Energy & Fuels 2008, 22, 967–971
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Experimental Study on Premixed Combustion of Dimethyl Ether– Hydrogen–Air Mixtures Zuohua Huang,* Gen Chen, Chaoyang Chen, Haiyan Miao, Xibin Wang, and Deming Jiang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an, People’s Republic of China ReceiVed October 23, 2007. ReVised Manuscript ReceiVed January 9, 2008
The laminar combustion characteristics of DME–hydrogen–air premixed mixture were studied at different equivalence ratios, ratios of hydrogen to DME plus hydrogen, and initial pressures by using a constant volume combustion bomb. The influences of the equivalence ratio, the hydrogen addition, and the initial pressure on flame speed and combustion characteristics are analyzed. The results show that the flame speed, the laminar burning velocity, and the mass burning rate increase with the increase of hydrogen addition. Increasing the ratio of hydrogen to DME plus hydrogen will shorten the combustion duration. In the case of small hydrogen addition, the Markstein length decreases with the increase of equivalence ratio, and this reveals that lean mixture has better flame front stability compared with that of the rich mixture. In the case of large hydrogen addition, the Markstein length increases with the increase of equivalence ratio, and this indicates that the rich mixture has better flame front stability than the lean mixture. Maximum combustion pressure increases with the increase of initial pressure, and initial pressure has larger influence on combustion pressure than that from hydrogen addition. The flame development duration decreases with the increase of hydrogen addition and the increase of equivalence ratio.
Introduction With increasing concern about energy shortage and environmental protection, the researches on improving engine fuel economy and reduction of exhaust emissions have become the major research aspect in the combustion community and engine development. Due to limited crude oil reserves, the development of alternative fuel engines has attracted more and more concern in the engine community. Alternative fuels are usually clean fuels compared to diesel fuel and gasoline fuel in the engine combustion process; thus, the introduction of these alternative fuels is beneficial to the slowing-down of fuel shortage and reduction of engine exhaust emissions. Recently, dimethyl ether (DME) with its high cetane number and rich oxygen content has attracted increasing attention in engine development (see Table 1).1,2 Up to now, most work on DME was concentrated on the engine application,3,4 and some researches on the * Corresponding author. Address for correspondence: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China. E-mail:
[email protected]. (1) Sorenson, S. C.; Mikkelsen, S. E. Performance and emissions of a 0.273 liter direct injection diesel engine fueled with neat dimethyl ether. SAE Trans. 1995, 104 (4), 80–90. (2) Huang, Z. H.; Wang, H. W.; Chen, H. Y. Study of combustion characteristics of a compression ignition engine fueled with dimethyl ether. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 1999, 213 (D3), 647– 652. (3) Wang, H. W.; Huang, Z. H.; Zhou, L. B.; Jiang, D. M. Investigation on emission characteristics of a compression ignition engine with oxygenated fuels and exhaust gas recirculation. Proc. Inst. Mech. Eng. Part D: J. Automobile Eng. 2000, 214 (D5), 503–508. (4) Maroteaux, F.; Descombes, G.; Sauton, F.; Jullien, J. Investigation on exhaust emissions of a common rail high-speed direct injection diesel engine running with dimethyl ether. Int. J. Engine Res. 2001, 2 (3), 199– 207.
Table 1. Fuel Properties of Dimethyl Ether fuel
dimethyl ether
hydrogen
chemical formula molecular weight/g boiling point/°C Reid vapor pressure/MPa density/g/cm3 liquid viscosity/cP low heating value/MJ/kg explosion limit in air/vol % ignition temperature/K stoichiometric air/fuel ratio latent heat of evaporation/kJ/kg
CH3–O–CH3 46.07 -24.9 0.51 (20 °C) 668 kg /m3(liquid) 0.15 28.43 3.4–17 235 9 460 (-20 °C)
H2 2 -252 – 0.083 kg/m3(gas) – 119.7 4–75.6 858 34.4 –
premixed combustion of DME–air mixtures were reported,5–9 including aspects of flame speed and combustion behavior. Addition of hydrogen into the gaseous fuel can increase the burning velocity and enhance the combustion as well as reduce the emissions, and this approach is widely used in the enhancement of the mixture combustion for the gaseous fuels.10–12 However, no report was founded for DME–hydrogen–air (5) Zhao, Z.; Kazakov, A.; Dryer, F. L. Laminar flame speed study of dimethyl ether-air mixtures by using particle image velocimetry. Combust. Flame 2004, 139 (1–2), 52–60. (6) Daly, C. A.; Simmie, J. M.; Würmel, J. Burning velocities of dimethyl ether and air. Combust. Flame 2001, 125 (4), 1329–1340. (7) Huang, Z. H.; Wang, Q.; Yu, J. R.; Zhang, Y.; Zeng, K.; Miao, H. Y.; Jiang, D. M. Measurement of laminar burning velocity of dimethyl ether-air premixed mixtures. Fuel 2007, 86 (15), 2360–2366. (8) Huang, Z. H.; Wang, Q.; Miao, H. Y.; Wang, X. B.; Zeng, K.; Liu, B.; Jiang, D. M. Study on dimethyl ether-air premixed mixture combustion with a constant volume vessel. Energy Fuels 2007, 21 (4), 2013–2017. (9) Qin, X.; Ju, T. G. Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures. Proc. Combust. Inst. 2005, 30, 233–240. (10) Yu, G.; Law, C. K.; Wu, C. K. Laminar flame speeds of hydrocarbon + air mixtures with hydrogen addition. Combust. Flame 1986, 63 (1–2), 339–347.
