Study on Laminar Burning Characteristics of Premixed High-Octane

Dec 3, 2009 - Laminar burning characteristics of high-octane fuel-air premixed mixtures ... The flame propagation speed, the laminar burning velocity,...
0 downloads 0 Views 3MB Size
Download PDF
Energy Fuels 2010, 24, 965–972 Published on Web 12/03/2009

: DOI:10.1021/ef901117d

Study on Laminar Burning Characteristics of Premixed High-Octane Fuel-Air Mixtures at Elevated Pressures and Temperatures Jing Gong, Chun Jin, Zuohua Huang,* and Xuesong Wu State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic China Received October 1, 2009. Revised Manuscript Received November 11, 2009

Laminar burning characteristics of high-octane fuel-air premixed mixtures (ETBE, TBA, and ethanol) were studied in a constant-volume bomb at various equivalence ratios, initial temperatures, and initial pressures by using outwardly propagating spherical flames with a high-speed schlieren imaging system. The flame propagation speed, the laminar burning velocity, the Markstein length, the adiabatic flame temperature, the flame thickness, and the density ratio were obtained and the influence of equivalence ratio, initial temperature, and initial pressure on these parameters was analyzed. The experimental results show that both the unstretched flame propagation speed and unstretched laminar burning velocity increase with the increase of initial temperature and decrease with the increase of initial pressure. Thermal-diffusive instabilities (Markstein length) decrease at higher initial temperature and/or at lower initial pressure. Meanwhile, the unstretched flame propagation speed and the unstretched laminar burning velocity give maximum values on the rich mixture side. The onset of cellular instability was evaluated in terms of the hydrodynamic and diffusional-thermal instabilities, and the reasons for the onset of the cellular structure were analyzed. The results indicate that the propensity for cellular structure is enhanced due to the increase in hydrodynamic and diffusional-thermal instabilities at high initial pressure. Moreover, the flame instability is more sensitive to initial pressure compared to initial temperature.

the method of liquid phase synthesis from ethanol and isobutylene (IB).8-10 However, IB sources are limited to catalytic cracking and steam cracking fractions,11 and the cost of this kind of production method is high due to the purification process of high purity ETBE. These restrict the wide application of ETBE as the octane enhancer. From an economic and technological aspect, an alternative route to synthesis ETBE from tert-butyl alcohol (TBA) and ethanol is introduced.3 Without separating ETBE from others, the reaction product is a blend composed of ETBE, unreacted TBA, and unreacted ethanol. All three compounds (ETBE, TBA, and ethanol) are gasoline boosters. They not only improve the octane ratings12 but also reduce the emission of carbon monoxide, unburned hydrocarbons, and other exhaust emissions.13 It is predictable that the substitution of this high-octane mixture (ETBE, TBA, and ethanol) instead of ETBE can decrease the cost of octane improver, without lowering the engine performance. As the fundamental combustion characteristics are strongly related to engine combustion, it is necessary to understand the fundamental combustion characteristics and provide the guidance on engine operation. Laminar burning velocity is a fundamental characteristic of a mixture and is always used to validate the chemical kinetics mechanism and predict the performance and emissions of an internal combustion engine.14 There are various flame configurations in measuring the laminar burning velocity, such

