Experimental and Numerical Studies on Laminar Premixed Flames of

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Experimental and Numerical Studies on Laminar Premixed Flames of Ethanol−Water−Air Mixtures Junjie Liang, Gesheng Li,* Zunhua Zhang, Zhuang Xiong, Fan Dong, and Rui Yang Key Laboratory of High Performance Ship Technology of Ministry of Education, School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei 430063, People’s Republic of China ABSTRACT: Laminar burning velocities of premixed ethanol−water−air flames over a range of equivalence ratios from 0.7 to 1.6 at 0.1 MPa and 383 K were determined experimentally at different water contents in a combustion chamber with central ignition. An ethanol oxidation mechanism was selected to simulate one-dimensional planar flames of ethanol−water−air mixtures under the same conditions to observe the effect of the water addition on the planar flame structure, the sensitivity of laminar burning velocity, and the net reaction rates of the elementary reactions. The physical effect of water was separated from its chemical effect by designing a type of fictitious water in the simulation. Results show that unstretched flame speeds and laminar burning velocities of the flames decrease with increasing the water content. When the water content was elevated, the peaks of the mole fractions of the main radical species gradually decrease and the net reaction rates of the elementary reactions with positive sensitivity coefficients decrease more than those of the elementary reactions with negative sensitivity coefficients. Both physical and chemical effects of water suppress laminar burning velocities of hydrous ethanol−air mixtures, and the former dominates. The chemical effect of water promotes production of OH and has a much more remarkable influence on the reaction rates of the elementary reactions with negative sensitivity coefficients than on those of the elementary reactions with positive coefficients. The physical effect of water has an inhibiting effect on both the production of the radicals and the reaction rates of the elementary reactions. dependence of laminar burning velocities of ethanol−air flames with a constant-volume combustion chamber. However, in Gülder’s research work, the effect of stretch on spherical flame propagation is not taken into account, possibly leading to significant error in laminar burning velocities.20 Thus, from the above, it is seen that the water dependence of laminar burning velocities of ethanol−air flames lacks accurate determination. The ethanol oxidation mechanism has also been a subject for many researchers. In 1999, Marinov21 proposed a detailed ethanol oxidation mechanism after summarizing the experimental and numerical studies on mechanisms of ethanol oxidation and decomposition. A similar summary was also made by Li,22 and some improvements were proposed for the mechanism by Marinov.21 Then, on the basis of mechanisms by Marinov21 and Norton et al.,23 Li proposed a comprehensive ethanol oxidation mechanism, which was improved in 2007,24 as shown below. In this mechanism, the ethanol submechanism was taken from the Marinov mechanism with modification and updated in parameters of many elementary reactions. Saxena25,26 also proposed an ethanol kinetic scheme. The ethanol oxidation part of the Saxena mechanism was taken from the Li mechanism.22 In 2007, according to the newest information about thermodynamic data, reaction rate constant, and branching ratio of ethanol dissociation, Li et al.24 improved and perfected the mechanism proposed previously. Meanwhile, a comparison was conducted by Li between experimental data from a variable pressure flow reactor (VPRF) and predicted

