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Effect of a small amount of NOx on extinction limit of lean premixed counterflow flame
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 33 (2011) 1179–1186
www.elsevier.com/locate/proci
Effect of a small amount of NOx on extinction limit of lean premixed counterflow flame Kenichi Takita *, Akihiro Morinaga, Taku Someya Department of Aeronautics and Space Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Sendai 980-8579, Japan Available online 6 August 2010
Abstract The effect of a small amount of NOx (NO and NO2) on the extinction of CH4 and C2H4 flames was experimentally and numerically investigated. The extinction stretch rates of counterflow premixed flames when NO2 was added to the mixture were measured at pressures ranging from 0.1 to 0.3 MPa. Calculations by using one-dimensional flame code with full chemistry were also conducted to clarify the effect of NOx on the extinction limit and burning velocity. Extinction stretch rates of mixtures near the flammability limit increased by adding NOx at high pressure in the experiments and the calculations. The catalytic effect of NOx for CH4 was via the low temperature chemistry involving CH3, CH3O, CH3O2, and HO2, and that for C2H4 was via reactions that included HO2 species. The change in extinction stretch rates with NOx addition reflected the behavior of the burning velocity with NOx addition. The burning velocities of mixtures near the flammability limit increased with the addition of NOx at high pressure, the same as the extinction stretch rate. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Extinction; Counterflow flame; NOx catalytic effect; Hydrocarbon fuel; High pressure
1. Introduction A small amount of NOx (NO and NO2) drastically accelerates the oxidation of hydrogen [1,2] and hydrocarbon fuels [3–5]. The authors [6,7] have been developing a new ignition enhancement method in a scramjet combustor by using N2/O2 plasma jet that can supply a suitable amount of NOx for ignition in the engine. In such an engine for high-speed vehicles, flame-holding after ignition in a high-speed flow is also important for successful development of the engine. When the flame-holding aspect is considered, the extinction limit or the blow-off limit of the flame, which is
*
Corresponding author. Fax: +81 22 795 7009. E-mail address: [email protected] (K. Takita).
strongly related to flame propagation characteristics such as burning velocity, becomes another important factor. Therefore, effects of NOx on the extinction limit must be investigated for the new ignition and combustion enhancement systems. However, reports on the effect of NOx on extinction were very limited, though many studies about the effect of that on ignition has been conducted as previously mentioned. Quite recently, Lee et al. [8,9] have investigated the effect of NO on a local extinction and re-ignition of non-premixed hydrogen-air counterflow flames. Flame propagation in a flow containing a small amount of NOx appears in many situations. In a combustion test facility for the engine of a highspeed vehicle, a very high total temperature of the flow is required to realize flight conditions at high altitude, and therefore a heating system such as a shock-tunnel, arc-heater, or vitiation heater that
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.045
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Nomenclature E.R. or / equivalence ratio inflow temperature T0 a mole fraction of NO2 in N2/NO2 mixture
heats the air up to an extremely high temperature is indispensable. As such, NOx must be included in the flow in such a high-speed engine test. For another example, in an automobile engine, increased exhaust gas recirculation (EGR) has resulted from the pursuit of a high-efficiency, lowemission engine. Therefore, the effect of NOx contamination in the exhaust gas must be understood for precise engine control. Considering the above situation, this study experimentally and numerically investigated the effect of a small amount of NOx on the extinction limit of counterflow lean hydrocarbon/N2/O2 flames. The detailed mechanisms of the catalytic effect of NOx on the extinction of CH4 and C2H4 flames and their dependence on ambient conditions were discussed. 2. Experimental method Figure 1 schematically illustrates the experiment setup. The basic system was the same as that used in the authors’ previous studies on extinction of counterflow flames at high pressure [10] and high temperature [11]. The inner diameter of the high-pressure chamber was 333.4 mm, and its length was 580.0 mm. Two counterflow burners with nozzle diameters (D) of 9.0 mm were installed in the chamber. The separation distance (L) between the two burners was set at 10.0 mm (L/D = 1.1). Shroud flows of N2 were issued from the coaxial outer nozzles to eliminate effects from the surrounding area and flame tails. Mixtures of fuel (CH4 or C2H4) and oxidizer (N2/O2 or N2/ O2/NO2) mixed in a mixing chamber were issued
e ee
stretch rate stretch rate at extinction
from the two nozzles, and twin counterflow flames were established across the stagnation plane. Mass flow controllers were used to measure the flow rates. Their accuracy was 2% of full scale of each mass flow controller (5, 10 and 50 l/min. for N2, fuel, and O2, respectively). When NO2 was added, an N2/NO2 mixture was substituted for N2. The authors’ calculation of equilibrium composition of the N2/O2 PJ in the previous study [5] showed a local NOx concentration reached at several percentages in the whole mixture. Therefore, two volume ratios of NO2 in the N2/NO2 mixture (1% (a = 0.01) and 1.5% (a = 0.015)) were tested. When the equivalence ratio of the mixture was 0.6, the mole fractions of NO2 in the overall mixture were 0.74% and 1.11%, respectively. The change in the Lewis number of the mixture due to adding NO2 was less than 0.005 throughout the experiments. The mixtures were ignited by an electrically heated wire, which was remotely controlled. After stable twin flames were established, the flow rates of the mixtures were gradually increased until the flames disappeared. During the experiment under high pressure, the pressure in the chamber was maintained at a constant level by controlling the exhaust valve. The fluctuation of pressure was kept within 2 kPa. The global stretch rate at extinction (ee) was defined as 2U e ð1Þ L where Ue is the average velocity at the exit of the nozzle. Considering the slight change in the Lewis number of the mixtures with and without adding NO2, the global stretch rate was sufficient for investigating the overall effect of NOx addition on extinction phenomena. ee ¼
3. Numerical method
Fig. 1. Schematic of experiment setup.
