Energy Fuels 2009, 23, 4355–4362 Published on Web 07/27/2009
: DOI:10.1021/ef900454v
Measurements of Laminar Burning Velocities and Markstein Lengths of 2,5Dimethylfuran-Air-Diluent Premixed Flames Xuesong Wu, Zuohua Huang,* Chun Jin, Xiangang Wang, Bin Zheng, Yingjia Zhang, and Lixia Wei State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, China Received May 14, 2009. Revised Manuscript Received July 9, 2009
Laminar burning velocities and Markstein lengths of 2,5-dimethylfuran (DMF)-air-N2/CO2 premixed mixtures at the atmospheric pressure and initial temperature of 393 K over three different dilution ratios were obtained by using the outwardly propagating spherical flame and high-speed schlieren photograph system. Addition of diluent was used to simulate the effects of exhaust gas recirculation on the flame propagation. The results show that both unstretched flame propagation speed and laminar burning velocity decrease with the increase of dilution ratio. Markstein length is increased with the increase of dilution ratio, indicating that addition of diluent will improve the stability of the flame. The dilution effects of CO2 as diluent on the flame propagation and the flame stability are stronger than those of N2 as diluent. For a specific equivalence ratio, the laminar burning velocity shows a linear decreasing trend with the increase of dilution ratio. The ratio of the laminar burning velocities with and without diluent gas (the normalized laminar burning velocity) demonstrates good linear trend versus the dilution ratio, and a linear formula is developed to express this relationship based on experimental data.
ultraviolet synchrotron radiation photoionization and molecular-beam mass spectrometry. Possible reaction pathways of DMF, 2-methylfuran, and furan are proposed based on the intermediates identified. The data of laminar burning velocities of DMF have not been reported yet. EGR (exhaust gas recirculation) can effectively reduce NOx emission by lowering combustion temperature and diluting oxygen concentration. EGR ratio can be as high as 30% in engine. Flame propagation speed and flame stability will be affected when diluent is added. Therefore, it is important to measure the laminar burning velocities of the fuel-airdiluent mixtures and to study the dilution effects on the flame propagation and flame stability. Qiao et al.9 studied the hydrogen-air-diluent premixed flame and found that diluents became more effective in the order helium, argon, nitrogen, and carbon dioxide. The addition of diluent can decrease the Markstein numbers, especially for CO2, which made the flames more susceptible to preferential-diffusion instability. Hermanns et al.10 investigated the laminar adiabatic burning velocities of hydrogen-oxygen-nitrogen mixtures by heat flux method and found that laminar adiabatic burning velocity decreased with the decrease of the oxygen content in the oxidizer. Stone et al.11 gave the laminar burning velocity of methane-air-diluent mixtures. The diluent were carbon dioxide, nitrogen, and the mixture of 15% carbon dioxide and 85% nitrogen. Han et al.12 measured the laminar burning velocity of methane-air with variations of EGR diluent, reformer gas, temperature, and pressure. The diluent reduced the effective heating value of the mixture, which resulted in a
1. Introduction Laminar burning velocity is a physicochemical property of a combustible mixture. It embodies fundamental information on diffusivity, reactivity, and heat release rate of the mixture and is widely used to validate the kinetic mechanism.1,2 The laminar burning velocity is dependent on fuel type, equivalence ratio, pressure, temperature, and diluent gases. Recently, a new type of oxygenated biofuel, 2,5-dimethylfuran (DMF), has attracted the worldwide attention. It can be produced in large-scale from sugar or cellulose by the methods an-Leshkov et al.,4 and Zhao reported by Mascal