ARTICLE pubs.acs.org/EF
Additive Effects on the Burning Velocity of EthyleneAir Mixtures Codina Movileanu,*,† Domnina Razus,† and Dumitru Oancea‡ † ‡
“I. G. Murgulescu” Institute of Physical Chemistry, Splaiul Independentei 202, Post Office Box 12-194, 060021, Bucharest, Romania Faculty of Chemistry, University of Bucharest, Bulevardul Elisabeta 4-12, 030018 Bucharest, Romania ABSTRACT: Using a simple correlation based on the cubic law of early pressure rise during a closed vessel explosion, the laminar burning velocities of the stoichiometric ethyleneair mixture diluted with several additives (Ar, N2, and CO2) (concentrations between 4 and 40 vol %) were calculated from pressuretime records, at total initial pressures between 0.2 and 1.2 bar and ambient initial temperature. These are important input properties for numerical modeling of explosion propagation in closed or vented enclosures and for safety recommendations concerning the necessary inert gas addition. The decrease of burning velocities determined by the increase of the total initial pressure and inert gas addition is examined. A computational study of free laminar premixed flames of ethyleneair in the presence of the same additives based on the use of 53 species and an extended reaction mechanism with 592 elementary reactions delivered the temperature and species profiles in the flame front, together with the flame width and burning velocity. Both the computed and the experimental burning velocities versus the additive concentration follow the same trend, explained by the influence of additives on the overall reaction rate between fuel and oxidant in the reaction zone and heat- and mass-transfer rates between the burned and unburned gases. The differences observed between computed and experimental burning velocities are small and can be assigned to both unavoidable scattering of available measurements and the mechanism used for numerical simulations.
1. INTRODUCTION The design and efficient control of practical combustion systems is increasingly based on the knowledge of the chemical mechanism of fuel combustion in wide ranges of initial conditions,13 because the chemical modeling of combustion provides the basic parameters, such as the normal burning velocity, the maximum temperature reached in a flame, or the ignition delay and ignition temperature of self-ignition. The interest toward the development and improvement of detailed mechanisms of hydrocarbon oxidation at high temperatures prompted numerous experimental studies on flames in various conditions, either deflagrations propagating in laminar conditions or selfignitions in shock tubes. The mechanism validation could be made by a comparison between the computed and measured values of a few global parameters, such as the normal burning velocity and/or the ignition delay and ignition temperature, for representative fuels under extensive variations of the thermodynamic parameters.415 Ethylene, as a primary fuel and a key intermediate in the oxidation of higher hydrocarbons, was among the most studied pure fuels. In addition, the need for reducing soot or other pollutant concentrations in combustion systems required the study of ethylene chemistry, especially in ethylene-rich mixtures.1618 Normal burning velocities for ethyleneair and ethyleneairdiluents have been reported by many researchers for various fuel/ oxygen and oxygen/diluent ratios.1625 Such data are essential for the design and construction of venting devices, modeling of turbulent combustion, optimization of internal combustion engines, and flame inhibition and/or suppression. Measurements have been made using flames stabilized over a burner,23,2527 flames propagating in tubes or in spherical vessels with central ignition,19,28,29 or counter-flow twin flames.17,2022,24 Measured or calculated normal burning velocities were also used for the r 2011 American Chemical Society
determination of overall kinetic parameters (activation energy and reaction order) of the oxidation process in flames.30,31 The present paper aims to provide additional data on the normal burning velocity of ethyleneair mixtures in the presence of diluents, at ambient initial temperature and various initial pressures and dilution degrees. The burning velocities are obtained from both experimental measurements of pressure variation, in the early stage of closed vessel explosions, and a detailed modeling of a free laminar premixed flame. The experiments were made using a stoichiometric ethylene/air mixture diluted with various amounts of Ar, N2, or CO2 ([additive] = 040 vol %) at various initial pressures (p0 = 0.21.2 bar) in a spherical and two cylindrical vessels with central ignition. The results are used to examine the behavior of studied additives and to assign the observed burning velocity variation to physical and/or chemical factors. They have a great practical interest for mitigating the effects associated with gaseous explosions and for setting safer conditions of running reactors or plants where flammable mixtures are formed.