“…Lengthening also correlates with fuel richness. For CH, this reflects the lengthening reported previously in low-pressure flames. − 6 Measured relative concentrations of CH radicals as a function of downstream time in the early burned gases of the seven acetylene flames together with their temperatures at 0.2 ms. Curves are labeled by their individual unburned C 2 H 2 /O 2 /N 2 flame ratios. 7 Corresponding measurements to the previous figure but for the relative concentrations of C 2 in the early burned gases of the seven acetylene flames.…”
Section: Experimental Results and Preliminary Implicationssupporting
confidence: 60%
“…For CH, this reflects the lengthening reported previously in lowpressure flames. [24][25][26] Another notable component is the fact that, at 0.1 ms, the closest point to the flat reaction zone where measurements could be made, the initial concentrations for CH start out at similar concentration levels and also with C 2 but to a lesser exactness. On calculating the total carbon content of a unit volume of the burned gases for each of these seven flames, a total variation of only a factor of 2 is noted.…”
Section: Experimental Results and Preliminary Implicationsmentioning
confidence: 99%
“…There is a slight pressure and temperature dependence, 24 but all measurements obtaining concentration profiles through the flame have been made at reduced pressures of from 3 to 130 mbar (2-100 Torr). 18,20,21,[23][24][25][26][27][28] Of these studies, four with reduced pressure acetylene flames did obtain profiles of both CH and C 2 . 18,20,21,23 More will be said later of this prior body of work in connection with the new data presented herein, and on the previous limited efforts to kinetically model the observed profiles, which have mainly emphasized CH measurements in CH 4 flames.…”
Section: Previous Measurements Of Ch and C 2 In Flamesmentioning
Measured CH and C2 profiles show a striking resemblance as a function of time in a series of seven well-characterized fuel-rich (phi=1.2-2.0) non-sooting acetylene flames. This implied commonality and interrelationship are unexpected as these radicals have dissimilar chemical kinetic natures. As a result, a rigorous examination was undertaken of the behavior of each of the hydrocarbon species known to be present, C, CH, CH2, CH3, CH4, CHO, CHOH, CH2O, CH2OH, CH3O, CH3OH, C2, C2H, C2H2, CHCO, CH2CO, and C2O. This emphasized the main region where CH and C2 are observed (50-600 micros) and reduced the kinetic reactions to only those that operate efficiently and are dominant. It was immediately apparent that this region of the flame reflects the nature of a hydrogen flame heavily doped with CO and CO2 and containing traces of hydrocarbons. The radical species, H, OH, O, along with H2, H2O, and O2, form an important controlling radical pool that is in partial equilibrium, and the concentrations of each of the hydrocarbon radicals are minor to this, playing secondary roles. As a result, the dominant fast reactions are those between the hydrocarbons and the basic hydrogen/oxygen radicals. Hydrocarbon-hydrocarbon reactions are unimportant here at these equivalence ratios. CH and C2 are formed and destroyed on a sub-microsecond time scale so that their flame profiles are the reflection of a complex kinetically dynamic system. This is found to be the case for all of the hydrocarbon species examined. As might be expected, these rapidly form steady-state distributions. However, with the exceptions of C, CHO, CHOH, and CH2O, which are irreversibly being oxidized, the others all form an interconnected hydrocarbon pool that is under the control of the larger hydrogen radical pool. The hydrocarbon pool can rapidly adjust, and the CH and C2 decay together as the pool is drained. This is either by continuing oxidation in less rich mixtures, or in richer flames where this is negligible by the onset of hydrocarbon-hydrocarbon reactions. The implications of such a hydrocarbon pool are significant. It introduces a buffering effect on their distribution and provides the indirect connection between CH and C2. Moreover, because they are members of this radical pool, flame studies alone cannot answer questions concerning their specific importance in combustion other than their contributing role to this pool. The presence of such a pool modifies the exactness that is needed for kinetic mechanisms, and knowledge of every species in the system no longer is necessary. Furthermore, as rate constants become refined, it will allow for the calculation of the relative concentrations of the hydrocarbon species and facilitate reduced kinetic mechanisms. It provides an explanation for previous isotopically labeled experiments and illustrates the difficulty of exactly identifying in flames the role of individual species. It resolves the fact that differing kinetic models can show similar levels of accuracy and has implications for sensitivity analyses...