et al.,3 Rom et al.5 Compared to ethanol (the only renewable liquid fuel currently produced in large quantities), DMF has the higher energy density, the higher boiling point, and the higher research octane number.4 DMF is insoluble in water, thus it will not pollute the groundwater as methyl t-butyl ether (MTBE) does. The above advantages make DMF an attractive alternative for ethanol and octane improver. Up to now, the published work mainly reported the thermal decomposition of DMF.6,7 Recently, Wu et al.8 studied the low-pressure premixed laminar DMF/O2/Ar flame with tunable vacuum *To whom correspondence should be addressed. Fax: 0086-02982668789. E-mail:
[email protected]. (1) Law, C. K.; Sung, C. J. Prog. Energy Combust. Sci. 2000, 26, 459– 505. (2) Zhao, Z.; Kazakov, A.; Li, J.; Dryer, A. L. Combust. Sci. Technol. 2004, 176, 1705–1723. (3) Mascal, M.; Nikitin, E. B. Angew. Chem., Int. Ed. 2008, 47, 7924– 7926. (4) Rom an-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Nature 2007, 447, 982–985. (5) Zhao, H.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Science 2007, 316, 1597–1600. (6) Lifshitz, A.; Tamburu, C.; Shashua, R. J. Phys. Chem. A 1998, 102, 10655–10670. (7) Grela, M. A.; Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1985, 89, 38–41. (8) Wu, X.; Huang, Z.; Yuan, T.; Zhang, K.; Wei, L. Combust. Flame 2009, 156, 1365–1376. r 2009 American Chemical Society
(9) Qiao, L.; Gu, Y.; Dahm, W. J. A.; Oran, E. S.; Faeth, G. M. Combust. Flame 2007, 151, 196–208. (10) Hermanns, R. T. E.; Konnov, A. A.; Bastiaans, R. J. M.; deGoey, L. P. H. Energy Fuels 2007, 21, 1977–1981. (11) Stone, R.; Clarke, A.; Beckwith, P. Combust. Flame 1998, 114, 546–555. (12) Han, P.; David Checkel, M.; Fleck, B. A.; Nowicki, N. L. Fuel 2007, 86, 585–596.
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Figure 1. Experimental setup.
lower flame temperature and consequently decreased the burning velocity. Tang et al.13 studied the effects of nitrogen dilution on laminar burning velocities and Markstein lengths of propane-air mixtures. Their results showed that with the increase of dilution ratio, the burning velocity decreased and, for equivalence ratio less than 1.4, Markstein length increased with the increase of the dilution ratio. In this study, N2 and CO2 were chosen as diluents because of their availability and more approaching to species in exhaust gas of engines. The objective of the study is to obtain the laminar burning velocity and Markstein length of the DMF-air-N2/CO2 premixed mixtures at different equivalence ratios and dilution ratios and to clarify the effects of dilution on flame propagation speed and flame stability. On the basis of the experimental data, a correlated formula is provided to characterize the relationship between the ratio of the laminar burning velocities with and without diluent (the normalized laminar burning velocity) and dilution ratio. These data can be used in developing the chemical kinetics of DMF-air flame.
experiment. The purity levels of liquid fuel and gases used in present study are as follows: DMF, 99%; and O2, N2, and CO2, 99.99%. Liquid fuel is injected into the chamber by the syringe through the liquid fuel injection valve. Gases are introduced into the chamber through the inlet/outlet value. After the mixtures preparation, the chamber is left undisturbed for at least 5 min before ignition. After the combustion, the chamber is vacuumed and flushed with fresh air to avoid the influence of the residual gas. The experiments were conducted on premixed DMF-air-N2/ CO2 mixtures at initial pressure of 0.1 MPa, initial temperature of 393 K, equivalence ratios of 0.9-1.5 and dilution ratios of 0-15%. The initial temperature of 393 K was selected to ensure that DMF can be completely vaporized in the chamber (boiling point of DMF is 365 K). The results showed in this study are the averaged value of three repeated experiments.
