Low-pressure Flames Research Articles

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography – Part II: 2-Methylfuran

This is Part II of a series of three papers which jointly address the combustion chemistry of furan and its alkylated derivatives 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) under premixed low-pressure flame conditions. Some of them are considered to be promising biofuels. With furan as a common basis studied in Part I of this series, the present paper addresses two laminar premixed low-pressure (20 and 40mbar) flat argon-diluted (50%) flames of MF which were studied with electron–ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) for equivalence ratios ϕ=1.0 and 1.7, identical conditions to those for the previously reported furan flames. Mole fractions of reactants, products as well as stable and reactive intermediates were measured as a function of the distance above the burner. Kinetic modeling was performed using a comprehensive reaction mechanism for all three fuels given in Part I and described in the three parts of this series. A comparison of the experimental results and the simulation shows reasonable agreement, as also seen for the furan flames in Part I before. This set of experiments is thus considered to be a valuable additional basis for the validation of the model. The main reaction pathways of MF consumption have been derived from reaction flow analyses, and differences to furan combustion chemistry under the same conditions are discussed.

The predictive capability of an automatically generated combustion chemistry mechanism: Chemical structures of premixed iso-butanol flames

The chemical compositions of four low-pressure premixed flames of iso-butanol are investigated with an emphasis on assessing the predictive capabilities of an automatically generated combustion chemistry model. This kinetic model had been extensively tested against earlier experimental data [S.S. Merchant, E.F. Zanoelo, R.L. Speth, M.R. Harper, K.M. Van Geem, W.H. Green, Combust. Flame (2013), http://dx.doi.org/10.1016/j.combustflame.2013.04.023.] and also shows impressive capabilities for predicting the new flame data presented here. The new set of data consists of isomer-resolved mole fraction profiles for more than 40 species in each of the four flames and provides a comprehensive benchmark for testing of any combustion chemistry model for iso-butanol. Isomer-specificity is achieved by analyzing flames, which are burner-stabilized at equivalence ratios of ϕ=1.0–1.5 and at pressures between 15 and 30Torr, with molecular-beam mass spectrometry and single-photon ionization by tunable vacuum-ultraviolet synchrotron radiation. Predictions of the C2H4O, C3H6O, and C4H8O enol–aldehyde–ketone isomers are improved compared to the earlier work by Hansen et al. [N. Hansen, M. R. Harper, W. H. Green, Phys. Chem. Chem. Phys. 13 (2011) 20262-20274] on similar n-butanol flames. A reaction path analysis identifies prominent fuel-consumption and oxidation sequences. Almost all of the species mole fraction data reported here are predicted within the measurement uncertainties of a factor of two to three. Some significant differences with previous published models are highlighted.

Laser-induced incandescence technique to identify soot nucleation and very small particles in low-pressure methane flames

This paper presents the study we carried out on the formation of soot particles in low-pressure premixed CH4/O2/N2 flames by using Laser-Induced Incandescence (LII). Flames were stabilised at 26.6 kPa (200 torr). Four different equivalence ratios were tested (Φ = 1.95, 205, 2.15 and 2.32), Φ = 1.95 corresponding to the equivalence ratio for which LII signals begin to be measurable along the flame. The evolution of the LII signals with laser fluence (fluence curve), time (temporal decay) and emission wavelength is reported at different heights above the burner. We specifically took advantage of the low-pressure conditions to probe with a good spatial resolution the soot inception zone of the flames. Significant different behaviours of the fluence curves are observed according to the probed region of the flames and Φ. In addition, while the surface growth process is accompanied by an increase in the LII decay-times (indicator of the primary particle diameter) at higher Φ, decay-times become increasingly short at lower Φ reaching a constant value along the flame at Φ = 1.95. These behaviours are consistent with the detection of the smallest incandescent particles in the investigated flames, these particles having experienced very weak surface growth. Flame modelling including soot formation has been implemented in flames Φ = 2.05 and 2.32. Experimental quantitative soot volume fraction profiles were satisfactorily reproduced by adjusting the fraction of reactive soot surface available for reactions. The qualitative variation of the computed soot particle diameter and the relative weight of surface growth versus nucleation were consistent with the experimental observations.