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. The experimental facility consists of three combustion chambers (a spherical and two cylindrical vessels), the gas handling system, and a data acquisition system. The spherical chamber consists of two hemispheric heads made from stainless steel that are bolted together to make a 10 cm inner diameter sphere. The chamber is designed to withstand pressures up to 40 bar. It is fitted with ports for filling and evacuating the chamber, spark electrodes, and ionization probes. The characteristics of the two cylindrical vessels: are Received: February 2, 2011 Revised: April 27, 2011 Published: April 27, 2011 2444
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Energy & Fuels vessel C1 with diameter Φ = 10 cm and height h = 15 cm and vessel C2 with Φ = h = 6 cm. Vessels C1 and C2 had a transparent window made from synthetic glass (3 cm in diameter) mounted in the center of the upper lid, which enabled the visual observation of flame appearance and propagation. The gas handling system consists of a vacuum pump for evacuating the chamber and a valve manifold connected to gas cylinders for the preparation of the fuel/oxidizer/additive mixtures. The combustible mixtures in the closed vessel experiments were centrally ignited. Ignition was made with inductivecapacitive sparks produced between stainless-steel electrodes with the spark gap of a constant width (3 mm). An electronic ignition system provides a spark with the desired energy, between 2 and 5 mJ. A Kistler 601A piezoelectric pressure transducer coupled with a Kistler 5001SN charge amplifier is used to obtain dynamic pressure versus time records from which the burning velocity was determined. Two ionization probes mounted 3 and 5 mm away, respectively, from the sidewall are used to measure the arrival time of the flame at the wall and check for spherical symmetry and buoyant rise. The data acquisition system consists of a TestLab Tektronix 2505, by means of an acquisition card type AA1, usually at 104 signals per second. Other details concerning the experimental setup were previously given.3234 The instrumentation ensured a good accuracy of measurements. For the stoichiometric ethyleneair mixture, sets of five experiments were conducted in identical conditions, in each explosion vessel. The average standard error observed in explosion pressures was 2%; the average standard error in the corresponding burning velocities was under 3.5%. 2.2. Procedure. The experimental procedure consists of evacuating the explosion vessel to 0.1 mbar. The fuelair or fuelairadditive mixture was then introduced at the desired pressure, allowed to become quiescent, and then ignited, and the signals of the acquisition system were captured, stored, and evaluated. The fuelair gaseous mixtures were obtained by the partial pressure method and used 24 h after mixing the components, at a total pressure of 4 bar. The initial pressures of fuelair mixtures were measured by a strain gauge manometer (Edwards-type EPS-10HM). 2.3. Examined Systems. The combustible used in all of this work was ethylene, and the stoichiometric (6.54 vol %) ethyleneair mixture was diluted with Ar, N2, or CO2 (concentrations up to 40 vol %). The reagents, ethylene (99.97% purity), nitrogen (99.00%), argon (99.99%), and carbon dioxide (99.7%), were purchased from SIAD, Italy, and used without further purification.
3. COMPUTING PROGRAMS The calculation of adiabatic flame temperatures and adiabatic explosion pressures of ethyleneair mixtures in the presence of additives was made with the program ECHIMAD, previously described.35 The program was developed for solving chemical equilibrium problems in various conditions, by minimizing the free enthalpy (in the case of isobaric combustion) or the free energy (in the case of isochoric combustion) of the system under consideration. A total of 15 compounds (among them, one solid compound) were considered as products: Cgraphite, CO2, CO, H2O, O2, N2, CH4, C2H2, C2H4, C3H6, H2, NO, H, OH, and O. Their heat capacities (expressed as function of the temperature with the form Cp = a þ bT þ cT2 þ dT2), the standard enthalpies of formation at 298 K, and the standard entropies at 298 K were taken from refs 36 and 37. Laminar burning velocities of gaseous mixtures were computed by means of the program INSFLA, developed by Warnatz, Maas, and co-workers,2,3840 for kinetic modeling of fuelair flames in various conditions. In the present case, we used the mechanism CH4C4 based on 53 chemical species, which participate in 592 elementary reactions.