1. INTRODUCTION In recent years, concern on the depletion of fossil fuels is increasing, and many environmental issues occur as a result of the heavy use of fossil fuels, leading to extensive attention on ethanol, which is a renewable alternative fuel. The ethanol widely used is mainly anhydrous, which means some energy is required to distill and purify ethanol in the process of deriving fuel-grade anhydrous ethanol from biomass. According to the investigation by Marinez-Frias et al.,1 such energy increases exponentially when the ethanol volumetric fraction in the hydrous ethanol is greater than 90%, occupying a significant share in the whole life of ethanol. Therefore, if some water is allowed to exist in the ethanol fuel, considerable energy savings will be reached, consequently increasing the cycle energy efficiency of ethanol. As a matter of fact, Martinez-Frias et al.1 and Mack et al.2 investigated the feasibility of hydrous ethanol as an alternative fuel in homogeneous charge compression ignition (HCCI) engines, and their studies1,2 show that the engines can operate on hydrous ethanol at high water content by volume. With regard to hydrous ethanol as a potential alternative fuel, water dependence of laminar burning velocities of ethanol−air mixtures is less known, as seen from the following. The laminar burning velocity is an important parameter for mechanism validation and analysis of turbulent flame propagation. Much attention has been concentrated on the measurement of laminar burning velocities of ethanol and its blends.3−19 The measurement methods vary from the counterflow twin-flame technique4 to outwardly expanding spherical flames3,5,7−9,11,12,14,15,17−19 as well as the heat flux method.10,13 The species mixed with ethanol−air mixtures involve N2,12 CO2,12 isooctane,11 etc. In 1985, Gülder19 studied the water © 2014 American Chemical Society

Received: December 8, 2013 Revised: June 17, 2014 Published: June 18, 2014 4754

dx.doi.org/10.1021/ef4024178 | Energy Fuels 2014, 28, 4754−4761

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Figure 1. Schematic of the experimental facility.

data derived from the Li mechanism,22,24 Marinov mechanism,21 and Saxena mechanism,25,26 and the result showed that the Li mechanism was predicted more accurately. Konnov27 also put forward a C/H/O/N mechanism for combustion of small hydrocarbons involving H2, CO, formaldehyde, methanol, methane, C2−C3 hydrocarbons and their oxygenated derivatives, etc. This mechanism involves far more species and elementary reactions than the above ethanol mechanisms mentioned. Ethanol can mix with water in any proportion, and water is also one of the products of ethanol combustion. The effect of water in combustion of hydrous ethanol can be divided into two parts, i.e., the physical effect and the chemical effect.28 The physical effect of water is contributed by its dilution in the concentration of reactants and absorption of heat for its thermal capacity, while the chemical effect is defined as how the water works as a species directly participating in the ethanol oxidation reaction. Under the assistance of simulation, the two effects of water can be separated from each other to understand how it works, as shown in detail in sections 3 and 4.3. Therefore, the objective of the present study is to determine the water dependence of laminar burning velocities of ethanol− water−air mixtures at 0.1 MPa and 383 K with equivalence ratios ranging from 0.7 to 1.6 by employing outwardly propagating spherical flames, to conduct a kinetic analysis for the water effect by simulating freely propagating one-dimensional planar flames of ethanol−water−air mixtures under the same conditions, and to separate the physical effect of water addition to ethanol from its chemical effect.

system for heating, a high-voltage spark generator, a data acquisition system, and a schlieren optical system for visualizing flame propagation cooperating with a high-speed digital camera. The cylindrical combustion chamber is made of stainless steel with two quartz optical windows of 80 mm diameter mounted to provide access for viewing the flames. Two electrodes of 1.5 mm diameter are fitted in the chamber to generate a spark at the center. A capture rate of 10 000 frames per second is selected for the camera in the present study. At the start of experiments, the combustion chamber was preheated to 383 K through the heating system, and this temperature was maintained using a temperature control system and the thermal insulation material mounted outside the chamber. The combustion chamber was then vacuumed. The reactant mixture was prepared by filling the chamber at the appropriate partial pressure, depending upon the desired mixture composition and test pressure. About 10 min waited before the ignition started, to ensure homogeneity and motionless of the mixtures in the chamber. Each experimental is repeated at least 3 times for the same condition, and excellent repeatability was achieved. The water content is defined as the volumetric fraction of water in hydrous ethanol at a temperature of 293 K and atmospheric pressure. In this paper, the water content varies from 0 to 30%. The ethanol used was graded at 99.98% purity. 2.2. Data Processing. The laminar burning velocity is derived from the schlieren photography. In the combustion chamber, electrodes were mounted in the horizontal direction, thus affecting the spherical flame propagation in this direction. Consequently, half of the flame front diameter in the vertical direction was taken as the flame radius when data processing. Flame radius measurements were limited to a range of 6−25 mm.17 The lower limit was selected to avoid disturbances because of the transient ignition process. The upper limit was to guarantee that the pressure rise inside the combustion chamber was negligible; i.e., the flame propagation in the chamber could be considered as a quasi-isobaric process. For outwardly propagating spherical flames, the stretched flame speed, Sn, is defined as the rate of change of the flame front radius by