1. Introduction Nowadays, our society faces the problems of energy shortage and environmental pollution. To solve these problems, many efforts have been undertaken. One is to improve combustion efficiency and reduce emissions of traditional internal combustion engines. The other is to develop the renewable energy and alternative fuels. The research on alternative fuel has made great development in recent years. Gasoline additives are always used to improve the performance and reduce the emissions of automotive vehicles.1,2 Ethyl tert-butyl ether (ETBE) is one kind of octane improver. The demand of ETBE has increased rapidly in place of Methyl tert-butyl ether (MTBE) in recent years3 because MTBE will contaminate the groundwater whereas ETBE will not.4-7 Many reports concentrated on the production of ETBE using *To whom correspondence should be addressed. Telephone: þ0086 29 82665075. Fax: þ0086 29 82668789. E-mail: [email protected]. cn. (1) Al-Hasan, M. Energy Convers. Manage. 2003, 44 (9), 1547–1561. (2) Rosell, M.; Lacorte, S.; Ginebreda, A.; Barcel, D. J. Chromatogr. A 2003, 995 (1-2), 171–184. (3) Yang, B. L.; Yang, S. B.; Yao, R. Q. React. Funct. Polym. 2000, 44 (2), 167–175. (4) Squillace, P. J.; Zogorski, J. S.; Wilber, W. G.; Price, C. V. Environ. Sci. Technol. 1996, 30 (5), 1721–1730. (5) da Silva, R.; Cataluna, R.; Menezes, E. W. d.; Samios, D.; Piatnicki, C. M. S. Fuel 2005, 84 (7-8), 951–959. (6) Caprino, L.; Togna, G. I. Environ. Health Persp. 1998, 106 (3), 115–125. (7) Shih, T.; Rong, Y.; Harmon, T.; Suffet, M. Environ. Sci. Technol. 2004, 38 (1), 42–48. (8) de Menezes, E. W.; Cataluna, R. Fuel Process. Technol. 2008, 89 (11), 1148–1152. (9) Gomez, C.; Cunill, F.; Iborra, M.; Izquierdo, F.; Tejero, J. Ind. Eng. Chem. Res. 1997, 36 (11), 4756–4762. r 2009 American Chemical Society

(10) Streicher, C.; Malmaison, R. U. S. Patent 5607557, 03-1997. (11) Yang, B. L.; Goto, S. Sep. Sci. Technol. 1997, 32 (5), 971–981. (12) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33 (3), 233–271. (13) Perry, R.; Gee, I. L. Sci. Total Environ. 1995, 169 (1-3), 149–156. (14) Bradley, D.; Hicks, R. A.; Lawes, M.; Sheppard, C. G. W.; Woolley, R. Combust. Flame 1998, 115 (1-2), 126–144.

965

pubs.acs.org/EF

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d 15,16

Gong et al. 17,18

31

as the stagnation plane flame, the heat flux method, and the outwardly propagating spherical flame.19-22 Stagnation plane flame method is difficult to draw a clear flame front and to stabilize the flame under the high-pressure conditions. The heat flux method needs interpolation to determine the adiabatic burning velocity.23 For outwardly propagating spherical flame, the stretch is well-defined and there is a linear relationship between flame speeds and flame stretches. Meanwhile, the Markstein length, which characterizes the effect of stretch on flame propagation and reflecting the flame stability, can be obtained easily. For this reason, the outwardly propagating spherical flame has been widely used to measure the laminar burning velocity.24-27 In this study, the laminar burning velocity of the premixed high-octane fuel composing of 65.3% ethanol, 11.9% TBA, and 22.8% ETBE by mass (result of reaction with an EtOH/TBA molar ratio of 2) and air mixture was measured at various initial temperatures and pressures over a wide range of equivalence ratios by using the outwardly propagating spherical flames. Many previous researches concentrated on combustion characteristics of ethanol and/or ETBE air mixtures, respectively. Egolfopoulos et al.28 measured the laminar burning velocities of ethanol-air mixture at atmospheric pressure, different equivalence ratios, and temperatures ranging from 363 to 453 K in the counter-flow flames. Liao et al.29 obtained the laminar burning velocity of ethanol-air mixtures at atmospheric pressure and at temperatures between 358 and 480 K over a wide range of equivalence ratios in a cuboid bomb. According to Liao, there existed a power law correlation between the unstretched laminar burning velocity and initial temperature and/or equivalence ratio over the ranges studied. More recently, Bradley et al.30 studied the laminar burning velocities and Markstein numbers of ethanol-air mixtures using a spherical explosion bomb at pressures up to 1.4 MPa. The results showed that the occurrence of unstable flame was more possible with the increase of initial pressure and the flame speed was enhanced by the flame wrinkling arising from the instabilities. For ETBE-air

