Experimental and Numerical Study on the Laminar Flame Speed of n

Apr 14, 2014 - expanding spherical flame and high-speed schlieren photography over a wide range of equivalence ratios, nitrogen dilution ratios, and D...
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Experimental and Numerical Study on the Laminar Flame Speed of n‑Butane/Dimethyl Ether−Air Mixtures Hao Wu, Erjiang Hu,* Huibin Yu, Qianqian Li, Zihang Zhang, Yizhen Chen, 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 n-butane/dimethyl ether (DME)−air mixtures were first measured using the outwardly expanding spherical flame and high-speed schlieren photography over a wide range of equivalence ratios, nitrogen dilution ratios, and DME blending ratios at elevated temperatures and pressures. The measured flame speeds were compared to the calculated flame speeds with some representative chemical kinetic models. Results show that laminar flame speeds of n-butane/DME blends slightly increase with the increase of the DME blending ratio. Reaction pathway analysis was performed, and the effect of the DME blending ratio on the laminar flame speed was interpreted with the latest Aramco Mech 1.3 model. A correlation of the laminar flame speed of n-butane/DME blends as a function of the equivalence ratio, temperature, pressure, and DME blending ratio is provided.

1. INTRODUCTION To solve the problem of energy shortage and environmental pollution, many engine researches have concentrated on the new engine combustion technology and use of alternative fuels. Homogeneous charge compression ignition (HCCI) is a promising combustion model for the internal ignition engine. It can achieve high thermal efficiency and barely produces NOx and soot. However, the ignition and combustion of the HCCI engine cannot be controlled by a simple mechanical way, such as spark ignition (SI) and compression ignition (CI) engines, but by chemical kinetics. Fuel blending is an efficient way to control the HCCI combustion. Fuel ignition properties can be adjusted by blending a high octane fuel and a high cetane fuel to meet the requirements under different operation conditions. In this study, dimethyl ether (DME) and n-butane blends were selected as the testing fuels. DME has a high cetane number and includes an oxygen atom but no C−C bonds in its structure. As one important component of natural gas, n-butane (n-C4H10) has a high octane number. Thus, it is expected that changing the blending ratio of DME and n-butane can control the combustion of the HCCI engine. Some investigations have been conducted on DME/hydrocarbon blends. Tsutsumi et al.1 and Konno et al.2 studied the combustion characteristics of the DME/methane mixture on the HCCI engine. They found that DME has the dominant influence on the ignition timing but varying the DME/methane mixing ratio could not control the rapidity of combustion. Kong3 numerically investigated the combustion characteristics of natural gas/DME blends using a computational fluid dynamics (CFD) model. Yao and co-workers4−7 conducted both experimental and numerical studies on combustion of DME/ methanol under a HCCI engine. They suggested that the combustion efficiency could be controlled by changing the DME concentration. Upon DME and n-butane blending, Iida et al.8 studied the autoignition and combustion of n-butane/DME/air mixtures in a HCCI engine. They found that the HCCI combustion at © 2014 American Chemical Society

different loads could be realized by changing the blending ratio. Hu et al.9 and Jiang et al.10 studied the ignition delay time of DME/butane mixtures on both experimental and numerical methods. Their works provided the experimental ignition delay times of DME/butane blends to validate the chemical kinetic models. The laminar flame speed, which is another fundamental parameter of fuel, embodies the reactivity, diffusivity, and exothermicity. It is the base of the turbulent flame speed and has been widely used for the validation of the chemical kinetic mechanism. Although the laminar flame speed of pure n-butane and DME have been extensively studied,11−22 the studies on n-butane/DME fuel blends are scarcely reported. The objective of this study is (1) to measure the laminar flame speed of n-butane/DME blend−air mixtures at various equivalence ratios, nitrogen dilution ratios, initial temperatures, and initial pressures using high-speed schlieren photography, (2) to validate the available chemical kinetic models, (3) to interpret the effect of DME addition to the laminar flame speed of n-butane, and (4) to provide the correlated equations on laminar flame speeds of DME/n-butane blends.

