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Experimental and kinetic studies of acetylene flames at elevated pressures
Available online at www.sciencedirect.com
ScienceDirect Proceedings of the Combustion Institute 35 (2015) 721–728
Proceedings of the
Combustion Institute www.elsevier.com/locate/proci
Experimental and kinetic studies of acetylene flames at elevated pressures Xiaobo Shen a,b, Xueliang Yang a,⇑, Jeffrey Santner a, Jinhua Sun b, Yiguang Ju a a
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, PR China
b
Available online 25 June 2014
Abstract The kinetic effects of CO2 and H2O dilution on the laminar flame speed of acetylene at elevated pressures have been investigated experimentally using outwardly propagating spherical flames in a nearly constant pressure chamber from 1 to 20 atm. The flame speeds of C2H2/air mixtures at atmospheric pressure agree with recent measurements reasonably well. Detailed analysis on the combustion chemistry of acetylene reveals that C2H2 + O, HCCO + O2, HCO + O2, CH3 + HO2, H + C2H3, CO + OH, CH2 (S) + C2H2, and HCO decomposition are among the most important reactions, which leads to a new kinetic model (HP Mech) that incorporates the recent understanding of elementary reactions. The effects of CO2 dilution on acetylene flame speeds are found to be small for both fuel rich and lean conditions due to the direct CO2 formation pathway (HCCO + O2) in acetylene oxidation. Water dilution effects are more pronounced, especially at lean conditions, because the radical pool composition is altered by shifting the equilibrium of H2O + O = OH + OH. Comparing to USC Mech II, HP Mech has much better performance compared to the current experimental measurements as well as the shock tube and flow reactor data. Ó 2014 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Acetylene; Laminar flame speed; Elevated pressure; HP Mech; CO2 and H2O dilution effects
1. Introduction Modern engines utilize exhaust gas recirculation (EGR) to reduce NOx emissions and increase thermal efficiency, but these systems often cause increased soot emission [1]. Acetylene, recognized ⇑ Corresponding author. Address: D225 E-Quad, Princeton University, Princeton, NJ 08544, United States. Tel.: +1 609 258 1411. E-mail address: [email protected] (X. Yang).
as an important precursor for soot formation [2] in hydrocarbon fuel combustion, has drawn continuous interest for decades [3–9]. Acetylene flame chemistry is not only important in understanding soot formation mechanisms, but is also an indispensable step in developing predictive hierarchical kinetic models [10,11]. Despite its importance in both fundamental science and application, the laminar flame speed of acetylene has only been measured at pressures up to 2 atm due to safety considerations and experimental limitations, and the limited data are in poor agreement with each. More importantly, most models
http://dx.doi.org/10.1016/j.proci.2014.05.106 1540-7489/Ó 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
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have not incorporated recent progress in elementary reaction rate determinations associated with acetylene combustion [12–18], and many reaction rates used in the models are based on empirical estimations or are optimized to fit particular global combustion properties. Therefore, there are large discrepancies between model predictions and experimental measurements of the acetylene flame speed and it is unlikely that these models can accurately predict combustion properties at engine conditions. In EGR engines, large quantities of CO2 and H2O are present in the unburned gas and engine performance is strongly affected by the thermal, dilution, transport, and chemical effects introduced by H2O and CO2. The thermal, transport, and dilution effects are easy to understand; however, the chemical effects are harder to quantify as they are fuel-specific. These effects are particularly important at high pressure, where practical engines operate, and three-body termination reactions become more important. This work will investigate the chemical effects of CO2 and H2O dilution on the laminar flame speed of acetylene at elevated pressures. The laminar flame speed of C2H2/air with equivalence ratios from 0.6 to 2.0 at atmospheric pressure was measured to compare with the available experimental data and widely used USC Mech II. Additionally, the flame speed was measured at pressures up to 20 atm with equivalence ratios of 0.8 and 1.6 at fixed adiabatic flame temperatures of 1800 K and 1900 K. The effects of CO2 and water dilution were examined at both equivalence ratios. Meanwhile, a kinetic model including the most recent advances in elementary reaction kinetics was developed and used to examine the flame structure and improve predictions. 2. Experimental methods Experiments were conducted in high pressure constant volume cylindrical and heated spherical bombs. The chambers are filled with a combustible mixture and centrally spark ignited. Flame propagation to a 3 cm radius was recorded using high speed schlieren photography (up to 15,000 frames per second). Both chambers utilize custom one way valves to release the burned gas pressure after the flame has passed the edge of the viewing window. Complete details of the experimental system and procedure have been described elsewhere [19–22]. Mixtures were prepared using the partial pressure method from acetylene (>99%, Atomic Absorption Grade), oxygen (99.5%), helium (99.995%), nitrogen (99.99%), carbon dioxide (99.9%), water vapor, and synthetic air. All experiments with gaseous compounds were conducted at room temperature (measured as 298 ± 3 K), and experiments with water dilution
were performed at 400 K to prevent condensation. Deionized liquid water was vaporized at 400 K in a separate container before filling the heated spherical chamber through heated tubing. The C2H2/O2/N2/He mixtures were prepared at initial total pressures from 1 and 20 atm with constant calculated adiabatic flame temperature (1800 K or 1900 K) for equivalence ratios of 0.8 and 1.6. The O2/N2 ratio was always kept the same as that in air. Then, mixtures with 20% (mole fraction in mixtures) CO2 or 10% water vapor were also used for both specified equivalence ratios at the same adiabatic flame temperatures. For all the experiments, the helium concentration was adjusted to control the flame temperature and the Lewis number in order to prevent cellular instability, buoyancy effects, ignition difficulty, and extrapolation error. The precision of the measurements was frequently checked by repeating the same experiments a few times. Experimental details can be found in Table 1. 2.1. Data evaluation For ease of processing, an automated flame edge detection and circle fitting program was built to locate the flame front and obtain the smoothed flame radius Rf. The instantaneous flame front velocity Su and stretch rate j were calculated from the flame radius and flame front velocity using, Su ¼
1
1 dRf r dt þ r1 r
ð1Þ
and j¼
2 dRf Rf dt
ð2Þ
where the flow correction term, -, accounts for the actual burned gas velocity induced by asymmetrical confinement at large flame radius (Rf/Rchamber > 30%) [20]. Experiments with water vapor were performed with a large chamber such that Rf/Rchamber < 30%, so - is assumed as zero for these measurements. The ratio of unburned to burned gas densities, r, was calculated using PREMIX [23]. The unstretched flame propagation speed, S 0u , is then determined through the nonlinear extrapolation method (for Le < 1) and linear extrapolation method (for Le > 1), as suggested by Wu et al. [24]. No data analyses were performed for flames that were observed to be wrinkled due to cellular or spiraling instabilities, affected by buoyancy, or influenced by transient effects. All flame speed data presented in this paper are plotted with error bars calculated from the RMS sum of the uncertainties corresponding to the measurements of flame radius, flow correction, and uncertainties resulted from initial conditions (i.e. temperature, pressure, mixture concentration) estimated using USC Mech II [10].