10.1021/ef700629j CCC: $40.75 2008 American Chemical Society Published on Web 02/15/2008
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Figure 1. Schematic diagram of the experiment.
Huang et al. resolution of 0.01 kPa. A high-speed digital camera (HG-100K) is used to record the flame pictures during combustion. In the experiments, the initial pressure is set from 0.08 to 0.15 MPa and the initial temperature is 293 K. Dimethyl ether is in gaseous state at room temperature and the pressure is less than 0.5 MPa; thus, the homogeneous dimethyl ether–hydrogen–air premixed mixture can be prepared in the combustion chamber by regulating the partial pressures of dimethyl ether, hydrogen, and air. Experiments were conducted at room temperature of 293 K, and the initial pressures were strictly regulated at the specified initial pressures. To avoid the influence of wall temperature on mixture temperature, there was a sufficient interval between two experiments, and this can provide enough time for the wall to cool down and maintain the same initial temperature. As the flame develops in a spherical pattern, the flame radius is scaled from the flame photo recorded by the high-speed camera. Laminar Burning Velocity and Markstein Length. Laminar burning velocity and Markstein length can be calculated from the formula given by Bradley et al.13 For a spherically expanding flame, the stretched flame velocity, Sn, reflecting the flame propagation speed, is derived from the flame radius versus time data as Sn )
dru dt
(1)
where ru is the radius of the flame in the Schlieren photographs and t is the time, and Sn can be directly obtained from flame photo. Flame stretch rate, R, representing the expanding rate of flame front area, in a quiescent mixture is defined as Figure 2. Schematic of the constant volume vessel.
premixed combustion, and the burning velocity and combustion behaviors of the DME–air mixture with the addition of hydrogen are still unknown. To supplement the basic data for the DME– hydrogen–air premixed mixture combustion, the study on fundamental characteristics of the DME–hydrogen–air premixed mixture combustion is worth investigation. The objectives of this paper are to measure the burning velocities of the DME–hydrogen–air premixed mixtures at various equivalence ratios and hydrogen fractions with spherically propagated flame in a constant volume vessel, and to analyze the DME–hydrogen–air premixed mixture combustion based on the information of flame propagation photos and recorded pressure. The study is expected to provide the new information for DME–hydrogen–air premixed mixture combustion. Experimental Setup and Procedures Schlieren photography combined with a high-speed camera is used to measure the flame propagation as shown in Figure 1. The experiment was conducted in a constant volume vessel as shown in Figure 2. The combustion bomb is cylinder type with a diameter of 130 mm and length of 130 mm. Two sides of this vessel are transparent to make the inside observable and optically accessible. The DME–hydrogen–air premixed mixtures with different hydrogen volume fractions are prepared in advance in a fuel bomb. The combustible mixture is prepared within the chamber by adding DME, hydrogen, and air at the specified partial pressures. The mixtures are ignited by centrally located electrodes, and a standard capacitive discharge ignition system is used for producing the spark. In this study, the ignition energy is 45 mJ. The pressure is recorded by a piezoelectric Kistler absolute pressure transducer with a (11) 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, 302–311. (12) Halter, F.; Chauveau, C.; Gokalp, I. Characterization of the effects of hydrogen addition in premixed methane-air flames. Int. J. Hydrogen Energy 2007, 32 (13), 2585–2592.
