Burak Atakan

Effects of a sampling quartz nozzle on the flame structure of a fuel-rich low-pressure propene flame

The perturbation of a flat, premixed, fuel-rich propene/oxygen/argon flame at 50 mbar by a sampling nozzle used in typical molecular beam mass spectrometry (MBMS) experiments was analyzed. Two-dimensional nozzle effects were visualized with laser-induced fluorescence (LIF) of OH, and the influence of the nozzle on the temperature profile along the burner centerline was examined using LIF of seeded NO, both in the flame with and without the nozzle. Temperature deviations of the order of 500 K were observed. For the nozzle-perturbed flame, two strategies for the determination of temperature effects were pursued. First, temperatures were only measured directly in front of the nozzle for different nozzle positions, as in previous work. Secondly, individual temperature profiles at different heights above the burner were determined for each nozzle position, thus simulating the conditions for a nonreacting species sampled by the nozzle along the burner centerline. To investigate the influence of the nozzle on the flame structure, the latter was modeled using the measured flame temperature profiles, with and without the nozzle. Peak concentrations and the positions where they are a maximum are compared for a variety of species, and concentration profiles are analyzed for selected examples, in association with the experimental results. The consequences for a meaningful comparison of MBMS data with flame model calculations are discussed with particular attention to the formation of C6 species (benzene) in this fuel-rich propene flame.

Fuel-rich flame chemistry in low-pressure cyclopentene flames

Laminar premixed flat cyclopentene/oxygen/argon Barnes with different stoichiometries (C/O = 0.6. 0.77, and 0.94) were studied at 50 mbar under fuel-rich non-sooting conditions. This study was motivated by the scarcity of information on C-5 fuel combustion which is ill contrast with the potential importance of C-5 species reactions in benzene formation. Concentrations as a function of height above the burner were measured for more than 30 stable and radical species using molecular beam mass spectrometry Temperature was measured in the unperturbed flame with laser-induced fluorescence of seeded NO. Stable species concentrations in the burned gases were found in good agreement with equilibrium calculations. For information on the flame structure in the reaction zone, species profiles for intermediates of relevance in the formation of aromatics were inspected regarding in particular several CxHy compounds with 2 less than or equal to x less than or equal to 10. The measured data was analyzed with respect to the formation of C-6 species, in particular of benzene as a key species in the soot formation mechanism. A reaction flow analysis has been performed which reveals striking differences to other fuels, including acetylene and propene. It does not seem feasible to rely on a single dominant pathway to benzene in cyclopentene flames. Reactions of C5H5 and C5H6 were found of importance, that of C5H6 + CH3 being of similar influence on C6H6 formation as the propargyl recombination, a result of interest for detailed flame modeling.

Kinetic studies of the gas-phase reactions of CN with O2 and H2 from 294 to 1000 K

Abstract Absolute rate coefficients for the reactions of CN(ν=1, 0) radicals with O2 (k1) and H2 (k2) were measured from 294 to 1000 K in a heatable quartz reactor by means of a laser photolysis/laser-induced fluorescence technique. The data fit the expressions k1 (T) (CN(ν=0)) = (8.7±1.0) × 1012 exp[ + (1.8±0.4) kJ/RT] cm3 mol−1 s−1, k1 (T) (CN(ν=1)) = (8.4±1.2) × 1012 exp [+(1.2± 0.4) kJ/RT] cm3 mol−1 s−1 and k2 (T) (CN(ν=0)) = (3.1±0.3) × 105T2.45 exp[− (9.3±0.2) kJ/RT] cm3 mol−1 s−1.

