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Access and use of this website and the material on it are subject to the Terms and Conditions set forth at An updated detailed mechanism for dimethyl ether combustion
AN UPDATED DETAILED MECHANISM FOR
DIMETHYL ETHER COMBUSTION
F. Liu, Y. Ju
†, X. Qin
†, and G. J. Smallwood
Institute for Chemical Process and Environmental Technology,
National Research Council, Building M-9, 1200 Montreal Road, Ottawa, Ontario Canada †Mechanical and Aerospace Engineering, Princeton University,
Engineering Quad, Princeton, NJ 08544
ABSTRACT
In this study, the current detailed DME reaction mechanism (v2.4) developed by Curran et al. was improved by incorporating the C1 mechanism recently developed by Li et al. The updated
DME mechanism developed in this study predicts the laminar burning velocities of DME/O2/N2
mixtures and CH4/air mixtures in much better agreement with available experimental data or
other well-established mechanism. In addition, the updated DME mechanism differs only slightly from DME v2.4 in the prediction of ignition delay.
INTRODUCTION
Combustion and pollutant emission characteristics of oxygenated fuels such as methanol, methyl ethyl ketone, and dimethyl ether, have received increased research attention in recent years as they posses many desirable properties as alternative transportation fuels. The simplest aliphatic ether, dimethyl ether (DME), has no carbon-carbon bonds and can be mass-produced from natural gas, coal or biomass [1]. The physical and chemical properties make DME an attractive alternative fuel to achieve low emission combustion in diesel engines. It has also been suggested to use DME as an additive to control ignition timing in natural gas fueled homogeneous change compression ignition (HCCI) engines [2] or even use DME as the primary fuel for HCCI engines [3]. Therefore, detailed knowledge of DME combustion mechanism is of importance to model the ignition, combustion, and pollutant emission characteristics of DME or DME/natural gas fueled engines.
A detailed reaction mechanism of DME oxidation was developed earlier by Curran et al. [4] (here after DME 1998) and updated later by Fischer et al. [5] and Curran et al. [6] (here after DME v2.4). Both mechanisms can be obtained from [7]. Many studies indicate that the DME
mechanisms developed by Curran et al. are fundamentally sound. Some drawback of the DME
mechanisms was revealed in the prediction of the laminar burning velocities of DME/air and methane/air mixtures. Laminar burning velocity is an important property of a combustible mixture and is determined by both physical and chemical properties of species and reactions involved such as heat and mass diffusivity, reaction rate, and heat release rate. Therefore, the accuracy in the prediction of laminar burning velocity is an essential aspect in reaction mechanism validation. In the case of DME, there are only perhaps four experimental measurements of the burning velocities of this fuel reported in the literature [8,9,10,11]. Using DME v2.4, which differs only slightly from DME 1998 for burning velocity calculations, and sufficiently fine computational grid, Zhao et al. [9] showed that the predicted burning velocities
of DME are in overall good agreement with their own experimental results obtained using particle image velocimetry in a stagnation flame burner configuration in the equivalence ratio range of 0.7 to 1.4. Both their prediction and data indicate that the peak burning velocity of about 54 cm/s occurs at an equivalence ratio of about 1.2. In contrast, Daly et al. [8] reported peak burning velocities of about 62 cm/s and 47.5 cm/s from prediction and measurement, respectively. The measured burning velocities of Zhao et al. [9] are consistently higher than those of Daly et al. [8]. Another recent experimental and numerical study conducted by Qin and Ju [10] measured the burning velocities of DME-air mixtures at both atmospheric pressure and elevated pressures using the spherical bomb technique, which was also employed in the study of Daly et al. [8]. At atmospheric pressure, the burning velocities of DME-air mixtures measured by Qin and Ju [10] are in very good agreement with those of Daly et al. [8] for equivalence ratios below 1.2, but are somewhat higher than those of Daly et al. [8] at larger equivalence ratios. Nevertheless, the recent measurement by Qin and Ju [10] supports the earlier results of Daly et al. [8]. The agreement between the data of Qin and Ju [10] and Daly et al. [8], to certain extent, demonstrates that the experimental results of Zhao et al. [9] might be subject to larger uncertainty, especially for rich mixtures. The studies of Daly et al. [8] and Qin and Ju [10] found that the DME 1998 and DME v2.4 predicted the burning velocities of DME-air mixtures that were uniformly and significantly higher than their experimental data, over the equivalence ratio of 0.7 to 1.7 investigated. Through sensitivity analysis, Daly et al. and Qin and Ju revealed that the burning velocity of DME-air mixtures is sensitive to reactions in the C1 mechanism. On the other hand, Zheng et al. [12] recently found that the ignition temperatures of nitrogen-diluted DME by heated air in counterflow predicted by DME v2.4 are in much better agreement with experimental data than DME 1998.