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Table 1 The equivalence ratio, mole fraction of each compound in the mixture, and the adiabatic flame temperature used in the high pressure experiments. (/: Stoichiometry, Tu: ambient temperature; Tf: adiabatic flame temperature). /
C2H2
O2
N2
He
CO2
H2O
Tu
Tf
0.8 0.8 0.8 0.8 1.6 1.6 1.6 1.6
0.034 0.036 0.028 0.026 0.063 0.069 0.058 0.055
0.106 0.113 0.089 0.081 0.098 0.108 0.091 0.086
0.400 0.000 0.000 0.000 0.369 0.000 0.376 0.429
0.460 0.650 0.783 0.893 0.470 0.624 0.376 0.429
0.000 0.200 0.000 0.000 0.000 0.200 0.000 0.000
0.000 0.000 0.100 0.000 0.000 0.000 0.100 0.000
298 298 400 400 298 298 400 400
1800 1800 1800 1800 1900 1900 1800 1800
3. Combustion chemistry of C2H2 and kinetic model development Unlike saturated hydrocarbon fuels, at elevated temperature acetylene is mostly consumed by reactions with O radicals, not the usual H-abstraction reactions: C2 H2 þ O $ HCCO þ H
ðR1Þ
3
ðR2Þ
C2 H2 þ O$ CH2 þ CO
However, the branching ratio (R1) over (R2) used in the different models varies from 0.45:0.55 to 0.8:0.2, causing large discrepancies in flame speed predictions [8]. Recent experimental and theoretical studies have confirmed that the branching ratio is 0.8:0.2 independent of temperature [12–14,25]. The subsequent oxidation reactions CH2 + O2 and HCCO + O2 have many product channels and the branching ratios of these product channels are very difficult to determine. In combustion conditions the ketenyl radical HCCO is partially consumed by reactions: HCCO þ O2 $ H þ CO þ CO2 HCCO þ O2 $ OH þ CO þ CO
ðR3Þ ðR4Þ
(R3) is confirmed to be the dominant product channel [15,16] by recent experimental and theoretical studies but (R4) is still emphasized in many models [10,26], which may yield uncertainties and discrepancies in model predictions. The CH2 + O2 system is even more complicated since it is associated with multiple potential energy surfaces leading to many possible product channels. The reaction rates and branching ratios reported in the literature often disagree with each other significantly, however recent flow reactor and shock tube studies have found that the branching ratio of CO2 production is 50% [27,28]. Singlet methylene CH2(S) produced by HCCO + H can quickly react with C2H2 forming propargyl radical (C3H3), one of the main precursors for the formation of polycyclic aromatic hydrocarbons [29]. Meanwhile, singlet methylene can also be quenched to its ground state by different colliders, suppressing propargyl formation. Detailed understanding of the competition between CH2(S)
quenching and its reaction with C2H2 is crucial for soot formation prediction. Therefore, these reaction rates must be carefully determined to create a predictive kinetic model. The present work is part of an ongoing effort to develop a reaction mechanism based on recent advances in elementary reaction rate determinations. The model, High Pressure Mech (HP Mech) incorporates the H2/O2 sub mechanism of Burke et al. [30] and recent collaborative work on methanol combustion kinetics [31]. For elementary reaction rates with multiple literature determinations, each source was carefully examined and evaluated based on our understanding of experimental and theoretical uncertainties. For the elementary reactions lacking experimental or theoretical determinations, rates were estimated from ab initio calculations with uncertainties below a factor of 3. Extra care was taken for chemically activated systems in which the pressure dependence of the reaction rates may be complicated to make sure that reaction rates and branching ratios are in the reasonable range. Unlike other models, no optimization or scaling of the reaction rates was attempted to fit any experimental data. The details of HP Mech used in this work are provided in Supplemental file. The reaction set provided here should only be used for high temperature combustion of acetylene as many reactions that are important for combustion of other fuels but irrelevant to acetylene chemistry are not included. Numerical simulations were obtained using the freely propagating flame module in CHEMKIN Pro software [32]. Calculations included multicomponent molecular diffusion and thermal diffusion. For the flame speed calculations, gradient and curvature tolerances were set to 0.04, producing converged, grid-independent solutions. 4. Results and discussion 4.1. C2H2/air flames at atmospheric pressure Present measurements of the laminar flame speed of acetylene in air at 1 atm and those from the literature [6,7] are plotted in Fig. 1, as well
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as simulations based on USC Mech II and HP Mech. The current results agree with Jomaas et al. [6] within 10% over the whole equivalence ratio range. The results in [6] do not account for confinement induced flow, which may account for the difference observed for lean conditions. USC Mech II is able to predict the laminar flame speeds at fuel lean conditions reasonably well, but underestimates the flame speeds at some fuel rich conditions by 10–15 cm/s. A careful examination of USC Mech II shows that many important reactions used in the model have very large uncertainties. Aside from those highlighted in the H2/O2 sub mechanism [30], the rate of CO + OH M CO2 + H used in USC Mech II is 7% higher than the recommended value [33] and may result in overprediction of the flame speeds; however, the lower predicted flame speed indicates the effect of this reaction may be cancelled by other reactions. The competing reactions HCO(+M) M H + CO(+M) and HCO + O2 M HO2 + CO are also important for flame speed simulation and both reaction rates used in USC Mech II are roughly 50% higher than the recent suggested values [34,35]. Additionally, the CH3 + HO2 reaction rate in USC Mech II is a factor of 10 higher than the recent experimental and theoretical determinations [36,37]. As mentioned above, (R4) is the only HCCO + O2 channel used in USC Mech II. USC Mech II also underestimates the CH2(S) + C2H2 reaction rate by a factor of 3 compared to the recommendation of Ref. [18]. On the contrary, the simulations based on HP Mech, which incorporates new, more accurately determined elementary reaction rates, agree with the experiments reasonably well. The major causes of improved model performance are the updated HCCO + O2 reaction rates. Other updated
160
Laminar Flame Speed / (cm/s)
C2H2/Air 298K
140
1 atm
120
100 This work
80
Egolfopoulos et al. (1991) Jomaas et al. (2005) USCMech II
60
HP Mech
40 0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
Equivalence Ratio
Fig. 1. Experimentally measured premixed laminar flame speeds of C2H2/air mixtures at 1 atm and 298 K compared with simulations based on USC Mech II [10] and HP Mech. Experiments: h Egolfopoulos et al. [7]; e Jomaas et al. [6]; d present measurements. Simulations: ––, USC Mech II; — HP Mech.