REMPI temperature measurement in molecular beam sampled low-pressure flames

The cooling effect in molecular beam (MB) sampling from low-pressure flames and gas mixtures was investigated. Although the MB method is often used to study the flame structure of low-pressure flames, typically combined with mass spectrometric (MS) detection, it is poorly characterized. The temperature of the MB must be known, especially if species concentrations are to be measured with spectroscopic methods, like resonance-enhanced multiphoton ionization (REMPI) spectroscopy. In the present study, two independent MBMS instruments, which are very similar to those used previously by different groups, were investigated starting with pressures of 40 and 50 mbar in the burner chamber. The rotational temperatures of NO and benzene were determined using REMPI spectroscopy for different initial conditions: the ions were separated by time-of-flight mass spectrometers. Two REMPI excitation schemes were applied: for NO the first step was always the excitation of the A-X transition near 225 nm, while benzene was excited and ionized at wavelengths near 259 nm. Unexpectedly, molecular beams from cold-gas flows were cooled very slightly by 10%–25%. In the molecular beams derived from low-pressure flames, the cooling effect was stronger, with final rotational temperatures of 300–400 K, but the MB temperature was virtually independent of the initial temperature. A possible explanation of this finding would be that the cooling takes place to a large extent by wall collisions within the nozzle and to a lesser degree by intermolecular collisions.

Fuel-rich propene and acetylene flames: a comparison of their flame chemistries

Abstract Laminar premixed flat propene and acetylene flames with different C/O ratios have been investigated at 50 mbar. Profiles of a variety of stable and radical species were obtained by molecular beam sampling mass spectrometry (MBMS). The temperature profiles were measured with laser-induced fluorescence (LIF) of OH and NO. In the high-temperature regime, the OH radical was probed, whereas NO was added to the fresh gases as a temperature indicator for the cooler regions. In evaluating the measurements, emphasis was laid on the formation of C 6 species, in particular of C 6 H 6 , as one of the key species in soot formation. Concentration profiles for more than 30 species have been analyzed, and the observed differences in the concentrations of several intermediates, in particular potential precursors of benzene, are discussed. A reaction flow analysis has been performed to reveal that striking differences exist in the chemistry of fuel-rich propene and acetylene flames, a result which is of value for flame modeling. For the flames investigated, the observed differences in the concentration profiles of some potential precursors of benzene are discussed in comparison with the predictions of flame modeling.

An experimental study of fuel-rich 1,3-pentadiene and acetylene/propene flames

Abstract Fuel-rich laminar premixed flat flames of an acetylene/propene (1:1) mixture and of 1,3-pentadiene were investigated at 50 mbar in order to compare their flame chemistries for identical C/H (0.625) and C/O (0.77) ratios; under these conditions, observed differences in the reaction pathways should be related to fuel structure. Concentrations of the most important species for benzene formation were obtained by molecular beam mass spectrometry (MBMS). Temperature was measured with laser-induced fluorescence (LIF) of seeded NO. The burnt gas temperatures in both flames were similar with maximum values of 2250 K and 2100 K, respectively. The data were analyzed with respect to the formation of C 6 species, in particular to that of benzene as a key species in the soot formation mechanism. As a consequence of different fuel-specific primary decomposition reactions, the two flames show a strikingly different pattern of intermediate compounds, enhancing different possible benzene formation pathways. Relative reaction flows were also calculated from the experimental results which confirm this observation; however, large uncertainties in some important rate coefficients are noted. While the C 3 H 3 recombination reaction contributes an important fraction of benzene formed in each flame, the contribution of e.g. C 4 H 5 + C 2 H 2 cannot be overlooked in the 1,3-pentadiene flame, C 4 H 5 being an important pyrolysis product of 1,3-pentadiene. The present results can also be compared to those obtained under similar conditions in pure acetylene and pure propene flames as well as in flames burning further C 5 fuels including 1-pentene and cyclopentene. The consistent data sets provided here as part of this series of systematic investigations should be valuable for testing flame models that include the fuel-rich chemistry of higher hydrocarbons.

Kinetic measurements and product branching ratio for the reaction NH2+NO AT 294–1027 K

Abstract Absolute rate coefficients for the reaction NH 2 +NO were measured by following the OH radical formation at 294–1027 K in a heatable quartz reactor by means of the laser photolysis/laser-induced fluorescence technique. The data fit the expression k 1 = (7.9±1.0) × 10 15 T −1.17±0.25 cm 3 mol −1 s −1 . In addition, the branching ratio β into the OH product c to be 0.1±0.025 at 300 K; β increases with increasing temperature and reaches a value of 0.19±0.05 at 1000 K.