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The runs were performed for premixed laminar-free flames, propagating in adiabatic conditions at ambient initial temperature and pressure. As input data, we used thermochemical properties from JANAF, as given in ref 41. The transport coefficients were calculated according to Warnatz et al.2
4. RESULTS AND DISCUSSION 4.1. Experimental Burning Velocities. The test mixture for experiments in all three explosion vessels was the stoichiometric ethyleneair mixture ([C2H4] = 6.54 vol %), at ambient initial conditions: p0 = 1.013 bar and T0 = 298 K. The normal burning velocities, Su, were calculated from the cubic law coefficients of pressure rise, k, using eq 1, derived after assuming an isothermal compression of unburned gas in the early stage of explosion propagation32,42 !1=3 !2=3 k p0 Su ¼ R ð1Þ Δpmax pmax
where R is the radius of the explosion vessel, Δpmax is the maximum pressure rise during explosion, and pmax = p0 þ Δpmax. As input values for eq 1, we used the experimental values of p0, Δpmax, and pmax. The computation was restricted to the early stage of explosions, i.e., to a pressure range p0 e p e 2p0 for all experiments. The coefficient k, given by equation42,43 Δp ¼ Kp0
Su 3 t 3 ¼ kt 3 V0
ð2Þ
was determined for each experiment by a nonlinear regression method, applied to a relationship of the form Δp ¼ R þ kðt βÞ3
ð3Þ
where R and β are pressure and time corrections, respectively, meant to eliminate the signal shift of pressure transducer and any possible delay in signal recording. The constant K, defined as ! !2 4π pmax p0 pmax ð4Þ K ¼ 3 p0 p0 is a dimensionless constant dependent upon the nature and state of the explosive mixture.32 This method allows for the application of the cubic law for evaluation of the normal burning velocity from data recorded in a single experiment. For the test mixture, data measured in the spherical vessel S delivered Su = 70.0 ( 2.5 cm/s, data measured in cylindrical vessel C1 delivered Su = 56.1 ( 3.2 cm/s, and data measured in cylindrical vessel C2 delivered Su = 66.0 ( 2.0 cm/s. All measured burning velocities, especially those obtained from measurements in symmetrical vessels S and C2, fit well into the range of values reported in the literature, measured by other experimental methods: 58 cm/s (spherical bomb technique),29 65.8 and 69.5 cm/s (counter-flow twin-flame technique,17,24 respectively), 58.0 and 64 cm/s (outwardly propagating spherical flames, at constant pressure,19,21 respectively). The burning velocities obtained from cubic law coefficients of pressure rise in asymmetrical vessel C1 are systematically lower, as compared to burning velocities obtained from measurements in vessels S and C1, for all compositions. The deviations observed between the data obtained when the combustion chamber is varied appear 2445
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Figure 1. Data referring to the stoichiometric ethyleneair mixture diluted with argon, at ambient initial temperature and pressure, measured in spherical vessel S. [Ar]: circle, 0%; upward triangle, 5%; downward triangle, 10%; diamond, 15%; tilted triangle, 20%; and square, 30%.