2. EXPERIMENTAL SETUP AND NUMERICAL APPROACH The experimental apparatus has been described in details in previous studies.23,24 Here, only a brief description is given. The whole system consists of six parts, which are a combustion chamber, a heating system, a mixture preparation system, an ignition system, a data acquisition system, and a high-speed schlieren photography system. During experiment, n-butane, DME, O2, and N2 are introduced at specified partial pressures. The SI with two thin electrodes was initiated at the chamber center. The whole flame propagation images were taken with 10 000 f/s. Before each experiment, the chamber was Received: December 19, 2013 Revised: April 12, 2014 Published: April 14, 2014 3412

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Figure 1. Stretched flame propagation speed versus stretch rate. flushed with fresh air for many times to eliminate the effect of residual gas. For the spherically expanding flame, the stretched flame propagation speed (Sb) can be derived from the flame radius (rf) versus time (t) as Sb = drf/dt, The stretch rate (α) is derived by α = 2Sb/rf. At the early stage of flame expansion, the flame propagation speed and stretch rate are linearly correlated as S0b = Sb − Lbα, where S0b is the unstretched flame propagation speed and Lb is the burned gas Markstein length. Because of the mass conservation across the flame front, the unburned gas laminar flame speed (S0u) can be calculated as S0u = ρbS0b/ρu, where ρu and ρb are the densities of unburned and burned mixtures, respectively. The ignition energy and pressure rise can affect the accuracy of the measured laminar flame speed. Therefore, to avoid these effects in the combustion chamber, photos with the flame radius range of 8−25 mm were used in this study. In addition, the maximum flame radius was also restricted by the occurrence of the cellular structure.25 The adiabatic temperature (Tad) and gas density are calculated through thermal equilibrium of the Chemkin package.26 The experimental uncertainties are mainly from experimental operation, temperature and pressure measurement, and data process. In this study, each experimental condition was performed at least 3 times to reduce the random error and ensure the results within the experimental uncertainty (95% confidence level). The variation of the initial temperature is about ±3 K. Therefore, the maximum relative error in the initial temperature is 1.00% at Tu = 300 K. The initial pressure is measured by both a mercury manometer and a high accuracy pressure transmitter (Rosemount 3051) with the accuracy of 1 mmHg and 50 Pa, respectively. Therefore, the maximum relative

Figure 2. Measured laminar flame speeds compared to the literature and calculations. error in the initial pressure is 0.133% (for mercury manometer) and 0.05% (for high accuracy pressure transmitter) at Pu = 0.1 MPa. Meanwhile, the error of the partial pressure of the fuel and oxidizer caused an error of the equivalence ratio. The fitted laminar flame speed (S0u) of n-butane/DME−air mixtures is a function of the equivalence ratio, initial temperature, and initial pressure. Therefore, the systematic errors were calculated by the error propagation formula (ΔS0u = |∂S0u/∂ϕ|Δϕ + |∂S0u/∂T|ΔT + |∂Su0/∂P|ΔP). The random errors were calculated by standard deviations of three 3413

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Figure 3. Experimental laminar flame speeds at different initial conditions. Simulations on laminar flame speeds were performed using the Chemkin and Premix codes.26,27 Premix uses a hybrid time-integrating/ Newton iteration technique to solve the steady-state mass, species, and energy conservation equations and can simulate the freely propagating flame. All calculations were performed using seven continuation options, and the values of adaptive grid parameters (GRAD and CURV) varied from 0.9 to 0.02 for each case to obtain grid-independent solutions. The final solutions (GRAD = 0.02, and CURV = 0.02) were usually obtained

measurements. The error bars were composed of systematic and random errors. In this study, six fuel blends with different blending ratios (f vb = 0, 20, 40, 60, 80, and 100%; f vb = XDME/(XDME + Xbutane), where XDME and Xbutane are mole fractions of DME and n-butane, respectively), were tested at initial temperatures (T) of 300, 320, 345, 370, and 395 K, initial pressures (P) of 0.1, 0.3, 0.5, and 0.7 MPa, and equivalence ratios from 0.7 to 1.7. 3414

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Figure 5. Reaction pathway of n-butane and DME (values out of parentheses, DME/butane blends with f vb = 60%; values in parentheses, pure butane and pure DME).

