Structure and kinetics of CH4/N2O flames (original) (raw)
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Study of the Chemical Structure of Laminar Premixed H2/CH4/C3H8/O2/Ar Flames at 1–5 atm
Energy & Fuels, 2017
The paper presents an experimental and modeling study of the chemical structure of laminar premixed stoichiometric H 2 /CH 4 /C 3 H 8 /O 2 /Ar flames stabilized on a flat burner at 1, 3 and 5 atm. The flames structure was simulated using four different detailed chemical kinetic mechanisms proposed in the literature for oxidation of small hydrocarbons. The width of the zone of consumption of the fuel components was shown to differ appreciably at the three pressures. Hydrogen was shown to have the largest consumption zone, while propane has the smallest one. The kinetic analysis provided an explanation for the observed phenomenon, which assumes the formation of additional pathways for hydrogen and methane production in the flames of ternary fuel mixtures. Comparison of the measured and simulated flame structures shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, products and some intermediates at atmospheric and elevated pressures. It is noteworthy that the mechanisms adequately predict the spatial variations in the mole fractions of free radicals, including the H, OH and CH 3 radicals, within the pressure range. However, some drawbacks of the mechanisms used have been identified. The mechanisms were shown to overpredict the mole fractions of some unsaturated hydrocarbons, including ethylene and acetylene, at elevated pressures. Therefore, the rate constants of the crucial reactions responsible for production/consumption of these species, as well as their pressure dependences, should be specified, and the mechanisms should be refined. To provide a deeper insight into the combustion chemistry of ternary fuel mixtures, one should focus on the structure of rich flames.
Combustion and Flame, 1999
As part of an ongoing investigation of an exhaust NO x emission index minimum measured for partially premixed flames, radial temperature profiles and CH radical locations were measured in atmospheric-pressure, partially premixed, coflow, methane/air flames with fuel-side equivalence ratios of 1.6, 2.0, and 3.5, at three axial heights above the burner. The work was undertaken because of the importance of temperature and CH radical behavior in NO formation chemistry. Thin-filament pyrometry was found to be more appropriate than thermocouple thermometry for temperature measurements in partially premixed flames. Results demonstrated that the 1.6-equivalence-ratio flame exhibited classical double-flame structure, the 2.0-equivalence-ratio flame was a merged flame, and the 3.5-equivalence-ratio flame exhibited diffusion-flame structure. Signals from CH* chemiluminescence and CH laser-induced fluorescence provide evidence that, for the present measurement locations, double flames exhibit single CH peaks which can be associated with their premixed component flames. Double CH radical peaks, which were predicted to occur in low-strain-rate flames, were not found for the limited number of flame conditions and locations studied. In the near-burner region, the premixed and nonpremixed component flames of the ⌽ B ϭ 1.6 double flame diverge radially with increasing downstream distance and merge together for larger values of ⌽ B .
Chemical Engineering Research & Design, 2018
The flame propagation in nitrogen-diluted CH 4-N 2 O mixtures was monitored by pressure measurements during explosions in a spherical vessel with central ignition. The burning velocities were obtained from experimental measurements of pressure variation during closed vessel explosions and from the detailed modelling of free laminar premixed flames. Lean-and stoichiometric methane-nitrous oxide mixtures (equivalence ratios: 0.8 and 1.0) diluted by various amounts of nitrogen between 40 and 60 vol% were studied at various initial pressures between 0.3 and 1.8 bar and ambient initial temperature. Nitrogen addition to each CH 4-N 2 O mixture results in the decrease of laminar burning velocity and flame temperature, along with the increase of flame width. Examination of burning velocity variation against average flame temperature in experiments at constant initial pressure and various inert concentrations allowed the determination of the overall activation energy; examination of burning velocity dependence on pressure, at constant inert concentration, allowed the determination of the overall reaction orders. For all CH 4-N 2 ON 2 mixtures, the temperature, volumetric rate of heat release and reactive species concentration profiles across the flame front were examined versus similar data characteristic to stoichiometric methane-air mixtures. The most important elementary reactions in CH 4-N 2 ON 2 and CH 4-air mixtures were identified by means of sensitivity analysis.
An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure
Combustion and Flame, 2009
An experimental and modeling study of 11 premixed NH 3 /CH 4 /O 2 /Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt-13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH 3 /CH 4 (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH 4 /NH 3 mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H 2 O, NO and N 2 increase while those of H 2 , CO, CO 2 and NO 2 have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH 4 and NH 3 oxidation and their mutual interaction.
Combustion and Flame, 2007
Experimental measurements of adiabatic burning velocity and NO formation in (CH 4 + H 2 ) + (O 2 + N 2 ) flames are presented. The hydrogen content in the fuel was varied from 0 to 35% and the oxygen content in the air from 20.9 to 16%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The NO concentrations in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. In lean flames, enrichment by hydrogen has little effect on [NO], while in rich flames, the concentration of nitric oxide decreases significantly. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. Different responses of stretched and nonstretched flames to enrichment by hydrogen are demonstrated and discussed.