The revised PREMIX code [12] in the CHEMKIN library [13] was used to calculate burning velocities (SL), flame thicknesses (d) of nonstretched flames, and extinction stretch rates of stretched flames. A plug flow and L of 10 mm between the two burners were assumed in correspondence with the experiment. GRI-Mech. Ver.3.0 [14], with the following low temperature chemistry (R1)–(R3) [3–5], was used as a kinetic model for the CH4 flames. Rate constants for
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
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(R1)–(R3) [3] are shown in Table 1. It is well known that simulation by using original GRI mechanism [14] cannot predict the NOx catalytic effect because of a lack of the low temperature chemistry. CH3 þ NO2 ¼ CH3 O þ NO CH3 O2 þ NO ¼ CH3 O þ NO2 CH3 þ O2 þ M ¼ CH3 O2 þ M
ðR1Þ ðR2Þ ðR3Þ
The San Diego Mechanism [15] was used for the C2H4 flames because the GRI mechanism was not intended for analysis of the C2H4 flames. 4. Experimental results 4.1. Luminescence from flames Fig. 2. Effect of NO2 addition on extinction stretch rate of CH4/O2/N2 counterflow flames (/ = 0.75) at p = 0.1 MPa.
The flame color changed from blue to white following the addition of NO2 to the CH4/N2/O2 mixture. Also, the color of the burned gas changed to red-brown when NO2 was added, indicating existence of the NO2.
the extinction stretch rate decreased by more than 15%. This result indicates that including the low temperature chemistry is important for precisely analyzing the extinction stretch rate of CH4 fuel when the NOx is added to the mixture. Figure 3 shows the effect of NO2 addition on the extinction stretch rates of the mixture with / = 0.55 at elevated pressures. Numerical results obtained by using the GRI mechanism with the low temperature chemistry are also plotted in Fig. 3. Addition of a small amount of NO2
4.2. Extinction stretch rates The effect of adding a small amount of NO2 on the extinction stretch rates of lean CH4/O2/N2 mixtures with different equivalence ratios (/) was investigated at elevated pressures. Figure 2 shows the result of measured extinction stretch rates for the mixture with / = 0.75 at atmospheric pressure. Numerical results are also plotted in Fig. 2. Estimated error level which included the error in measurement of flow rates of mixtures were noted with experimental data. Though the error level gradually increased with ambient pressure because of the flame instability, it never affected the tendency of experimental data and conclusions in this study. To validate the effect of the low temperature chemistry (R1)–(R3) [3–5], both of GRI mechanisms with and without the low temperature chemistry were tested as a kinetic model. Figure 2 clearly indicates that the extinction stretch rates in the experiment were insensitive to the amount of NO2 added. This result agrees well with the numerical result based on the kinetic model with the low temperature chemistry. When the original GRI mechanism was used, the extinction stretch rates were considerably decreased by the addition of NO2. For example, when NO2 (a = 0.015) was added,
Fig. 3. Effect of NO2 addition on extinction stretch rate of CH4/O2/N2 counterflow flames (/ = 0.55) at elevated pressures.
Table 1 Rate constants for low temperature chemistry ((R1)–(R3)) [3] (in the form kf = ATBexp(Ea/RT); units are mol, cm, s, K, and cal/mol). Reaction
A
B
Ea
R1. CH3 + NO2 = CH3O + NO R2. CH3O2 + NO = CH3O + NO2 R3. CH3 + O2 + M = CH3O2 + M
1.39E13 2.53E12 1.01E08
0 0 1.6
0 358 0
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decreased the extinction stretch rate of CH4/O2/N2 flame at atmospheric pressure. However, NO2 addition increased the extinction stretch rate under high pressure conditions. This reverse of the effect of the NO2 addition on extinction in the experiments was well reproduced in the numerical results based on the kinetic model with the low temperature chemistry. Though NO2 addition tended to increase the extinction stretch rate for the mixture with / = 0.60 under high pressure in the authors’ additional experiments, the magnitude of increase was smaller than that for the mixture with / = 0.55 in Fig. 3. It was noted that the combustion enhancement by NO2 addition was stronger for lean mixtures and under high pressure conditions. This trend in the experimental results agreed with the authors’ additional numerical results. Figure 4 shows the effect of NO2 addition on the extinction stretch rates of C2H4/O2/N2 mixtures at elevated pressures. NO2 addition increased the extinction stretch rate of the mixture with / = 0.50 at high pressure and slightly decreased that of the same mixture at atmospheric pressure. For the mixture with / = 0.55, however, the effect of NO2 addition was not clear even at 0.3 MPa. The results for C2H4 fuel tended to be the same as those for CH4 fuel. The extinction stretch rates in Fig. 4 gradually decreased with pressure irrespective of the existence of NO2 in the mixture. However, the decrease in the extinction stretch rate did not indicate a decrease in the reactivity of the mixture. The relevant quantity representing convective transport in basic equations for flame analysis is the mass flux, and therefore the density-weighted stretch rate should be used in estimating the pressure effect [16,17]. Figure 5 depicts the densityweighted extinction stretch rates of C2H4/O2/N2 mixture and C2H4/O2/N2/NO2 mixtures in Fig. 4. Basically, increasing the pressure increased the density-weighted extinction stretch rates of all
Fig. 5. Density-weighted extinction stretch rates of C2H4/O2/N2 counterflow flames and C2H4/O2/N2/NO2 counterflow flames (/ = 0.50, 0.55) at elevated pressures.