reactions that have a large effect on flame speed simulation include HCCO + H, CH2(S) + C2H2 and the quenching reaction of CH2(S). CH2 (S) + C2H2 and the quenching reaction of CH2(S) have been well studied [17,18] and the uncertainties of these reaction rates are no more than 30%. The elementary reaction rate of HCCO + H = CH2(S) + CO has only been measured once at room temperature [38] and most of the current models use that rate assuming no temperature dependence. However, a slightly decreasing reaction rate is often observed in the low temperature range [39,40] for this type of radical–radical association–elimination reaction and the rate used in HP Mech is decreased by 30% accordingly. In the following sections, the comparisons between the simulation results of HP Mech and USC Mech II and experimental results are provided, but the mechanism discussion will be only based on HP Mech as HP Mech provides more accurate predictions and is based on recent accurate reaction rate determinations. 4.2. Effects of CO2 dilution on acetylene flame speed at elevated pressures The effects of CO2 dilution on acetylene flame speed were investigated at equivalence ratios of 0.8 and 1.6 up to 20 atm. The mole fractions of each species in the initial C2H2/O2/N2/He/CO2 mixtures are shown in Table 1. The calculated adiabatic flame temperature was held constant for each equivalence ratio to remove thermal effects caused by the high heat capacity of CO2. Therefore, the observed effects of CO2 dilution are mainly caused by chemistry and third body effects. As shown in Fig. 2a and b, the flame speed monotonically decreases with increasing pressure, but there are almost no changes in the measured flame speeds with CO2 addition for both lean and rich conditions, unlike many other studies [30,41]. In the combustion system, CO2 decreases the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2(+M) because it has a higher collisional efficiency than typical bath gases such as N2 and He. The presence of CO2 in the system can also shift the equilibrium of CO + OH M CO2 + H to the reverse direction so that the heat release rate is suppressed and the radical pool content is altered. Both effects are expected to result in decreased flame speed [30,41]. However, the combustion of acetylene can bypass CO + OH M CO2 + H. Unlike most hydrocarbons, O atom addition dominates the acetylene destruction pathway rather than H-abstraction, forming HCCO + H and 3CH2 + CO. Reactions of HCCO and CH2 with O2 can form CO2 directly. At fuel rich conditions with / = 1.6, the association of HCCO with H producing CH2(S) + CO is dominant and 50% of the quenched CH2 is converted to CO2 directly. The
X. Shen et al. / Proceedings of the Combustion Institute 35 (2015) 721–728
Fig. 2. Pressure dependence of the premixed laminar flame speeds of acetylene with (20%) and without (0%) CO2 dilution at unburned gas temperature Tu = 298 K. Experiments (symbols) at an adiabatic flame temperature (a) Tf = 1900 K for / = 1.6 and (b) Tf = 1800 K for / = 0.8 are compared against the predictions based on HP Mech and USC Mech II.
conversion of CO to CO2 plays a minimal role in CO2 formation due to the relatively low concentration of OH at rich conditions. Additionally, the presence of CO2 can enhance HCO(+M) M H + CO(+M), increasing H production and the flame speed. Due to these chemical effects, modeling and experimental results both show that CO2 dilution has almost no effect on flame speed (Fig. 2a). Flame structure analysis provides more insight into the CO2 dilution effects. The changes in mole fraction of the major radicals at 20 atm due to CO2 dilution are plotted in Fig. 3a for / = 1.6. H and OH concentrations change significantly on the opposite directions due to the equilibrium shift of CO + OH M CO2 + H, and their effects are therefore cancelled. O atom concentration is not significantly affected. As discussed earlier, C2H2 oxidation is driven by reactions with O radicals, so the flame speed is not significantly affected by CO2 dilution. At lean conditions, the mole fractions of OH and O atom are much higher than for rich conditions. HCCO + O2 and CH2 + O2 reactions still
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Fig. 3. Effect of 0% CO2 and 20% CO2 addition on the high-pressure flame structure at 20 atm for / = 1.6 (a) and / = 0.8 (b) conditions calculated with HP Mech. —: 0% CO2; ––: 20% CO2.
control the first step of CO2 formation, but further oxidation of CO by OH is more important than for fuel rich conditions. The addition of CO2 will suppress the O atom generation by decreasing the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2(+M). H atom concentration decreases with CO2 addition due to the increased rate of H + O2(+M) M HO2(+M) and the equilibrium shift of CO + OH M CO2 + H. But OH concentration will not change as much as H and O, as the suppression effect through the branching reactions is counteracted by the chemical effect through CO + OH M CO2 + H. As shown in Fig. 3b, the peak concentrations of O, H and OH all decrease, however the change of OH mole fraction is smaller than that for other radicals. Nevertheless, O atom concentration has been decreased due to CO2 dilution and as a result the flame speeds should become slower. However, the CO2 dilution effect is not as strong as findings in DME combustion [41] because at least 50% of the CO2 is not produced from CO + OH M CO2 + H for acetylene flames. Note that although the simulations agree with the experimental measurements, the measured flame speeds shown in Fig. 2b are
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weakly affected by CO2 addition while the simulations predict a larger effect. This could result from the insufficient understanding of HO2 chemistry in the base H2/O2 sub mechanism [30], and further careful evaluations of the experimental procedure and the reaction set in HP Mech must be carried out in the future to resolve this problem. 4.3. Effects of water dilution on acetylene flame speed at elevated pressures Acetylene flame speeds with and without water dilution were also measured up to 10 atm at equivalence ratios of 0.8 and 1.6, but initial temperature of the mixture was increased to 400 K to ensure that water would not condense in the chamber. The experimental results are compared with model predictions in Fig. 4. In Fig. 4, much higher flame speeds are observed at / = 0.8 (Fig. 4b) than at / = 1.6 (Fig. 4a) with the same unburnt gas temperature and adiabatic flame temperature and this can be attributed to differences in density and diffusivity between He and N2. This set of experimental data have more uncertainties than the 300 K measurements and it can be attributed to a few reasons. The experimental uncertainties in the mixture preparation process are larger than for pure gas mixtures at room temperature, as water vapor can condense/evaporate or be adsorbed on/ emitted from surfaces during mixture preparation and the temperature nonuniformity of the mixture may be larger. There could also be some extrapolation uncertainties in the evaluation of unstretched flame propagation speed. The larger scattering of the experimental dataset seems to support this suspect. HP Mech generally agrees with the experimental determination very well with slightly over-prediction on the flame speeds at rich conditions. Besides the experimental uncertainties, the defects of HP Mech may also be responsible for the discrepancies. At fuel rich conditions / = 1.6, the higher concentrations of H and OH than O atom favor the molecular growth pathways starting with CH + C2H2 M C3H2 + H, CH2 + C2H2 M C3H3 + H, C2H + C2H2 M C4H2 + H, and C2H3 + C2H2 M C4H4 + H. The association reactions of C3 and C4 species are much less understood and have larger uncertainties in term of pressure and temperature dependences. These molecular growth pathways generally contribute negatively to the flame speed. With increasing pressure and the addition of water vapor, which has a very high collisional efficiency, the association reactions are expected to be more important in flame speed predictions. Many reaction rates used in HP Mech are written in PLOG format to capture the irregular pressure dependence of C3 and C4 association reactions, but this format assumes the collisional efficiency of every species to be equal. This prevents modeling the collisional
Fig. 4. Pressure dependence of the premixed laminar flame speeds of acetylene with (10%) and without (0%) water dilution at unburned gas temperature Tu = 400 K. Experiments (symbols) at an adiabatic flame temperature Tf = 1800 K (a) / = 1.6 and (b) / = 0.8 are compared against the predictions based on HP Mech and USC Mech II.