Shock tube measurements and model development for morpholine pyrolysis and oxidation at high pressures

Fuel decomposition time-history measurements during morpholine pyrolysis and ignition delay time measurements during morpholine oxidation were carried out behind reflected shock waves in a high-pressure shock tube. For the pyrolysis studies, morpholine concentrations of 5000ppm in argon were employed, with experiments covering the temperature range 1086–1404K and pressure range 20–23.6atm. For the ignition delay time measurements, experiments covered the temperature range 866–1197K with pressures near 15 and 25atm, oxygen mole fractions of 4% and 21%, and equivalence ratios of 0.5, 1, and 2. A morpholine reaction set developed to simulate low-pressure flames was updated based on the current shock tube data, recently published rate constants for relevant reactions, and newly calculated thermochemistry. The simulations from the updated set were in good agreement with the current shock tube experiments.

Reinvestigation of the spectroscopy of the [formula omitted] transition of the NCN radical at high temperature: Application to quantitative NCN measurement in flames

The initiation reaction of the prompt-NO mechanism in flames (CH+N2=NCN+H) involves the NCN radical. This radical is a very minor species reaching only a few hundreds of ppb in mole fraction in hydrocarbon/air flames, which makes the experimental determination of its absolute concentration profile challenging. The aim of this study is (1) to show that unambiguous assignment of absorption spectral lines of the NCN radical can be obtained in selected flames and (2) to provide the temperature-dependent NCN absorption cross sections needed to determine the absolute NCN concentration. In this work, NCN spectroscopy of the A–X band is fully revisited using theoretical spectroscopic calculations. Assessment of the spectroscopic constants is based on comparison of the theoretical spectrum, simulated with the pgopher program, with an extended LIF spectrum obtained in a low-pressure acetylene/oxygen/nitrogen flame. From the calculated electronic transition moments and the partition function, the temperature-dependent absorption cross section of NCN σ(T) has been calculated at the most intense bandhead wavelength (329.13nm). Cross-section temperature dependence is in very good agreement with that experimentally obtained by Dammeier and Friedrichs [J. Phys. Chem. A 114 (2010) 12963] using NCN3 pyrolysis in a shock tube followed by single-pass NCN absorption measurement, though their values differ by a factor of 2.6. The first NCN absorption spectrum between 329.0 and 329.3nm obtained by CRDS at high temperature in a flame is provided here, from which the bandhead is clearly identified. Occurrence of possible interferences has been examined. Finally, peak mole fractions obtained by CRDS are given in three low-pressure premixed flames by reconsidering previous published data and adding a new datum. The experimental absorption cross section required for this determination is derived from the theoretical one, taking into account the laser bandwidth. The accuracy of the NCN measurements is discussed and highlights the benefit of coupling CRDS and LIF.

Nanostructuration of carbonaceous dust as seen through the positions of the 6.2 and 7.7μm AIBs

Context. Carbonaceous cosmic dust is observed through infrared spectroscopy either in absorption or in emission. The details of the spectral features are believed to shed some light on its structure and finally enable the study of its life cycle. Aims. The goal is to combine several analytical tools in order to decipher the intimate nanostructure of some soot samples. Such materials provide interesting laboratory analogues of cosmic dust. In particular, spectroscopic and structural characteristics that help to describe the polyaromatic units embedded into the soot, including their size, morphology, and organisation are explored. Methods. Laboratory analogues of the carbonaceous interstellar and circumstellar dust were produced in fuel-rich low-pressure, premixed and flat flames. The soot particles were investigated by infrared absorption spectroscopy in the 2−15 μm spectral region. Raman spectroscopic measurements and high-resolution transmission electron microscopy were performed, which offered complementary information to better delineate the intimate structure of the analogues. Results. These laboratory analogues appeared to be mainly composed of sp 2 carbon, with a low sp 3 carbon content. A cross relation between the positions of the aromatic C=C bands at about 6.2 micron and the band at about 8 micron is shown to trace differences in shapes and structures of the polyaromatic units in the soot. Such effects are due to the defects of the polyaromatic structures in the form of non-hexagonal rings and/or aliphatic bridges. The role of these defects is thus observed through the 6.2 and 7.7 μm aromatic infrared band positions, and a distinction between carriers composed of curved aromatic sheets and more planar ones can be inferred. Based on these nanostructural differences, a scenario of nanograin growth and evolution is proposed.