mixtures. Of course, the tendency of the NO2 effect does not change by such a re-plot. 5. Detailed numerical analysis 5.1. NOx addition to CH4 flame As seen in Fig. 2 in the previous section, the main mechanism for increased extinction stretch rates of CH4 flames when a small amount of NO2 was added is considered to be the catalytic effect of that via the low temperature chemistry (R1)–(R3) [3–5]. These reactions accelerated the oxidation of CH3 to CH3O which was a very slow step in CH4 oxidation. In addition to the low temperature chemistry, the following reaction also enhanced combustion reactions in the presence of NO. Conversion of the relatively inactive HO2 to OH radicals strongly enhanced the combustion reactions. HO2 þ NO ¼ NO2 þ OH
Fig. 4. Effect of NO2 addition on extinction stretch rate of C2H4/O2/N2 counterflow flames (/ = 0.50, 0.55) at elevated pressures.
ðR4Þ
NO was quickly produced by reaction (R1) when NO2 was added to the mixture. It was confirmed by sensitivity analysis for extinction stretch rates that sensitivity coefficients of reaction (R1) and reaction (R4) for NO2 added CH4/O2/N2 flames (a = 0.01) were much larger than those for CH4/ O2/N2 flames (a = 0). Figures 6 and 7 depict NO2 and NO distributions in front regions of one-dimensional nonstretched planar CH4 flames (/ = 0.50, p = 0.1 MPa) with NO2 addition and NO addition, respectively. The origin of the x axis was set at the flame position where the concentration of CH radicals reached at the maximum. Conversion of NO2 to NO occurs further upstream of the flame surface and a large amount of NO exists in the pre-heat zone, when NO2 was added to the mixture. In the case of NO addition, a reverse
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
Fig. 6. Distributions of NO and NO2 when NO2 is added to CH4/O2/N2 non-stretched planar flame (/ = 0.50, p = 0.1 MPa).
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Fig. 8. Distributions of intermediate species in CH4/O2/ N2 and CH4/O2/N2/NO2 (a = 0.01) non-stretched planar flames (/ = 0.50, p = 0.1 MPa).
higher, the other CH3 oxidation paths becomes more rapid than that via (R1)–(R3) reactions. Therefore, the effect of NOx addition becomes strong with the decrease of the equivalence ratio. The extinction stretch rate is in proportional to the reverse of the chemical characteristic time (sc) for the stretched flame [18]. The chemical characteristic time is usually evaluated using the burning velocity (SL) as follows: S L S 2L ð2Þ d a where, d and a are a flame thickness and thermal diffusivity of a mixture, respectively. The burning velocity (SL) of a CH4 flame with NO2 addition was investigated. Figure 9 shows the effect of NO2 addition on the burning velocities of CH4 non-stretched planar flames at 0.1 and 0.5 MPa. The NO2 addition increased the burning velocity in two regions, the rich side, where NO2 played the role of oxidizer, and the region of very low equivalence ratio, near the flammability limit. This increase in the burning velocities resulted in increase of the extinction eext / s1 c
Fig. 7. Distributions of NO and NO2 when NO is added to CH4/O2/N2 non-stretched planar flame (/ = 0.50, p = 0.1 MPa).
conversion of NO to NO2 was also observed. As a result, a difference in effectiveness between NO addition and NO2 addition became small. Figure 8 shows distributions of intermediate species related to reactions (R1)–(R3) in onedimensional non-stretched planar flame of CH4/ O2/N2 and CH4/O2/N2/NO2 (a = 0.01) mixtures with / = 0.50 at atmospheric pressure. Figure 8 clearly demonstrates that the CH3O and CH2O, which are typical intermediate species in the next step of CH3 in CH4 oxidation process, drastically increased and the CH3O2 considerably decreased with the NO2 addition. It was also confirmed in calculation that CH3 began to form further upstream, where the temperature was low, when NO2 was added. The NO2 addition also drastically reduced the mole fraction of HO2 via the consumption reaction of (R4). The NOx catalytic effect via the low temperature chemistry was remarkable for low-temperature flames. When the flame temperature becomes
Fig. 9. Calculated burning velocities of NO2 added CH4/O2/N2 non-stretched planar flame at p = 0.1 MPa and p = 0.5 MPa (T0 = 300 K).
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stretch rate. When the original GRI mechanism [14] was used as the kinetic model, the burning velocity of mixtures on the lean side decreased with the addition of NO2. A relative magnitude of increase in the burning velocity by the NO2 addition at 0.5 MPa was larger than that at atmospheric pressure. Figure 10 shows results of extinction stretch rates for NO2 added counterflow flames with different equivalence ratios at elevated pressures. The behavior of extinction stretch rates with NO2 addition well reflected the effect of NO2 addition on the burning velocity. NO2 addition extended the extinction stretch rate of the lean mixture near the flammability limit at high pressure and its impact increased with pressure. The effect of NO2 addition on the extinction stretch rate disappeared for mixtures with a moderate equivalence ratio, and it appeared again for the stoichiometric mixture at almost all pressures because NO2 played the role of an oxidizer. Additional calculation revealed that the effect of NO addition on the extinction stretch rate was almost the same as that of NO2 addition for the CH4 flame. The rapid conversion of NO2 to NO shown in Fig. 6 is a reason for this result. 5.2. NOx addition to C2H4 flame As seen in Fig. 4, NO2 addition increased the extinction stretch rates of the C2H4/O2/N2 mixtures near the flammability limit at high pressure, the same as those of the CH4/O2/N2 mixtures. However, the catalytic effect of NOx via the low temperature chemistry (R1)–(R3) was not important for the C2H4 fuel. The following reactions related to the HO2 species were dominant in the
Fig. 10. Calculated extinction stretch rates of NO2 added CH4/O2/N2 counterflow flames with different equivalence ratios and at elevated pressures.