effects of water vapor and other species, therefore the model may predict a higher flame speed as observed here for / = 1.6. In addition to collisional effects, water dilution is expected to alter the flame speed through a few important reactions [42–44]. First, the presence of H2O will shift the reaction H2O + O M OH + OH in the forward direction, resulting in lower O atom mole fraction as can be observed in Fig. 5a and b. For most hydrocarbon combustion processes, two OH radicals are certainly more reactive than one O atom and the flame speed may be increased due to this effect. However, the subsequent reaction of OH with C2H2 is fairly slow and the possible products CH3 + CO, H + CH2CO, HCCOH + H, and C2H + H2O are much less reactive than the products of the very fast reaction C2H2 + O M HCCO + H. Therefore, the equilibrium shift may decrease the flame speed. Moreover, the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2 (+M) can be significantly decreased by the very high collisional efficiency of water (14 relative to N2 in the model), inhibiting the flame propagation. At fuel lean conditions where O and OH
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the collisional effect of H + O2(+M) M HO2 (+M). The drastic change of the radical pool composition, particularly the decreased O radical concentration, leads to a large drop of flame speeds. Finally, HP Mech was further examined against the acetylene oxidation measurement in shock tube and flow reactor and the details are provided in Supplemental file II. Excellent agreement has been found between the experiments and simulations, demonstrating the success of HP Mech. 5. Conclusion
Fig. 5. Effect of 10% water addition on the highpressure flame structure at 4 atm for (a) / = 1.6 and (b) / = 0.8 calculated with HP Mech. —: 0% H2O; ––, 10% H2O.
mole fractions are high, the inhibiting effects of water dilution are more pronounced than for rich conditions (Fig. 4) in the experiments, flame speed predictions, and flame structure modeling. The mole fraction changes of O, H, and OH due to water dilution in Fig. 5 reveal more details regarding flame chemistry. At / = 1.6, the slight decrease in O atom mole fraction and increase of OH mole fraction are expected and have been discussed. However, the change of H atom mole fraction is more complex. For many hydrocarbons, oxidation proceeds through CH2O ! HCO ! CO, thus water vapor should enhance the H atom formation due to the collisional effect in HCO(+M) M H + CO(+M). As mentioned above, a significant amount of C2H2 is converted to CO2 directly, therefore little H atom is produced from HCO. On the contrary, water vapor can enhance the association of H with other species forming larger molecules due to its large collisional efficiency. The decrease of O and H atom mole fractions leads to slower flame speeds as shown in Fig. 4. At / = 0.8, a dramatic change in the major radical concentrations is observed. The changes in O and OH concentrations are mainly due to H2O + O = OH + OH, and the decrease of H mole fraction can be attributed to
The analysis of laminar flame speeds of C2H2/ air at atmospheric pressure reveals the importance of a series of reactions and the need for a new predictive kinetic model. Comparing with USC Mech II, the proposed HP Mech incorporates recent fundamental understanding and new, more accurately determined reaction rates. HP Mech can accurately predict the flame speed over a wide range of pressures and equivalence ratios. The effects of CO2 and water dilution on the flame speeds of acetylene at elevated pressures have been carefully investigated. The effects of CO2 dilution are experimentally found to be small. A careful analysis of the reaction pathways suggests that the effect of CO2 on the equilibrium of CO2 + H M CO + OH is minimized by the direct formation of CO2 via HCCO + O2 and CH2 + O2. In addition, acetylene is mainly consumed by O radicals instead of OH or H attack, further reducing the chemical effect of CO2 addition through the equilibrium shift. In the combustion of acetylene, CO2 acts more as a third body in the reactions. Therefore, the laminar flame speed of acetylene is less sensitive to CO2 dilution than other hydrocarbons. The predictions based on HP Mech for the effects of CO2 dilution on the flame speed are satisfactory. On the contrary, water dilution effects are stronger than CO2 effects, especially at fuel lean conditions, since the addition of water will inhibit O atom but enhance OH production through H + O2(+M) M HO2(+M) and H2O + O M OH + OH and much slower flame speed can be expected with less O atom to initiate the main acetylene destruction pathway. Acknowledgements This work was supported by an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences with Grant No. DESC0001198 and DOE National Energy Technology Laboratory UTSR grant DE-FE0011822. Xiaobo Shen and Jinhua Sun would like to thank the financial support from National Natural
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Science Foundation of China (No. 51376174) and National Basic Research Program of China (973 program, No. 2012CB719702). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.proci.2014.05.106. References [1] A. Maiboom, X. Tauzia, J.F. Hetet, Energy 33 (2008) 22–34. [2] J.A. Miller, M.J. Pilling, J. Troe Proc. Combust. Inst. 30 (2005) 43–88. [3] E. Bastin, J.-L. Delfau, M. Reuillon, C. Vovelle, J. Warnatz, Symp. (Int.) Combust. 22 (1989) 313–322. [4] H. Wang, M. Frenklach, Combust. Flame 110 (1997) 173–221. [5] Y. Li, L. Zhang, Z. Tian, T. Yuan, K. Zhang, B. Yang, F. Qi, Proc. Combust. Inst. 32 (2009) 1293– 1300. [6] G. Jomaas, X.L. Zheng, D.L. Zhu, C.K. Law, Proc. Combust. Inst. 30 (2005) 193–200. [7] F.N. Egolfopoulos, D.L. Zhu, C.K. Law, Symp. (Int.) Combust. 23 (1991) 471–478. [8] O. Park, P.S. Veloo, F.N. Egolfopoulos, Proc. Combust. Inst. 34 (2013) 711–718. [9] R.P. Lindstedt, G. Skevis, Combust. Sci. Technol. 125 (1997) 73–137. [10] H. Wang, X. You, A.V. Joshi, S.G. Davis, A. Laskin, F. Egolfopoulos, C.K. Law, 2007,. [11] W.K. Metcalfe, S.M. Burke, S.S. Ahmed, H.J. Curran, Int. J. Chem. Kinet. 45 (2013) 638–675. [12] J.V. Michael, A.F. Wagner, J. Phys. Chem. 94 (1990) 2453–2464. [13] T.L. Nguyen, L. Vereecken, J. Peeters, J. Phys. Chem. A 110 (2006) 6696–6706. [14] N. Balucani, F. Leonori, P. Casavecchia, Energy 43 (2012) 47–57. [15] S.J. Klippenstein, J.A. Miller, L.B. Harding, Proc. Combust. Inst. 29 (2002) 1209–1217. [16] D.L. Osborn, J. Phys. Chem. A 107 (2003) 3728– 3732. [17] K.L. Gannon, M.A. Blitz, C.H. Liang, M.J. Pilling, P.W. Seakins, D.R. Glowacki, J. Phys. Chem. A 114 (2010) 9413–9424. [18] D. Polino, S.J. Klippenstein, L.B. Harding, Y. Georgievski, J. Phys. Chem. A 117 (2013) 12677– 12692. [19] X. Qin, Y. Ju, Proc. Combust. Inst. 30 (2005) 233– 240.