mixtures, Yahyaoui et al. studied the laminar burning velocities in a spherical bomb at room temperature and atmospheric pressure over wide range of equivalence ratios (0.5-1.5), and experimental results from both shock tube and spherical bomb were compared to those computed using a detailed chemical kinetic reaction mechanism. 2. Experimental Setup and Procedures In this experiment, a constant-volume bomb (a cylinder-type vessel 180 mm in diameter and 210 mm in length) is used, of which the two sides have quartz windows to make the inside observable and to provide optical access. The details of the experimental setup have been reported in ref 32. When the initial pressure is less than the atmospheric pressure a U-tube mercury manometer is used to ensure the initial pressure with high precision, and a pressure transmitter is used when the initial pressure is higher than the atmospheric pressure. The measuring accuracy of the U-tube mercury manometer is 66 Pa. The high-octane fuel-air mixture is ignited by centrally located electrodes and a standard capacitive discharge ignition system. A high-speed digital camera (HG-100K) operating at 10 000 frames/s is employed to record the flame picture, and a Kistler pressure transducer is used to record the pressure with a resolution of 0.01 KPa during combustion process. In the present experiment, the mixtures of oxygen and nitrogen (molar ratio 1:3.76) are employed to simulate “dry air”. The liquid fuel mixtures are injected into the chamber by a microliter syringe through the liquid fuel injection valve, and gases are introduced into the chamber through the inlet/outlet valve by the calculation in advance according to the designed equivalence ratio, initial temperature, and initial pressure. Then, waiting for 10 min before the ignition is necessary to ensure the well-mixed and motionless mixture. On the other hand, to escape from the influence of wall temperature on mixture temperature, a sufficient interval is set between two experiments to cool down and preserve the same initial temperature. In the experiment, the initial temperature is set at 373, 423, and 473 K; the initial pressures are 0.10, 0.25, 0.50, and 0.75 MP; and the equivalence ratios ranges from 0.7 to 1.4. The total equivalence ratio (φ) is defined as φ = (F/A)/(F/A)st, where F/A is the fuel-air ratio and (F/A)st refers to the stoichiometric value of F/A.

3. Determination of the Laminar Burning Velocity

(15) Chao, B. H.; Egolfopoulos, F. N.; Law, C. K. Combust. Flame 1997, 109 (4), 620–638. (16) Yu, G.; Law, C. K.; Wu, C. K. Combust. Flame 1986, 63 (3), 339– 347. (17) Hermanns, R. T. E.; Konnov, A. A.; Bastiaans, R. J. M.; de Goey, L. P. H.; Lucka, K.; Kohne, H. Fuel 2010, 89 (1), 114–121. (18) Vanmaaren, A.; Thung, D. S.; Degoey, L. P. H. Combust. Sci. Technol. 1994, 96 (4-6), 327–344. (19) Gu, X. J.; Haq, M. Z.; Lawes, M.; Woolley, R. Combust. Flame 2000, 121 (1-2), 41–58. (20) Marley, S. K.; Roberts, W. L. Combust. Flame 2005, 141 (4), 473– 477. (21) Miao, H. Y.; Ji, M.; Jiao, Q.; Huang, Q.; Huang, Z. H. Int. J. Hydrogen Energy 2009, 34 (7), 3145–3155. (22) Law, C. K.; Jomaas, G.; Bechtold, J. K. Proc. Combust. Inst. 2005, 30, 159–167. (23) Bosschaart, K. J.; de Goey, L. P. H. Combust. Flame 2004, 136 (3), 261–269. (24) Tang, C.; Huang, Z.; Jin, C.; He, J.; Wang, J.; Wang, X.; Miao, H. Int. J. Hydrogen Energy 2008, 33 (18), 4906–4914. (25) Lamoureux, N.; Djebaili-Chaumeix, N.; Paillard, C. E. Exp. Therm. Fluid Sci. 2003, 27 (4), 385–393. (26) Vu, T. M.; Park, J.; Kwon, O. B.; Kim, J. S. Int. J. Hydrogen Energy 2009, 34 (16), 6961–6969. (27) Tseng, L. K.; Ismail, M. A.; Faeth, G. M. Combust. Flame 1993, 95 (4), 410–426. (28) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. In Twenty-Fourth Symp. (Int) on Combust., 1992; The Combustion Institute: Pittsburgh, 1992; pp 833-841. (29) Liao, S. Y.; Jiang, D. M.; Huang, Z. H.; Zeng, K.; Cheng, Q. Appl. Therm. Eng. 2007, 27 (2-3), 374–380. (30) Bradley, D.; Lawes, M.; Mansour, M. S. Combust. Flame 2009, 156 (7), 1462–1470.

