Experimental Study on Combustion Characteristics of N2-Diluted

deviation of the temperature gradient from the normal direction of streamline, ...... Unpublished work presented at Pairs, presented at La Techniq...
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Energy Fuels 2009, 23, 5798–5805 Published on Web 10/02/2009

: DOI:10.1021/ef900633z

Experimental Study on Combustion Characteristics of N2-Diluted Diethyl Ether-Air Mixtures Ni Zhang, Yage Di, Zuohua Huang,* Bin Zheng, and Zhiyuan Zhang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Received June 22, 2009. Revised Manuscript Received September 3, 2009

Combustion characteristics of diethyl ether-air-N2 diluent are studied at different initial temperatures, equivalence ratios, and dilution ratios using an outwardly expanding spherically flame and high-speed schlieren photography system. The effects of these parameters on laminar burning velocity, Markstein length, and Zeldovich number are analyzed. The results show that laminar burning velocity is increased with the increase of the initial temperature and is decreased with the increase of the dilution ratio. The maximum value of the laminar burning velocity is presented at the equivalence ratio of 1.0. The Markstein length is decreased with the increase of the initial temperature and equivalence ratio. Except for the equivalence ratio of 1.2, the Markstein length is increased with the increase of the dilution ratio, indicating that the increase of the initial temperature and equivalence ratio enhances the diffusional-thermal instability of the flame front but the increase of the dilution ratio reduces the diffusional-thermal instability. The density ratio is decreased, and the flame thickness is increased with the increase of the dilution ratio, indicating that the hydrodynamic instability is suppressed. The Zeldovich number is increased with the increase of the dilution ratio and is decreased with the increase of the initial temperature. The Zeldovich number gives the lowest value at the equivalence ratio of 1.0, where the adiabatic temperature gives the maximum value. A formula of laminar flame velocity is correlated with initial conditions based on the experimental data.

reduce smoke obviously at the cost of NOx emissions.1 Nowadays, one of the most effective methods to decrease NOx emission is the introduction of exhaust gas recirculation (EGR) by reducing the flame temperature and oxygen concentration as residual gas is added into the combustible mixture.3,6-9 Dilution has an important influence on fuel-air combustion. However, the fundamental combustion characteristics of DEE-air mixtures in the presence of diluent are not fully understood. Therefore, fundamental combustion characteristics of DEE-air diluent mixtures are necessarily investigated. These parameters, such as laminar burning velocity, laminar flame thickness, and stability response, can make an in-depth understanding to DEE combustion. The laminar burning velocity is a fundamental parameter of laminar premixed flames. An accurate knowledge of the laminar burning velocity, together with the influence of other properties on it, is significant in any combustion studies. From the practical view, the laminar burning velocity plays an important role in determining several aspects of the combustion process in spark-ignition engines, such as the fuel burning rate and the emissions.10 From the fundamental view, it is the

1. Introduction Investigations on alternative fuels have received more and more attention because of the increasing crude oil price and more stringent government regulation on exhaust emissions. Diethyl ether (DEE) has good fuel-air mixing potential, which may result in low hydrocarbon (HC) and CO emissions because of oxygen molecules1 and has better ignition characteristics because of a higher cetane number compared to that of diesel.2-4 Thus, DEE can be used not only as an ignition enhancer5,6 but also as the oxygenated alternative to decrease engine smoke emission.1,2 In addition, DEE is liquid under normal atmospheric conditions, which is facilitated to be stored and handled on board.3 Previous experiments in the diesel engine found that the addition of DEE into diesel could *To whom correspondence should be addressed. Telephone: þ008629-82665075. Fax: þ0086-29-82668789. E-mail: [email protected]. edu.cn. (1) Kapilan, N.; Mohanan, P.; Reddy, R. P. Performance and emission studies of diesel engine using diethyl ether as oxygenated fuel additive. SAE Tech. Pap. 2008-01-2466, 2008. (2) Bailey, B.; Eberhardt, J.; Goguen, S.; Erwin, J. Diethyl ether (DEE) as a renewable diesel fuel. SAE Tech. Pap. 972978, 1997. (3) Anand, R.; Mahalakshmi, N. V. Simulataneous reduction of NOx and smoke from a direct-injection diesel engine with exhaust gas recirulation and diethyl ether. Proc. Inst. Mech. Eng., Part D 2007, 221 (1), 109–116. (4) Ashok, M. P.; Saravanan, C. G. Effect of diethyl ether with emulsified fuel in a direct injection diesel engine. SAE Tech. Pap. 2007-01-2126, 2007. (5) Miller Jothi, N. K.; Nagarajan, G.; Renganarayanan, S. Experimental studies on homogeneous charge CI engine fueled with LPG using DEE as an ignition enhancer. Renewable Energy 2007, 32 (9), 1581–1593. (6) Miller Jothi, N. K.; Nagarajan, G.; Renganarayanan, S. LPG fueled disesl engine using diethyl ether with exhaust gas recirclation. Int. J. Therm. Sci. 2008, 47 (4), 450–457. r 2009 American Chemical Society