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Proceedings of the
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Experimental and kinetic studies of acetylene flames at elevated pressures Xiaobo Shen a,b, Xueliang Yang a,⇑, Jeffrey Santner a, Jinhua Sun b, Yiguang Ju a a
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, PR China
b
Available online 25 June 2014
Abstract The kinetic effects of CO2 and H2O dilution on the laminar flame speed of acetylene at elevated pressures have been investigated experimentally using outwardly propagating spherical flames in a nearly constant pressure chamber from 1 to 20 atm. The flame speeds of C2H2/air mixtures at atmospheric pressure agree with recent measurements reasonably well. Detailed analysis on the combustion chemistry of acetylene reveals that C2H2 + O, HCCO + O2, HCO + O2, CH3 + HO2, H + C2H3, CO + OH, CH2 (S) + C2H2, and HCO decomposition are among the most important reactions, which leads to a new kinetic model (HP Mech) that incorporates the recent understanding of elementary reactions. The effects of CO2 dilution on acetylene flame speeds are found to be small for both fuel rich and lean conditions due to the direct CO2 formation pathway (HCCO + O2) in acetylene oxidation. Water dilution effects are more pronounced, especially at lean conditions, because the radical pool composition is altered by shifting the equilibrium of H2O + O = OH + OH. Comparing to USC Mech II, HP Mech has much better performance compared to the current experimental measurements as well as the shock tube and flow reactor data. Ó 2014 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Acetylene; Laminar flame speed; Elevated pressure; HP Mech; CO2 and H2O dilution effects
1. Introduction Modern engines utilize exhaust gas recirculation (EGR) to reduce NOx emissions and increase thermal efficiency, but these systems often cause increased soot emission [1]. Acetylene, recognized ⇑ Corresponding author. Address: D225 E-Quad, Princeton University, Princeton, NJ 08544, United States. Tel.: +1 609 258 1411. E-mail address: [email protected] (X. Yang).
as an important precursor for soot formation [2] in hydrocarbon fuel combustion, has drawn continuous interest for decades [3–9]. Acetylene flame chemistry is not only important in understanding soot formation mechanisms, but is also an indispensable step in developing predictive hierarchical kinetic models [10,11]. Despite its importance in both fundamental science and application, the laminar flame speed of acetylene has only been measured at pressures up to 2 atm due to safety considerations and experimental limitations, and the limited data are in poor agreement with each. More importantly, most models
http://dx.doi.org/10.1016/j.proci.2014.05.106 1540-7489/Ó 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
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have not incorporated recent progress in elementary reaction rate determinations associated with acetylene combustion [12–18], and many reaction rates used in the models are based on empirical estimations or are optimized to fit particular global combustion properties. Therefore, there are large discrepancies between model predictions and experimental measurements of the acetylene flame speed and it is unlikely that these models can accurately predict combustion properties at engine conditions. In EGR engines, large quantities of CO2 and H2O are present in the unburned gas and engine performance is strongly affected by the thermal, dilution, transport, and chemical effects introduced by H2O and CO2. The thermal, transport, and dilution effects are easy to understand; however, the chemical effects are harder to quantify as they are fuel-specific. These effects are particularly important at high pressure, where practical engines operate, and three-body termination reactions become more important. This work will investigate the chemical effects of CO2 and H2O dilution on the laminar flame speed of acetylene at elevated pressures. The laminar flame speed of C2H2/air with equivalence ratios from 0.6 to 2.0 at atmospheric pressure was measured to compare with the available experimental data and widely used USC Mech II. Additionally, the flame speed was measured at pressures up to 20 atm with equivalence ratios of 0.8 and 1.6 at fixed adiabatic flame temperatures of 1800 K and 1900 K. The effects of CO2 and water dilution were examined at both equivalence ratios. Meanwhile, a kinetic model including the most recent advances in elementary reaction kinetics was developed and used to examine the flame structure and improve predictions. 2. Experimental methods Experiments were conducted in high pressure constant volume cylindrical and heated spherical bombs. The chambers are filled with a combustible mixture and centrally spark ignited. Flame propagation to a 3 cm radius was recorded using high speed schlieren photography (up to 15,000 frames per second). Both chambers utilize custom one way valves to release the burned gas pressure after the flame has passed the edge of the viewing window. Complete details of the experimental system and procedure have been described elsewhere [19–22]. Mixtures were prepared using the partial pressure method from acetylene (>99%, Atomic Absorption Grade), oxygen (99.5%), helium (99.995%), nitrogen (99.99%), carbon dioxide (99.9%), water vapor, and synthetic air. All experiments with gaseous compounds were conducted at room temperature (measured as 298 ± 3 K), and experiments with water dilution
were performed at 400 K to prevent condensation. Deionized liquid water was vaporized at 400 K in a separate container before filling the heated spherical chamber through heated tubing. The C2H2/O2/N2/He mixtures were prepared at initial total pressures from 1 and 20 atm with constant calculated adiabatic flame temperature (1800 K or 1900 K) for equivalence ratios of 0.8 and 1.6. The O2/N2 ratio was always kept the same as that in air. Then, mixtures with 20% (mole fraction in mixtures) CO2 or 10% water vapor were also used for both specified equivalence ratios at the same adiabatic flame temperatures. For all the experiments, the helium concentration was adjusted to control the flame temperature and the Lewis number in order to prevent cellular instability, buoyancy effects, ignition difficulty, and extrapolation error. The precision of the measurements was frequently checked by repeating the same experiments a few times. Experimental details can be found in Table 1. 2.1. Data evaluation For ease of processing, an automated flame edge detection and circle fitting program was built to locate the flame front and obtain the smoothed flame radius Rf. The instantaneous flame front velocity Su and stretch rate j were calculated from the flame radius and flame front velocity using, Su ¼
1
1 dRf r dt þ r1 r
ð1Þ
and j¼
2 dRf Rf dt
ð2Þ
where the flow correction term, -, accounts for the actual burned gas velocity induced by asymmetrical confinement at large flame radius (Rf/Rchamber > 30%) [20]. Experiments with water vapor were performed with a large chamber such that Rf/Rchamber < 30%, so - is assumed as zero for these measurements. The ratio of unburned to burned gas densities, r, was calculated using PREMIX [23]. The unstretched flame propagation speed, S 0u , is then determined through the nonlinear extrapolation method (for Le < 1) and linear extrapolation method (for Le > 1), as suggested by Wu et al. [24]. No data analyses were performed for flames that were observed to be wrinkled due to cellular or spiraling instabilities, affected by buoyancy, or influenced by transient effects. All flame speed data presented in this paper are plotted with error bars calculated from the RMS sum of the uncertainties corresponding to the measurements of flame radius, flow correction, and uncertainties resulted from initial conditions (i.e. temperature, pressure, mixture concentration) estimated using USC Mech II [10].