3. Laminar Burning Velocity and Markstein Length The stretched flame propagation speed (Sn) is the velocity of the flame related to the burned gases. For the outwardly propagating spherical flame, Sn is derived from the instantaneous flame radius versus time, dru Sn ¼ ð1Þ dt
2. Experimental Setup and Procedures The experimental system is the same as that in literatures.14,15 Figure 1 shows the schematic diagram of the experimental setup. It consists of a constant-volume combustion chamber, heating and temperature control system, mixture preparation system, ignition system, data acquisition system, and high-speed schlieren photography system. The entire chamber is heated by a 2.4 kW heating-tape wrapped outside the chamber body. Two quartz windows of 80 mm diameter are mounted on the two sides of the chamber to allow the optical access. A high-speed digital camera (HG-100K) with a frame speed of 10 000 frames/s is used to record the flame images during the combustion. A thermocouple is used to measure the initial temperature of mixtures in the chamber with an accuracy of 1 K. The partial pressures of each component are regulated by a mercury manometer. Pressure transducer (Kistler 7001) is used to measure the pressure history during the combustion. The mixtures of oxygen and nitrogen (molar ratio 1:3.76) are used to simulate “dry air” in the present
where ru is the instantaneous flame radius measured from schlieren images of the flame, and t is the elapsed time after the ignition. For an outwardly propagating spherical flame, the stretch rate, the Lagrangian time derivative of the logarithm of the area A of any infinitesimal element of the surface, is calculated according to1,16 dðln AÞ 1 dA 2 dru 2 ð2Þ ¼ ¼ ¼ Sn R ¼ dt A dt ru dt ru At the early stage of flame propagation, there exists a linear relationship between the flame propagation speed and the flame stretch rate,1,17,18 Sl -Sn ¼ Lb R ð3Þ
(13) Tang, C.; Huang, Z.; He, J.; Jin, C.; Wang, X.; Miao, H. Energy Fuels 2009, 23, 151–156. (14) Zhang, Z.; Huang, Z.; Wang, X.; Xiang, J.; Wang, X.; Miao, H. Combust. Flame 2008, 155, 358–368. (15) Tang, C.; Huang, Z.; Jin, C.; He, J.; Wang, J.; Wang, X.; Miao, H. Int. J. Hydrogen Energy 2008, 33, 4906–4914.
(16) Bradley, D.; Hicks, R. A.; Lawes, M.; Sheppard, C. G. W.; Woolley, R. Combust. Flame 1998, 115, 126–144. (17) Gu, X. J.; Haq, M. Z.; Lawes, M.; Woolley, R. Combust. Flame 2000, 121, 41–58. (18) Chen, Z.; Burke, M. P.; Ju, Y. Proc. Combust. Inst. 2009, 32, 1253–1260.
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Figure 2. Schlieren images of the stoichiometric DMF-air-N2/CO2 mixtures.
the equation1,16
where Sl is the unstretched flame propagation speed obtained by extrapolating Sn to the zero flame stretch rate. The burned gas Markstein length (Lb) is the negative of the slope of the straight-line fit between the stretched flame propagation speed and stretch rate, which indicates the effect of stretch rate on flame propagation speed and characterizes the diffusional-thermal instability of the flame front. A positive Markstein length indicates the flame is stable to the diffusional-thermal effect, whereas a negative Markstein length indicates the flame surface will be distorted due to diffusional-thermal instability, leading to the acceleration of flame speed and the formation of a cellular structure. Two other types of instabilities;the hydrodynamic instability (intrinsic instability, for all values of Lb), and the buoyancy-driven instability;are observed under stable diffusional-thermal conditions when the flame diameters become large.1,16,19-22 The presence of diffusionalthermal instability could be identified by irregular (chaotic) distortions of the flame surface relatively early in the flame propagation process only when Lb < 0. Hydrodynamic instability is caused by density jump across the flame, and it could be identified by the development of a somewhat regular cellular disturbance pattern on the flame surface, and this instability becomes obvious only when flame diameter is large. The increase in density ratio across the flame and the decrease in flame thickness will promote this instability. The buoyancy-driven instability can be observed in slow flames near lean-burn limit and can be identified by the appearance of distortion of the flame surface from a spherical shape. To avoid both the effect of ignition disturbance and pressure rise, the flame images with the radius between 6 and 25 mm are used in the analysis in this study.1,14,16,18,23,24 The laminar burning velocity (ul) is calculated from the unstretched flame propagation speed and density ratio using
ul ¼
Fb Sl Fu
ð4Þ