combustion enhancement by NOx addition in the oxidation of C2H4. These reactions are well known as the catalytic mechanism of NOx for H2 [1,2]. Therefore, the oxidation path of H2 is important in the C2H4 combustion reaction. HO2 þ NO ¼ NO2 þ OH
ðR4Þ
NO2 þ H ¼ NO þ OH
ðR5Þ
Figure 11 depicts numerical results for the effect of NO2 addition on the extinction stretch rates of C2H4/O2/N2 mixtures with different equivalence ratios at elevated pressures. Data for the mixture with / = 0.45 was not obtained in the experiment (Fig. 4). The extinction stretch rate in the numerical result tended to increase, when the pressure increased. This tendency in numerical result did not agree with the experimental result. As described in the previous section, however, the density-weighted stretch rate was more essential as a parameter when the pressure changed. Both the density-weighted extinction stretch rates in the experiment and calculation increased monotonically with pressure. As a consequence, the difference between the experimental and numerical results was just the slope of increase of the density-weighted extinction stretch rates with pressure, and there was no essential difference. The pressure range, over which the extinction stretch rate increased with NO2 addition, became larger with decreasing equivalence ratio of the mixture. This result agreed well with the experiment result. The catalytic effect of NOx via reactions (R4) and (R5) is readily apparent on the lower temperature and higher pressure side than those at the second explosion limit of the H2 flame in the P–T plane, where more HO2 is formed via the chain-termination reaction (R6) below than is consumed via the chain-branching reaction (R7).
Fig. 11. Calculated extinction stretch rates of NO2 added C2H4/O2/N2 counterflow flames with different equivalence ratios and at elevated pressures.
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
H þ O2 þ M ¼ HO2 þ M H þ O2 ¼ O þ OH
ðR6Þ ðR7Þ
A decrease in the equivalence ratio of the mixture resulted in a low flame temperature, and therefore the NO2 effect was stronger in the mixture near the flammability limit. Figure 12 shows the effect of NO2 addition on the burning velocity of the C2H4 flame, focused on the lean side. As discussed in the previous section, the behavior of the burning velocity with NO2 addition agreed well with that of the extinction stretch rate. The burning velocity of the C2H4 flame increased for mixtures near the flammability limit under high pressure. In addition, the NO2 became an oxidizer in a fuel rich mixture, and therefore, adding NO2 increased the burning velocities of the mixtures with large equivalence ratios. Basically, the effect of NO2 addition to
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the C2H4 flame coincided with that for the CH4 flame. However, in the case of the C2H4 flame, 1.5% NO2 addition resulted in a decrease of the burning velocity at atmospheric pressure (0.1 MPa). Recombination reactions with radicals such as O, H, and OH become significant at high concentration of NOx as reported in [3] and the NOx has a role of an inhibitor. Figure 13 shows the effect of NO addition on the extinction stretch rate of a C2H4 flame. The impact of NO addition on the extinction stretch rate appeared greater than that of NO2 addition regardless of pressure. This result differed from that for the CH4 flame. The enhanced conversion of HO2 to OH via reaction (R4) by directly adding NO must be dominant in such a case. 6. Conclusion The extinction stretch rates and burning velocities of a lean CH4/O2/N2 mixture and a C2H4/O2/ N2 mixture increased at high pressure by adding a small amount of NOx. The catalytic effect of NOx on CH4 was via the low temperature chemistry involving CH3, CH3O, CH3O2, and HO2, and the effect on C2H4 was via reactions involving HO2. These catalytic effects were more apparent under low temperature and high pressure conditions. Acknowledgements The authors are grateful to Professor Goro Masuya and Dr. Toshinori Kouchi for their valuable comments.
Fig. 12. Calculated burning velocities of NO2 added C2H4/O2/N2 non-stretched planar flames at p = 0.1 MPa and p = 0.5 MPa (T0 = 300 K).
References
Fig. 13. Comparison of effects of NO and NO2 addition on extinction stretch rates of C2H4/O2/N2 counterflow flames (/ = 0.45, 0.50) at elevated pressures.
[1] M. Slack, A. Grillo, NASA Report CR-2896, 1997. [2] B. Han, C.J. Sung, M. Nishioka, Combust. Sci. Technol. 176 (2004) 305–330. [3] J.H. Bromly, F.J. Burnes, S. Muris, X. You, B.S. Haynes, Combust. Sci. Technol. 115 (1996) 259–296. [4] Y. Tan, C.G. Fotache, C.K. Law, Combust. Flame 119 (1999) 346–355. [5] T. Amano, F.L. Dryer, Proc. Combust. Inst. 27 (1998) 397–404. [6] K. Takita, N. Abe, G. Masuya, Y. Ju, Proc. Combust. Inst. 31 (2007) 2489–2496. [7] K. Takita, R. Ohashi, N. Abe, J. Propul. Power 25 (2009) 565–570. [8] U.D. Lee, C.S. Yoo, J.H. Chen, J.H. Frank, Proc. Combust. Inst. 32 (2009) 1059–1066. [9] U.D. Lee, C.S. Yoo, J.H. Chen, J.H. Frank, Combust. Flame 157 (2010) 217–229. [10] K. Takita, H. Yamazaki, T. Uchida, G. Masuya, Combust. Sci. Technol. 178 (2006) 1649–1688. [11] K. Takahashi, P. Pramoonjaggo, K. Takita, Combust. Sci. Technol. 181 (2009) 1065–1077.