For the spherically propagating flame, the stretched flame propagation speed, Sn, is derived from the flame radius versus time:33-36 dru ð1Þ Sn ¼ dt where ru is the radius of the flame in schlieren photograph and t is the elapsed time from spark ignition. The flame stretch rate, R, indicates the expending rate of the flame front area. In a quiescent mixture, a general definition of stretch at any point on the flame surface is shown in eq 2, dðln AÞ 1 dA ð2Þ ¼ R ¼ dt A dt (31) Yahyaoui, A.; Djebaili-Chaumeix, N.; Dagaut, P.; Paillard, C. E. Energy Fuels 2008, 22 (6), 3701–3708. (32) Zhang, Z. Y.; Huang, Z. H.; Wang, X. G.; Xiang, J.; Wang, X. B.; Miao, H. Y. Combust. Flame 2008, 155 (3), 358–368. (33) Bradley, D.; Gaskell, P. H.; Gu, X. J. Combust. Flame 1996, 104 (1-2), 176–198. (34) Huang, Z. H.; Wang, Q.; Yu, J. R.; Zhang, Y.; Zeng, K.; Miao, H. Y.; Jiang, D. M. Fuel 2007, 86, 2360–2366. (35) Serrano, C.; Hernandez, J. J.; Mandilas, C.; Sheppard, C. G. W.; Wbolley, R. Int. J. Hydrogen Energy 2008, 33 (2), 851–862. (36) Huang, Z.; Zhang, Y.; Zeng, K.; Liu, B.; Wang, Q.; Jiang, D. Combust. Flame 2006, 146 (1-2), 302–311.

966

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

where A is the area of flame. For an outwardly propagating spherical flame, the flame stretch rate can be simplified as, 1 dA 2 dru 2 ð3Þ R ¼ ¼ ¼ Sn A dt ru dt ru And a linear correlation is occurred between the flame speed and the flame stretch rate with respect to the early stage of flame propagation,33 that is, ð4Þ Sl -Sn ¼ Lb R where Sl is the unstretched flame propagation speed and Lb is the Markstein length of burned gas. The unstretched flame speed, Sl, can be obtained as the intercept value at R = 0 in the plot of Sn against R. The burned gas Markstein length Lb is the negative value of the slope of a Sn-R curve according to eq 4. Positive values of Lb demonstrate that the flame speed decreases with the increase of flame stretch rate and the flame is stable to diffusional-thermal instability. In this case, the flame speed in the flame protruding position will be suppressed if any kinds of protuberances appear at the flame front. On the contrary, negative values of Lb indicate the flame speed is increased with the increase of the flame stretch rate. In this case, the flame speed in the flame protruding position will be creased if any kinds of protuberances appear at the flame front, and the instability of the flame front will be increased.37 When Lb is positive and the absolute value is small, the local flame speed in the flame protruding position will not be suppressed effectively if any kinds of protuberances appear at the flame front (stretch rate is increased). This is attributed to the weak influence of stretch rate on flame speed. Thus, the flame instability in this case is more obvious compared with that of positive Lb with large absolute value. The laminar burning velocity can be obtained by the following equation, ð5Þ ul ¼ Fb Sl =Fu where Fb and Fu are the densities for burned gas and unburned gas, which can be received from initial state and thermal equilibrium calculation, respectively; and density ratio, σ, is defined as σ = Fu/Fb. Mass burning flux is defined as f0 = Fuul.38 Owing to the finite thickness, there exist two possible definitions for the stretched laminar burning velocity.14 One is the stretched laminar burning velocity un, which is defined at the unburned gas side and related to the entrainment of the unburned gas. The other is stretched mass burning velocity unr, which is involved in the production of the burned gas. " # Fb un ¼ S Sn ð6Þ Fu unr

Fb ¼ ðun -Sn Þ Fb -Fu

Figure 1. Stretched flame propagation speed vs stretch rate at different equivalence ratios, initial temperatures, and pressures.