R)
d(lnA) 1 dA ) dt A dt
(2)
where A is the area of flame. For a spherically outwardly expanding flame front, the flame stretch rate can be simplified as R)
1 dA 2 dru 2 ) ) Sn A dt ru dt ru
(3)
In respect to the early stage of flame expansion, there exists a linear relationship between the flame speeds and the flame stretch rates;14 that is Sl - Sn ) LbR
(4)
where Sl is the unstretched flame speed and Lb is the Markstein length of burned gases. From eqs 1 and 3, the stretched flame speed, Sn, and flame stretch rate, R, can be calculated. The unstretched flame speed, Sl, can be obtained as the intercept value at R ) 0, in the plot of Sn against R, and the burned gas Markstein length Lb is the slope of Sn–R curve. The Markstein length can reflect the stability of flame. A positive value of Lb indicates that the flame speed decreases with the increase of flame stretch rate; if any kind of protuberances appear at the flame front (stretch increasing), the flame speed at the flame protruding position will be suppressed, and this makes the flame stable. In contrast to this, a negative value of Lb means that the flame speed increases with the increase of flame stretch rate; in this case, if any kind of protuberances appear at the flame front, the flame speed at the flame protruding position will be increased, and this increases the instability of the flame.15 When the observation is limited to the initial part of the flame expansion, where the pressure does not vary significantly yet, then a simple relationship links the spatial flame velocity, Sl, to the unstretched laminar burning velocity, ul, is given as follows: (13) Bradley, D.; Hicks, R. A.; Lawes, M. Measurement of laminar burning velocities and Markstein numbers for iso-octane-air and iso-octanen-heptane-air mixtures at elevated temperatures and pressures in an explosion bomb. Combust. Flame 1998, 115 (1–2), 126–144. (14) Gu, X. J.; Haq, M. Z.; Lawes, M. Burning velocity and Markstein lengths of methane–air mixtures. Combust. Flame 2000, 121 (1–2), 41–58. (15) Law, C. K.; Jomaas, G.; Bechtold, J. K. Cellular instabilities of expanding hydrogen/propane spherical flames at elevated pressures: theory and experiment. Proc. Combust. Inst. 2005, 30 (1), 159–167.
Combustion of Dimethyl Ether–Hydrogen–Air Mixtures ul ) FbSl ⁄ Fu
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where Fb and Fu are the densities for burned gases and unburned gases. Equation 6 is used to determine the stretched laminar burning velocity, un, and the stretched mass burning velocity, unr, proposed by Bradley,13 is calculated from eq 7:
[ ]
(6)
Fb (u - Sn) Fb - Fu n
(7)
un ) S Sn unr )
Fb Fu
in which S is a rectified function that depends upon the flame radius and the density ratio and accounts for the effect of the flame thickness on the mean density of the burned gases. The expression of S in the present study used the formula given by Bradley et al.:13
[()] [()]
S ) 1 + 1.2
δl Fu ru Fb
2.2
- 0.15
δl Fu ru Fb
Figure 3. Flame speed versus radius at various ignition energies.
2.2 2
(8)
Here δl is laminar flame thickness given by δl ) ν/ul, in which ν is the kinetic viscosity of unburned mixture.
Results and Discussion The relationship between the stretched flame speed (of natural gas–air mixture), Sn, and flame radius was reported by Huang et al. at various ignition energies.16 As shown in Figure 3, the influence of ignition energy on the flame propagation speed is larger at the initial stage, and the influence is decreased as flame radius increases. Liao et al.16 found that ignition energy has little influence on the flame propagation speed when flame radius is larger than 5 mm, which can be regarded as the critical flame radius. The existence of the critical flame radius was also observed by Bradley et al.13 and Lamoureux et al.17 In their studies, this value falls in the range between 5 and 6 mm. In addition, the study by Liao et al.16 showed that no observable pressure rise was found within the flame radius less than 30 mm. In this study, to avoid the influence from the ignition energy and the pressure, the data in analyzing the premixed combustion and burning velocities uses the values for the flame radius within 5 and 25 mm. Figure 4 gives the stretched flame speed versus stretch rate of the stoichiometric equivalence ratio for various hydrogen fractions (in this paper, RH2 is hydrogen fraction; for example, RH2 ) 20% refers to the case of 20% hydrogen volume fraction and 80% DME volume fraction) at the initial pressure of 0.097 MPa and initial temperature of 293 K. The linear relationship between the stretched flame speed and the stretch rate is demonstrated for all mixtures with different hydrogen fractions. In this figure, the large value of the stretch rate corresponds to the small flame radius while the small value of the stretch rate corresponds to the large flame radius. The stretched flame speed increases with the decrease of the stretch rate, indicating the increase of stretched flame speed along with flame development. Increasing the hydrogen fraction leads to the increase in the stretched flame speed, and the increment in the stretched flame speed is increased with the increase of hydrogen fraction. The unstretched flame speed, Sl, is obtained by extrapolating the Sn–R curve to the position of R ) 0, and the value of the (16) Liao, S. Y.; Jiang, D. M.; Gao, J. Measurements of Markstein numbers and laminar burning velocities for natural gas-air mixtures. Energy Fuels 2004, 18 (2), 316–326. (17) Lamoureux, N.; Djebayli-Chaumeix, N.; Paillard, C. E. Laminar flame velocity determination for H2–air–He–CO2 mixtures using the spherical bomb method. Exp. Thermal Fluid Sci. 2003, 27 (4), 385–393.