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[12] Y. Ju, H. Guo, K. Maruta, F. Liu, J. Fluid Mech. 342 (1997) 315–339. [13] R.J. Kee, J.F. Grcar, M.D. Smooke, J.A. Miller, Sandia National Laboratories Report, SAND858240, 1985. [14] G.P. Smith, D.M. Golden, M. Franklach, et al., GRI-Mech 3.0, 1999, URL: http://www.me.berkeley.edu/gri_mech/.
[15] San Diego Mechanism, URL: http://maemail.uscd.edu/combustion/cermech/, 2005. [16] C.J. Sun, C.J. Sung, D.L. Zhu, C.K. Law, Proc. Combust. Inst. 26 (1996) 1111–1119. [17] C.K. Law, Combust. Sci. Technol. 178 (2006) 335– 360. [18] N. Peters, Proc. Combust. Inst. 21 (1986) 1231– 1250.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 33 (2011) 1179–1186
www.elsevier.com/locate/proci
Effect of a small amount of NOx on extinction limit of lean premixed counterflow flame Kenichi Takita *, Akihiro Morinaga, Taku Someya Department of Aeronautics and Space Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Sendai 980-8579, Japan Available online 6 August 2010
Abstract The effect of a small amount of NOx (NO and NO2) on the extinction of CH4 and C2H4 flames was experimentally and numerically investigated. The extinction stretch rates of counterflow premixed flames when NO2 was added to the mixture were measured at pressures ranging from 0.1 to 0.3 MPa. Calculations by using one-dimensional flame code with full chemistry were also conducted to clarify the effect of NOx on the extinction limit and burning velocity. Extinction stretch rates of mixtures near the flammability limit increased by adding NOx at high pressure in the experiments and the calculations. The catalytic effect of NOx for CH4 was via the low temperature chemistry involving CH3, CH3O, CH3O2, and HO2, and that for C2H4 was via reactions that included HO2 species. The change in extinction stretch rates with NOx addition reflected the behavior of the burning velocity with NOx addition. The burning velocities of mixtures near the flammability limit increased with the addition of NOx at high pressure, the same as the extinction stretch rate. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Extinction; Counterflow flame; NOx catalytic effect; Hydrocarbon fuel; High pressure
1. Introduction A small amount of NOx (NO and NO2) drastically accelerates the oxidation of hydrogen [1,2] and hydrocarbon fuels [3–5]. The authors [6,7] have been developing a new ignition enhancement method in a scramjet combustor by using N2/O2 plasma jet that can supply a suitable amount of NOx for ignition in the engine. In such an engine for high-speed vehicles, flame-holding after ignition in a high-speed flow is also important for successful development of the engine. When the flame-holding aspect is considered, the extinction limit or the blow-off limit of the flame, which is
*
Corresponding author. Fax: +81 22 795 7009. E-mail address: [email protected] (K. Takita).
strongly related to flame propagation characteristics such as burning velocity, becomes another important factor. Therefore, effects of NOx on the extinction limit must be investigated for the new ignition and combustion enhancement systems. However, reports on the effect of NOx on extinction were very limited, though many studies about the effect of that on ignition has been conducted as previously mentioned. Quite recently, Lee et al. [8,9] have investigated the effect of NO on a local extinction and re-ignition of non-premixed hydrogen-air counterflow flames. Flame propagation in a flow containing a small amount of NOx appears in many situations. In a combustion test facility for the engine of a highspeed vehicle, a very high total temperature of the flow is required to realize flight conditions at high altitude, and therefore a heating system such as a shock-tunnel, arc-heater, or vitiation heater that
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.045
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K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
Nomenclature E.R. or / equivalence ratio inflow temperature T0 a mole fraction of NO2 in N2/NO2 mixture
heats the air up to an extremely high temperature is indispensable. As such, NOx must be included in the flow in such a high-speed engine test. For another example, in an automobile engine, increased exhaust gas recirculation (EGR) has resulted from the pursuit of a high-efficiency, lowemission engine. Therefore, the effect of NOx contamination in the exhaust gas must be understood for precise engine control. Considering the above situation, this study experimentally and numerically investigated the effect of a small amount of NOx on the extinction limit of counterflow lean hydrocarbon/N2/O2 flames. The detailed mechanisms of the catalytic effect of NOx on the extinction of CH4 and C2H4 flames and their dependence on ambient conditions were discussed. 2. Experimental method Figure 1 schematically illustrates the experiment setup. The basic system was the same as that used in the authors’ previous studies on extinction of counterflow flames at high pressure [10] and high temperature [11]. The inner diameter of the high-pressure chamber was 333.4 mm, and its length was 580.0 mm. Two counterflow burners with nozzle diameters (D) of 9.0 mm were installed in the chamber. The separation distance (L) between the two burners was set at 10.0 mm (L/D = 1.1). Shroud flows of N2 were issued from the coaxial outer nozzles to eliminate effects from the surrounding area and flame tails. Mixtures of fuel (CH4 or C2H4) and oxidizer (N2/O2 or N2/ O2/NO2) mixed in a mixing chamber were issued
e ee
stretch rate stretch rate at extinction
from the two nozzles, and twin counterflow flames were established across the stagnation plane. Mass flow controllers were used to measure the flow rates. Their accuracy was 2% of full scale of each mass flow controller (5, 10 and 50 l/min. for N2, fuel, and O2, respectively). When NO2 was added, an N2/NO2 mixture was substituted for N2. The authors’ calculation of equilibrium composition of the N2/O2 PJ in the previous study [5] showed a local NOx concentration reached at several percentages in the whole mixture. Therefore, two volume ratios of NO2 in the N2/NO2 mixture (1% (a = 0.01) and 1.5% (a = 0.015)) were tested. When the equivalence ratio of the mixture was 0.6, the mole fractions of NO2 in the overall mixture were 0.74% and 1.11%, respectively. The change in the Lewis number of the mixture due to adding NO2 was less than 0.005 throughout the experiments. The mixtures were ignited by an electrically heated wire, which was remotely controlled. After stable twin flames were established, the flow rates of the mixtures were gradually increased until the flames disappeared. During the experiment under high pressure, the pressure in the chamber was maintained at a constant level by controlling the exhaust valve. The fluctuation of pressure was kept within 2 kPa. The global stretch rate at extinction (ee) was defined as 2U e ð1Þ L where Ue is the average velocity at the exit of the nozzle. Considering the slight change in the Lewis number of the mixtures with and without adding NO2, the global stretch rate was sufficient for investigating the overall effect of NOx addition on extinction phenomena. ee ¼
3. Numerical method
Fig. 1. Schematic of experiment setup.