(7) Ladommatos, N.; Abdelhalim, S.; Zhao, H. Control of oxides of nitrogen from diesel engines using diluents while minimising the impact on paniculate pollutants. Appl. Therm. Eng. 1998, 18 (11), 963–980. (8) Abd Alla, G. H. Using exhaust gas recirculation in internal combustion engines: A review. Energy Convers. Manage. 2002, 43 (8), 1027–1042. (9) Zheng, M.; Reader, G. T.; Hawley, J. G. Diesel engine exhaust gas reciculation;A review on advanced and novel concepts. Energy Convers. Manage. 2004, 45 (6), 883–900. (10) De, A.; Ting, D. S. K.; Checkel, M. D. The effects of temperature and pressure on stretched, freely propagating, premixed, laminar methane-air flame. SAE Tech. Pap. 2006-01-0494, 2006.

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base data in validating the kinetic mechanics of fuels. More work has been made on determining laminar burning velocity both experimentally and theoretically. Various methods have been employed to measure the laminar burning velocity, including the outwardly propagating spherical flame,12-22 the counterflow or stagnation flame,23-27 the Bunsen flame,28 and the burner-stabilized flat flame.29,30 The counterflow method is difficult to implement at high pressure because of instability caused by flow Reynolds number,31 at which the

stretch rate must be measured. It is widely accepted that an outwardly propagating spherical flame in a constant bomb is an appropriate approach in measuring the laminar burning velocity because of the well-defined stretch rate, configurationally simple flame, and wide range of initial temperatures and pressures. Meanwhile, other fundamental parameters can be easily obtained.12,14,19,21,22 It is well-known that the addition of diluents will reduce the laminar flame velocity because of the reduction of the mixture heating value and flame temperature.23,32 However, studies of the diluent effect on combustion characteristics are not fully conducted. Kwon and Faeth17 experimentally and computationally studied the unstretched laminar burning velocities of mixtures of hydrogen and oxygen, in which nitrogen, argon, and helium were used as diluents. They found that, at lean-fuel conditions, increasing flame temperatures tended to reduce flame sensitivity to stretch and, at low pressures, heliumdiluted flames reduced the tendencies toward diffusionalthermal instability behavior. Zhao et al.30 studied the N2 dilution effect on the laminar flame velocity of propane-air and revealed a quasi-linear relationship between the laminar flame velocity and dilution ratio. Qiao et al.33 investigated the suppression effect of diluents on laminar premixed hydrogenair flames and demonstrated that diluents became more effective in the order of helium, argon, nitrogen, and carbon dioxide. Tang et al.34 reported the effects of N2 dilution on laminar burning characteristics of propane-air premixed mixtures and found that the Markstein length was increased with the increase of the dilution ratio because of the suppression of preferential-diffusion instability. In addition, the hydrodynamic instability was decreased with the decrease of the density ratio and the increase of flame thickness. Previous research on laminar burning velocities and Markstein lengths of DEE-air mixtures are reported in ref 35. However, no publication on dilution effects on combustion characteristics of DEE-air mixtures is reported. This paper will study the combustion characteristics of DEE-air-diluent mixtures by measuring laminar burning velocities and analyzing flame stabilities in the presence of diluent.