Figure 6. Adiabatic temperature at different equivalence ratios and blending ratios.

Figure 4. Measured and calculated laminar flame speeds at different initial conditions (symbols, experimental data; lines, calculated data with the Aramco Mech 1.3 model).

The chemical kinetic models used in the study are the Aramco Mech 1.3 model,28 USC Mech II model,29 and Zhao DME model.30 The Aramco Mech 1.3 model was developed by Prof. Curran at the National University of Ireland in 2013, which includes the oxidation mechanism of C1−C4 hydrocarbons and a DME submodel. It consists of 253 species and 1542 elementary reactions. USC Mech II is a H2/CO/C1−C4 kinetic model (including 111 species and 784 elementary

with about 300 mesh points. For most cases, calculated laminar burning velocities for final grids are within a 2 cm s−1 interval compared to the former solution. This level of convergence is accurate enough for the calculated results. The mixture-averaged transport option was used to determine the species ordinary diffusion coefficients in our calculations. 3415

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reactions) raised by Prof. Wang at the University of Southern California in 2007, which can be used to simulate high-temperature oxidation of hydrogen, carbon monoxide, and C1−C4 hydrocarbons. The Zhao DME model was developed by Prof. Dryer at Princeton University in 2007, on the basis of C1−C2 hydrocarbons and H2/O2 kinetics with a hierarchical nature of reacting systems. It is a hightemperature DME pyrolysis and oxidation model (including 55 species and 290 elementary chemical reactions).

3. RESULTS AND DISCUSSION 3.1. Flame Propagation Speed. Figure 1a shows the stretched flame propagation speeds of n-butane−air mixtures versus stretch rate at different equivalence ratios. A large stretch rate refers to a small flame radius. At ϕ = 1.0, the stretched flame propagation speed shows a higher value than others, which indicates that the stretched flame propagation speed decreases at both rich and lean mixtures. Additionally, at large flame ratios, flame acceleration occurs at the lean side and deceleration occurs at the rich side. The stretched flame propagation speeds versus stretch rate at different DME blending ratios are illustrated in Figure 1b. With the increase of the DME blending ratio, the stretched flame speed increases. All original experimental results on the stretched flame propagation speed (Sb) as a function of the stretch rate (α) have been presented in the Supporting Information. 3.2. Validation of the Experimental System and Chemical Models. The experimental apparatus has been used to measure the laminar flame speeds at different initial temperatures and pressures.21−24 Figure 2a shows the measured laminar flame speeds of n-butane−air mixtures compared to the previous studies.13−16 The present experimental data agree well with the measurements of Tang’s Chamber J and Bosschaart at lean mixtures. Tang’s Chamber P results are slightly lower than other researchers’ measurements at the lean side, while present measurements are slightly lower than others at the rich side. The measurements of Davis and Hirasawa present the highest values overall because of the different method. The Aramco Mech 1.3 model and the USC Mech II model can well-predict the experimental data. Figure 2b gives the comparison of the measured laminar flame speeds of DME to those in the literature.17−22 There are large discrepancies among the laminar flame speeds measured by different researchers, and these discrepancies also exist in other fuels, such as methane. The reason has yet to be quantitatively explained.31 The measurements in this study agree fairly well with the data by Chen over a wide range of equivalence ratios. Additionally, the Zhao DME model yields better performance than the Aramco Mech 1.3 model on the prediction of the laminar flame speed, especially at the stoichiometric and rich conditions. Figure 2c gives the comparison between the measured laminar flame speeds and the calculations with the Aramco Mech 1.3 model at f vb = 20 and 80%. Because of the lack of experimental data of DME/ n-butane blends, only the calculated results are provided in Figure 2c. Results indicate that the Aramco Mech 1.3 model can well-simulate the laminar flame speed of DME/n-butane blends at lean mixtures but give a relatively poor prediction for the stoichiometric and rich mixtures. This is different to the prediction on the ignition delay time, because the Aramco Mech 1.3 model can well-predict the ignition delay time of DME/n-butane blends32 over all equivalence ratio ranges. This requires further optimization of the model. 3.3. Measured Laminar Flame Speeds. Figure 3 gives the laminar flame speed of the fuel blend−air mixtures at