2. EXPERIMENTAL METHOD 2.1. Experimental Setup. In the present study, outwardly expanding spherical flames were employed to determine laminar burning velocities of ethanol−water−air mixtures. A schematic of the experimental facility is shown in Figure 1. Detailed information about this experimental facility can be found in ref 17, and the related experimental error analysis can be referred to refs 17, 29, and 30. Here, only a brief description is presented. The major components of the experimental facility include a constant-volume combustion chamber, a

Sn =

dR dt

(1)

where R is the flame front radius, which can be determined from the schlieren pictures recording the flame propagation process. The stretch rate acting on the flame, α, is defined as the relative rate of change of the flame surface area, i.e. 4755


Energy & Fuels α≡

1 dA 2 dR 2 = = Sn A dt R dt R

Article

To separate the physical effect of water from its chemical effect, one-dimensional planar flames of three types of mixtures, as shown in Figure 3, were modeled. Mixture 1 contained no

(2)

where A is the flame surface area. A nonlinear relationship between the stretched flame speed Sn and the stretch rate α is used to derive the unstretched flame speed Sl31,32 ⎛ Sn ⎞ ⎛ Sn ⎞2 2L α ⎜ ⎟ln⎜ ⎟ = − b Sl ⎝ Sl ⎠ ⎝ Sl ⎠

(3)

where Lb is the Markstein length. The laminar burning velocity, ul, is deduced from Sl using ρ ul = b S l ρu (4) where ρu is the density of the unburned mixture under initial conditions and ρb is assumed to be the density of the burned mixture in the chemical equilibrium state.33 The thermal radiation is not taken into consideration when determining ρb.14,34

Figure 3. Three types of mixtures designed to separate the physical effect of water from its chemical effect in the simulation.

water, while mixture 2 contained an amount of water. A type of fictitious water (expressed as F-H2O), which has the same thermodynamic and transport properties as real water, was designed to be added to mixture 3.38−40 The fictitious water did not appear in the elementary reactions in the oxidation mechanism, except as a third body with the same enhanced coefficient as the real water. Corresponding modifications were made to files of thermodynamic data, transport data, and mechanism. One-dimensional planar flames of these types of mixtures are simulated at the same initial temperature, pressure, and equivalence ratio. The overall effect of water can be derived from the comparison of the modeled results between mixture 1 and mixture 2 and the physical effect of water derived from the comparison between mixture 1 and mixture 3. The chemical effect is equal to the overall effect minus the physical effect. In the following, the two effects of water are presented through specific parameters. For simplicity, HE100 denotes hydrous ethanol at a water content of 0% by volume and FHE100 denotes hydrous ethanol at a fictitious water content of 0% in the paper, etc.

3. NUMERICAL METHOD PREMIX code 35 in CHEMKIN-PRO software 36,37 was employed to simulate freely propagating one-dimensional planar flames of ethanol−water−air mixtures. The interval of equivalence ratio is 0.05, with the equivalence ratio range from 0.6 to 1.6 in the simulation. The effect of thermal diffusion is considered. A minimum of 600 grid points was imposed in the calculation for a fully converged solution of laminar burning velocity to ensure simulation accuracy.22 On the basis of the description of the ethanol oxidation mechanisms in the Introduction, the Li mechanism including 38 species and 238 elementary reactions was selected for the present numerical study. Figure 2 shows the comparison of