(11) Aung, K. T.; Hassan, M. I.; Faeth, G. M. Flame stretch interactions of laminar premixed hydrogen-air flames at normal temperature and pressure. Combust. Flame 1997, 109 (1-2), 1–24. (12) Dowdy, D. R.; Smith, D. B.; Taylor, S. C. The use of expanding spherical flames to determine burning velocities and stretch effects in hydrogen/air mixtures. Proceedings of the 24th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1990; pp 325-332. (13) Rozenchan, G.; Zhu, D. L.; Law, C. K.; Tse, S. D. Outward propagation, burning velocities, and chemical effects of methane flames up to 60 atm. Proc. Combust. Inst. 2002, 29 (2), 1461–1470. (14) Sun, C. J.; Sung, C. J.; Law, C. K. Dynamics of weakly stretched flames: Quantitative description and extraction of global flame parameters. Combust. Flame 1999, 118 (1-2), 108–128. (15) Bradley, D.; Hicks, R.; Lawes, M.; Sheppard, C. G. W.; Woolley, R. The measurement of laminar burning for iso-octane-air and isooctane-n-heptane-air mixtures at elevated temperatures and pressures in an explosion bomb. Combust. Flame 1998, 115 (1-2), 126–144. (16) Gu, X. J.; Haq, M. Z.; Laws, M.; Woolley, R. Laminar burning velocity and Markstein lengths of methane-air mixtures. Combust. Flame 2000, 121 (1-2), 41–58. (17) Kwon, O. C.; Faeth, G. M. Flame/stretch interactions of premixed hydrogen-fueled flames: Measurements and predictions. Combust. Flame 2001, 124 (4), 590–610. (18) Verhelst, S.; Woolley, R.; Lawes, M.; Sierens, R. Laminar and unstable burning velocities and Markstein lengths of hydrogen-air mixtures at engine-like conditions. Proc. Combust. Inst. 2004, 30 (1), 209–216. (19) Huang, Z. H.; Zhang, Y.; Zeng, K.; Liu, B.; Wang, Q.; Jiang, D. M. Measurements of laminar burning velocities for natural gas-hydrogen-air mixtures. Combust. Flame 2006, 146 (1-2), 302–311. (20) Jomaas, G.; Law, C. K.; Bechtold, J. K. On transition to cellularity in expanding spherical flames. J. Fluid Mech. 2007, 583, 1–26. (21) 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. (22) Tang, C. L.; Huang, Z. H.; Jin, C.; He, J. J.; Wang, J. H. Laminar burning velocities and combustion characteristics of propanehydrogen-air premixed flames. Int. J. Hydrogen Energy 2008, 33 (18), 4906–4914. (23) Park, J.; Kim, S. G.; Lee, K. M.; Kim, T. K. Chemical effect of diluents on flames structures and NO emission characteristics in methane-air counterflow diffusion flame. Int. J. Energy Res. 2002, 26 (13), 1141–1160. (24) Jomaas, G.; Zheng, X. L.; Zhu, D. L.; Law, C. K. Experimental determination of counterflow iginition temperatures and laminar flames speeds of C2-C3 hydrocarbons at atomspheric and elevated pressures. Proc. Combust. Inst. 2005, 30 (1), 193–200. (25) Chao, B. H.; Egolfopoulos, F. N.; Law, C. K. Structure and propagation of premixed flame in nozzle-generated counterflow. Combust. Flame 1997, 109 (4), 620–638. (26) Vagelopoulos, C. M.; Egolfopoulos, F. N. Direct experimental determination of laminar speeds. Proceedings of the 27th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; pp 513-519. (27) Kumar, K.; Sung, C. J. Laminar flame speeds and extinction limits of preheated n-decane/O2/N2 and n-dodecane/O2/N2 mixtures. Combust. Flame 2007, 151 (1-2), 209–224. (28) Sankaran, R.; Hawkes, E. R.; Chen, J. H.; Lu, T.; Law, C. K. Structure of a spatially developing turbulent lean methane-air Bunsen flame. Proc. Combust. Inst. 2007, 31 (1), 1291–1298. (29) van Maaren, A.; Thung, D. S.; de Goey, L. P. H. Measurement of flame temperature and adiabatic burning velocity methane/air mixtures. Combust. Sci. Technol. 1994, 96 (4-6), 327–344. (30) Zhao, Z. W.; Kazakov, A.; Li, J.; Dryer, F. L. The initial temperature and N2 dilution effect on the laminar flame speed of propane/air. Combust. Sci. Technol. 2004, 176 (10), 1–19. (31) Tse, S. D.; Zhu, D. L.; Law, C. K. Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atm. Proc. Combust. Inst. 2000, 28 (2), 1793–1800.

