 1-pentanol is higher than those of 2- and 3-pentanol. Almost no difference between 2- and 3-pentanol is demonstrated. The behavior of the adiabatic flame temperature among three pentanol isomers is consistent with the behavior of the laminar flame speed among three pentanol isomers. The molecular structure plays an important role in the laminar flame speed. Previous studies covered some kinds of hydrocarbon and alcohol isomers.13,17,34,35 In comparison to OH branched-chain alcohols, OH straight-chain alcohols give a higher reaction rate.13,17,34,35 Their influences on the following reactions contribute to the higher reaction rate of OH straightchain alcohols: (a) In OH straight-chain alcohol flames, notably higher concentrations of formaldehyde are formed, which, in turn, lead to the formation of more formyl radicals. Formyl radicals are a very important chain-branching reaction through R1 to generate H and CO and contribute to the increase in the reaction rate.

Figure 9. Experimental and computed species concentration profiles at ϕ = 0.5, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line, modified model; dashed line, Dagaut model.

HCO + M = H + CO + M

(R1)

(b) OH branched-chain alcohols produce more CH3 radicals, which, in turn, consume H through recombination reaction R2. CH3 + H + M → CH4 + M

(R2)

(c) Carbons are easily consumed through molecular dehydration and form stable species of alkene and water when hydroxy (OH) attaches on β-carbon and γ-carbon compared to α-carbon. This will decrease the reaction rate. 3.3. Flame Instability. Both diffusional−thermal instability and intrinsic hydrodynamic instability were analyzed in this study. Intrinsic hydrodynamic instability caused by a density jump across the flame front becomes more important at an elevated pressure. It can be analyzed by the density ratio and flame front thickness.36,37 The flame front thickness and density ratio are calculated via equations lf = v/S0u and σ = ρu/ρb. A decrease in the flame front thickness and an increase in the density ratio enhance the hydrodynamic instability. Diffusional−thermal instability caused by non-equal diffusion between heat and mass can be assessed by the Lewis number (Le), the ratio of mixture thermal diffusivity/mass diffusivity.28,38−41 Markstein length, Lb, is a parameter characterizing the effect of stretch on flame propagation and reflects flame instability. A negtive value of Markstein length corresponds to an unstable

Figure 10. Experimental and computed species concentration profiles at ϕ = 2.0, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line, modified model; dashed line, Dagaut model.

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Figure 12. Laminar flame speeds versus equivalence ratio at elevated pressures for three pentanol isomers.

Figure 14. Markstein lengths versus equivalence ratio for three pentanol isomers.

Figure 13. Adabatic flame temperature versus equivalence ratio for three pentanol isomers.

Table 3. Instability Parameters of Pentanol Isomers at 393 K and 0.1 MPa

flame front structure, and a positive value of Markstein length corresponds to a stable flame front structure. A high Markstein length indicates a highly stable flame front as the flame develops. Figure 14 gives the Markstein lengths of three pentanol isomers at 0.1 MPa and 393 K. The Markstein length decreases monotonically with the increase of the equivalence ratio. Markstein lengths of three pentanol isomers give close values, and this indicates the same flame front stability and/or instability behavior for the three pentanol isomers. Table 3 gives the parameters that reflect the flame instability for three pentanol isomers at 393 K, 0.1 MPa, and various equivalence ratios. They are flame front thickness lf, density ratio σ of unburned mixture/burned mixture, and Lewis number. Close values in parameters are presented at a fixed equivalence ratio among three pentanol isomers, especially for 2- and 3-pentanol. 1-Pentanol gives a smaller flame thickness and higher density ratio than those of the other two isomers,

ϕ 0.8

1.2

1.6

fuel

lf (mm)

σ

Le

1-pentanol 2-pentanol 3-pentanol 1-pentanol 2-pentanol 3-pentanol 1-pentanol 2-pentanol 3-pentanol

0.0471 0.0527 0.0513 0.039 0.0424 0.0414 0.1076 0.1106 0.1217

5.6624 5.6404 5.6379 6.4661 6.4334 6.4297 6.1691 6.1188 6.1131

2.4137 2.4249 2.4268 0.9248 0.9311 0.9322 0.8840 0.8917 0.8931

reflecting its higher hydrodynamic instability behavior than those of the other two isomers. Meanwhile, Le of 1-pentanol is smaller, which indicates its higher diffusional−thermal instability behavior. Thus, 1-pentanol has a higher flame front 1147

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instability tendency than those of the other two isomers. The flame front just reflects the diffusional−thermal instability because hydrodynamic instability is not significant at atmospheric pressure. The difference is small, which is also demonstrated in the photos. Figure 15 gives the flame

Figure 16. Schlieren photos of three pentanol isomers at elevated pressures (Tu = 433 K and ϕ = 1.1).

cellular structure at the flame front increases the flame front area, leading to a rapid increase in flame speed. This phenomenon easily occurs at an elevated pressure. Bechtold and Matalon42 defined the onset radius of the cellular flame front structure as the critical radius, Rcr. In this study, we adopt the method by Jomaas et al.43 to give the Rcr. A dimensionless critical Peclet number, Pecr, which is the ratio of Rcr to flame thickness, lf, is also used to reflect flame instability. Figure 17