because of additional effects: buoyancy (which determines flamefront distortion in larger vessels) and early heat transfer to walls, when cylindrical vessels are used. At the same time, the assumption of a thin and smooth flame front, used in eq 2, does not hold any more in some cases (i.e., low initial pressures). Most likely, the model is less adequate for measurements in asymmetrical vessels (h > Φ). A set of representative data referring to ethyleneairAr mixtures is given in Figure 1a, where the cubic law coefficients obtained in spherical vessel S and the burning velocities calculated by means of eq 1 are plotted against the initial pressure p0 of the mixture. For each composition, the normal burning velocities decrease with the increase of the initial pressure, p0, as shown in Figure 1b. Similar diagrams were obtained for all examined mixtures, in the three explosion vessels. The decrease of burning velocity can be explained after examination of eq 5 given by the thermal theory of flame propagation vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u u λ u u FCp u ð5Þ Su u t Ea n þ 1 C0 exp RTb where the burning velocity Su is expressed as a function of thermal diffusivity D = λ/FCp (with λ being the thermal conductivity, F being the density, and Cp being the heat capacity), initial fuel concentration, the end temperature in the flame front, Tb, and overall kinetic parameters of the oxidation (n, overall reaction order; Ea, overall activation energy). Thus, the pressure dependence of burning velocity originates primarily in the unburned gas density variation; when pressure increases, the density increases and the burning velocity decreases. In the present case, the pressure dependence of the burning velocity was analyzed according to the empirical power-law equation !ν p ð6Þ Su ¼ Su, ref pref where Su,ref is the normal burning velocity at reference pressure pref and ν is the baric coefficient. With pref = 1 bar, the baric coefficients
Table 1. Baric Coefficients and Overall Reaction Orders for the Stoichiometric EthyleneAir Mixture Diluted with Ar, N2, and CO2 argona
nitrogenb
carbon dioxidea
[diluent] (vol %)
ν
n
ν
n
0
0.094
1.81
0.094
1.81
5.0 8.0
0.108
1.78
10.0
0.136
15.0
0.127
0.094
1.81 1.68
1.73
0.168
1.66
1.75
0.150
1.70
0.266
1.47
0.121
1.76
0.270
0.151
32.0
0.101
1.46
0.076
1.85
0.074
1.85
0.068
1.86
1.70
24.0 30.0
n
0.160
16.0 20.0
ν
1.80
a
Data were obtained from measurements in vessel S. b Data were obtained from measurements in vessel C2.
of normal burning velocities for ethyleneair and ethyleneair additive mixtures were calculated by a nonlinear regression analysis of Su = f(p0) data. Negative values of the baric coefficients were obtained for ethyleneairadditive mixtures. Some values for the examined mixtures are given in Table 1, together with the overall reaction orders obtained using the equation44 n ¼ 2ðν þ 1Þ
ð7Þ
The baric coefficients of ethyleneair and ethyleneairdiluent are within the range found for mixtures of other lower hydrocarbons with air (0.30 for stoichiometric methaneair,45 0.11 for stoichiometric butaneair,46 between 0.26 and 0.12 for propaneair mixtures,4749 and between 0.28 and 0.17 for propyleneair mixtures32). For propyleneairadditive mixtures, baric coefficients between 0.16 and 0.26 were calculated.50 In the present case, the additive presence influences both the baric coefficients and the overall reaction orders. An increase of absolute values of baric coefficients when the additive concentration increases and lower baric coefficients for ethyleneairCO2 mixtures, as compared to ethyleneairAr, at 2446
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Figure 2. Burning velocities of ethyleneair mixtures against the CO2 concentration at various initial pressures in spherical vessel S.
Figure 3. Burning velocities for stoichiometric ethyleneair mixture diluted with Ar, N2, and CO2 at p0 = 1 bar, obtained from experiments in spherical vessel S and in cylindrical vessel C2.
identical additive content were found. The corresponding values of the overall reaction orders range between 1.50 and 1.80. The plots of burning velocities versus the additive concentration, at a constant initial pressure, are similar for Ar, N2, and CO2. Sets of typical data are given in Figures 2 and 3. As expected, the increase of the additive concentration determines the decrease of the burning velocity, at all initial pressures. Examination of eq 5 explains the dilution effect by the decrease of the initial fuel concentration at constant pressure. At the same time, the thermal diffusivity D of unburned mixtures also influences the burning velocity. Examination of C2H4airadditive mixtures containing the same amount of additive (16%) showed that, among the examined additives, CO2 has the most important inhibiting effect, followed by Ar and N2. The same order is found for the thermal diffusivities of corresponding fuelairadditive mixtures: D = 1.94 105 m2 s1 (mixtures containing N2), 1.91 105 m2 s1 (mixtures containing Ar), and 1.73 105 m2 s1 (mixtures containing CO2). Experimentally, it was observed that a CO2 concentration higher than 36% determines a complete inertization of the system at any initial pressure, while the systems ethyleneairN2 and ethyleneairAr are still flammable, even at 44 vol % inert. A comparison of the present data obtained at ambient initial conditions in cylindrical vessel C2 with literature results25 is given in Figure 4, where normalized
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Figure 4. Relative burning velocities for C2H4airN2 mixtures; experimental and computed data.