In this study, the latest version of the detailed DME mechanism, DME v2.4, was updated in a systematic way by replacing its C1 mechanism with the C1 mechanism recently developed by Li
et al. [13]. Laminar burning velocities of DME/O2/N2 mixtures and methane/air mixtures at
atmospheric pressure calculated using the updated DME mechanism were compared with those from DME v2.4, experimental data, and GRI Mech3.0 (for methane/air mixtures).
UPDATED DME OXIDATION MECHANISM
The drawback of DME v2.4 mechanism was revealed in the prediction of the burning velocities of DME/air mixtures in the studies of Daly et al. [8] and Qin and Ju [10]. It was shown in these studies that the laminar burning velocities of DME-air mixtures predicted by this mechanism are about 20% higher than the experimental measurements. The approach of making some ad hoc modification to some individual reactions in the detailed DME mechanism in order to improve the prediction of DME burning velocity, such as in the study of Daly et al. [8], cannot produce a robust DME mechanism, since such modification is likely to deteriorate the performance of the detailed DME mechanism in calculations of other flame properties [10]. Therefore, a systematic approach should be used to improve the DME v2.4 mechanism. As indicated in the study by Qin and Ju through sensitivity analysis, the burning velocities of DME are primarily sensitive to reactions in the C1 chemistry. Therefore, the deficiency of DME v2.4 mechanism is likely in the C1 mechanism of the overall reaction scheme. This conclusion is further supported by the poor
agreement between the numerical laminar burning velocities of rich CH4-air mixtures predicted
C1 mechanism plays an important role in the calculation of the burning velocity of DME and other hydrocarbon fuels. Based on these observations, the DME v2.4 mechanism developed by
Curran et al. was updated by replacing the entire C1 sub-mechanism (including reactions for H2
-O2 system) by the newly developed C1 mechanism [13]. Reactions that are in the C1 chemistry
of Li et al. [13] but not in the DME v2.4 mechanism are retained in the updated DME mechanism. Other reactions in the DME v2.4 mechanism are left unchanged. The updated DME mechanism consists of 79 species (the same as the current DME mechanism) and 371 reactions (compare to 351 reactions in the DME v2.4 mechanism). The thermal and transport databases are the same as those used in DME v2.4 and were obtained from [7].
RESULTS AND DISCUSSION
The laminar burning velocities of DME/air mixtures at 1 atm and 298 K were calculated using the updated DME mechanism and DME v2.4 of Curran et al. [7]. The CHEMKIN codes were used in these calculations. The numerical results are compared with experimental data in Fig.1. As mentioned in the Introduction, the experimental data of Zhao et al. [9] are likely to subject to large uncertainty for rich mixtures. It is evident that the updated DME mechanism predicts the laminar burning velocities of DME/air mixtures in much better overall agreement with the experimental data of Daly et al. [8] and Qin and Ju [10] over the entire range of equivalence ratio considered. Also plotted in Fig.1 are the results calculated using the reduced DME mechanism (based on DME v2.4) of Qin and Ju [10] (40 species and 168 reactions) and the updated reduced DME mechanism (40 species and 189 reactions). The reduced DME mechanisms consist of only 40 species (including He), much smaller than the full DME mechanisms (79 species). The laminar burning velocities of DME/air mixtures predicted by the reduced mechanisms are in good agreement with those from their respective full mechanisms.
Equivalence Ratio, φ 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Lam inar Bu rning V e locit y , cm /s 10 20 30 40 50 60 Qin and Ju (2004) Daly et al. (2001) Zhao et al. (2004) Gibbs and Calcote (1959) DME v2.4
Updated detailed DME mech. Reduced DME mech. Updated reduced DME mech. DME-Air
Fig.1 Comparison between the predicted and measured laminar burning velocities of DME/air mixtures at 1 atm and 298 K.
The calculated and experimentally measured laminar burning velocities of DME/19%O2/81%N2
in oxygen concentration on the burning velocity, results for DME/air mixtures are also plotted in this figure. It is clear that the updated DME mechanism performs significantly better than the DME v2.4 mechanism. A 10% decrease in oxygen mole fraction in the mixture leads to about 20% decrease in the burning velocity.
Equivalence Ratio, φ 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Lam in ar Bu rnin g Ve loci ty , cm /s 0 10 20 30 40 50 60 DME-Air DME-(19%O2/81%N2) DME v2.4
Updated DME mech.
Fig.2 Comparison between the predicted and measured laminar burning velocities of
DME/19%O2/81%N2 mixtures at 1 atm and 298 K.