Laminar Burning Velocities of C4–C7 Ethyl Esters in a Spherical Combustion Chamber: Experimental and Detailed Kinetic Modeling

To better understand the combustion characteristics of ethyl esters that could become second-generation biofuels, new experimental data were obtained for the combustion of a series of small ethyl esters. Unstretched laminar burning velocities of C4–C7 ethyl ester–air premixed laminar flames were measured in a spherical combustion chamber over a range of pressures (1–10 bar), initial temperatures (323–473 K), and equivalence ratios (0.7–1.5). These experiments were simulated as well as low-pressure flame structures, jet-stirred reactor species profiles, and ignition delays in shock tubes and rapid compression machines. A new detailed chemical kinetic reaction mechanism (1845 reactions versus 232 species) was developed and used in this work. The present model shows good agreement with the present experimental data and those taken from the literature. To interpret the results, sensitivity analyses were performed.

Experimental and Kinetic Modeling Study ofn-Butanol Pyrolysis and Combustion

n-Butanol pyrolysis in a flow reactor was investigated at the pressures of 5, 30, 80, 200, and 760 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for the identification of pyrolysis species and the measurement of their mole fractions. A detailed kinetic model consisting of 121 species and 658 reactions was developed to simulate the n-butanol pyrolysis. To enhance the accuracy of the model, the rate constants of unimolecular reactions of n-butanol and β-scission reactions of four n-butanol radicals (C4H8OH) were calculated with the variable reaction coordinate–transition-state theory (VRC–TST) and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory coupled with the master equation method. These rates are very sensitive to the mole fractions of pyrolysis species and have been well-validated by the pyrolysis experiment. The model was further validated by the low-pressure premixed flames at different equivalence ratios, oxidation data from the jet-stirred reactor, and ignition del...

Investigation of the effect of ethanol additives on the structure of low-pressure ethylene flames by photoionization mass spectrometry

Molecular beam mass spectrometry with photoionizationby synchrotron radiation was used to study the structure of flat premixed flames of C2H4/O2/Ar and C2H4/C2H5OH/O2/Ar at a pressure of 30 torr (4 kPa) with equivalence ratios ϕ = 1 and 2. It was shown that the replacement of part of ethylene with ethanol in the starting mixture lowered the concentration of a number of intermediate flame species, including soot precursors C3H3 and C6H6. These data are important for the development, analysis, and further improvement of detailed kinetic mechanisms for combustion of mixtures of hydrocarbon and oxygenated fuels.

Measurement of OH concentration profiles by laser diagnostics and modeling in high-pressure counterflow premixed methane/air and biogas/air flames

Detailed kinetic mechanisms validated under atmospheric or low-pressure flame conditions cannot generally be directly extrapolated toward high pressure, due to the lack of experimental data or to uncertainties concerning the rate constants of elementary reactions. It is thus necessary to complete the experimental database and to extend the validation domain of combustion mechanisms at high pressure. In this work, OH concentration profiles were measured in high-pressure methane/air and biogas/air laminar premixed counterflow flames by linear laser-induced fluorescence at different equivalence ratios (0.7–1.2) and pressures (0.1–0.7MPa). Each flame was calibrated in absolute OH concentration by a combination of laser absorption and planar laser-induced fluorescence measurements. Experimental results were compared with simulations using the OPPDIF code and three detailed kinetic mechanisms: two versions of the Gas Research Institute Mechanism, GRI-Mech 2.11 and GRI-Mech 3.0, and the recently updated GDFkin®3.0_NCN mechanisms. Results show that the flame front positions are very well predicted by modeling with the three mechanisms for lean and stoichiometric CH4/air flames and stoichiometric CH4/CO2/air flames. However, discrepancies appear at a higher equivalence ratio (Φ=1.2) for the CH4/air flames and at a lower equivalence ratio (Φ=0.7) for the CH4/CO2/air flames. OH mole fractions are quantitatively well predicted at high pressure in all cases, while systematic overestimation by modeling is observed at atmospheric pressure. A kinetic analysis of the results is also presented.

Kinetic effect of hydrogen addition on natural gas premixed flames

The chemical structure of laminar premixed natural gas flame with and without 60% of hydrogen on the fuel has been studied experimentally and numerically at atmospheric pressure. This work completes our recent results reported for low-pressure premixed flames. Both flames have been stabilized under similar conditions (similar cold velocity and C/O). Stable mole fraction profiles have been obtained by gas chromatography, FTIR and GC–MS. Temperature profiles have been measured by thermocouple and corrected from radiation losses. Comparatively to low pressure, more traces species have been observed at atmospheric pressure. These new results have been modeled and a revised version of GDF-Kin®3.0-NCN is reported. This version is able to predict the present data (mole fraction profiles and recent burning velocities of natural gas and natural gas/hydrogen flames) without alteration of previous predictions. Reactions path analysis confirms that hydrogen enhances H-abstraction reactions by H atoms under lean conditions. It also enhances the oxidation of C1 sequence and disfavors the C2 sequence. Consequently, a reduction of C2 intermediate specie mole fraction could be observed.