Figure 4. Stretched flame speed versus stretch rate at different ratios of hydrogen to DME plus hydrogen.
Figure 5. Unstretched flame speed versus equivalence ratio at different ratios of hydrogen to DME plus hydrogen.
Markstein length, Lb, is the negative value of the gradient of Sn–R curve. Figure 5 shows the unstretched flame speed versus the equivalence ratio at different ratios of hydrogen to DME plus hydrogen. For a specified mixture, the unstretched flame speed increases with the increase of equivalence ratio, and this reveals the high unstretched flame speed for rich mixture combustion. For a specified equivalence ratio, the unstretched flame speed increases with the increase of hydrogen addition, reflecting an enhancement of flame propagation by hydrogen addition. Similar to the stretched flame speed, an increase in hydrogen fraction leads to the increase in the increment of the unstretched flame speed, and this indicates that large hydrogen addition can get remarkable increase in the unstretched flame speed while small hydrogen addition can only bring the limited increase in the unstretched flame speed. Figure 6 illustrates the Markstein length versus the equivalence ratio for the DME–hydrogen–air premixed mixtures with different ratios of hydrogen to DME plus hydrogen. Different
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Figure 6. Markstein length versus equivalence ratio at different ratios of hydrogen to DME plus hydrogen.
Figure 8. Combustion pressure versus time at different ratios of hydrogen to DME plus hydrogen.
Figure 7. Stretched laminar burning velocity versus stretch rate at different ratios of hydrogen to DME plus hydrogen.
Figure 9. Maximum pressure versus the ratio of hydrogen to DME plus hydrogen at different initial pressures.
behaviors are presented for DME–air premixed mixture combustion and DME–hydrogen–air premixed mixture combustion. Markstein length gives the decreasing trend with the increase of equivalence ratio in the case of the DME–air premixed mixture combustion, and this reveals the increase in flame front stability with increase of equivalence ratio. In contrast to this, the Markstein length shows an increasing trend with the increase of equivalence ratio in the case of the DME–hydrogen–air premixed mixture combustion, and this indicates that the flame front stability increases with the increase of equivalence ratio. Addition of hydrogen will change the variation trend of Markstein length to the equivalence ratio. Small addition of hydrogen shows behavior similar to that of the DME–air mixture premixed combustion while large addition of hydrogen demonstrates the behavior similar to that of the hydrogen–air mixture premixed combustion. Generally speaking, the DME–hydrogen– air premixed mixture combustion with large hydrogen addition gives lower value of Markstein length than that with small hydrogen addition; thus, the flame front stability will decrease with the increase of the ratio of hydrogen to DME plus hydrogen in the DME–hydrogen–air premixed mixture combustion. Figure 7 gives the stretched laminar velocity (un) and the stretched mass burning velocity (unr) versus the stretch rate at the stoichiometric mixtures of different ratios of hydrogen to DME plus hydrogen at the initial pressure of 0.097 MPa and the initial temperature of 293 K. The difference between the stretched laminar burning velocity and the stretched mass burning velocity can be clearly observed. The stretched laminar burning velocity un, which denotes the rate of mixture entrainment, always increases as the stretch rate increases. In contrast, the mass burning velocity unr, which is the burning velocity related to the formation of burned gas or consumption of
unburned gas, usually decreases as the stretch rate increases. The difference between the stretched laminar velocity and the stretched mass burning velocity, un - unr, increases with the increase of the stretch rate and this is regarded as from the influence of flame thickness on burning velocities. Large value of un - unr is presented at small radii (corresponding to high stretch rate), where the flame thickness is the same order as the flame radius. As the definition indicated, a high value of stretch rate corresponds to a small flame radius; thus, the influence by flame thickness becomes great. When stretch rate reaches zero, the flame radius becomes infinity, and the influence of flame thickness can be negligible, and thus un and unr will get the same value; that is, both un and unr will reach the unstretched laminar burning velocity ul. This is consistent with the results in the previous study.11,16 Figure 8 gives the combustion pressure in the chamber versus time for the stoichiometric DME–hydrogen–air mixtures at different ratios of hydrogen to DME plus hydrogen at initial pressure of 0.097 MPa and initial temperature of 293 K. Fast pressure rise is observed in the case of hydrogen–air premixed mixture combustion while slow pressure rise is demonstrated in the case of DME–air mixture premixed combustion. Increasing the hydrogen fraction will advance the timing of peak pressure and makes a fast pressure rise. For stoichiometric mixture combustion, little variation in the peak value of pressure is found for various hydrogen fractions in constant volume vessel experiment, and this phenomenon is also observed in the natural gas–hydrogen air premixed mixture combustion.11 Figure 9 shows the maximum pressure versus the ratio of hydrogen to DME plus hydrogen for the stoichiometric DME– hydrogen–air premixed mixture combustion at different initial pressures. Except for hydrogen–air premixed mixture combus-
Combustion of Dimethyl Ether–Hydrogen–Air Mixtures
Figure 10. Mass burning rate versus time at different ratios of hydrogen to DME plus hydrogen.