Figure 7. Computed mole fraction profiles of OH and H.

different DME blending ratios, equivalence ratios, temperatures, pressures, and nitrogen dilution ratios. The laminar flame speed shows a slight increase with the increase of the DME blending ratio, and the tendency becomes more obvious at rich mixtures. As expected, the laminar flame speed increases significantly with the increase of the temperature. The tendency seems linear because the studied temperature range is narrow (300−395 K), and we can expect that, at a larger temperature range, it would be exponential, as shown in ref 33. The laminar flame speed gives a marked drop from 0.1 to 0.3 MPa and then drops steadily. The laminar flame speed decreases linearly with the increase of the nitrogen dilution ratio for all DME/n-butane mixtures. 3.4. Comparison of Measurements and Calculations. Figure 4 gives the comparison of the measured and calculated laminar flame speeds using the Aramco Mech 1.3 model. This model overpredicts the experimental data, especially for high DME blending ratios. For the stoichiometric mixtures, the calculated results are a little higher than the experimental data at different temperatures, pressures, and nitrogen dilution ratios, but the tendencies are consistent. 3.5. Reaction Pathway Analysis. Figure 5 shows the reaction pathway of DME/butane blends ( f vb = 60%) and pure n-butane and pure DME at ϕ = 1.0. It is noted that DME produces a lot of HCO; hence, (R30: HCO + M ⇔ H + CO + M) is more sensitive in DME than n-butane. The reactions C2H4 + H ( +M)⇔C2H5 ( +M) 3416

(R201)

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Figure 8. Correlation of the laminar flame speed of n-butane/DME−air mixtures (symbols, experimental data; lines, correlated data).

CH3 + CH3 ⇔ H + C2H5

of n-butane/DME−air mixtures and the equivalence ratio, initial temperature, and initial pressure is

(R206)

are more sensitive in n-butane because n-butane produces large C2H5. Little difference on branching ratios between the blending fuel and pure fuel is found by the reaction pathway analysis, which indicates that these two fuels react independently. This is consistent with the result of high-temperature ignition of DME/n-butane blends.9 3.6. Interpretation of the Effect of the DME Blending Ratio on the Laminar Flame Speed. Figure 6 shows that the adiabatic temperature of the binary fuel−air mixtures is slightly increased with the increase of the DME blending ratio. In most cases, the adiabatic temperature has a dominant influence on the laminar flame speed, and its tendency is consistent with that of the laminar flame speed, as shown in Figure 3a. Padley and Sugden34 and Butler and Hayhurst35 pointed out that a relationship existed between the laminar flame speed and Hfree radical peak. As shown in Figure 7, the computed mole factions of OH and H increase with the increase of the DME blending ratio. There is a linear relationship between the laminar flame speed and peak value of H and OH. 3.7. Laminar Flame Speed Correlation. Correlation of laminar flame speeds of fuel−air mixtures versus pressure and temperature are useful for the practical applications and simulations. As shown in Figure 8, the fitted laminar flame speeds of DME/n-butane blend−air mixtures well-capture the experimental data. The correlation of the laminar flame speed