HCN quantitative measurement in a laminar low pressure flame at 1036 nm using pulsed CRDS technique

The present work aims to quantitatively measure HCN in a low pressure premixed CH4/N2O/O2/N2 flame after gas probe sampling. For that purpose, the (003)–(000) band of HCN has been selected for the first time for HCN monitoring. This band was chosen because it lies in a wavelength range (1.036μm) easily reachable by commercial dye laser combined with Raman cell shifter. The line strengths of its main R branches are 30 times lower than the transitions lying at 1.5μm generally used. The sensitivity was achieved using pulsed CRDS technique in a long absorption cell (1m). The required wavelength is generated from the first stokes radiation of hydrogen after 724nm pumping. First, absorption spectra of HCN diluted in N2 have been obtained and compared to simulated spectra of the R-branch. The very good agreement between the experimental and simulated spectra, both in line position and absolute intensity, allows to validate our strategy and the experimental setup. Second the HCN profile in the flame was determined by measuring the integrated spectral absorptivity around the R(7) transition line of HCN. The profile of the HCN mole fraction along the flame has been measured with a peak value equal to 600ppm. The technique implemented allows detecting HCN in the limit of 2.5×1013moleculecm−3 with an accuracy of 20%.

Photoionization mass spectrometry and modeling study of premixed flames of three unsaturated C5H8O2 esters

The detailed chemical structures of low-pressure premixed laminar flames fueled by three simple unsaturated C5H8O2 esters, the methyl crotonate (MC), methyl methacrylate (MMA), and ethyl propenoate (EPE), are investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Significant differences in the compositions of key reaction intermediates between these flames under similar flame conditions are observed. The results enable further refinement and validation of a detailed chemical kinetic reaction mechanism, which is largely based on a previous model for saturated esters. Detailed kinetic modeling describes how these differences are related to molecular structures, leading to unique fuel destruction pathways for each of these isomers. Meanwhile, the effect of carbon carbon double bonds on the combustion chemistry of small fatty acid esters is addressed.

Experimental and numerical study of chemiluminescent species in low-pressure flames

Chemiluminescence has been observed since the beginning of spectroscopy, nevertheless, important facts still remain unknown. Especially, reaction pathways leading to chemiluminescent species such as OH∗, CH∗, \(\mathrm{C}_{2}^{*}\), and \(\mathrm{CO}_{2}^{*}\) are still under debate and cannot be modeled with standard codes for flame simulation. In several cases, even the source species of spectral features observed in flames are unknown. In recent years, there has been renewed interest in chemiluminescence, since it has been shown that this radiation can be used to determine flame parameters such as stoichiometry and heat release under some conditions.

Simulation of Microgravity Diffusion Flames Using Sub-Atmospheric Pressures

Ethylene/air laminar diffusion flames were studied at sub-atmospheric pressures to simulate a non-buoyant environment and at super-atmospheric pressures for comparison, at fuel flow rates of 0.48 mg/s and 1.16 mg/s. Flame properties including flame geometry, soot formation and temperature field of the flames were studied. Overall, luminous flame height decreased with decreasing pressure to the point of visible luminosity disappearance, resulting in blue flames at a near vacuum. Flame width increased with decreasing pressure until the flame was almost spherical. Soot formation was also found to decrease with decreasing pressure and existed at very negligible concentrations in a near vacuum. Subatmospheric peak soot volume fractions ranged from about 0.1 ppm to 0.93 ppm at 0.48 mg/s, whereas at 1.16 mg/s, peak soot volume fractions were substantially higher. At subatmospheric pressures, higher fuel flow rates produced flames with higher soot concentrations. Soot production was restricted to an annular region with this annular region shifting closer to the flame centerline with increasing height from the burner. At locations about halfway between the burner rim and the flame tip, soot volume fraction decreased with increasing radial distance from the flame centerline. These results are consistent for both high and low pressure flames. The annular soot formation region was also located further from the flame centerline for the higher flow rate flames because of the difference in luminous flame shape. At 0.48 mg/s, the fraction of carbon in the fuel converted into soot was between 0.1 % and 1.2 % in the sub-atmospheric pressure range, from 0.2 atm to 1 atm.