(relative) burning velocities (defined as Su,rel = Su/Su0, where Su is the normal burning velocity of the fuelairadditive mixture and Su0 is the normal burning velocity of the fuelair mixture without additive) of ethyleneairnitrogen mixtures are plotted against the nitrogen concentration, together with results of INSFLA modeling for ethyleneairnitrogen flames. The plots in Figures 3 and 4 are linear only at low additive concentrations, below 10 vol %, in agreement with previous data found for other fuelairadditive systems, e.g., syngas (50% H2 and 50% CO)airN2,8 dissociated methanol (66.7% H2 and 33.3% CO)airN2,11 propaneairN2,15,51 or methane hydrogenairN2.52 Dilution by CO2 has a greater impact on laminar flame speed in comparison to N2 and Ar, as observed for many other fuelairadditive mixtures.911,14,53 Dilution of ethyleneair mixture has, as a first consequence, the diminution of fuel and oxygen contents and the amount of evolved heat, able to sustain the flame propagation. As a consequence, both the maximum flame temperature in the reaction zone and the burning velocity decrease. The typical examples are ethylene airN2 and the ethyleneairAr mixtures (the upper plots in Figure 3). Dilution by CO2 maintains the diminution of fuel content, adding the specific influence of this additive: its ability to dissociate and to dissipate heat by radiation. To estimate the impact of carbon dioxide dissociation of the normal burning velocity, Halter et al.10 have performed the modeling of CH4 airadditive adiabatic laminar flames by introducing as an additive a chemically inactive molecule, having the thermal and transport characteristics of carbon dioxide. This molecule does not react, and its concentration remains constant during the entire combustion process. The “false” CO2 molecule replaced CO2 in the simulations. The laminar burning velocities calculated with this new component were located between those corresponding to nitrogen and carbon dioxide dilution. Their conclusion was that the dissociation of CO2 is not negligible; however, the effect of carbon dioxide dissociation is less important when the additive amount is increased because the flame temperature is lower and does not support an extensive dissociation. 4.2. Computed Burning Velocities. The computed normal burning velocity of the stoichiometric ethyleneair mixture, obtained with INSFLA package using the Warnatz mechanism, was 54.8 cm/s at ambient initial conditions. It is a quite low value compared to results of other computations: 65.769.1 cm/s,17 81.0 cm/s,2 and 81.8 cm/s,54 obtained according to various kinetic schemes. A set of representative results obtained in the 2447
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Table 2. Characteristic Parameters of a Free Laminar Premixed Flame, Propagating in a Stoichiometric EthyleneAir Gaseous Mixture at Ambient Pressure and Temperature, as Compared to Parameters Computed for EthyleneAir in the Presence of 8% Additive additive no additive
argon
nitrogen
carbon dioxide
Tmax (K)
2403.4
2349.3
2322.5
2242.0
dfl (mm) Su (cm/s)
0.649 54.84
0.736 46.31
0.791 43.41
0.975 31.18
Figure 6. Relative flame temperatures of the stoichiometric ethyleneair mixture diluted with Ar, N2, and CO2 at p0 = 1 bar: computed values.
Figure 5. Relative normal burning velocities of the stoichiometric ethyleneair mixture diluted with Ar and CO2 at p0 = 1 bar: experimental and computed values.