2. Experiment Setup and Procedures In this study, laminar combustion characteristics of N2-diluted DEE-air mixtures are investigated at different initial temperatures (Tu = 323, 343, and 363 K), equivalence ratios (φ = 0.8, 1.0, and 1.2), and dilution ratios of 0, 5, 10, 15, and 20%, respectively. The experimental setup consists of five parts, including the combustion vessel, the heating system, the ignition system, the data acquisition system, and the high-speed schlieren photography system. In the propagating spherical flame method, a quiescent and homogeneous combustible mixture in a closed chamber is centrally ignited by an electrical spark that results in an outwardly propagating spherical flame. The flame front history is recorded by a high-speed digital camera (HG-100K, (32) Elia, M.; Ulinski, M.; Metghalchi, M. Laminar burning velocity of methane-air-diluent mixtures. J. Eng. Gas Turbines Power 2001, 123 (1), 190–196. (33) Qiao, L.; Kim, C. H.; Faeth, G. M. Suppression effects of diluents on laminar premixed hydrogen/oxygen/nitrogen flames. Combust. Flame 2005, 143 (1-2), 79–96. (34) Tang, C. L; Huang, Z. H.; He, J. J.; Jin, C.; Wang, X. B.; Miao, H. Y. Effects of N2 dilution on laminar burning characteristics of propane-air premixed flames. Energy Fuels 2009, 23 (1), 151–156. (35) Di, Y. G.; Huang, Z. H.; Zhang, N.; Zheng, B.; Wu, X. S.; Zhang, Z. Y. Measurement of laminar burning velocities and Markstein lengths for diethyl ether-air mixtures at different initial pressures and temperature. Energy Fuels 2009, 23 (5), 2490–2497.

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Figure 1. Diagram of the constant volume combustion vessel.

American Redlake Corporation) operating at 5000 frames/s. The flame propagation speed is calculated using the flame radii through theoretical models.15,16,36 Figure 1 shows the diagram of the cylinder-type constant volume vessel, with a diameter of 180 mm and length of 210 mm. Two sides of the vessel are mounted with the quartz window for optical observation. Mixtures were prepared according to the partial pressure of each component. A mercury manometer is used to regulate the initial and partial pressure when it is smaller than the atmospheric pressure. A pressure transmitter with the accuracy of 0.001 MPa is used to measure the initial pressure and partial pressure when it is larger than the atmospheric pressure. The whole vessel is heated by a 24 kW heating tape wrapped outside the chamber body. A thermocouple is employed to measure the initial temperature of the mixtures with an accuracy of 1 K. The combustion pressure is recorded by a digital acquisition system (DL750, Yokogawa Corporation). Once the combustion was completed, the combustion vessel is flushed with fresh air for several times to eliminate the influence of residual gas on the next experiment.

unr ¼

Fb ðun -unr Þ Fb -Fu

ð3Þ

where S is a function that depends upon the flame radius and density ratio. Bradley et al.15,36 provided an expression for S, shown in eq 7. 2 2 !2:2 3 !2:2 32 δ F δ F l u u 5 -0:154 l 5 S ¼ 1 þ 1:24 ð4Þ ru Fb ru Fb Here, δl is the characteristic laminar flame thickness, obtained by δl = ν/ul. ν is the kinetic viscosity of the unburned mixture. Fb is the density of burned gas. The lower limitation of flame radii is chosen to avoid the effects of initial spark ignition on the flame propagating speed.15,36 Bradley et al.15,36 found that the region influenced by spark energy and electrodes was less than 5 mm of flame radius. The upper limitation of flame radii is to ensure that the increment of pressure is less than 1% and the flame speed is not affected by the confinement of the cylinder chamber.37 Thus, the data used in this paper is from 6 to 25 mm. In the absence of body forces, two major sources of intrinsic instabilities in premixed flames are originated from hydrodynamic effects and diffusional-thermal effects.20 The deviation of the temperature gradient from the normal direction of streamline, which results from thermal expansion of the flame front, gives rise to hydrodynamic instability.38-40 Hydrodynamic instability is presented first as the onset of crack and then cellular structure. The hydrodynamic factor influences flame instability when the flame radius is large enough, and the hydrodynamic instability is influenced by variation of flame thickness.40 The presence of diffusional-thermal instability could be identified by irregular distortions of the flame surface relatively early in the flame propagation process.40,41 The diffusional-thermal instability results from the competing effects of heat conduction from the flame