Here, δl is laminar flame thickness achieved by δl = ν/ul,39 where ν is the kinetic viscosity of the unburned mixtures.

ð7Þ

4. Results and Discussions

where S is a rectified function depending on the flame radius and the density ratio and accounting for the effect of the flame thickness on the mean density of the burned gas. Bradley et al.14 provided a formula for S, 2 2 !2:2 3 !2:2 32 δ F δ F l l u u 5 -0:154 5 S ¼ 1 þ 1:24 ð8Þ ru Fb ru Fb

4.1. Flame Propagation Speed and Markstein Length. During the early stage of flame propagation, there exists a tendency for the flame to quench due to the high stretch rate. A spark produces a flame kernel and results in a very apparent flame expansion. Then, there is a limitation of minimum ignition energy for the continuous propagation of the flame. The introduction of the minimum ignition energy can influence the measured value. Bradly et al.,33

(37) Liao, S. Y.; Jiang, D. M.; Gao, J.; Huang, Z. H. Energy Fuels 2004, 18 (2), 316–326. (38) Law, C. K., Combustion Physics; Cambridge University Press: New York, 2006; pp 275-283.

(39) Bradley, D.; Lawes, M.; Liu, K.; Verhelst, S.; Woolley, R. Combust. Flame 2007, 149 (1-2), 162–172.

967

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

Figure 2. Markstein length and unstretched flame propagation speed versus equivalence ratio at different initial temperatures.

Figure 3. Markstein length and unstretched flame propagation speed vs equivalence ratio at different initial pressures.

Liao et al.,40 and Lamoureux et al.41 indicated that the flame speeds were independent of the ignition energy when flame radius is beyond 6 mm. With respect to the isobaric combustion, the maximum flame radius is limited to 25 mm, where the variation of the pressure can be discounted.42 Thus, the following study uses the flame radius between 6 mm and 25 mm to avoid the influence of ignition energy and pressure variation, which has been testified suitably for the constantvolume combustion chamber used in this study.32,36,43,44 Figure 1 shows the stretched flame propagation speed (Sn) versus the stretch rate (R) at different equivalence ratios, initial temperatures, and pressures. There exists a linear relationship between the stretched flame propagation speeds and flame stretch rates. The stretched flame propagation speed is decreased with the increase of stretch rate in all the cases, indicating a positive value of Lb. In Figure 1a, Sn gives its maximum value at the stoichiometric equivalence ratio and the slope of the Sn-R curve is increased with the increase of equivalence ratio, indicating the decrease of Lb. That means the instability of the flame front is increased at rich mixture side. As shown in Figure 1b, stretched flame propagation speed is increased with the increase of initial tempera-

ture. Increasing initial temperature will promote the mixture combustion and chemical reaction rate. Figure 1c shows the influence of initial pressure on stretched flame propagation speed. The stretched flame propagation speed is decreased with the increase of initial pressure. The slopes of Sn-R lines present the negative values, reflecting the positive values of Markstein lengths. This indicates that the flame front is stable to the diffusional-thermal instability. Figure 2 illustrates the Markstein length and unstretched flame propagation speed versus equivalence ratio at different initial temperatures. The Markstein length decreases monotonously with the increase of equivalence ratio and the decrease of initial temperature. Markstein length reflects the influence of diffusional-thermal and flame stretch on the explosion flame front, which results from the competing effects of heat conduction from the flame and reactant diffusion toward the flame.43 Thus, it reveals that lean mixtures and/or high initial temperature mixtures are more stable to diffusional-thermal instability. The unstretched flame propagation speed increases with the increase of initial temperature due to the increase of chemical reaction rate. The peak value of the unstretched flame propagation speed appears at an equivalence ratio between 1.0 and 1.1, and it shifts to the rich mixture side slightly as the increase of the initial temperature. Figure 3 gives Markstein length and unstretched flame propagation speed versus equivalence ratio at different initial pressures. The Markstein length decreases with the increase of initial pressure, and this indicates the flame becomes more unstable with the increase of initial pressure.