0 Su0 = Su,ref (T /T 0)α (P /P 0)β

(1)

where is the reference laminar flame speed, T and P are the reference temperature and pressure, which are 300 K and 0.1 MPa in this study, respectively, and α and β are determined by 0

S0u,ref

0

0 α = ln(Su0/Su,ref )/ln(T /T 0)

(2)

0 β = ln(Su0/Su,ref )/ln(P /P 0)

(3)

where α, β, and were fitted on the basis of experimental data using the least-squares method. for the n-butane−air mixtures S0u,ref

0 Su,ref = −2.34135 + 6.50317ϕ − 4.98745ϕ2 + 1.17577ϕ3

α = −26.84296 + 0.15239T − 0.000203966T 2 β = −0.30433 + −0.0218P

for the DME−air mixtures 0 Su,ref = −1.30293 + 3.64925ϕ − 2.34556ϕ2 + 0.40776ϕ3

α = −9.52178 + 0.05528T − 0.0000694477T 2 3417

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β = = −0.35837 + 0.19336P

(4) Yao, M. F.; Chen, Z.; Zheng, Z. Q.; Zhang, B.; Xing, Y. Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol. Fuel 2006, 85, 2046−2056. (5) Zheng, Z. Q.; Yao, M. F.; Chen, Z.; Zhang, B. Experimental study on HCCI combustion of dimethyl ether (DME)/methanol dual fuel. SAE [Tech. Pap.] 2004, DOI: 10.4271/2004-01-2993. (6) Yao, M. F.; Chen, Z.; Zheng, Z. Q.; Zhang, B.; Xing, Y. Effect of EGR on HCCI combustion fuelled with dimethyl ether (DME) and methanol dual fuels. SAE [Tech. Pap.] 2005, DOI: 10.4271/2005-013730. (7) Yao, M. F.; Zheng, Z. L.; Liang, X. Numerical study on the chemical reaction kinetics of DME/methanol for HCCI combustion process. SAE [Tech. Pap.] 2006, DOI: 10.4271/2006-01-1521. (8) Iida, N.; Igarashi, T. Auto-ignition and combustion of n-butane and DME/air mixtures in a homogeneous charge compression ignition engine. SAE [Tech. Pap.] 2000, DOI: 10.4271/2000-01-1832. (9) Hu, E. J.; Jiang, X.; Huang, Z. H.; Zhang, J. X.; Zhang, Z. H.; Man, X. J. Experimental and kinetic studies on ignition delay times of dimethyl ether/n-butane/O2/air mixtures. Energy Fuels 2013, 27, 530− 536. (10) Jiang, X.; Zhang, Y. J.; Man, X. J.; Pan, L.; Huang, Z. H. Shock tube measurements and kinetic study on ignition delay times of lean DME/n-butane blends at elevated pressures. Energy Fuels 2013, 27, 6238−6246. (11) Wang, Y. L.; Feng, Q.; Egolfopoulos, F. N.; Tsotsis, T. T. Studies of C4 and C10 methyl ester flames. Combust. Flame 2011, 158 (8), 1507−1519. (12) Veloo, P. S.; Wang, Y. L.; Egolfopoulos, F. N.; Westbrook, C. K. A comparative experimental and computational study of methanol, ethanol, and n-butanol flames. Combust. Flame 2010, 157 (10), 1989− 2004. (13) Bosschaart, K. J.; de Goey, L. P. H. The laminar burning velocity of flames propagating in mixtures of hydrocarbons and air measured with the heat flux method. Combust. Flame 2004, 136 (3), 261−269. (14) Davis, S. G.; Law, C. K. Determination of and fuel structure effects on laminar flame speeds of C1 to C8 hydrocarbons. Combust. Sci. Technol. 1998, 140 (1−6), 427−449. (15) Hirasawa, T.; Sung, C. J.; Joshi, A.; Yang, Z.; Wang, H.; Law, C. K. Determination of laminar flame speeds using digital particle image velocimetry: Binary fuel blends of ethylene, n-butane, and toluene. Proc. Combust. Inst. 2002, 29 (2), 1427−1434. (16) Tang, C. L.; Huang, Z. H.; Law, C. K. Determination, correlation, and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarbon−air mixtures. Proc. Combust. Inst. 2011, 33 (1), 921−928. (17) Qin, X.; Ju, Y. Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures. Proc. Combust. Inst. 2005, 30 (1), 233−240. (18) Zhao, Z.; Kazakov, A.; Dryer, F. L. Measurements of dimethyl ether/air mixture burning velocities by using particle image velocimetry. Combust. Flame 2004, 139 (1−2), 52−60. (19) Daly, C. A.; Simmie, J. M.; Wurmel, J.; Djebaili, N.; Paillard, C. Burning velocities of dimethyl ether and air. Combust. Flame 2001, 125 (4), 1329−1340. (20) Wang, Y. L.; Holley, A. T.; Ji, C.; Egolfopoulos, F. N.; Tsotsis, T. T.; Curran, H. J. Propagation and extinction of premixed dimethyl ether/air flames. Proc. Combust. Inst. 2009, 32 (1), 1035−1042. (21) Chen, Z. Y.; Wei, L. J.; Huang, Z. H.; Miao, H. Y.; Wang, X. B.; Jiang, D. M. Measurement of laminar burning velocities of dimethyl ether−air premixed mixtures with N2 and CO2 dilution. Energy Fuels 2009, 23 (2), 735−739. (22) Huang, Z. H.; Wang, Q.; Yu, J. R.; Zhang, Y.; Zeng, K.; Miao, H. Y.; Jiang, D. M. Measurement of laminar burning velocity of dimethyl ether−air premixed mixtures. Fuel 2007, 86 (15), 2360−2366. (23) Zhang, Z. Y.; Huang, Z. H.; Wang, X. G.; Xiang, J.; Wang, X. B.; Miao, H. Y. Measurements of laminar burning velocities and Markstein lengths for methanol−air−nitrogen mixtures at elevated pressures and temperatures. Combust. Flame 2008, 155 (3), 358−368.