3. Parameters to Combustion Analysis The dilution ratio is defined as the partial pressure of the diluent over the total pressure (diluent þ air þ fuel) and is expressed as follows: Pdiluent ð1Þ φr ¼ Pdiluent þ Pair þ Pfuel where Pdiluent, Pair, and Pfuel are the partial pressures of the diluent, air, and fuel, respectively. For an outwardly propagating spherical flame, the stretched flame propagation speed, Sn, and laminar burning velocity, u1, can be obtained using the method in previous studies.15,16,36 Because of the finite flame thickness, the two possible definitions for the stretched laminar burning velocity depend upon whether the burning velocity is defined at the unburned gas side or burned gas side. Bradley et al.15,36 proposed these two burning velocities, which are stretched laminar burning velocity un and stretched mass burning velocity unr. The former expresses the rate of entrainment of cold unburned gas by the flame front. The latter is associated with the rate of appearance of completely burned gas behind the front.36 The definitions of velocities un and unr are as follows: " # Fb ð2Þ un ¼ S Sn Fu

(37) Qin, X.; Ju, Y. G. Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures. Proc. Combust. Inst. 2005, 30 (1), 233–240. (38) Darrieus, G. Propagation d’un front de flame. Unpublished work presented at Pairs, presented at La Technique Moderne (Paris) and in 1945 at Congres de Mechanique Appliquee, Paris, France, 1938. (39) Landau, L. D. On the theory of slow combustion. Acta Physicochim. URSS 1944, 19, 77–88. (40) Law, C. K.; Sung, C. J. Structure, aerodynamics, and geometry of premixed flamelets. Prog. Energy Combust. 2000, 26 (4-6), 459–505. (41) Clavin, P. Dynamic behavior of premixed flame fronts in laminar and turbulent flows. Prog. Energy Combust. 1985, 11 (1), 1–59.

(36) Bradley, D.; Gaskell, P. H.; Gu, X. J. Burning velocities, Markstein lengths, and flame quenching for spherical methane-air flames: A computational study. Combust. Flame 1996, 104 (1-2), 176–198.

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Figure 2. Flame radii versus time.

ratios, and initial temperatures. There is a linear relationship between the stretched flame propagation speed and the stretch rate at asymptotically weak stretch rates.41 As shown in Figure 3, Sn is increased with the increase of the equivalence ratio. There is a sharp falling position (marked as A) at the equivalence ratio of 0.8. Bradley et al.,15 Tang et al.,34 and Di et al.35 reported a similar phenomenon. Bradley et al.15 pointed out that the flame before this regime was not fully developed; thus, the data before this position cannot be used in the determination of laminar burning velocity. The transition point B marks the onset of the cellular structure. Sn is remarkably increased as the increase of the flame surface. This result agrees well with those observed by Zhang et al.,21 Bradley et al.,15 Gu et al.,16 and Di et al.35 To eliminate the effect of the undeveloped flame region, the flame radius should be larger than that of point A. To eliminate the effect of the cellular structure on the flame propagation speed, the flame radius should be smaller than that of point B. Panels b and c of Figure 3 illustrate the effects of the N2 dilution ratio and initial temperature on Sn. It clearly shows that diluent addition will significantly decrease the stretched flame propagation speed. The reasons for the substantial reduction of flame propagation speed because of N2 addition to fuel-air mixtures at a given equivalence ratio are the decrease in heat release and the increase in specific heat capacity of the mixtures.33,43 Shrestha and Karim44 analyzed the thermodynamic effects of CO2 and N2 dilution on the methane-air mixtures, and they suggested that diluents reduced the heating value of the mixture, lowered flame temperature, and decreased the flame propagation speed. An increase of the initial temperature leads to an increase of Sn, and this is due to the enhancement of the chemical reaction at increased temperature. The slope of Sn - R linear fitting line is the negative value of the burned gas Markstein length, Lb.15,16 Lb represents the sensitivity of laminar premixed flames to the stretch rate and

and reactant diffusions, which mainly depend upon the fast diffusive component in the mixture. If the fast diffusive component is dominant, the flame is stable. Otherwise, the flame is unstable.42 In practice, it is convenient to derive the activation temperature, Ea/R, from the linear plot of 2 ln( f 0) against 1/Tad,15,16 as suggested by the following function:14-16 Ea d lnð f 0 Þ ¼ -2 ð5Þ dð1=Tad Þ R where Ea is the activation energy, R is the universal gas constant, and Tad is the adiabatic flame temperature. f 0 is the mass burning flux, which can be obtained from the following equation: f 0 ¼ Fu ul