present computations, referring to a stoichiometric ethyleneair mixture diluted with 8% additive (Ar, N2, or CO2), is given in Table 2, together with data characteristic of the stoichiometric ethyleneair mixture: the maximum flame-front temperature (Tmax), the flame-front width (dfl), and the normal burning velocity (Su). As expected, additive presence determines the decrease of the maximum flame temperatures and burning velocities and the increase of the flame-front width. Among the used additives, CO2 has the largest influence on flame propagation. Data on all examined compositions are given in Figures 5 and 6, as plots of relative burning velocities and relative maximum flame temperatures against the additive concentration. Good correlations were found between the normal burning velocities and the maximum flame temperatures for all examined diluents, as seen from Figure 7. In the range of high flame temperatures, all data lie practically on the same plot; they differentiate in the low-temperatures range. Such correlations were reported for other systems as well, e.g., CH4O2N2CO2 or H2O2N2CO2, where oxygen-enriched air was used, with different enrichment factors.9 On the basis of these data, the authors considered that additives influence mostly the thermal properties of fuelairadditive mixtures, through the average heat capacity, thus, the flame temperature and, eventually, the reaction rate.9 Additional information on the influence of CO2 dissociation on flame propagation is obtained from examination of the total mass fraction of H and OH radicals, which are the main chain
Figure 7. Relative normal burning velocities against relative flame temperatures of the stoichiometric ethyleneair mixture diluted with Ar, N2, and CO2 at p0 = 1 bar: computed values.
carriers and influence most significantly the fuel consumption reaction.2,3 In Figure 8, a plot of the total mass fractions of H and OH against the additive concentration is given; as expected, the dilution with Ar, N2, or CO2 decreases the mass fractions of examined radicals. The effect is larger in flames diluted by carbon dioxide, as compared to flames diluted by nitrogen or argon. In Figure 9, the normal burning velocities are plotted versus the total mass fractions of H and OH, for each examined additive. A comparison of ethyleneairadditive systems reveals that, for an identical maximum flame temperature, the mixtures containing CO2 have the lowest normal burning velocity and the highest mass fraction of radicals. A set of representative data, given in Table 3, confirms the combined influence of carbon dioxide on flame propagation, through a modified heat dissipation rate and extended dissociation in the flame front, entailing modified normal burning velocities. 4.3. Overall Activation Energy of Ethylene Combustion. The normal burning velocity is directly influenced by the overall reaction rate in the flame front; therefore, the good correlation of normal burning velocities with flame temperatures, illustrated by data in Figure 7, can be used to evaluate an overall activation 2448
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Figure 8. Total mass fractions of H and OH in the flame front of the stoichiometric ethyleneair mixture diluted with various amounts of Ar, N2, and CO2 at p0 = 1 bar: computed values.
Figure 10. Correlations of normal burning velocities and average flame temperatures for ethyleneairadditive mixtures.
Table 4. Activation Energies of Ethylene Oxidation in the Presence of Various Additives Ea (kJ/mol)
mixture
from experimental
from computed
burning velocities
burning velocities
ethyleneairAr
134.4 ( 4.8
154.7 ( 3.3
ethyleneairCO2
158.6 ( 12.5
215.0 ( 5.7
97.2 ( 7.6
158.9 ( 3.5
ethyleneairN2
examined mixture. The average flame temperature was calculated with the relationship55 Tfl, av ¼ T0 þ 0:74ðTfl T0 Þ Figure 9. Burning velocities for the stoichiometric ethyleneair mixture diluted with Ar, N2, and CO2 at p0 = 1 bar, in correlation with the total mass fractions of H and OH.
Table 3. Total Mass Fractions of Radicals and Normal Burning Velocities of EthyleneAirAdditive Flames Characterized by the Same Flame Temperature Tmax = 2163 K [additive] (vol %)
xH þ xOH (104)
Su (cm/s)
argon
28.5
1.50
26.0
nitrogen
20.3
1.59
26.5
carbon dioxide
12.0
1.95
24.3
additive
energy of the combustion process. The overall activation energy of the ethyleneoxygen reaction within the flame front, in the presence of different additives, was calculated with the equation 1 n Ea ln Su þ ln Tfl, av ln Y ¼ constant 2 2 2RTfl
ð8Þ
derived by Burke et al.55 for any fueloxygeninert mixture where the flame temperature is varied by dilution. In eq 8, Tfl,av is the average temperature within the flame front and Y is the mole fraction of reactive components (fuel þ oxidant) in the
ð9Þ