The binary fuel−air mixture is correlated by the mixing rule based on Le Chatelier’s rule36 1 0 Su,blend (ϕ) = n X ∑i = 1 0 i Su, i(ϕ)

(4)

where n is the total species in fuel blends, and in this study, n = 2. Xi is the percentage of i fuel in fuel blends, and Su,i0(ϕ) is the laminar flame speed of i fuel at an equivalence ratio of ϕ.

4. CONCLUSION Experimental and numerical studies on laminar flame speeds of n-butane/DME blend−air mixtures were conducted at elevated pressures and temperatures and different nitrogen dilution ratios. The effect of the DME blending ratio on the laminar flame speed was analyzed. Laminar flame speeds of n-butane/ DME blends were measured, which provide fundamental data for the blended fuel. The Aramco Mech 1.3 model overpredicts the experimental data. Reaction pathway analysis reveals the dominant role of H and OH radicals on the determination of laminar flame speeds, and the reaction of the two fuels are independent. Laminar flame speeds linearly vary with the peak value of H and OH radicals. In addition, the laminar flame speed as a function of the equivalence ratio, initial pressure, initial temperature, and DME blending ratio is correlated on the basis of experimental data.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Stretched flame propagation speeds (Sb) versus stretch rate (α) given at different equivalence ratios, temperatures, pressures, dilution ratios, and DME blending ratios (Figures 1S−4S). This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Authors

*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. *Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Natural Science Foundation of China (51306144), the State Key Laboratory of Engines at Tianjin University (SKLE201302), and the National Basic Research Program (2013CB228406). The support from the Fundamental Research Funds for the Central Universities is also appreciated.

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REFERENCES

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