4. RESULTS AND DISCUSSION 4.1. Effect of the Water Addition on Premixed Ethanol−Water−Air Flames. Schlieren pictures of spherical flames of hydrous ethanol−air mixtures at 0.1 MPa and 383 K with a water content ranging from 0 to 30% were obtained via the experimental facility mentioned above. After data processing, here are the results. Figure 4 shows the variations of the measured unstretched flame speed of the hydrous ethanol−air mixture with the equivalence ratio at different water contents. As seen in the figure, the unstretched flame speed increases and then decreases when elevating the equivalence ratio and decreases monotonously with increasing the water content. Laminar burning velocities of hydrous ethanol−air mixtures are derived from unstretched flame speeds, and the result is presented in Figure 5. The modeled values of laminar burning velocities are also presented in the figure represented by solid lines. It is seen that the variations of the laminar burning velocity with the equivalence ratio and water content are similar to those of the unstretched flame speed in Figure 4. The experimental values of laminar burning velocities have a reasonably good agreement with the modeled values. The laminar burning velocity is related to the constantpressure adiabatic flame temperature. The flame temperature decrease tends to suppress the laminar burning velocity. Figure

Figure 2. Comparison between experimental values of laminar burning velocities for ethanol−air mixtures at 0.1 MPa and 383 K and the modeled values derived from different mechanisms.

experimental values of laminar burning velocity of ethanol−air flames at 0.1 MPa and 383 K to the modeled values derived from ethanol mechanisms of Li, Marinov, and Saxena, respectively. The experimental values were obtained using the experimental setup mentioned above. As seen in the figure, the discrepancy between the experimental and modeled data becomes larger in the order of the Li mechanism,22,24 the Saxena mechanism,25,26 and the Marinov mechanism,21 therefore indicating that the Li mechanism behaves better. 4756


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Figure 6. Variations of a constant-pressure adiabatic flame temperature with the equivalence ratio at different water contents.

Figure 4. Measured unstretched flame speeds of ethanol−water−air mixtures at different equivalence ratios and water contents.

4.2.1. Effect of the Water Addition on the Planar Flame Structure and the Radical Production. Shown in Figure 7 are computed planar flame structures of stoichiometric ethanol− water−air mixtures at different water contents. Here, the initial mole fractions of reactants C2H5OH and O2 decrease when elevating the water content, which results from the dilution effect of water. The final mole fraction of H2O gradually increases with increasing the water content, while the final mole fraction of CO2 remains approximately unchanged and the final mole fraction of CO decreases. The change in the mole fractions of these three species is the total result of dilution, heat absorption, and chemical effect of water addition to ethanol. In the ethanol oxidation process, atom C first appears in CO, which then mainly reacts with OH to produce CO2 via the elementary reaction CO + OH = CO2 + H. The reaction rate of CO + OH = CO2 + H is lower than that of OH reacting with other typical hydrocarbon species.41 Therefore, a high concentration of hydrocarbon will strongly inhibit the oxidation of CO, leading to a high CO concentration in the ethanol oxidation process, as observed in Figure 7. When the initial hydrocarbon molecules and easily decomposing intermediate hydrocarbon molecules are fully consumed, enabling the OH concentration to rise to a certain level, rapid oxidation of CO into CO2 occurs, leading to the decrease of the CO concentration with distance after it rises to a certain level. According to the modeled result of one-dimensional planar flames, the first five radical species, which have the maximum mole fraction peaks are OH, H, O, CH3, and HO2 in order. Shown in Figure 8 are the mole fraction profiles of these five radical species. As seen in the figure, the peaks of mole fractions of OH, H, and O are greater than those of CH3 and HO2 by an order of magnitude. When elevating the water content, the peaks of the mole fractions of all of these five radical species decrease. In other words, to add water to ethanol suppresses the radical production. However, water does not always suppress the production of the radical species. According to the study by Singh et al.,42 adding up to 20% water to the H2− CO (5:95)−air flames will promote the production of the radical species OH and H, and then, when the water content continually increases, the peaks of mole fractions of OH and H decrease.