where Fu is the density of unburned gases, and Fb is the density of burned gases. In this study, the dilution ratio is defined as the volumetric fraction of dilution addition in the mixtures, Vdiluent ð5Þ φr ¼ Vfuel þVair þVdiluent 4. Results and Discussion 4.1. Flame Morphology. Flame propagation is likely affected by the cooling effect of the electrodes, so the flame radius in vertical direction is used. Almost all the flames in this experiment have the smooth front surfaces. Thus, the flame acceleration due to the cellular structure of the flame will not be considered. The maximum flame radius used in the determination of flame speed is restricted by the pressure rise (the pressure rise in the flame chamber during the measurement period is less than 1% increasing). Timeresolved schlieren images are shown in Figure 2 for the stoichiometric DMF-air-N2/CO2 mixtures. In the early stage of flame propagation, the cooling effect of the electrodes is demonstrated, and this leads to a slower flame propagation speed along the direction of the electrodes than that in the vertical direction. Flame front keeps smooth as flame is propagating and flame propagation speed decreases as addition of diluent in the mixtures. Flame radius at any specific instant for the same equivalence ratio becomes smaller in the order no diluent, N2 as diluent, and CO2 as diluent. This indicates that the dilution effects of CO2 are stronger than that of N2. Figure 3 shows the schlieren images of DMF-air-N2/CO2 mixtures as a function of flame radius at φ = 1.3. In the case of φr = 0, cracks is appearing on the flame surface when the flame radius is larger than 20 mm. In the case of N2 and CO2 dilution, the flame front always keeps smooth during flame propagation, and no cracks are observed on the flame surface. This indicates that flame stability is increased as the mixture is diluted. With the increase of dilution ratio, the Markstein length is increased, as shown later in Figure 7. It can be concluded that the stabilizing effect of thermo-diffusion becomes stronger with the increase of dilution ratio.
(19) Prathap, C.; Ray, A.; Ravi, M. R. Combust. Flame 2008, 155, 145–160. (20) Bradley, D.; Sheppart, C. G. W.; Woolley, R.; Greenhalgh, D. A.; Lockett, R. D. Combust. Flame 2000, 122, 195–209. (21) Qiao, L.; Kim, C. H.; Faeth, G. M. Combust. Flame 2005, 143, 79–96. (22) Aung, K. T.; Hassan, M. I.; Faeth, G. M. Combust. Flame 1997, 109, 1–24. (23) Huang, Z.; Zhang, Y.; Zeng, K.; Liu, B.; Wang, Q.; Jiang, D. Combust. Flame 2006, 146, 302–311. (24) Burke, M. P.; Chen, Z.; Ju, Y.; Dryer, F. L. Combust. Flame 2009, 156, 771–779.
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Figure 3. Schlieren images of DMF-air-N2/CO2 mixtures at φ = 1.3.
4.2. Flame Propagation Speeds. Figure 4a gives the stretched flame propagation speed versus flame radius at different equivalence ratios. The stretched flame propagation speed increases monotonously with the increase of flame radius at all equivalence ratios. Meanwhile, the stretched flame propagation speed is increased with the increase of equivalence ratio. Figures 4b and 4c show the stretched flame propagation speed versus flame radius at different dilution ratios with N2 and CO2 as diluent, respectively. With the increase of dilution ratio, the stretched flame propagation speed is decreased, and the reduction of stretched flame propagation speed with the increase of dilution ratio is more obvious in the case of CO2 as diluent than that in the case of N2 as diluent. Figure 5 shows the stretched flame propagation speed versus stretch rate at different equivalence ratios, dilution ratios, and diluents. For a specific equivalence ratio or a specific dilution ratio, the stretched flame propagation speed is decreased with the increase of stretch rate (the decrease of flame radius). As shown in Figure 5a, the slope of the fitted straight-line for Sn-R curve is increased with the increase of equivalence ratio. Since Markstein length corresponds to the negative value of the slope of the fitted straight-line for Sn-R curve, the Markstein length is decreased with the increase of equivalence ratio. With the increase of dilution ratio, the stretched flame propagation speed is decreased. Moreover, the slope of the fitted straight-line for Sn-R curve (or Markstein length) is decreased (or increased) with the increase of dilution ratio. This indicates that the flame front becomes more stable with the addition of diluent. The unstretched flame propagation speed versus equivalence ratio at different dilution ratios is shown in Figure 6. The unstretched flame propagation speed gives its peak value at the equivalence ratio of 1.1-1.2 at all dilution ratios, and it monotonously decreases with the increase of dilution ratio. With the addition of diluent, the amount of fuel and O2 per unit volume is reduced, thus lowering the heat release of the mixtures per unit volume and the probability of the fuel molecule to meet with the oxidizer molecule. This causes a corresponding reduction of temperatures within the reaction zone of the flames with the associated reduction of flame propagation speed. With the increase of dilution ratio, the unstretched flame propagation speed gives lower value in the case of CO2 as diluent than that in the case of N2 as diluent. 4.3. Markstein Lengths. According to the asymptotic theory, the resulting Markstein length depends only on the
Figure 4. Stretched flame propagation speed vs flame radius.