The revised PREMIX code [12] in the CHEMKIN library [13] was used to calculate burning velocities (SL), flame thicknesses (d) of nonstretched flames, and extinction stretch rates of stretched flames. A plug flow and L of 10 mm between the two burners were assumed in correspondence with the experiment. GRI-Mech. Ver.3.0 [14], with the following low temperature chemistry (R1)–(R3) [3–5], was used as a kinetic model for the CH4 flames. Rate constants for
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
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(R1)–(R3) [3] are shown in Table 1. It is well known that simulation by using original GRI mechanism [14] cannot predict the NOx catalytic effect because of a lack of the low temperature chemistry. CH3 þ NO2 ¼ CH3 O þ NO CH3 O2 þ NO ¼ CH3 O þ NO2 CH3 þ O2 þ M ¼ CH3 O2 þ M
ðR1Þ ðR2Þ ðR3Þ
The San Diego Mechanism [15] was used for the C2H4 flames because the GRI mechanism was not intended for analysis of the C2H4 flames. 4. Experimental results 4.1. Luminescence from flames Fig. 2. Effect of NO2 addition on extinction stretch rate of CH4/O2/N2 counterflow flames (/ = 0.75) at p = 0.1 MPa.
The flame color changed from blue to white following the addition of NO2 to the CH4/N2/O2 mixture. Also, the color of the burned gas changed to red-brown when NO2 was added, indicating existence of the NO2.
the extinction stretch rate decreased by more than 15%. This result indicates that including the low temperature chemistry is important for precisely analyzing the extinction stretch rate of CH4 fuel when the NOx is added to the mixture. Figure 3 shows the effect of NO2 addition on the extinction stretch rates of the mixture with / = 0.55 at elevated pressures. Numerical results obtained by using the GRI mechanism with the low temperature chemistry are also plotted in Fig. 3. Addition of a small amount of NO2
4.2. Extinction stretch rates The effect of adding a small amount of NO2 on the extinction stretch rates of lean CH4/O2/N2 mixtures with different equivalence ratios (/) was investigated at elevated pressures. Figure 2 shows the result of measured extinction stretch rates for the mixture with / = 0.75 at atmospheric pressure. Numerical results are also plotted in Fig. 2. Estimated error level which included the error in measurement of flow rates of mixtures were noted with experimental data. Though the error level gradually increased with ambient pressure because of the flame instability, it never affected the tendency of experimental data and conclusions in this study. To validate the effect of the low temperature chemistry (R1)–(R3) [3–5], both of GRI mechanisms with and without the low temperature chemistry were tested as a kinetic model. Figure 2 clearly indicates that the extinction stretch rates in the experiment were insensitive to the amount of NO2 added. This result agrees well with the numerical result based on the kinetic model with the low temperature chemistry. When the original GRI mechanism was used, the extinction stretch rates were considerably decreased by the addition of NO2. For example, when NO2 (a = 0.015) was added,
Fig. 3. Effect of NO2 addition on extinction stretch rate of CH4/O2/N2 counterflow flames (/ = 0.55) at elevated pressures.
Table 1 Rate constants for low temperature chemistry ((R1)–(R3)) [3] (in the form kf = ATBexp(Ea/RT); units are mol, cm, s, K, and cal/mol). Reaction
A
B
Ea
R1. CH3 + NO2 = CH3O + NO R2. CH3O2 + NO = CH3O + NO2 R3. CH3 + O2 + M = CH3O2 + M
1.39E13 2.53E12 1.01E08
0 0 1.6
0 358 0
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decreased the extinction stretch rate of CH4/O2/N2 flame at atmospheric pressure. However, NO2 addition increased the extinction stretch rate under high pressure conditions. This reverse of the effect of the NO2 addition on extinction in the experiments was well reproduced in the numerical results based on the kinetic model with the low temperature chemistry. Though NO2 addition tended to increase the extinction stretch rate for the mixture with / = 0.60 under high pressure in the authors’ additional experiments, the magnitude of increase was smaller than that for the mixture with / = 0.55 in Fig. 3. It was noted that the combustion enhancement by NO2 addition was stronger for lean mixtures and under high pressure conditions. This trend in the experimental results agreed with the authors’ additional numerical results. Figure 4 shows the effect of NO2 addition on the extinction stretch rates of C2H4/O2/N2 mixtures at elevated pressures. NO2 addition increased the extinction stretch rate of the mixture with / = 0.50 at high pressure and slightly decreased that of the same mixture at atmospheric pressure. For the mixture with / = 0.55, however, the effect of NO2 addition was not clear even at 0.3 MPa. The results for C2H4 fuel tended to be the same as those for CH4 fuel. The extinction stretch rates in Fig. 4 gradually decreased with pressure irrespective of the existence of NO2 in the mixture. However, the decrease in the extinction stretch rate did not indicate a decrease in the reactivity of the mixture. The relevant quantity representing convective transport in basic equations for flame analysis is the mass flux, and therefore the density-weighted stretch rate should be used in estimating the pressure effect [16,17]. Figure 5 depicts the densityweighted extinction stretch rates of C2H4/O2/N2 mixture and C2H4/O2/N2/NO2 mixtures in Fig. 4. Basically, increasing the pressure increased the density-weighted extinction stretch rates of all
Fig. 5. Density-weighted extinction stretch rates of C2H4/O2/N2 counterflow flames and C2H4/O2/N2/NO2 counterflow flames (/ = 0.50, 0.55) at elevated pressures.