tion which has a low volumetric heating value, the maximum value of pressure gives little variation or slight decrease with the increase of the ratio of hydrogen to DME plus hydrogen. For a specified ratio of hydrogen to DME plus hydrogen, the maximum value of pressure gives an increase with the increase of initial pressure. The normalized mass burning rate is defined as 1 dmb m dt where m is the mass of combustible mixture and mb refers to the mass of burned gas. It reflects the rate of the whole combustion process. The normalized mass burning rate is calculated with the method in ref8 based on the pressure data and two-zone model. Figure 10 gives the normalized mass burning rate versus time for the stoichiometric DME–hydrogen– air premixed mixture combustion with different ratios of hydrogen to DME plus hydrogen. Fast and high value of the normalized mass burning rate is presented for the hydrogen–air premixed mixture combustion while slow rate and low value of the normalized mass burning rate is demonstrated for the DME–hydrogen–air premixed mixture combustion and the DME–air premixed mixture combustion. The hydrogen–air premixed mixture combustion completes at 0.005 s after the electrode sparking, where at this moment, heat release is not observed or is just at its early stage for the DME–hydrogen–air premixed mixture combustion and the DME–air premixed mixture combustion. Increasing the ratio of hydrogen to DME plus hydrogen leads to the increase in mass burning rate and decrease in heat release process. The flame development duration is defined as the time interval from the spark ignition to the timing of 10% accumulated mass burned.11 This parameter can reflect the flame early development. Figure 11 gives the flame development duration versus the equivalence ratio for the DME–hydrogen–air premixed mixtures with different ratios of hydrogen to DME plus hydrogen. For a specified equivalence ratio, the flame development duration decreases with the increase of the ratio of
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Figure 11. Flame development duration versus equivalence ratio at different ratios of hydrogen to DME plus hydrogen.
hydrogen to DME plus hydrogen; the improvement in ignition and flame speed leads to this phenomenon. For a specified DME–hydrogen–air premixed mixture combustion, the flame development duration decreases with the increase of equivalence ratio, indicating good mixture ignitability and fast flame propagation at rich mixture combustion. Conclusions The laminar combustion characteristics of DME–hydrogen–air premixed mixture were studied at different equivalence ratios, ratios of hydrogen to DME plus hydrogen, and initial pressures by using a constant volume combustion bomb. The main results are summarized as follows: 1. The flame speed, the laminar burning velocity, and the mass burning rate increase with the increase of the ratio of hydrogen to DME plus hydrogen. Increasing the hydrogen fraction will advance the timing of peak pressure and shorten the combustion duration. 2. In the case of small hydrogen addition, the Markstein length decreases with the increase of equivalence ratio, and this reveals that lean mixture has better flame front stability compared with that of rich mixture. In the case of large hydrogen addition, the Markstein length increases with the increase of equivalence ratio, and this indicates that rich mixture has better flame front stability than the lean mixture. 3. Maximum combustion pressure increases with the increase of initial pressure, and initial pressure has larger influence on combustion pressure than that by hydrogen addition. 4. For a specified equivalence ratio, the flame development duration decreases with the increase of hydrogen addition. For a specified DME–hydrogen–air mixture, the flame development duration decreases with the increase of equivalence ratio. Acknowledgment. The study is supported by the National Natural Science Foundation of China (50636040, 50521604). EF700629J