Figure 5. Variations of the laminar burning velocity of the ethanol− water−air mixture with the equivalence ratio at different water contents. The symbols represent experimental values, and the lines represent modeled values.

6 presents the relationship between the flame temperature, which are calculated using the element potential method, and the equivalence ratio, similar to the relationship between the laminar burning velocity and the equivalence ratio. At a fixed water content, the flame temperature first increases and then decreases with increasing the equivalence ratio. When water is added into the ethanol, the flame temperature decreases. The flame temperature decreases more with the water content at the equivalence ratio above 1.0 than below 1.0, indicating that, at the rich-fuel side, water has a more remarkable influence on the flame temperature. As shown in Figure 5, a more remarkable influence of water is also observed on the laminar burning velocity at the rich-fuel side than at the lean-fuel side, indicating that a relationship exists between the laminar burning velocity and the flame temperature. 4.2. Chemical Kinetic Analysis of the Effect of Water Addition. To better understand the effect of water addition on laminar premixed flames of ethanol−water−air mixtures, a kinetic analysis is conducted via simulating freely propagating one-dimensional planar flames of ethanol−water−air mixtures. 4757


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Figure 7. Computed one-dimensional flame structures of the stoichiometric ethanol−water−air mixtures at different water contents.

Figure 8. Mole fraction profiles of the radical species in stoichiometric ethanol−water−air planar flames at different water contents.

4.2.2. Sensitivity Analysis of the Laminar Burning Velocity. The sensitivity analysis of laminar burning velocities of stoichiometric ethanol−air mixtures at different water contents was conducted, as shown in Figure 9. From the figure, it is seen that laminar burning velocities are relatively insensitive to reactions related to the ethanol oxidation but sensitive to the elementary reactions involving oxidation of the H atom and CO reaction with OH radicals. When the water content is increased, sensitivity coefficients of H + O2 = O + OH, H + OH + M = H2O + M, and CH3 + H (+M) = CH4 (+M) have a relatively remarkable change but those of more than half of the selected reactions remain approximately unchanged. The elementary reaction H + O2 = O + OH has the most significant influence on the laminar burning velocity, and its influence increases gradually with elevating the water content. The elementary reactions that consume H atoms have negative sensitivity coefficients, tending to inhabit the combustion, while those reactions that produce H atoms have positive coefficients, tending to promote the combustion. There is a strong influence of water addition to ethanol on the net reaction rates of the elementary reactions, as shown in Figure 10. Eight reactions are selected from Figure 9 to show the influence. Here, regardless of the water content, the net reaction rates of the reactions that have positive sensitivity

Figure 9. Sensitivity analysis of the laminar burning velocity for the stoichiometric ethanol−water−air mixtures.

coefficients are greater than those of the reactions that have negative sensitivity coefficients, thus enabling the radical pool 4758


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Figure 10. Profiles of the net reaction rates of the main elementary reactions in the stoichiometric ethanol−water−air planar flames.

to increase gradually. When the water content is elevated, the net reaction rates of these selected reactions decrease, except two reactions: H + OH + M = H2O + M and CH3 + H (+M) = CH4 (+M), both of which have negative sensitivity coefficients. The net reaction rates of these two reactions remain approximately unchanged when increasing the water content. Consequently, the net reaction rates of the elementary reactions that have positive sensitivity coefficients decrease with increasing the water content, leading to reducing the radical production, while the reaction rates of the elementary reactions that have negative sensitivity coefficients remain nearly unchanged, indicating no remarkable change in the radical consumption. The interaction between the two types of reactions results in the decrease of the amount of the radicals, thus decreasing the burning velocity. 4.3. Physical and Chemical Effects of Water Addition. Parameters including laminar burning velocities and mole fractions of five radicals, including OH, H, O, CH3, and HO2, as well as the net reaction rates of elementary reactions are selected to present the physical and chemical effects of water addition. Shown in Figure 11 is the variation of the physical and chemical effects of water on laminar burning velocities for ethanol−water−air mixtures at a water content of 20% with the equivalence ratio. Here, both physical and chemical effects of water suppress the laminar burning velocity over the range of the equivalence ratio and reach the peak of the suppression effect at the equivalence ratio of about 1.15. The suppression effect is greater at high equivalence ratios than at low equivalence ratios. The physical effect of water is much stronger than the chemical effect on the laminar burning velocity. At higher and lower equivalence ratios, the chemical effect of water is nearly zero. Shown in Figure 12 are the physical and chemical effects of water on the radical production. From the figure, it is observed that the physical and chemical effects of water on the production of the radicals OH, H, and O are at the same order of magnitude, and the chemical effect suppresses the production of H and O but promotes the OH production, while the physical effect always shows inhibition on the production of these three radicals. On the production of CH3 and HO2, the chemical effect of water is negligible, much