Diamond deposition in low-pressure acetylene flames : In situ temperature and species concentration measurements by laser diagnostics and molecular beam mass spectrometry

Abstract Diamond deposition in a flat, premixed acetylene–oxygen–argon flame at 50 mbar was investigated to characterize the reactive gas phase in the vicinity of the substrate. For this, flames with and without a substrate present were analyzed as a function of stoichiometry; also, the distance between the substrate and the burner was varied. Optimal conditions for the deposition of diamond films were found for oxygen–acetylene ratios of 1.3 and 1.4 and at distances between substrate and burner of 8, 9, and 10 mm. The flame structure in this region was investigated. In particular, gas temperature and OH radical concentrations were measured by laser-induced fluorescence (LIF). Furthermore, hydrogen atoms were monitored using three-photon excitation and subsequent fluorescence detection. Molecular beam mass spectrometry was employed to obtain an overview of stable species and hydrocarbon intermediates. The results provide a substantial experimental basis for comparison with theoretical models and are consistent with earlier observations, which stress the importance of H and CH 3 for the diamond deposition process. In addition, the observations indicate the participation of hydrocarbon species with more than 2 carbon atoms, e.g., C 3 H 3 , C 4 H 3 , and C x H 2 with x = 4, 6, or 8, in the gas-phase reactions controlling the deposition of diamond; an active role for these species in diamond chemical vapor deposition (CVD) has not been discussed before. As a first interpretation, diamond formation seems to be controlled by a counterbalance between OH and hydrocarbon intermediates at a position in the flame where sufficient H-atoms and CH 3 radicals are present to support diamond film growth.

Light emitting diode excitation of Cr3+:Al2O3 as thermographic phosphor: experiments and measurement strategy

Light emitting diode (LED) excitation of thermographic phosphors for temperature measurements was investigated in the present work. A near-UV and a green LED excited the phosphorescence of sol?gel deposited chromium-doped alumina (Cr3+:Al2O3, ruby) on silicon in different experiments. The influence of the pulse length on signal intensity was measured and is discussed theoretically. From this, measurement strategies are recommended in order to obtain high signal levels. The temperature-dependent phosphorescence between room temperature and 800 K is compared to previously obtained laser excited phosphorescence. The phosphorescence decay time of the ruby films is found to be slightly dependent on the excitation pulse duration but neither on the excitation wavelength nor on the oxygen partial pressure in the atmosphere. In many situations the less expensive LED excitation appears to be a good alternative to the laser excitation methods. In addition, a strategy is proposed for measuring two-dimensional surface temperatures using two gated cameras; the validity was proven for pointwise measurements.

Contributions to the investigation of reaction pathways in fuel-rich flames

Development and validation of detailed reaction mechanisms for fuel-rich combustion have a continuing need for quantitative experimental flame data. In this study, an overview is presented of recent experimental investigations of a series of fuel-rich premixed low-pressure flames burning acetylene, propene, linear and cyclic C5-alkenes and C5-alkanes with a combination of laser spectroscopy and molecular beam mass spectrometry (MBMS). Particular attention was devoted to the reaction pathways leading to the first aromatic ring. Fuel-specific aspects with respect to benzene formation are discussed. The potential of resonance-enhanced multi-photon ionisation (REMPI) MBMS as a quantitative technique for the measurement of stable species is examined for benzene as an example. Also, first results of the investigation of a fuel-rich ethanol flame under similar conditions are given. Advantages and potential drawbacks of the applied diagnostic methods are discussed in view of the importance of reliable, quantitative measurements for the understanding of fuel-rich chemistry preceding polycyclic aromatic hydrocarbon (PAH) and soot formation as well as for the related modelling of these chemical processes.