Flame-ion probe of the reaction zone in a CH 4 –O 2 –Ar flame with added HCN, NH 3 and NO
Canadian Journal of Chemistry, 1981
The addition of 0.3% of the fuel-nitrogen (fuel-N) compounds HCN, NH3, or NO to a premixed, fuel-rich, CH4–O2–Ar flame burning at atmospheric pressure demonstrated the rapid interconversion of nitrogenous intermediates in the reaction zone. The nitrogenous species (HCN/CN, HNCO/NCO, NH3, NH2, NH, NO, NO2) were observed as ions (CN−, H2CN+, NCO−, H2NCO+, NH4+, NH3+, NH2+, NO+, NO2−, and hydrate ions) formed in chemical ionization processes discussed previously (1). The ions were sampled directly into a flame-ion mass spectrometer which had sufficient spatial resolution for the measurement of ion concentration profiles through the reaction zone. The study bears on Fenimore's suggestion for the formation of "prompt NO" in fuel-rich hydrocarbon flames. These additive results were compared with previous results involving nitrogenous species present in a similar CH4–O2 flame doped with 10% N2. The increased sensitivity of the additive approach confirmed many of the mass assi...
Chemical Structures of Fuel-Rich, Premixed, Laminar Flames of 1, 1- C2H4Cl2/CH4/O2/Ar
Combustion Science and Technology, 1995
Temperature and species mole fraction profiles were determined in atmospheric-pressure, premixed, laminar, flat flames of I, 1-C,H,CI, and CH, under fuel-rich conditions at different CI/H ratios. Samples were withdrawn from within the flame using a heated microprobe followed by gas analysis by on-line capillary gas chromatography-mass spectrometry (GC/MS). The mole fraction profiles were determined for all the reactants and for CO,
The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames
Proceedings of the Combustion Institute, 2013
Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH 4) and nitrous oxide (N 2 O), and ethane (C 2 H 6) and N 2 O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O 2), nitrogen (N 2), and N 2 O, and the other a mixture of CH 4 and N 2 or C 2 H 6 and N 2. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N 2 O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H 2 + O 2 2H 2 O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N 2 O + H 2 N 2 + H 2 O where stable products are formed. Inhibition of the flames by N 2 O is attributed to competition between these two overall steps.
Modeling of Rich Premixed C2H4/O2/Ar and C2H4/ Dimethoxymethane/O2/Ar Flames
Two rich premixed ethylene/oxygen/argon and ethylene/dimethoxymethane/oxygen/argon flat flames burning at 50 mbar were investigated experimentally by using molecular beam mass spectrometry to study the effect of methylal (dimethoxymethane) addition on species concentration profiles (Renard C, Van Tiggelen P.J. and Vandooren J., Proc. Combust. Inst., 29, 1277-1284, 2002. The replacement of 5.7% C 2 H 4 by 4.3% C 3 H 8 O 2 , keeping the equivalence ratio equal to 2.50, is responsible for a decrease of the maximum mole fractions of most of the detected intermediate species. If this phenomenon is barely noticeable for C 2 to C 4 intermediates, it becomes more efficient for C 5 to C 10 species. Previously, a reaction mechanism has been validated against a premixed rich C 2 H 4 /O 2 /Ar flame (φ = 2.50) which describes in detail the formation of soot precursors and more precisely the main pathways involving benzene (Dias, V., Renard, C., Van Tiggelen, P.J. and Vandooren, J., European Combustion Meeting, Orléans -France, p.221, 2003). The aim of this work is to extend this original model by building a sub-mechanism taking into account the formation and the consumption of oxygenated species involved in dimethoxymethane combustion. The new mechanism contains 474 elementary reactions and involves 90 chemical species in order to simulate both ethylene flames with and without methylal addition. The model leads to a good simulation for all species detected in these flames, and underlines the effect of methylal addition on species concentration profiles. According to this mechanism, the two main degradation pathways of methylal (CH 3 OCH 2 OCH 3 ) in C 2 H 4 /methylal/oxygen/argon flame are: 1)
Experimental studies of nitromethane flames and evaluation of kinetic mechanisms
Combustion and Flame, 2018
The present work reports new experimental data for premixed flames of nitromethane, CH 3 NO 2 , at atmospheric pressure, and an evaluation of two contemporary kinetic mechanisms based on these new flame studies as well as previously published experimental data on laminar burning velocity and ignition. Flames of nitromethane + air at lean (φ = 0.8) and rich (φ = 1.2) conditions were stabilized on a flat-flame burner, where profiles of CH 2 O, CO and NO were obtained using laser-induced fluorescence and temperature profiles using coherent anti-Stokes Raman spectroscopy. Laminar burning velocities for nitromethane + O 2 + CO 2 were measured using the heat flux method for φ = 0.8-1.3 at 348 K and φ = 0.8-1.6 at 358 K, and an oxidizer composition of 35% O 2 and 65% CO 2. In addition, the effect of the oxidizer composition was examined for a stoichiometric flame at 358 K by varying oxygen fraction from 30% to 40%. The mechanism by Mathieu et al. (Fuel 2016, 182, 597), previously not validated for flames, was able to reproduce experimental laminar burning velocities for nitromethane + air, but under predicted new results for CH 3 NO 2 + O 2 + CO 2 mixtures. The mechanism by Brequigny et al. (Proc. Combust. Inst. 2014, 35, 703) under predicted experimental laminar burning velocities significantly at all investigated conditions. Previous studies have shown that none of the mechanisms can accurately predict ignition delay time over a wide range of conditions with respect to pressure, temperature, diluent and dilution ratio. The evaluation of the mechanisms reveals that the understanding of nitromethane combustion is at the present time not sufficient to produce a widely applicable mechanism.