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Table 1 The equivalence ratio, mole fraction of each compound in the mixture, and the adiabatic flame temperature used in the high pressure experiments. (/: Stoichiometry, Tu: ambient temperature; Tf: adiabatic flame temperature). /
C2H2
O2
N2
He
CO2
H2O
Tu
Tf
0.8 0.8 0.8 0.8 1.6 1.6 1.6 1.6
0.034 0.036 0.028 0.026 0.063 0.069 0.058 0.055
0.106 0.113 0.089 0.081 0.098 0.108 0.091 0.086
0.400 0.000 0.000 0.000 0.369 0.000 0.376 0.429
0.460 0.650 0.783 0.893 0.470 0.624 0.376 0.429
0.000 0.200 0.000 0.000 0.000 0.200 0.000 0.000
0.000 0.000 0.100 0.000 0.000 0.000 0.100 0.000
298 298 400 400 298 298 400 400
1800 1800 1800 1800 1900 1900 1800 1800
3. Combustion chemistry of C2H2 and kinetic model development Unlike saturated hydrocarbon fuels, at elevated temperature acetylene is mostly consumed by reactions with O radicals, not the usual H-abstraction reactions: C2 H2 þ O $ HCCO þ H
ðR1Þ
3
ðR2Þ
C2 H2 þ O$ CH2 þ CO
However, the branching ratio (R1) over (R2) used in the different models varies from 0.45:0.55 to 0.8:0.2, causing large discrepancies in flame speed predictions [8]. Recent experimental and theoretical studies have confirmed that the branching ratio is 0.8:0.2 independent of temperature [12–14,25]. The subsequent oxidation reactions CH2 + O2 and HCCO + O2 have many product channels and the branching ratios of these product channels are very difficult to determine. In combustion conditions the ketenyl radical HCCO is partially consumed by reactions: HCCO þ O2 $ H þ CO þ CO2 HCCO þ O2 $ OH þ CO þ CO
ðR3Þ ðR4Þ
(R3) is confirmed to be the dominant product channel [15,16] by recent experimental and theoretical studies but (R4) is still emphasized in many models [10,26], which may yield uncertainties and discrepancies in model predictions. The CH2 + O2 system is even more complicated since it is associated with multiple potential energy surfaces leading to many possible product channels. The reaction rates and branching ratios reported in the literature often disagree with each other significantly, however recent flow reactor and shock tube studies have found that the branching ratio of CO2 production is 50% [27,28]. Singlet methylene CH2(S) produced by HCCO + H can quickly react with C2H2 forming propargyl radical (C3H3), one of the main precursors for the formation of polycyclic aromatic hydrocarbons [29]. Meanwhile, singlet methylene can also be quenched to its ground state by different colliders, suppressing propargyl formation. Detailed understanding of the competition between CH2(S)
quenching and its reaction with C2H2 is crucial for soot formation prediction. Therefore, these reaction rates must be carefully determined to create a predictive kinetic model. The present work is part of an ongoing effort to develop a reaction mechanism based on recent advances in elementary reaction rate determinations. The model, High Pressure Mech (HP Mech) incorporates the H2/O2 sub mechanism of Burke et al. [30] and recent collaborative work on methanol combustion kinetics [31]. For elementary reaction rates with multiple literature determinations, each source was carefully examined and evaluated based on our understanding of experimental and theoretical uncertainties. For the elementary reactions lacking experimental or theoretical determinations, rates were estimated from ab initio calculations with uncertainties below a factor of 3. Extra care was taken for chemically activated systems in which the pressure dependence of the reaction rates may be complicated to make sure that reaction rates and branching ratios are in the reasonable range. Unlike other models, no optimization or scaling of the reaction rates was attempted to fit any experimental data. The details of HP Mech used in this work are provided in Supplemental file. The reaction set provided here should only be used for high temperature combustion of acetylene as many reactions that are important for combustion of other fuels but irrelevant to acetylene chemistry are not included. Numerical simulations were obtained using the freely propagating flame module in CHEMKIN Pro software [32]. Calculations included multicomponent molecular diffusion and thermal diffusion. For the flame speed calculations, gradient and curvature tolerances were set to 0.04, producing converged, grid-independent solutions. 4. Results and discussion 4.1. C2H2/air flames at atmospheric pressure Present measurements of the laminar flame speed of acetylene in air at 1 atm and those from the literature [6,7] are plotted in Fig. 1, as well
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as simulations based on USC Mech II and HP Mech. The current results agree with Jomaas et al. [6] within 10% over the whole equivalence ratio range. The results in [6] do not account for confinement induced flow, which may account for the difference observed for lean conditions. USC Mech II is able to predict the laminar flame speeds at fuel lean conditions reasonably well, but underestimates the flame speeds at some fuel rich conditions by 10–15 cm/s. A careful examination of USC Mech II shows that many important reactions used in the model have very large uncertainties. Aside from those highlighted in the H2/O2 sub mechanism [30], the rate of CO + OH M CO2 + H used in USC Mech II is 7% higher than the recommended value [33] and may result in overprediction of the flame speeds; however, the lower predicted flame speed indicates the effect of this reaction may be cancelled by other reactions. The competing reactions HCO(+M) M H + CO(+M) and HCO + O2 M HO2 + CO are also important for flame speed simulation and both reaction rates used in USC Mech II are roughly 50% higher than the recent suggested values [34,35]. Additionally, the CH3 + HO2 reaction rate in USC Mech II is a factor of 10 higher than the recent experimental and theoretical determinations [36,37]. As mentioned above, (R4) is the only HCCO + O2 channel used in USC Mech II. USC Mech II also underestimates the CH2(S) + C2H2 reaction rate by a factor of 3 compared to the recommendation of Ref. [18]. On the contrary, the simulations based on HP Mech, which incorporates new, more accurately determined elementary reaction rates, agree with the experiments reasonably well. The major causes of improved model performance are the updated HCCO + O2 reaction rates. Other updated
160
Laminar Flame Speed / (cm/s)
C2H2/Air 298K
140
1 atm
120
100 This work
80
Egolfopoulos et al. (1991) Jomaas et al. (2005) USCMech II
60
HP Mech
40 0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
Equivalence Ratio
Fig. 1. Experimentally measured premixed laminar flame speeds of C2H2/air mixtures at 1 atm and 298 K compared with simulations based on USC Mech II [10] and HP Mech. Experiments: h Egolfopoulos et al. [7]; e Jomaas et al. [6]; d present measurements. Simulations: ––, USC Mech II; — HP Mech.