(40) Liao, S. Y.; Jiang, D. M.; Gao, J.; Huang, Z. H.; Cheng, Q. Fuel 2004, 83 (10), 1281–1288. (41) Lamoureux, N.; Djebaili-Chaumeix, N.; Paillard, C. E. Exp. Therm. Fluid Sci. 2003, 27 (4), 385–393. (42) Hu, E.; Huang, Z.; He, J.; Jin, C.; Zheng, J. Int. J. Hydrogen Energy 2009, 34 (11), 4876–4888. (43) Hu, E.; Huang, Z.; He, J.; Zheng, J.; Miao, H. Int. J. Hydrogen Energy 2009, 34 (13), 5574–5584. (44) Chen, Z.; Wei, L.; Huang, Z.; Miao, H.; Wang, X.; Jiang, D. Energy Fuels 2009, 23 (2), 735–739.

968

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

Figure 4. Adiabatic flame temperature vs equivalence ratio at different initial temperatures and pressures.

The unstretched flame propagation speed decreases with the increase of initial pressure, and the maximum value is presented near the equivalence ratio of 1.1. Figure 4 shows the adiabatic temperature versus equivalence ratio at different initial temperatures and pressures. Adiabatic temperature, a parameter that characterizes the thermodynamic behavior of the combustible mixture, is deduced from combustion equilibrium. The peak value of Tad is presented at the equivalence between 1.0 and 1.1 in all cases, and the trend is similar to the unstretched flame propagation speed. For a given equivalence ratio, Tad increases with the increase of initial temperature. This phenomenon results from the enhancement of chemical reaction rate as initial temperature increases, leading to the increase of intermediate radicals such as OH, and ultimately increases the Tad. In case of specified initial temperature and equivalence ratio, Tad shows little variation at different initial pressures except for stoichiometric mixtures. Tad is increased obviously with the increase of initial pressure at equivalence ratios from 0.9 to 1.2. This is attributed to the reduced thermal dissociation at higher initial pressure.45 This behavior was also reported by Anupam et al.46 in methane-air flame and Di et al.47 in diethyl ether-air flame.

Figure 5. Stretched laminar burning velocity and stretched mass burning velocity vs stretch rate at different equivalence ratios, initial temperatures, and initial pressures (un: soild point; unr: hollow point).

4.2. Laminar Burning Velocity and Mass Burning Flux. Figure 5 shows the stretched laminar burning velocity (un) versus stretched mass burning velocity (unr) at different equivalence ratios, initial temperatures, and initial pressures. The stretched laminar burning velocity based on the rate of disappearance or entrainment of cold unburned gas, increases with the increase of stretch rate except for lean mixtures (φ = 0.8) at the reference condition, Pu = 0.10 MP, and Tu = 373 K. In contrast, the stretched mass burning velocity, unr, related to the rate of appearance of burned gas, decreases as stretch rate increases. The difference between un and unr is clearly illustrated in Figure 5, and this results from the influence of flame thickness on burning velocity. The

(45) Law, C. K.; Makino, A.; Lu, T. F. Combust. Flame 2006, 145 (4), 808–819. (46) De, A.; Ting, D.; Checkel, M. D. SAE Tech. Paper, 2006-01-0494, 2006. (47) Di, Y. G.; Huang, Z. H.; Zhang, N.; Zheng, B.; Wu, X. S.; Zhang, Z. Y. Energy Fuels 2009, 23, 2490–2497.

969

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

Figure 6. Unstretched laminar burning velocity versus equivalence ratio at different initial temperatures and initial pressures.