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Figure 6. Unstretched flame propagation speed vs equivalence ratio at different dilution ratios.
Lewis number of the deficient reactant.1,25,26 In fuel-lean mixtures, the Markstein length depends on the Lewis number of fuel, whereas in fuel-rich mixtures the Markstein length depends on the Lewis number of oxidizer. Since DMF belongs to the heavy hydrocarbon, it can be deduced that the Markstein length of DMF-air or DMF-airdiluent mixtures will decrease monotonously with the increase of equivalence ratio. Figure 7 shows the Markstein lengths versus equivalence ratio at different dilution ratios. The Markstein lengths of DMF-air-diluent mixtures decrease monotonously with the increase of equivalence ratio and increase with the increase of dilution ratio. This reveals that the fuel-lean
mixture maintains the higher stability compared to that of fuel-rich mixture and addition of diluent can increase the flame front stability. The increment of Markstein length with dilution ratios is larger in the lean mixture side than that in the rich mixture side. This suggests that addition of diluent has stronger influence on flame stability for lean flame. As shown in Figure 7, Markstein lengths within the experimental range give the positive values, indicating that flame propagation will accelerate with the decrease of stretch rate and DMF-air-diluent flames in this study are all stable to the diffusional-thermal effect. The perturbations at flame front due to spark and/or electrodes can be renovated by the strong stabilizing effect of a diminished flame stretch and thermo-diffusion.17,27,28 As shown in Figure 3, with the addition of diluent, no crack is presented on the flame surface, and the flame front keeps smooth as the flame is propagating. The increment of Markstein length with the addition of CO2 as diluent is larger than that with the addition of N2 as diluent, and this reveals that the stabilizing effect of CO2 as diluent is higher compared to that of N2 as diluent. 4.4. Laminar Burning Velocities. As described above, addition of diluent (N2 or CO2) lowers the heat release of the mixtures per unit volume and the probability of the fuel molecule to meet with the oxidizer molecule, this will bring a temperature drop within the reaction zone of the flames. According to the classical phenomenological theories of
(25) Matalon, M.; Matkowsky, B. J. J. Fluid Mech. 1982, 124, 239– 259. (26) Bechtold, J. K.; Matalon, M. Combust. Flame 2001, 127, 1906– 1913.
(27) Bradley, D.; Harper, C. M. Combust. Flame 1994, 99, 562–572. (28) Johnston, R. J.; Farrell, J. T. Proc. Combust. Inst. 2005, 30, 217– 224.
Figure 5. Stretched flame propagation speed vs stretch rate.
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Figure 8. Adiabatic temperature vs equivalence ratio at different dilution ratios. Figure 7. Markstein length vs equivalence ratio at different dilution ratios.
premixed laminar flame propagation,29 the reduction of temperature within the reaction zone of the flames associates with the reduction of laminar flame speed and laminar burning velocity. Figure 8 shows the adiabatic temperature versus equivalence ratio at different dilution ratios. The adiabatic temperature gives its peak value at the equivalence ratio of 1.1 at all dilution ratios, and it decreases with the increase of dilution ratio. For a specific equivalence ratio and dilution ratio, the reduction of adiabatic temperature is larger in the case of CO2 as diluent compared to N2 as diluent because of the bigger specific heat of CO2 and the chemical effect of CO2.30,31 Addition of diluent has no effect on the peak position of the adiabatic temperature. Figure 9 shows the density ratio and flame thickness versus equivalence ratio at different dilution ratios. The density ratio gives its peak value at equivalence ratio of 1.1-1.2 and it decreases with the increase of dilution ratio. Meanwhile, the flame thickness is increased with the increase of dilution ratio. Thus, the hydrodynamic instability is decreased with the increase of dilution ratio.32,33 Laminar burning velocity versus dilution ratio at φ = 1.0 is shown in Figure 10. Laminar burning velocity is decreased (29) Law, C. K. Proc. Combust. Inst. 1989, 22, 1381–1402. (30) Liu, F.; Guo, H.; Smallwood, G. J. Combust. Flame 2003, 133, 495–497. (31) Halter, F.; Foucher, F.; Landry, L.; Mounaim-Rousselle, C. Combust. Sci. Technol. 2009, 181, 813–827. (32) Kwon, O. C.; Rozenchan, G.; Law, C. K. Proc. Combust. Inst. 2002, 29, 1775–1783. (33) Jomaas, G.; Law, C. K.; Bechtold, J. K. J. Fluid Mech. 2007, 583, 1–26.