mixtures. Of course, the tendency of the NO2 effect does not change by such a re-plot. 5. Detailed numerical analysis 5.1. NOx addition to CH4 flame As seen in Fig. 2 in the previous section, the main mechanism for increased extinction stretch rates of CH4 flames when a small amount of NO2 was added is considered to be the catalytic effect of that via the low temperature chemistry (R1)–(R3) [3–5]. These reactions accelerated the oxidation of CH3 to CH3O which was a very slow step in CH4 oxidation. In addition to the low temperature chemistry, the following reaction also enhanced combustion reactions in the presence of NO. Conversion of the relatively inactive HO2 to OH radicals strongly enhanced the combustion reactions. HO2 þ NO ¼ NO2 þ OH
Fig. 4. Effect of NO2 addition on extinction stretch rate of C2H4/O2/N2 counterflow flames (/ = 0.50, 0.55) at elevated pressures.
ðR4Þ
NO was quickly produced by reaction (R1) when NO2 was added to the mixture. It was confirmed by sensitivity analysis for extinction stretch rates that sensitivity coefficients of reaction (R1) and reaction (R4) for NO2 added CH4/O2/N2 flames (a = 0.01) were much larger than those for CH4/ O2/N2 flames (a = 0). Figures 6 and 7 depict NO2 and NO distributions in front regions of one-dimensional nonstretched planar CH4 flames (/ = 0.50, p = 0.1 MPa) with NO2 addition and NO addition, respectively. The origin of the x axis was set at the flame position where the concentration of CH radicals reached at the maximum. Conversion of NO2 to NO occurs further upstream of the flame surface and a large amount of NO exists in the pre-heat zone, when NO2 was added to the mixture. In the case of NO addition, a reverse
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
Fig. 6. Distributions of NO and NO2 when NO2 is added to CH4/O2/N2 non-stretched planar flame (/ = 0.50, p = 0.1 MPa).
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Fig. 8. Distributions of intermediate species in CH4/O2/ N2 and CH4/O2/N2/NO2 (a = 0.01) non-stretched planar flames (/ = 0.50, p = 0.1 MPa).
higher, the other CH3 oxidation paths becomes more rapid than that via (R1)–(R3) reactions. Therefore, the effect of NOx addition becomes strong with the decrease of the equivalence ratio. The extinction stretch rate is in proportional to the reverse of the chemical characteristic time (sc) for the stretched flame [18]. The chemical characteristic time is usually evaluated using the burning velocity (SL) as follows: S L S 2L ð2Þ d a where, d and a are a flame thickness and thermal diffusivity of a mixture, respectively. The burning velocity (SL) of a CH4 flame with NO2 addition was investigated. Figure 9 shows the effect of NO2 addition on the burning velocities of CH4 non-stretched planar flames at 0.1 and 0.5 MPa. The NO2 addition increased the burning velocity in two regions, the rich side, where NO2 played the role of oxidizer, and the region of very low equivalence ratio, near the flammability limit. This increase in the burning velocities resulted in increase of the extinction eext / s1 c
Fig. 7. Distributions of NO and NO2 when NO is added to CH4/O2/N2 non-stretched planar flame (/ = 0.50, p = 0.1 MPa).
conversion of NO to NO2 was also observed. As a result, a difference in effectiveness between NO addition and NO2 addition became small. Figure 8 shows distributions of intermediate species related to reactions (R1)–(R3) in onedimensional non-stretched planar flame of CH4/ O2/N2 and CH4/O2/N2/NO2 (a = 0.01) mixtures with / = 0.50 at atmospheric pressure. Figure 8 clearly demonstrates that the CH3O and CH2O, which are typical intermediate species in the next step of CH3 in CH4 oxidation process, drastically increased and the CH3O2 considerably decreased with the NO2 addition. It was also confirmed in calculation that CH3 began to form further upstream, where the temperature was low, when NO2 was added. The NO2 addition also drastically reduced the mole fraction of HO2 via the consumption reaction of (R4). The NOx catalytic effect via the low temperature chemistry was remarkable for low-temperature flames. When the flame temperature becomes
Fig. 9. Calculated burning velocities of NO2 added CH4/O2/N2 non-stretched planar flame at p = 0.1 MPa and p = 0.5 MPa (T0 = 300 K).