Figure 7. Mass burning flux vs equivalence ratio at different initial temperatures and initial pressures.

difference is more obvious at small radius (corresponding to large stretch rate), and it reflects the large influence of flame thickness on burning velocity. As flame radius approaches to infinity and/or flame stretch rate becomes infinitesimal, the effect of flame thickness can be neglected, that is, un and unr tend toward a identical value, the unstretched laminar burning velocity. As shown in Figure 5a, at Pu = 0.10 MP and Tu = 373 K, the value of un-unr is larger at φ = 1.0 and φ = 1.2 compared with that at φ = 0.8. This suggests the effect of flame thickness on burning velocity is decreased at lean mixture combustion. Figure 5, panels b and 5c, illustrates the effect of initial temperature and initial pressure on un and unr. They all increase with the increase of initial temperature and the decrease of initial pressure. These variation tendencies are similar to those of Sn and Sl. Although these parameters are defined from different respects, they will reflect the same combustion characteristics. Therefore, they present the similar variation tendencies with the variation of initial temperature and pressure. Figure 6 gives the unstretched laminar burning velocity versus equivalence ratio at different initial temperatures and initial pressures. The unstretched laminar burning velocity is increased as initial temperature increases and initial pressure decreases. The peak value of unstretched laminar burning velocity occurs near φ = 1.0 in the mixtures. This behavior is similar to that of unstretched flame propagation speed. Figure 7 gives the mass burning flux (f0) versus equivalence ratio at different initial temperatures and initial pressures. The value of f0 is increased with the increase of initial temperature and initial pressure. This is different

to unstretched laminar burning velocity, which is increased with the increase of initial temperature and decreased with the increase of initial pressure. For a specified initial pressure, the enhanced chemical reaction rate caused by increasing initial temperature leads to the increase of flame propagation speed and laminar burning velocity. The density of combustible mixture is decreased, and this also results from the increased initial temperature. However, the rate of decreasing in density is less than the increasing in laminar burning velocity. This combined effect leads to the increase of f0 as initial temperature increases. For a given initial temperature, with the increase of initial pressure, the unstretched laminar burning velocity is decreased and the density of mixture is increased. As the increase rate in density of mixture is larger than the decrease rate in laminar burning velocity, f0 is ultimately increased at an elevated initial pressure as shown in Figure 7b. The different trend between f0 and ul reflects the influence of unburned mixture density at different initial pressures.38 4.3. Flame Stability and Cellular Structure. The occurrence of flame front instability depends on the combined influence of hydrodynamic and thermal-diffusive instabilities. Markstein length is employed to represent the thermal-diffusive instability, which is the dominant factor leading to the unstable flame front at the early stage of flame propagation. Hydrodynamic instability is originated from gas thermal expansion, and this instability becomes more obviously as the flame propagates outwardly and is characterized by density ratio, σ, and flame thickness, δl. Generally, 970

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

Figure 8. Schlieren images of flame front at flame radius of 30 mm at different initial pressures and initial temperatures (φ = 1.0). Table 1. Markstein Length, Flame Thickness, and Density Ratio at Different Initial Pressures (Tu = 473 K, φ = 1.0) initial pressure Pu (MPa) 0.10 0.25 0.50

Markstein length Lb (mm)

flame thickness δl (mm)

density ratio Fu/Fb

1.3700 0.3920 -0.1910

0.0441 0.0193 0.0105

5.3103 5.3652 5.3899

Table 2. Markstein Length, Flame Thickness, and Density Ratio at Different Initial Temperatures (Pu = 0.1 MP, φ = 1.0) initial temperature Tu (K) 373 423 473

Markstein length Lb (mm)

flame thickness δl (mm)

density ratio Fu/Fb

0.5960 1.0000 1.3700

0.0497 0.0470 0.0441

6.5826 5.8707 5.3103

Figure 9. Schlieren images of flame front at different equivalence ratios and flame radius. (Pu = 0.25 MP, Tu = 373 K).