ð6Þ

where Fu is the unburned gas density and u1 is the laminar burning velocity. Zeldovich number (Ze) is a dimensionless form of the overall activation energy. It reflects the mass burning flux reliance on the activation temperature,15,16,20 Ea Tad -Tu Ze ¼ ð7Þ R Tad 2 4. Results and Discussion 4.1. Stretched Flame Speed and Markstein Length. Figure 2 shows the flame radii versus time at different equivalence ratios, dilution ratios, and initial temperatures. Flame radii are increased monotonically with time. The mixtures of φ = 0.8 and 1.2 represent the lean-fuel flame and the rich-fuel flame, respectively. The rich mixture has a higher flame propagation speed, as illustrated in Figure 2a. As can be seen in Figure 2b, the addition of the diluent decreases the flame propagation speed. Increasing the initial temperature will increase the flame propagation speed, as shown in Figure 2c, but the influence is small. Figure 3 plots the stretched flame propagation speed, Sn, versus the stretch rate at different equivalence ratios, dilution

(43) Prathap, C.; Ray, A.; Ravi, M. R. Investigation of nitrogen dilution effects on the laminar burning velocity and flame stability of syngas fuel at atmospheric condition. Combust. Flame 2008, 155 (1-2), 145–160. (44) Shrestha, S. O. B.; Karim, G. A. Predicting the effects of the presence of diluents with methane on spark ignition engine performance. Appl. Therm. Eng. 2001, 21 (3), 331–342.

(42) Markstein, G. H. Cell structure of propane flames burning in tubes. J. Chem. Phys. 1949, 17 (4), 428–429.

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Figure 3. Stretched flame propagation speed versus the stretch rate.

the flame stability response.13,14,17,45 As shown in Figure 3, it is dependent upon the equivalence ratio and experimental conditions. Markstein lengths are positive in all cases, indicating that the flame is stable one. Here, the stretch has an adverse effect on the flame speed. A positive value of Lb indicates that the flame propagation speed is decreased with the increase of the flame stretch rate. In this case, the protuberance of the flame front is suppressed and the flame tends to be diffusional-thermal-stable. In contrast to this, a negative value of Lb indicates that the flame propagation speed is increased with the increase of the flame stretch rate, and here, any protuberance in the flame front tends to be unstable.19,46 Figure 4 illustrated the Markstein length versus the N2 dilution ratio at different initial temperatures and equivalence ratios. The Markstein length is increased with the increase of the N2 dilution ratio, indicating that diluent addition can reduce diffusional-thermal instability of the flame front. This phenomenon is consistent with that observed by Tang et al.34 and Miao et al.47 An increase in the initial temperature results in a decrease in Lb, revealing that increasing the initial temperature will enhance the diffusional-thermal instability of the flame front. Figure 4b shows that Lb increases with the increase of the dilution ratio for lean and stoichiometric mixtures but decreases with the increase of the dilution ratio for rich mixtures. This is consistent with the theory of Markstein;42 that is, if the faster diffusive component is dominant, the laminar flame is stable to diffusional-thermal instability. When the equivalence ratio is less than 1.0, the fast diffusive component (air) is dominant in the DEE-air flame. In addition, the nitrogen has similar transport properties with air. Thus, the addition

Figure 4. Burned gas Markstein length versus the dilution ratio.

of the diluent increases the concentration of the faster diffusive component, and the flame tends to more stable to the diffusional-thermal effect. In contrast, at the equivalence ratio of 1.2, the effect of the diluent becomes insignificant and this can also be observed from the slight variation of Lb versus the dilution ratio. The possible explanation is that the addition of diluent increases the specific heat capacity and decreases the flame temperature; however, the faster diffusive component is insufficient, and the flame tends to be unstable. The mixtures without diluent show small variation in Lb at three equivalence ratios, implying that the mixtures without diluent are less sensitive to the stretch rate.43 Addition of the diluent increases the value of Lb at lean mixtures and decreases the value at rich mixtures, reflecting strong sensitivity of the flame to stretch rate. 4.2. Laminar Burning Velocity and Hydrodynamic Instability. Unstretched laminar burning velocity, u1, versus the dilution ratio is plotted in Figure 5. u1 decreases monotonously with the increase of the dilution ratio regardless of the initial temperature and equivalence ratio. Zhang et al.,21

(45) Tseng, L. K.; Ismail, M. A.; Faeth, G. M. Laminar burning velocities and Markstein numbers of hydrocarbon/air flames. Combust. Flame 1993, 95 (4), 410–426. (46) Huang, Z. H.; Wang, Q.; Yu, J. R.; Zhang, Y.; Zeng, K.; Miao, H. Y.; Jiang, D. M. Measurement of laminar burning speed of dimethyl ether-air premixed mixtures. Fuel 2007, 86 (15), 2360–2366. (47) Miao, H. Y.; Ji, M.; Jiao, Q.; Huang, Q.; Huang, Z. H. Laminar burning velocity and Markstein length of nitrogen diluted natural gas/ hydrogen/air mixtures at normal, reduced and elevated pressures. Int. J. Hydrogen Energy 2009, 34 (7), 3145–3155.