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stretch rate. When the original GRI mechanism [14] was used as the kinetic model, the burning velocity of mixtures on the lean side decreased with the addition of NO2. A relative magnitude of increase in the burning velocity by the NO2 addition at 0.5 MPa was larger than that at atmospheric pressure. Figure 10 shows results of extinction stretch rates for NO2 added counterflow flames with different equivalence ratios at elevated pressures. The behavior of extinction stretch rates with NO2 addition well reflected the effect of NO2 addition on the burning velocity. NO2 addition extended the extinction stretch rate of the lean mixture near the flammability limit at high pressure and its impact increased with pressure. The effect of NO2 addition on the extinction stretch rate disappeared for mixtures with a moderate equivalence ratio, and it appeared again for the stoichiometric mixture at almost all pressures because NO2 played the role of an oxidizer. Additional calculation revealed that the effect of NO addition on the extinction stretch rate was almost the same as that of NO2 addition for the CH4 flame. The rapid conversion of NO2 to NO shown in Fig. 6 is a reason for this result. 5.2. NOx addition to C2H4 flame As seen in Fig. 4, NO2 addition increased the extinction stretch rates of the C2H4/O2/N2 mixtures near the flammability limit at high pressure, the same as those of the CH4/O2/N2 mixtures. However, the catalytic effect of NOx via the low temperature chemistry (R1)–(R3) was not important for the C2H4 fuel. The following reactions related to the HO2 species were dominant in the
Fig. 10. Calculated extinction stretch rates of NO2 added CH4/O2/N2 counterflow flames with different equivalence ratios and at elevated pressures.
combustion enhancement by NOx addition in the oxidation of C2H4. These reactions are well known as the catalytic mechanism of NOx for H2 [1,2]. Therefore, the oxidation path of H2 is important in the C2H4 combustion reaction. HO2 þ NO ¼ NO2 þ OH
ðR4Þ
NO2 þ H ¼ NO þ OH
ðR5Þ
Figure 11 depicts numerical results for the effect of NO2 addition on the extinction stretch rates of C2H4/O2/N2 mixtures with different equivalence ratios at elevated pressures. Data for the mixture with / = 0.45 was not obtained in the experiment (Fig. 4). The extinction stretch rate in the numerical result tended to increase, when the pressure increased. This tendency in numerical result did not agree with the experimental result. As described in the previous section, however, the density-weighted stretch rate was more essential as a parameter when the pressure changed. Both the density-weighted extinction stretch rates in the experiment and calculation increased monotonically with pressure. As a consequence, the difference between the experimental and numerical results was just the slope of increase of the density-weighted extinction stretch rates with pressure, and there was no essential difference. The pressure range, over which the extinction stretch rate increased with NO2 addition, became larger with decreasing equivalence ratio of the mixture. This result agreed well with the experiment result. The catalytic effect of NOx via reactions (R4) and (R5) is readily apparent on the lower temperature and higher pressure side than those at the second explosion limit of the H2 flame in the P–T plane, where more HO2 is formed via the chain-termination reaction (R6) below than is consumed via the chain-branching reaction (R7).
Fig. 11. Calculated extinction stretch rates of NO2 added C2H4/O2/N2 counterflow flames with different equivalence ratios and at elevated pressures.
K. Takita et al. / Proceedings of the Combustion Institute 33 (2011) 1179–1186
H þ O2 þ M ¼ HO2 þ M H þ O2 ¼ O þ OH
ðR6Þ ðR7Þ
A decrease in the equivalence ratio of the mixture resulted in a low flame temperature, and therefore the NO2 effect was stronger in the mixture near the flammability limit. Figure 12 shows the effect of NO2 addition on the burning velocity of the C2H4 flame, focused on the lean side. As discussed in the previous section, the behavior of the burning velocity with NO2 addition agreed well with that of the extinction stretch rate. The burning velocity of the C2H4 flame increased for mixtures near the flammability limit under high pressure. In addition, the NO2 became an oxidizer in a fuel rich mixture, and therefore, adding NO2 increased the burning velocities of the mixtures with large equivalence ratios. Basically, the effect of NO2 addition to
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the C2H4 flame coincided with that for the CH4 flame. However, in the case of the C2H4 flame, 1.5% NO2 addition resulted in a decrease of the burning velocity at atmospheric pressure (0.1 MPa). Recombination reactions with radicals such as O, H, and OH become significant at high concentration of NOx as reported in [3] and the NOx has a role of an inhibitor. Figure 13 shows the effect of NO addition on the extinction stretch rate of a C2H4 flame. The impact of NO addition on the extinction stretch rate appeared greater than that of NO2 addition regardless of pressure. This result differed from that for the CH4 flame. The enhanced conversion of HO2 to OH via reaction (R4) by directly adding NO must be dominant in such a case. 6. Conclusion The extinction stretch rates and burning velocities of a lean CH4/O2/N2 mixture and a C2H4/O2/ N2 mixture increased at high pressure by adding a small amount of NOx. The catalytic effect of NOx on CH4 was via the low temperature chemistry involving CH3, CH3O, CH3O2, and HO2, and the effect on C2H4 was via reactions involving HO2. These catalytic effects were more apparent under low temperature and high pressure conditions. Acknowledgements The authors are grateful to Professor Goro Masuya and Dr. Toshinori Kouchi for their valuable comments.
Fig. 12. Calculated burning velocities of NO2 added C2H4/O2/N2 non-stretched planar flames at p = 0.1 MPa and p = 0.5 MPa (T0 = 300 K).
References
Fig. 13. Comparison of effects of NO and NO2 addition on extinction stretch rates of C2H4/O2/N2 counterflow flames (/ = 0.45, 0.50) at elevated pressures.
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