Figure 9. Density ratio and flame thickness vs equivalence ratio at different dilution ratios.
linearly with the increase of dilution ratio. This phenomena has also been reported by Zhao et al.2 As shown in Figure 10, 4360
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Figure 10. Laminar burning velocity versus dilution ratio at φ = 1.0.
Figure 12. Normalized laminar burning velocity vs dilution ratio at different equivalence ratios.
effects, only four equivalence ratios are shown in Figure 11b in the case of CO2 as diluent with dilution ratio of 15%. For both CO2 and N2 dilution, the laminar burning velocities decrease with the increase of dilution ratio because of the temperature reduction within the reaction zone of the flames with the addition of diluent. The laminar burning velocities give their peak values at equivalence ratio of 1.1-1.2 for both CO2 and N2 dilution. Similarly, the dilution effect of CO2 as diluent on the reduction of laminar burning velocity is larger than that of N2 as diluent because of the bigger specific heat of CO2 and the chemical effect of CO2.30,31 Figure 12 gives the normalized laminar burning velocity versus dilution ratio at different equivalence ratios. For the specific diluent and dilution ratio, the normalized laminar burning velocity varies slightly versus the equivalence ratios. This indicates that the diluent has almost the same dilution effects on the laminar burning velocity at all equivalence ratios for a specific dilution ratio.13,19 As shown in Figure 12, the normalized laminar burning velocity shows a linear relationship with dilution ratio. On the basis of the experimental data, the correlation between the normalized laminar burning velocity and dilution ratio can be expressed as ul ðφr Þ ¼ aφr þb ul ð0Þ
ð6Þ
where a = -1.74 and b = 1.00 in the case of N2 as diluent, and a = -4.00 and b = 1.00 in the case of CO2 as diluent. The absolute value of a in the case of CO2 as diluent is larger than that of N2 as diluent, and this also reflects that the larger influence of CO2 as diluent on laminar burning velocity than that of N2 as diluent.
Figure 11. Laminar burning velocity versus equivalence ratio at different dilution ratios.
the slope of the ul-φr curve is smaller in the case of CO2 as diluent (absolute value is 1.82) compared to N2 as diluent (absolute value is 0.56), and this means that the dilution effects of CO2 as diluent on the reduction of laminar burning velocity is larger than that of N2 as diluent. This phenomenon is consistent with that of unstretched flame propagation speed to the diluent. Figure 11 shows the laminar burning velocities versus equivalence ratio at different dilution ratios. Effects of buoyancy were small when laminar burning velocities were larger than 0.15 m/s.21 To avoid the buoyancy
5. Conclusions Laminar burning velocities and Markstein lengths of DMF-air-diluent mixtures were studied. The main conclusions are summarized as follows: (1) Addition of diluent decreases the unstretched flame propagation speed and the laminar burning velocity. Addition of CO2 as diluent has the larger influence on laminar burning velocity and Markstein length than that of N2 as diluent. 4361
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(2) Markstein length and flame stability are increased with the increase of dilution ratio. Addition of diluent has larger improvement on flame stability of fuel-lean mixture compared with that of fuel-rich mixture. (3) Linear correlation is present between laminar burning velocity and dilution ratio. And linear correlation is demonstrated between the normalized laminar burn-
ing velocity and dilution ratio regardless of equivalence ratio. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC) (50876085 and 50821604) and China Postdoctoral Science Foundation (20080441170).
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