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Figure 5. Unstretched laminar burning velocity versus the dilution ratio.

Figure 6. Stretched laminar burning velocity and stretched mass burning velocity versus the stretch rate.

Zhao et al.,30 Qiao et al.,33 Tang et al.,34 Prathap et al.,43 Miao et al.,47 and Han et al.48 reported a similar behavior of u1 versus the dilution ratio. Park et al.23 conducted experimental and computational studies on the chemical effects of diluents on flame structures. They suggested that the effects of the addition of diluents on burning velocity are from both thermal and chemical sides. First, the effective heating value of the fuel-air mixture is reduced because of the presence of diluents. Second, the addition of diluents decreases the concentration of the radicals, such as H, O, and OH. The maximal value of u1 is presented at φ = 1.0 regardless of the dilution ratio. An increase in the initial temperature gives rise to an increase of u1. Han et al.48 reported that the increase of the initial temperature resulted in a growing mass fraction of combustion species, such as H2, H, CHO, and CH3, and further enhanced the chain branching reaction O2 þ H T O þ OH, which contributed to the increase of heat release. As a result, u1 was increased with the increase of the initial temperature. Figure 6 shows stretched laminar burning velocity and stretched mass burning velocity versus the stretch rate at different dilution ratios. Stretched laminar burning velocity always increases with the stretch rate. The stretched mass burning velocity relating to the production of burned gas decreases with the increase of the stretch rate. The processes of mixture entrainment and burned gas production occur at two sides of the flame front. The difference between un and unr resulted from the existence of flame thickness. The difference between un and unr is large at a large stretch rate, where the flame thickness is comparable in scale to the flame radius.16 It can be clearly seen that the difference between un and unr is increased with the increase of the dilution ratio. As illustrated in Figure 7, δl is increased with the increase of the dilution ratio, which leads to the increase of the difference between un and unr. Hydrodynamic instability depends upon density ratio and flame thickness. Hydrodynamic instability will be enhanced with the increase of the density ratio and the decrease of flame thickness.40 Figure 8 gives the density ratio, σ, versus

Figure 7. Flame thickness versus the dilution ratio.

the dilution ratio at different initial temperatures and equivalence ratios. For a specified condition, σ decreases lineally with the increase of the dilution ratio. From the behavior of δl and σ with the increase of the dilution ratio, the increase of flame thickness and the decrease of the density ratio all contribute to flame stability. It can be concluded that the hydrodynamic instability is suppressed with the increase of the dilution ratio. 4.3. Burning Mass Flux and Zeldovich Number. Figure 9 shows the burning mass flux, f 0, versus the dilution ratio at different initial temperatures and equivalence ratios. f 0 is decreased monotonously with the increase of the dilution ratio. For the investigated initial temperature and equivalence ratio, f 0 shows a slight variation at a given dilution ratio. The slope of 2 ln( f 0) versus 1/Tad at different initial temperatures yields the values of Ea/R.15,16 As shown in Figure 10, ottgens et al.49 thought the data of 2 ln( f 0) to 1/Tad is plotted. G€ that Ea/R is a function of the equivalence ratio. However, the (49) G€ ottgens, J.; Mauss, F.; Peters, N. Analytic approximations of burning velocities and flame thicknesses of lean hydrogen, methane, ethylene, ethane, acetylene and propane flames. Proceedings of the 24th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1992; pp 129-135.

(48) Han, P. F.; Checkel, M. D.; Fleck, B. A.; Nowicki, N. L. Burning velocity of methane/diluent mixture with reformer gas addition. Fuel 2007, 86 (4), 585–596.

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Figure 8. Density ratio versus the dilution ratio.

Figure 10. 2 ln( f 0) versus 1/Tad. φ = 1.0 and 1.2. Points are the experimental data. Solid lines represent linear fits from experimental data. Dashed lines represent burning mass flux correlation.