hydrodynamic instability is enhanced with increasing density ratio and decreasing flame thickness.48 For a specified equivalence ratio, the flame front instability is affected by initial temperature and initial pressure. Figure 8 shows the schlieren images of flame front at flame radius of 30 mm at different initial pressures and initial temperatures. The flame front maintains a smooth surface at initial pressure of 0.10 MP, and no crack or cellular structure is observed. At the initial pressure of 0.25 MP, the image of the flame front shows some cracks, and one of the possible reasons for these cracks is the influence of ignition and the centrally located electrodes at the early stage of flame propagation. At the elevated pressure (Pu = 0.50 MP), cellular structure is clearly presented and this reflects the occurrence of the unstable flame. These behaviors indicate that, at a specified equivalence ratio and initial temperature, the cellular flame structure will occur more easily with the increase of initial pressure. In other words, the instability of the flame front is increased as initial pressure increases. This is consistent to the analysis in Lb and δl shown in Table 1. As shown in Table 1, Fu/Fb varies little and δl is decreased with the increase of initial pressure, and this indicates an increase in hydrodynamic instability at elevated

initial pressure. In respect to thermal-diffusive instability, the flame is highly unstable with the increase of initial pressure. Schlieren images clearly show the instability of the flame front at Pu = 0.50 MP. The corresponding Markstein length gives the negative value. At different initial temperatures, as shown in data of table 2, both Fu/Fb and δl are decreased with the increase of initial temperature. The decrease of Fu/Fb results in the decrease of hydrodynamic instability and the decrease of δl signifies the increase of hydrodynamic instability. As thermal-diffusive instability (Markstein length) has little variation and maintains positive value, the flame front instability is insensitive to the initial temperature under the combined effects of hydrodynamic and thermal-diffusive instabilities. The analysis is consistent with the results demonstrated in schlieren images. This is also consistent with the results obtained by other researchers.43 Figure 9 shows the development of cellular flame structure with the expansion of the spherical flame at different equivalence ratios, an initial pressure of 0.25 MPa, and initial temperature of 373 K. A smooth flame front is presented at the equivalence of 0.7, even at large radius. With the increase of equivalence ratio, the cellular structure is observed, and the structure is developed gradually as the flame propagates, when the stretch is lessened (corresponding to

(48) Wu, X.; Huang, Z.; Jin, C.; Wang, X.; Zheng, B.; Zhang, Y.; Wei, L. Energy Fuels 2009, 23 (9), 4355–4362.

971

Energy Fuels 2010, 24, 965–972

: DOI:10.1021/ef901117d

Gong et al.

the increase of flame radius) the cellular instability can no longer be suppressed. Consequently, the cellular structure quickly cracks and develops over the entire flame surface. This phenomenon is more obvious and the onset of cellular flame structure becomes advancing at large equivalence ratio (φ = 1.3) compared with that at φ = 1.0. The results suggest that flame instability is sensitive to equivalence ratio. Increasing equivalence ratio leads to an increased instability of the flame. This is in consistent with the result of analysis from the Markstein length.

The main results are summarized as follows: (1) Unstretched flame propagation speed and unstretched laminar burning velocity are increased as initial temperature increases and initial pressure decreases. (2) Markstein length decreases with the increase of equivalence ratio and initial pressure, and it increases with the increase of initial temperature. This indicates that flame is stable at high initial temperature and low initial pressure at the early stage of flame propagation. (3) Hydrodynamic instability is influenced by density ratio and flame thickness. Hydrodynamic instability is increased with the increase of initial pressure. (4) Flame front instability is decreased with the decrease of equivalence ratio and initial pressure. The cellular flame structure advances with the increase of initial pressure and equivalence ratio. Flame instability is insensitive to initial temperature.

5. Conclusions Combustion characteristics of outwardly spherical laminar premixed flame of high-octane fuel-air mixtures were studied by a high-speed schlieren photography system in a constantvolume bomb at elevated temperatures and pressures. Flame front instability and laminar burning velocity were analyzed.

Acknowledgment. This study is supported by National Science Foundation of China (Nos. 50636040, 50821604).

972