Figure 11. Variation of the physical and chemical effects of water on laminar burning velocities for ethanol−water−air mixtures at a water content of 20% with the equivalence ratio.

weaker than the physical effect that strongly suppresses the production of CH3 and HO2. The behavior of the physical and chemical effects of water addition on the net reaction rates of the main elementary reactions is shown in Figure 13. These elementary reactions are selected from Figure 9. The physical effect of water always suppresses the reaction rates of the elementary reactions selected, while the chemical effect varies from different reactions. For the elementary reactions with positive sensitivity coefficients, the physical effect of water is much stronger than the chemical effect on the net reaction rates, as shown in Figure 13a. The chemical effect of water addition weakly suppresses the reactions H + O2 = O + OH and HO2 + H = OH + OH but slightly promotes the reactions CO + OH = CO2 + H and HCO + M = H + CO + M. For the elementary reactions with negative sensitivity coefficients, the chemical effect of water on the net reaction rate significantly heightens, suppressing the reactions HO2 + OH = H2O + O2 and HCO + H = CO + H2 while promoting H + OH + M = H2O + M and CH3 + H (+M) = CH4 (+M). 4759


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Figure 12. Physical and chemical effects of water on the production of the radical species for the stoichiometric ethanol−water−air mixtures.

Figure 13. Physical and chemical effects of water on the net reaction rates of the selected elementary reactions in the stoichiometric ethanol−water− air planar flames. Three lines exist for each elementary reaction. The first line corresponds to the ethanol−air planar flames at a water content of 0%; the second line corresponds to the ethanol−air planar flames at a water content of 20%, and the third line corresponds to the ethanol−air planar flames at a fictitious water content of 20%.

5. CONCLUSION Laminar burning velocities of hydrous ethanol−air mixtures at 0.1 MPa and 383 K were experimentally determined over the equivalence ratio range of 0.7−1.6 with the water content ranging from 0 to 30% by employing outwardly expanding spherical flames. Numerical study via PREMIX code on onedimensional planar flames of the mixtures at the same conditions accompanied the experimental study. A type of fictitious water was designed to separate the physical effect of water from its chemical effect. These two effects are shown via specific parameters. The major conclusions of the study are as follows: (1) When elevating the water content, unstretched flame speeds and laminar burning velocities for ethanol− water−air mixtures gradually decrease. The experimental values of laminar burning velocities have a reasonably good agreement with the modeled values. (2) In the one-dimensional planar flames, the mole fraction of CO increases and then decreases with distance and the final mole fraction of CO decreases with the water content, while the final mole fraction of CO2 remains unchanged at different water contents. Water addition to the

ethanol suppresses the production of the main radical species O, H, OH, CH3, and HO2. (3) According to the sensitivity analysis of the laminar burning velocity, only a few elementary reactions are significantly influenced by the water addition to ethanol on the sensitivity. The net reaction rates of the elementary reactions with positive sensitivity coefficients decrease more with the water content than the reactions that have negative sensitivity coefficients. (4) Both physical and chemical effects of the water addition to ethanol suppress the laminar burning velocity, but the former dominates. The physical and chemical effects of water on the production of the radicals OH, H, and O are at the same order of magnitude, while on the production of CH3 and HO2, the chemical effect of water is negligible. The chemical effect of water promotes the OH production. The physical effect of water always suppresses the production of the radical species mentioned here. (5) For the elementary reactions with positive sensitivity coefficients, the chemical effect of water has a much weaker influence on the reaction rates than the physical effect, while for the reactions with negative sensitivity coefficients, the chemical effect of 4760