“…Lengthening also correlates with fuel richness. For CH, this reflects the lengthening reported previously in low-pressure flames. − 6 Measured relative concentrations of CH radicals as a function of downstream time in the early burned gases of the seven acetylene flames together with their temperatures at 0.2 ms. Curves are labeled by their individual unburned C 2 H 2 /O 2 /N 2 flame ratios. 7 Corresponding measurements to the previous figure but for the relative concentrations of C 2 in the early burned gases of the seven acetylene flames.…”
Section: Experimental Results and Preliminary Implicationssupporting
confidence: 60%
“…For CH, this reflects the lengthening reported previously in lowpressure flames. [24][25][26] Another notable component is the fact that, at 0.1 ms, the closest point to the flat reaction zone where measurements could be made, the initial concentrations for CH start out at similar concentration levels and also with C 2 but to a lesser exactness. On calculating the total carbon content of a unit volume of the burned gases for each of these seven flames, a total variation of only a factor of 2 is noted.…”
Section: Experimental Results and Preliminary Implicationsmentioning
confidence: 99%
“…There is a slight pressure and temperature dependence, 24 but all measurements obtaining concentration profiles through the flame have been made at reduced pressures of from 3 to 130 mbar (2-100 Torr). 18,20,21,[23][24][25][26][27][28] Of these studies, four with reduced pressure acetylene flames did obtain profiles of both CH and C 2 . 18,20,21,23 More will be said later of this prior body of work in connection with the new data presented herein, and on the previous limited efforts to kinetically model the observed profiles, which have mainly emphasized CH measurements in CH 4 flames.…”
Section: Previous Measurements Of Ch and C 2 In Flamesmentioning
Measured CH and C2 profiles show a striking resemblance as a function of time in a series of seven well-characterized fuel-rich (phi=1.2-2.0) non-sooting acetylene flames. This implied commonality and interrelationship are unexpected as these radicals have dissimilar chemical kinetic natures. As a result, a rigorous examination was undertaken of the behavior of each of the hydrocarbon species known to be present, C, CH, CH2, CH3, CH4, CHO, CHOH, CH2O, CH2OH, CH3O, CH3OH, C2, C2H, C2H2, CHCO, CH2CO, and C2O. This emphasized the main region where CH and C2 are observed (50-600 micros) and reduced the kinetic reactions to only those that operate efficiently and are dominant. It was immediately apparent that this region of the flame reflects the nature of a hydrogen flame heavily doped with CO and CO2 and containing traces of hydrocarbons. The radical species, H, OH, O, along with H2, H2O, and O2, form an important controlling radical pool that is in partial equilibrium, and the concentrations of each of the hydrocarbon radicals are minor to this, playing secondary roles. As a result, the dominant fast reactions are those between the hydrocarbons and the basic hydrogen/oxygen radicals. Hydrocarbon-hydrocarbon reactions are unimportant here at these equivalence ratios. CH and C2 are formed and destroyed on a sub-microsecond time scale so that their flame profiles are the reflection of a complex kinetically dynamic system. This is found to be the case for all of the hydrocarbon species examined. As might be expected, these rapidly form steady-state distributions. However, with the exceptions of C, CHO, CHOH, and CH2O, which are irreversibly being oxidized, the others all form an interconnected hydrocarbon pool that is under the control of the larger hydrogen radical pool. The hydrocarbon pool can rapidly adjust, and the CH and C2 decay together as the pool is drained. This is either by continuing oxidation in less rich mixtures, or in richer flames where this is negligible by the onset of hydrocarbon-hydrocarbon reactions. The implications of such a hydrocarbon pool are significant. It introduces a buffering effect on their distribution and provides the indirect connection between CH and C2. Moreover, because they are members of this radical pool, flame studies alone cannot answer questions concerning their specific importance in combustion other than their contributing role to this pool. The presence of such a pool modifies the exactness that is needed for kinetic mechanisms, and knowledge of every species in the system no longer is necessary. Furthermore, as rate constants become refined, it will allow for the calculation of the relative concentrations of the hydrocarbon species and facilitate reduced kinetic mechanisms. It provides an explanation for previous isotopically labeled experiments and illustrates the difficulty of exactly identifying in flames the role of individual species. It resolves the fact that differing kinetic models can show similar levels of accuracy and has implications for sensitivity analyses...