Figure 15. Schlieren photos of three pentanol isomers at Tu = 393 K, pu = 0.1 MPa, and ϕ = 1.6.

propagation photos of three pentanol isomers at a rich mixture (ϕ = 1.6). The photos show similar flame front morphology at the same flame radius among three pentanol isomers. Some cracks appear at the beginning of flame propagation, but the number of cracks does not increase. Table 4 gives the instability characterizing parameters of three pentanol isomers at 433 K and ϕ = 1.1 at four initial Table 4. Instability Parameters of Pentanol Isomers at 433 K and ϕ = 1.1 Pu (MPa) 0.10

0.25

0.50

0.75

fuel

lf (mm)

σ

Le

1-pentanol 2-pentanol 3-pentanol 1-pentanol 2-pentanol 3-pentanol 1-pentanol 2-pentanol 3-pentanol 1-pentanol 2-pentanol 3-pentanol

0.0395 0.0431 0.0416 0.0189 0.0203 0.0199 0.0112 0.0118 0.0114 0.0092 0.0099 0.0088

5.9283 5.9059 5.9033 5.9727 5.9484 5.9456 5.9988 5.9731 5.9702 6.0112 5.9849 5.9819

0.9396 0.9433 0.9443 0.9396 0.9433 0.9443 0.9396 0.9433 0.9443 0.9396 0.9433 0.9443

pressures. The corresponding schlieren photos at a specified flame radius are provided in Figure 16. With the increase of the pressure, the flame thickness (lf) is decreased and the density ratio (σ) is increased, with little variation in the Lewis number (Le). This reflects the enhanced hydrodynamic instability and insusceptible diffusional−thermal instability as pressure is increased. At low pressure (0.1 MPa), a smooth flame front is presented during flame propagation. At a pressure of 0.25 MPa, cracks appear on the flame front surface. When the pressure is further increased (0.5 and 0.75 MPa), the flame front surface tends to be more unstable and a cellular structure occurs. At an elevated pressure, the cellular structure of 1pentanol is stronger than those of the other two isomers. During flame propagation, the changing on the flame front structure will result in the variation of flame characteristics, such as cellular structure and cracks. The occurrence of the

Figure 17. Stretched flame speed versus stretch rate.

gives the stretched flame speeds of 2- and 3-pentanol versus stretch rate at 0.5 MPa and three equivalence ratios. The onset of the cellular structure is presented for rich mixtures of these two pentanol isomers. Figure 18a gives the Rcr of three pentanol isomers versus equivalence ratio at elevated pressures (0.5 and 0.75 MPa). Rcr decreases with the increase of the equivalence ratio for the three pentanol isomers, indicating the 1148

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pentanol, 1-pentanol has a smaller flame thickness and a higher Le number and density ratio, suggesting its high flame instability behavior. The critical radius and Peclect number, which correspond to the onset of the cellular structure, are smaller for the 1-pentanol flame.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

All data of laminar flame speeds for pentanol isomers (Tables S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Telephone: 0086-29-82665075. Fax: 0086-29-82668789. Email: [email protected] (E.H.); [email protected] (Z.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51136005 and 51206132) and the National Basic Research Program (2013CB228406).

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REFERENCES

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Figure 18. Critical parameters versus equivalence ratio for three pentanol isomers: (a) critical radius and (b) critical Peclet number.

enhancement of cellular structure development when the mixture is enriched. For a lean mixture, the diffusional−thermal instability is suppressed (Le > 1) and the flame is hard to develop into the cellular structure, at least within the observable window. 2- and 3-pentanol give the approximate critical radius, while 1-pentanol gives a smaller critical radius. This indicates the easily unstable flame front for 1-pentanol, which is consistent with the observation in flame photos in Figure 17. The decrease of Rcr corresponds to the advancement of the onset of flame cellularity. Figure 18b gives the critical Peclet number versus equivalence ratio for three pentanol isomers at elevated pressures. Similar to Rcr, Pecr decreases monotonically with the increase of the equivalence ratio. 1-Pentanol gives a lower Pecr than those of 2- and 3-pentanol.

4. CONCLUSION Laminar flame speeds of three pentanol isomers (1-, 2-, and 3pentanol) were measured, and kinetic analysis on 1-pentanol was made by both Dagaut and modified models. Flame instabilities were analyzed. The main conclusions are summarized as follows: (1) Laminar flame speeds of 1-pentanol increase with the increase of the temperature and the decrease of the pressure. Predictions using the Dagaut model give lower values than the measurement for the lean mixture, and discrepancy is decreased with the increase of the pressure. Good agreement between model prediction and measurement is presented at stoichiometric and fuel-rich mixtures. (2) Modification on the Dagaut model gives good prediction on the laminar flame speed. (3) The laminar flame speed of 1-pentanol gives the highest value, followed by those of 3- and 2-pentanol. The difference between three isomers decreases with the increase of the pressure. (4) In comparison to 2- and 31149

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dx.doi.org/10.1021/ef301901c | Energy Fuels 2013, 27, 1141−1150