reactions that have a large effect on flame speed simulation include HCCO + H, CH2(S) + C2H2 and the quenching reaction of CH2(S). CH2 (S) + C2H2 and the quenching reaction of CH2(S) have been well studied [17,18] and the uncertainties of these reaction rates are no more than 30%. The elementary reaction rate of HCCO + H = CH2(S) + CO has only been measured once at room temperature [38] and most of the current models use that rate assuming no temperature dependence. However, a slightly decreasing reaction rate is often observed in the low temperature range [39,40] for this type of radical–radical association–elimination reaction and the rate used in HP Mech is decreased by 30% accordingly. In the following sections, the comparisons between the simulation results of HP Mech and USC Mech II and experimental results are provided, but the mechanism discussion will be only based on HP Mech as HP Mech provides more accurate predictions and is based on recent accurate reaction rate determinations. 4.2. Effects of CO2 dilution on acetylene flame speed at elevated pressures The effects of CO2 dilution on acetylene flame speed were investigated at equivalence ratios of 0.8 and 1.6 up to 20 atm. The mole fractions of each species in the initial C2H2/O2/N2/He/CO2 mixtures are shown in Table 1. The calculated adiabatic flame temperature was held constant for each equivalence ratio to remove thermal effects caused by the high heat capacity of CO2. Therefore, the observed effects of CO2 dilution are mainly caused by chemistry and third body effects. As shown in Fig. 2a and b, the flame speed monotonically decreases with increasing pressure, but there are almost no changes in the measured flame speeds with CO2 addition for both lean and rich conditions, unlike many other studies [30,41]. In the combustion system, CO2 decreases the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2(+M) because it has a higher collisional efficiency than typical bath gases such as N2 and He. The presence of CO2 in the system can also shift the equilibrium of CO + OH M CO2 + H to the reverse direction so that the heat release rate is suppressed and the radical pool content is altered. Both effects are expected to result in decreased flame speed [30,41]. However, the combustion of acetylene can bypass CO + OH M CO2 + H. Unlike most hydrocarbons, O atom addition dominates the acetylene destruction pathway rather than H-abstraction, forming HCCO + H and 3CH2 + CO. Reactions of HCCO and CH2 with O2 can form CO2 directly. At fuel rich conditions with / = 1.6, the association of HCCO with H producing CH2(S) + CO is dominant and 50% of the quenched CH2 is converted to CO2 directly. The
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Fig. 2. Pressure dependence of the premixed laminar flame speeds of acetylene with (20%) and without (0%) CO2 dilution at unburned gas temperature Tu = 298 K. Experiments (symbols) at an adiabatic flame temperature (a) Tf = 1900 K for / = 1.6 and (b) Tf = 1800 K for / = 0.8 are compared against the predictions based on HP Mech and USC Mech II.
conversion of CO to CO2 plays a minimal role in CO2 formation due to the relatively low concentration of OH at rich conditions. Additionally, the presence of CO2 can enhance HCO(+M) M H + CO(+M), increasing H production and the flame speed. Due to these chemical effects, modeling and experimental results both show that CO2 dilution has almost no effect on flame speed (Fig. 2a). Flame structure analysis provides more insight into the CO2 dilution effects. The changes in mole fraction of the major radicals at 20 atm due to CO2 dilution are plotted in Fig. 3a for / = 1.6. H and OH concentrations change significantly on the opposite directions due to the equilibrium shift of CO + OH M CO2 + H, and their effects are therefore cancelled. O atom concentration is not significantly affected. As discussed earlier, C2H2 oxidation is driven by reactions with O radicals, so the flame speed is not significantly affected by CO2 dilution. At lean conditions, the mole fractions of OH and O atom are much higher than for rich conditions. HCCO + O2 and CH2 + O2 reactions still
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Fig. 3. Effect of 0% CO2 and 20% CO2 addition on the high-pressure flame structure at 20 atm for / = 1.6 (a) and / = 0.8 (b) conditions calculated with HP Mech. —: 0% CO2; ––: 20% CO2.
control the first step of CO2 formation, but further oxidation of CO by OH is more important than for fuel rich conditions. The addition of CO2 will suppress the O atom generation by decreasing the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2(+M). H atom concentration decreases with CO2 addition due to the increased rate of H + O2(+M) M HO2(+M) and the equilibrium shift of CO + OH M CO2 + H. But OH concentration will not change as much as H and O, as the suppression effect through the branching reactions is counteracted by the chemical effect through CO + OH M CO2 + H. As shown in Fig. 3b, the peak concentrations of O, H and OH all decrease, however the change of OH mole fraction is smaller than that for other radicals. Nevertheless, O atom concentration has been decreased due to CO2 dilution and as a result the flame speeds should become slower. However, the CO2 dilution effect is not as strong as findings in DME combustion [41] because at least 50% of the CO2 is not produced from CO + OH M CO2 + H for acetylene flames. Note that although the simulations agree with the experimental measurements, the measured flame speeds shown in Fig. 2b are
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weakly affected by CO2 addition while the simulations predict a larger effect. This could result from the insufficient understanding of HO2 chemistry in the base H2/O2 sub mechanism [30], and further careful evaluations of the experimental procedure and the reaction set in HP Mech must be carried out in the future to resolve this problem. 4.3. Effects of water dilution on acetylene flame speed at elevated pressures Acetylene flame speeds with and without water dilution were also measured up to 10 atm at equivalence ratios of 0.8 and 1.6, but initial temperature of the mixture was increased to 400 K to ensure that water would not condense in the chamber. The experimental results are compared with model predictions in Fig. 4. In Fig. 4, much higher flame speeds are observed at / = 0.8 (Fig. 4b) than at / = 1.6 (Fig. 4a) with the same unburnt gas temperature and adiabatic flame temperature and this can be attributed to differences in density and diffusivity between He and N2. This set of experimental data have more uncertainties than the 300 K measurements and it can be attributed to a few reasons. The experimental uncertainties in the mixture preparation process are larger than for pure gas mixtures at room temperature, as water vapor can condense/evaporate or be adsorbed on/ emitted from surfaces during mixture preparation and the temperature nonuniformity of the mixture may be larger. There could also be some extrapolation uncertainties in the evaluation of unstretched flame propagation speed. The larger scattering of the experimental dataset seems to support this suspect. HP Mech generally agrees with the experimental determination very well with slightly over-prediction on the flame speeds at rich conditions. Besides the experimental uncertainties, the defects of HP Mech may also be responsible for the discrepancies. At fuel rich conditions / = 1.6, the higher concentrations of H and OH than O atom favor the molecular growth pathways starting with CH + C2H2 M C3H2 + H, CH2 + C2H2 M C3H3 + H, C2H + C2H2 M C4H2 + H, and C2H3 + C2H2 M C4H4 + H. The association reactions of C3 and C4 species are much less understood and have larger uncertainties in term of pressure and temperature dependences. These molecular growth pathways generally contribute negatively to the flame speed. With increasing pressure and the addition of water vapor, which has a very high collisional efficiency, the association reactions are expected to be more important in flame speed predictions. Many reaction rates used in HP Mech are written in PLOG format to capture the irregular pressure dependence of C3 and C4 association reactions, but this format assumes the collisional efficiency of every species to be equal. This prevents modeling the collisional
Fig. 4. Pressure dependence of the premixed laminar flame speeds of acetylene with (10%) and without (0%) water dilution at unburned gas temperature Tu = 400 K. Experiments (symbols) at an adiabatic flame temperature Tf = 1800 K (a) / = 1.6 and (b) / = 0.8 are compared against the predictions based on HP Mech and USC Mech II.