“…Flame structure measurements had been carried out by Kohse-HÖinghaus et al [34] who measureḋ H andȮH radical concentrations versus distance in a H 2 /O 2 /Ar flame, at a pressure of 95 mbar, in the equivalence ratio range 0.6 ≤ φ ≤ 1.4 and in the temperature range 1100-1350 K. Vandooren and Bian [35] [20], and Wang et al [21]. Simulations of the data of Asaba et al [11], Hasegawa and Asaba [13], and Koike [17] were not attempted in this study because of a lack of sufficient information.…”
A detailed kinetic mechanism has been developed to simulate the combustion of H 2 /O 2 mixtures, over a wide range of temperatures, pressures, and equivalence ratios. Over the series of experiments numerically investigated, the temperature ranged from 298 to 2700 K, the pressure from 0.05 to 87 atm, and the equivalence ratios from 0.2 to 6.Ignition delay times, flame speeds, and species composition data provide for a stringent test of the chemical kinetic mechanism, all of which are simulated in the current study with varying success. A sensitivity analysis was carried out to determine which reactions were dominating the H 2 /O 2 system at particular conditions of pressure, temperature, and fuel/oxygen/diluent ratios. Overall, good agreement was observed between the model and the wide range of experiments simulated. C
Abstract. Nitrogen atoms have been detected in stoichiometric flat premixed H2/O2/N2 flames at 33 and 96 mbar doped with small amounts of NH3, HCN, and (CN)2 using two-photon laser excitation at 211 nm and fluorescence detection around 870nm. The shape of the fluorescence intensity profiles versus height above the burner surface is markedly different for the different additives. Using measured quenching rate coefficients and calibrating with the aid of known N-atom concentrations in a discharge flow reactor, peak N-atom concentrations in these flames are estimated to be on the order of 1012-5 × 1013 cm -3 ; the detection limit is about 1 × 1011 cm -3. PACS: 82.40Py, 33.50Dq In the combustion of nitrogen-containing fuels, a variety of conditions have been identified under which atomic nitrogen is of importance. For example, N atoms can influence the product distribution in the conversion of fuelbound nitrogen to N2 or NOx. Depending on the particular nitrogen source and the temperature and equivalence ratio of the combustion process, the reactions of N atoms with e.g. OH, NO, or CH3 can lead to the formation of NO, N2, or HCN. A detailed treatment of the gas-phase nitrogen chemistry in combustion can be found in the recent review by Miller and Bowman [1].Nitrogen atom concentrations have been measured by ESR [2] and by atomic resonance absorption (ARAS) [3]. ESR spectroscopy has been used to detect N atoms in discharge flow reactors [2]. The ARAS technique requires vacuum UV radiation for the detection of N atoms; although it has proven most successful in shock tube experiments [3], flames usually are not transparent for these short wavelengths. Molecular beam sampling techniques coupled with mass spectrometry [4] were used in an investigation of the structure of a 45mbar NH3/O2/Ar flame. In this experiment, N concentrations were found to be below the detection limit (10 .5 ) of the apparatus throughout the flame. Their role in the kinetic mechanism was thus judged to be marginal [4]. The recent kinetic model of [1], however, predicts for this flame a mole fraction of N atoms which is about a factor of 30 higher than the upper limit stated in [4]. Upon addition of small amounts of HCN as N-containing fuel to a 33 mbar H2/O2/Ar flame, Miller etal. [5] find that the NO mole fraction is very sensitive to nitrogen atom reactions. Their conclusions on the importance of N atoms in the processes of NO formation and of conversion of NO to N2 under these conditions are in agreement with earlier work of Haynes [6] and Morley [7]. Although N atoms were not detected in their experiment, Miller et al. [5] predict N atom mole fractions of 5 x 10 .5 to 10 -4 in their flames. In the context of these different flame studies, the bxperimental det'ermination of N atom concentrations for specific combustion situations is expected to provide information which might be used to examine current chemical-kinetic models of the nitrogen chemistry in flames.Two-photon laser-induced fluorescence is a non-perturbing optical diagnostic tech...
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