Figure 9. Burning mass flux versus the dilution ratio.

Figure 11. Measured and fitted unstretched laminar burning velocities versus the dilution ratio.

functionality is weak and can be neglected over the small range of the equivalence ratio. In this study, Ea/R is correlated as Ea ¼ ð8:19Tu þ 9644:2ÞK ð8Þ R

where ul0 denotes the reference burning velocity. The reference condition in this paper is at Tu = 323 K and Pu = 0.10 MPa. Tu indicates the initial temperature. φr is the diluent ratio. φ is the equivalence ratio. RT is the fitting coefficient. g(φr) reflects the influence of the diluent ratio. The comparison between the measured and calculated unstretched laminar burning velocities is plotted in Figure 11. The fitted values agree well with the measured ones over the experimental ranges. The maximal deviation between measured and calculated unstretched laminar burning velocities is presented at φ = 0.8, where the relative error is 7.1%. Prathap et al.43 fitted the correlation of ul at φ = 0.6-20, Tu = 302 ( 3 K, and Pu = 0.10 MPa with the maximum variation of 5% in H2-CO-O2-N2 flame. The difference between this study and that of Prathap et al.43 lies in the difference in fuel, fitting data range, and whether to take the initial temperature into account. Figure 12 shows that the Zeldovich number, Ze, is decreased with the increase of the dilution ratio because of the decrease of the adiabatic flame temperature, illustrated in Figure 13, indicating that the diluent addition decreases the reaction speed. Tad is decreased with the increase of the

Up to now, the empirical formulas were proposed to correlate the burning velocities of flames, and these formulas were available in the literature.15,16,18,19,34,35,43,48 ul usually formulated as a function of Tu, Pu, and φ. The simple function related to the initial temperature and equivalence ratio based on the experimental data is given as follows:  RT Tu gðφr Þ ð9Þ ul ¼ ul0 323 ul0 ¼ -1:19983 þ 3:1997φ -1:54375φ2

ð10Þ

RT ¼ 4:43884 - 3:2358φ þ 0:0315φ2

ð11Þ

gðφr Þ ¼ 1:01818 - 4:74935φr þ 32:93802φr 2 - 98:88074φr 3 ð12Þ 5804

Energy Fuels 2009, 23, 5798–5805

: DOI:10.1021/ef900633z

Zhang et al.

small among different initial temperatures but is large at different equivalence ratios, as shown in Figure 13. With the increase of the initial temperature, Tad gives little variation; thus, an increase of the initial temperature yields a decrease of Ze. Ze gives its minimal value at φ = 1.0, and Tad gives its maximum value at φ = 1.0. The stoichiometric mixture has the highest adiabatic temperature and the lowest Zeldovich number. 5. Conclusions The influences of the initial temperature, equivalence ratio, and dilution ratio on the combustion characteristics of diethyl ether-air-diluent mixtures are studied. The main results are summarized as follows: (1) The laminar burning velocity increases with the increase of the initial temperature and decreases with the increase of the dilution ratio. The laminar burning velocity gives the maximum value at the stoichiometric mixture. (2) The Markstein length is decreased with the increase of the initial temperature and equivalence ratio. The Markstein length is increased with the increase of the dilution ratio at lean and stoichiometric mixtures, indicating that an increase of the dilution ratio decreases the diffusional-thermal instability of the flame front. (3) The difference between un and unr is increased with the increase of the dilution ratio because of the increase of flame thickness. (4) The density ratio is decreased, and flame thickness is increased with the increase of the dilution ratio. These indicate the suppression of flame hydrodynamic instability. (5) The Zeldovich number is increased with the increase of the dilution ratio. An increase in the initial temperature leads to the decrease in the Zeldovich number. The Zeldovich number reaches the lowest value at φ=1.0, where adiabatic temperature gives its maximum value. A formula of laminar burning velocity is correlated on the basis of the experimental data, and the fitting value agrees well with the experimental data.

Figure 12. Zeldovich number versus the dilution ratio.

Figure 13. Adiabatic flame temperature versus the dilution ratio.

dilution ratio regardless of the initial temperature and equivalence ratio. Figure 12a gives Ze versus the dilution ratio at different initial temperatures. Ze is decreased with the increase of the initial temperature. The variation of Tad is

Acknowledgment. The work is supported by the National Natural Science Foundation of China (50576070 and 50821604).

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