Detailed kinetic modeling of the combustion of the four butanol isomers in premixed low-pressure flames

With regard to the importance of butanol as a potential replacement or additive to fossil transportation fuels, a detailed understanding of butanol combustion chemistry is desirable. Routes to different isomers of butanol from biomass are becoming available, and it is known that the structure of fuel molecules can be of crucial importance with respect to the intermediate species pool and the nature and amount of potential pollutants, including regulated air toxics. Quantitative major and intermediate species profiles for the combustion of the four butanol isomers under low-pressure premixed flame conditions, measured with two different in situ mass spectrometric instruments have recently been reported (P. Oßwald et al., Combust. Flame 158 (2011) 2–15), and this large consistent dataset has motivated us to further develop a detailed and comprehensive chemical kinetic model which was used here to complement the experimental results with numerical simulations, including reaction flow analyses. The major differences in the overall chemical pathways in these flames will be discussed, especially with respect to the formation of undesired emissions, and from agreement and differences between experiment and model, suggestions for further work will be given.

Experimental and modeling study of the oxidation of n-butylbenzene

New experimental results for the oxidation of n-butylbenzene, a component of diesel fuel, have been obtained using three different devices. A rapid compression machine has been used to measure autoignition delay times after compression at temperatures in the range 640–960K, at pressures from 13 to 23bar, and at equivalence ratios from 0.3 to 0.5. Results show low-temperature behavior, with the appearance of cool flames and a negative temperature coefficient (NTC) region for the richest mixtures. To investigate this reaction at higher temperatures, a shock tube has been used. The shock tube study was performed over a wide range of experimental temperatures, pressures, and equivalence ratios, with air used as the fuel diluent. The ignition temperatures were recorded over the range 980–1740K, at reflected shock pressures of 1, 10, and 30atm. Mixtures were investigated at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in order to determine the effects of fuel concentration on reactivity over the entire temperature range. Using a jet-stirred reactor, the formation of numerous reaction products has been followed at temperatures from 550 to 1100K, at atmospheric pressure, and at equivalence ratios of 0.25, 1.0, and 2.0. Slight low-temperature reactivity (below 750K) with a NTC region has been observed, especially for the leanest mixtures. A detailed chemical kinetic model has been written based on rules similar to those considered for alkanes by the system EXGAS developed at Nancy. Simulations using this model have been compared to the experimental results presented in this study, but also to results in the literature obtained in a jet-stirred reactor at 10bar, in the same rapid compression machine for stoichiometric mixtures, in a plug flow reactor at 1069K and atmospheric pressure, and in a low-pressure (0.066bar) laminar premixed methane flame doped with n-butylbenzene. The observed agreement is globally better than that obtained with models from the literature. Flow rate and sensitivity analyses have revealed a preponderant role played by the addition to molecular oxygen of resonantly stabilized, 4-phenylbut-4-yl radicals.

Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.

Observation of Flame Stabilized at a Hydrogen-Turbojet-Engine Injector Installed into a Lab-Scale Combustion Wind Tunnel

A lab-scale combustion wind tunnel was developed for investigation of low-pressure ignition and flame holding in a sub-scale pre-cooled turbojet engine with hydrogen fuel in order to make engine start at high altitudes sure. The combustion wind tunnel is a blow-down type. A fuel injector of the sub-scale pre-cooled turbojet engine was installed into the combustion wind tunnel. Conditions in which a flame can be stabilized at the fuel injector were examined. The combustor pressure and equivalence ratio were varied from 10 to 40 kPa and from 0.4 to 0.8, respectively. The mean inlet air velocity was varied from 2 to 48 m/s. Flames stabilized at 20 kPa in pressure and 0.6 in equivalence ratio were observed. It was found that the decrease in the combustor pressure narrows the mean inlet air velocity range for successful flame holdings. Flame holding at lower combustor pressures is realized at the equivalence ratio of 0.4 in the low mean inlet air velocity range, and at the equivalence ratio of 0.6 in the high mean inlet air velocity range. Flame luminosity is the largest near the fuel injector. The flame luminosity distribution becomes flatter as the increase in the mean inlet air velocity.

813 サブスケール予冷ターボジェットエンジン燃料インジェククの液体燃料対応に関する低圧環境保灸肬能試験(G01-3 燃焼,G01 熱流体部門)

813 サブスケール予冷ターボジェットエンジン燃料インジェククの液体燃料対応に関する低圧環境保灸肬能試験(G01-3 燃焼,G01 熱流体部門)