effects of water vapor and other species, therefore the model may predict a higher flame speed as observed here for / = 1.6. In addition to collisional effects, water dilution is expected to alter the flame speed through a few important reactions [42–44]. First, the presence of H2O will shift the reaction H2O + O M OH + OH in the forward direction, resulting in lower O atom mole fraction as can be observed in Fig. 5a and b. For most hydrocarbon combustion processes, two OH radicals are certainly more reactive than one O atom and the flame speed may be increased due to this effect. However, the subsequent reaction of OH with C2H2 is fairly slow and the possible products CH3 + CO, H + CH2CO, HCCOH + H, and C2H + H2O are much less reactive than the products of the very fast reaction C2H2 + O M HCCO + H. Therefore, the equilibrium shift may decrease the flame speed. Moreover, the branching ratio of H + O2 M O + OH over H + O2(+M) M HO2 (+M) can be significantly decreased by the very high collisional efficiency of water (14 relative to N2 in the model), inhibiting the flame propagation. At fuel lean conditions where O and OH
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the collisional effect of H + O2(+M) M HO2 (+M). The drastic change of the radical pool composition, particularly the decreased O radical concentration, leads to a large drop of flame speeds. Finally, HP Mech was further examined against the acetylene oxidation measurement in shock tube and flow reactor and the details are provided in Supplemental file II. Excellent agreement has been found between the experiments and simulations, demonstrating the success of HP Mech. 5. Conclusion
Fig. 5. Effect of 10% water addition on the highpressure flame structure at 4 atm for (a) / = 1.6 and (b) / = 0.8 calculated with HP Mech. —: 0% H2O; ––, 10% H2O.
mole fractions are high, the inhibiting effects of water dilution are more pronounced than for rich conditions (Fig. 4) in the experiments, flame speed predictions, and flame structure modeling. The mole fraction changes of O, H, and OH due to water dilution in Fig. 5 reveal more details regarding flame chemistry. At / = 1.6, the slight decrease in O atom mole fraction and increase of OH mole fraction are expected and have been discussed. However, the change of H atom mole fraction is more complex. For many hydrocarbons, oxidation proceeds through CH2O ! HCO ! CO, thus water vapor should enhance the H atom formation due to the collisional effect in HCO(+M) M H + CO(+M). As mentioned above, a significant amount of C2H2 is converted to CO2 directly, therefore little H atom is produced from HCO. On the contrary, water vapor can enhance the association of H with other species forming larger molecules due to its large collisional efficiency. The decrease of O and H atom mole fractions leads to slower flame speeds as shown in Fig. 4. At / = 0.8, a dramatic change in the major radical concentrations is observed. The changes in O and OH concentrations are mainly due to H2O + O = OH + OH, and the decrease of H mole fraction can be attributed to
The analysis of laminar flame speeds of C2H2/ air at atmospheric pressure reveals the importance of a series of reactions and the need for a new predictive kinetic model. Comparing with USC Mech II, the proposed HP Mech incorporates recent fundamental understanding and new, more accurately determined reaction rates. HP Mech can accurately predict the flame speed over a wide range of pressures and equivalence ratios. The effects of CO2 and water dilution on the flame speeds of acetylene at elevated pressures have been carefully investigated. The effects of CO2 dilution are experimentally found to be small. A careful analysis of the reaction pathways suggests that the effect of CO2 on the equilibrium of CO2 + H M CO + OH is minimized by the direct formation of CO2 via HCCO + O2 and CH2 + O2. In addition, acetylene is mainly consumed by O radicals instead of OH or H attack, further reducing the chemical effect of CO2 addition through the equilibrium shift. In the combustion of acetylene, CO2 acts more as a third body in the reactions. Therefore, the laminar flame speed of acetylene is less sensitive to CO2 dilution than other hydrocarbons. The predictions based on HP Mech for the effects of CO2 dilution on the flame speed are satisfactory. On the contrary, water dilution effects are stronger than CO2 effects, especially at fuel lean conditions, since the addition of water will inhibit O atom but enhance OH production through H + O2(+M) M HO2(+M) and H2O + O M OH + OH and much slower flame speed can be expected with less O atom to initiate the main acetylene destruction pathway. Acknowledgements This work was supported by an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences with Grant No. DESC0001198 and DOE National Energy Technology Laboratory UTSR grant DE-FE0011822. Xiaobo Shen and Jinhua Sun would like to thank the financial support from National Natural
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Science Foundation of China (No. 51376174) and National Basic Research Program of China (973 program, No. 2012CB719702). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.proci.2014.05.106. References [1] A. Maiboom, X. Tauzia, J.F. Hetet, Energy 33 (2008) 22–34. [2] J.A. Miller, M.J. Pilling, J. Troe Proc. Combust. Inst. 30 (2005) 43–88. [3] E. Bastin, J.-L. Delfau, M. Reuillon, C. Vovelle, J. Warnatz, Symp. (Int.) Combust. 22 (1989) 313–322. [4] H. Wang, M. Frenklach, Combust. Flame 110 (1997) 173–221. [5] Y. Li, L. Zhang, Z. Tian, T. Yuan, K. Zhang, B. Yang, F. Qi, Proc. Combust. Inst. 32 (2009) 1293– 1300. [6] G. Jomaas, X.L. Zheng, D.L. Zhu, C.K. Law, Proc. Combust. Inst. 30 (2005) 193–200. [7] F.N. Egolfopoulos, D.L. Zhu, C.K. Law, Symp. (Int.) Combust. 23 (1991) 471–478. [8] O. Park, P.S. Veloo, F.N. Egolfopoulos, Proc. Combust. Inst. 34 (2013) 711–718. [9] R.P. Lindstedt, G. Skevis, Combust. Sci. Technol. 125 (1997) 73–137. [10] H. Wang, X. You, A.V. Joshi, S.G. Davis, A. Laskin, F. Egolfopoulos, C.K. Law, 2007,
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