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(24) Li, J.; Kazakov, A.; Chaos, M.; Dryer, F. L. Chemical kinetics of ethanol oxidation. Proceedings of the 5th U.S. Combustion Meeting; San Diego, CA, March 25−28, 2007. (25) Saxena, P. Numerical and experimental studies of ethanol flames and autoignition theory for higher alkanes. Ph.D. Thesis, University of California at San Diego, San Diego, CA, 2007. (26) Saxena, P.; Williams, F. A. Proc. Combust. Inst. 2007, 31, 1149− 1156. (27) Konnov, A. A. Combust. Flame 2009, 156, 2093−2105. (28) Fu, W.; Gong, J.; Hou, L. Combustion of Fuel with Water Content; Higher Education Press: Beijing, China, 2009; pp 22−26. (29) Liao, S.; Jiang, D.; Huang, Z.; Shen, W.; Yuan, C.; Cheng, Q. Energy Convers. Manage. 2007, 48, 857−863. (30) Huang, Y.; Sung, C.; Eng, J. A. Combust. Flame 2004, 139, 239− 251. (31) Kelley, A. P.; Law, C. K. Combust. Flame 2009, 156, 1844−1851. (32) Halter, F.; Tahtouh, T.; Mounaïm-Rousselle, C. Combust. Flame 2010, 157, 1825−1832. (33) Dayma, G.; Halter, F.; Foucher, F.; Mounaïm-Rousselle, C.; Dagaut, P. Energy Fuels 2012, 26, 6669−6677. (34) Santner, J.; Francis, M. H.; Ju, Y.; Dryer, F. L. Combust. Flame 2014, 161, 147−153. (35) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A.; Meeks, E. PREMIX: A Fortran Program for Modeling Steady Laminar OneDimensional Premixed Flames; Sandia National Laboratories: Livermore, CA, 1998; Report SAND85-8240. (36) Kee, R. J.; Rupley, F. M.; Meeks, E.; Miller, J. A. CHEMKIN-III: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical and Plasma Kinetics; Sandia National Laboratories: Albuquerque, NM, 1996; Report SAND96-8216. (37) http://www.reactiondesign.com/products/chemkin/chemkinpro/. (38) Mazas, A. N.; Fiorina, B.; Lacoste, D. A.; Schuller, T. Combust. Flame 2011, 158, 2428−2440. (39) Vancoillie, J.; Christensen, M.; Nilsson, E. J. K.; Verhelst, S.; Konnov, A. A. Fuel 2013, 105, 732−738. (40) Galmiche, B.; Halter, F.; Foucher, F.; Dagaut, P. Energy Fuels 2011, 25, 948−954. (41) Kuo, K. K. Principles of Combustion, 2nd ed.; Wiley: New York, 2005; pp 477−479. (42) Singh, D.; Nishiie, T.; Tanvir, S.; Qiao, L. Fuel 2012, 94, 448− 456.

water heightens. The physical effect of water always inhibits the net reaction rates of the elementary reactions, while the chemical effect varies from different reactions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-27-87665796. Fax: +86-27-86558095. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of Hubei Province under Grant 2011CDA058, the Fundamental Research Program of Application of Ministry of Transport of the People’s Republic of China under Grant 2013319811140, and the Fundamental Research Funds for the Central Universities under Grant Wuhan University of Technology (WUT) 2014-IV-031. The authors appreciate the “Alternative Fuel Research Group of WUT” for assistance in setting up and testing the experimental facilities.

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