To further validate and demonstrate the superiority of the updated DME mechanism, calculations
were conducted for the laminar burning velocities of CH4/air mixtures. The results are compared
in Fig.3. The updated DME mechanism predicts the burning velocities in much better agreement with those from GRI-Mech3.0, which was optimized for methane combustion, than DME v2.4, especially for equivalence ratios greater than about 0.9. The underprediction of the burning
velocities of CH4/air mixture by the C1 mechanism of Li et al. [13] is expected since it does not
include C2 mechanism. Equivalence Ratio, φ 0.6 0.8 1.0 1.2 1.4 Laminar B u rni n g Veloc ity , cm/s 0 10 20 30 40 50 GRI-Mech. 3.0 DME mech. v2.4 Updated DME mech. C1 mech. of Li. et al.
CH4-Air
Fig.3 Comparison between the predicted and measured laminar burning velocities of CH4/air
To investigate the effect of changing the C1 mechanism in DME v2.4 on its performance in the prediction of ignition delay, numerical calculations were conducted using the SENKIN codes for the ignition delay of the stoichiometric DME-air mixture at p = 13 and 40 bar over the temperature range of 667 to 1300 K. Experimental data under these conditions were obtained by Pfahl et al. [14]. It is evident from Fig.4 that the ignition delays predicted by DME v2.4 and the updated DME mechanisms are in very good agreement and are in better agreement with the experimental data (not shown) than those from DME 1998. The region of negative temperature coefficient (NTC) in Fig.4 (except results of DME 1998) can be observed, though less significant than the experimental data.
1000 K / T 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Ig nition de lay, m s 0.01 0.1 1 10 DME V2.4 Updated DME DME 1998 p = 13 bar p = 40 bar φ = 1
Fig.4 Ignition delay of the stoichiometric DME-air mixture predicted by three mechanisms. To demonstrate the distinct two-stage ignition of DME-air mixtures, the temporal evolution of OH mole fraction of the stoichiometric mixture at p = 13 bar and T = 781.25 K is shown in Fig.5. The modification of the C1 chemistry has no influence on the first (low temperature) ignition and only slightly enhances the second ignition (shorter delay).
Time, ms 0.0 0.5 1.0 1.5 2.0 2.5 O H M o le F ra c ti on 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 T e mper a tur e, K 1000 1500 2000 2500 Updated DME: OH DME v2.4: OH Updated DME: T DME v2.4: T τ1 τtot = 2.044 ms τtot = 2.187 ms T = 781.25 K P = 13 bar
Fig.5 Temporal evolution of OH mole fraction of the stoichiometric DME-air mixture at p = 13 bar and T = 781.25 K.
CONCLUSIONS
An updated DME mechanism was developed by incorporating the newly developed C1 mechanism into the DME mechanism of Curran et al. The updated DME mechanism predicts
more accurate laminar burning velocities of DME/O2/N2 and CH4/air mixtures. In addition, the
updated DME mechanism performs very similarly to DME v2.4 in the prediction of ignition delay. Further evaluation of the updated DME mechanism is required in the prediction of species distributions in both premixed and diffusion flames.
REFERENCES
1. Hansen, B. J., Voss, B., Joensen, F., and Siguroardottir, I. D., SAE Paper 950063 (1995).
2. Flowers, D., Aceves, S., Westbrook, C. K., Smith, J. R., and Dibble, R., ASME J. Eng. Gas Turb. Power 123:433 (2001).
3. Yamada, H., Yoshii, M., and Tezaki, A., Proc. Comb. Inst. 30:2773 (2005).
4. Curran, H. J., Pitz, W. J., Westbrook, C. K., Dagaut, P., Boettner, J.-C., and Cathonnet, M., Int. J. Chem. Kinet. 30:229 (1998).
5. Fischer, S. L., Dryer, F. L., and Curran, H. J., Int. J. Chem. Kinet. 32:713 (2000). 6. Curran, H. J., Fischer, S. L., and Dryer, F. L., Int. J. Chem. Kinet. 32:741 (2000).
7. Curran H. J. http://www-cms.llnl.gov/combustion/combustion2.html.
8. Daly, C. A., Simmie, J. M., Würmel, J., Djebaïli, N., and Paillard, C., Combust. Flame 125:1329 (2001).
9. Zhao, Z., Kazakov, A., and Dryer, F. L., Combust. Flame 139: 52 (2004). 10. Qin, X. and Ju, Y., Proc. Comb. Inst. 30:233 (2005).
11. Gibbs, G. J. and Calcote, H. F., J. Chem. Eng. Data 3:226 (1959).
12. Zheng, X. L., Lu, T. F., Law, C. K., C. K. Westbrook, and H. J. Curran, Proc. Comb. Inst. 30:233 (2005).
13. Li J., Zhao, Z., Kazakov, A., and Dryer, F. L.,
http://www.princeton.edu/~combust/database/files/symposium/.