Plots of the left member of eq 8 against the reciprocal value of the average flame temperature for the stoichiometric ethyleneair mixture diluted with Ar, N2, and CO2 are given in Figure 10, where experimental values of normal burning velocities were taken into account. The slopes of the linear correlations give the overall activation energies. A typical set of data calculated from both experimental and computed burning velocities is given in Table 4. The experimental burning velocities were examined against reciprocal values of flame temperatures, as obtained from equilibrium computations performed with the program ECHIMAD. The computed burning velocities were examined against reciprocal values of flame temperatures, as obtained from kinetic modeling with the program INSFLA. For each ethyleneairadditive system, specific values of reaction orders n were used, as given in Table 1. The overall activation energies of the ethyleneoxygen reaction determined from experimental burning velocities range within the typical values characteristic for hydrocarbon oxidation in flames, e.g., Ea = 146 kJ/mol for stoichiometric propylene airAr mixtures and Ea = 207 kJ/mol for stoichiometric propyleneairCO2 mixtures.50 Higher activation energies were determined from computed burning velocities, for each examined additive. In both sets of data, the influence of carbon dioxide is observed, because the largest activation energies are calculated for ethyleneairCO2 mixtures. For comparison, a much higher activation energy was reported for preheated ethyleneair mixtures 2449
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Energy & Fuels by Kumar et al.:24 Ea = 385 kJ/mol, obtained from examination of mass burning flux against the adiabatic flame temperature, close to results reported for preheated propaneair mixtures: Ea = 254 kJ/mol (for lean mixtures) and Ea = 365 kJ/mol (for the stoichiometric mixture).56
5. CONCLUSION The behavior of stoichiometric ethyleneairadditive mixtures (additives: argon, nitrogen, and carbon dioxide) was studied both experimentally and computationally, at different additive contents and various total initial pressures. The normal burning velocities were calculated from cubic law coefficients of pressure rise in the early stage of spherical vessel explosions following the central ignition. Adiabatic burning velocities of examined mixtures were obtained, together with temperature and species profiles along the flame front, from simulations of one-dimensional laminar flames using the Warnatz mechanism developed for combustion of C1C4 hydrocarbons with air. A strong influence of the inert additives on the normal burning velocity, maximum flame temperature, and concentration of active radical species in the flame front was observed. Dilution by increasing amounts of additives determines the decrease of burning velocity and maximum flame temperature, for all investigated compositions and initial pressures of the ethyleneair mixture. Among the studied additives, CO2 is the most efficient, followed by N2 and Ar. The explanation is that carbon dioxide has a larger influence on the total heat capacity and heat dissipation rate in comparison to nitrogen and argon. The dilution by Ar, N2, or CO2 of the stoichiometric ethylene air mixture influences the overall reaction orders, which range within 1.50 and 1.90. The dilution effect is also observed in the overall activation energy: 134.4 kJ/mol (C2H4airAr), 158.6 kJ/mol (C2H4airCO2), and 97.2 kJ/mol (C2H4airN2), relevant values on the inerting effect of carbon dioxide on flame propagation in ethyleneair mixtures. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ40-21-3167912. Fax: þ40-21-3121147. E-mail:
[email protected].
’ ACKNOWLEDGMENT 00 The authors gratefully thank Prof. U. Maas (Institut fur Technische Verbrennung, Karlsruhe, Germany) and Dr. D. Markus [Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany] for the permission to run the program INSFLA and for the provided assistance. The financial support of the Humboldt Foundation, Germany, to its former fellow, Dr. Domnina Razus, as a donation of scientific equipment, is also gratefully acknowledged. ’ NOMENCLATURE a, b, c, and d = constants C = heat capacity (J kmol1 K1) D = thermal diffusivity (m2 s1) E = energy (J) k = coefficient of the cubic law (bar s3) K = dimensionless constant of the cubic law n = overall reaction order
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p = pressure (bar) R = vessel radius (m) S = speed and velocity (m s1) t = time (s) T = temperature (K) V = volume (m3) Y = mole fraction of reactive components (fuel þ oxidant) in the fueloxidantdiluent Greek Letters
r = pressure correction (bar) β = time correction (s) λ = thermal conductivity (J s1 m1 K1) ν = baric coefficient of the normal burning velocity F = density (kg m3) Δ = variation Subscripts
a = activation av = average value fl = referring to a flame max = maximum value u = unburned gas p = referring to an isobaric value rel = relative value ref = reference value 0 = initial condition
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