N-butane Flames Research Articles

Sooting transition diagnostics in counter-flow flames of C4 isomer fuels

Macroscopic transition processes of soot formation for 1-butene, isobutene, n-butane and isobutane counterflow diffusion flames from sooting-free to heavy-sooting were studied by varying fuel concentration (XF) in the fuel stream. A novel optical diagnostic algorithm was proposed to classify different sooting transition stages in flames. Effects of saturated/unsaturated bonds and linear/branched structures of fuel molecules on the sooting transition were investigated in details. Flame species were detected and quantified by on-line gas chromatography and also kinetic analysis was performed. KL factor was obtained by optical diagnostics to qualitatively compare soot concentrations. Results showed that XF required for 1-butene flame to reach the critical state of macroscopic sooting transition was the lowest, followed by isobutene, isobutane and n-butane flames. Fuels with higher sooting tendencies could achieve entire sooting transition process with a narrower range of XF. Linear butane was more difficult to achieve the sooting transition than branched butane, while linear 1-butene was easier to form soot than branched butene, reflecting the difference between isomeric structures acting on C4 alkenes and alkanes. These might be largely due to the close association of allyl (C3H5-A) with 1-butene. Higher C3 species content in isobutane flame made it have higher sooting tendency than n-butane.

Spherical gas-fueled cool diffusion flames

An improved understanding of cool diffusion flames could lead to improved engines. These flames are investigated here using a spherical porous burner with gaseous fuels in the microgravity environment of the International Space Station. Normal and inverse flames burning ethane, propane, and n-butane were explored with various fuel and oxygen concentrations, pressures, and flow rates. The diagnostics included an intensified video camera, radiometers, and thermocouples. Spherical cool diffusion flames burning gases were observed for the first time. However, these cool flames were not readily produced and were only obtained for normal n-butane flames at 2 bar with an ambient oxygen mole fraction of 0.39. The hot flames that spawned the cool flames were 2.6 times as large. An analytical model is presented that combines previous models for steady droplet burning and the partial-burning regime for cool diffusion flames. The results identify the importance of burner temperature on the behavior of these cool flames. They also indicate that the observed cool flames reside in rich regions near a mixture fraction of 0.53.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

In a small range of rich equivalence ratios, premixed flat flames, so-called nucleation flames, were shown to produce very small-size (2–4 nm) soot particles, which undergo negligible growth with the residence time. In such flames, the contribution of the soot nucleation step with respect to the soot growth process is larger than in standard sooting flames, making nucleation flames ideal target flames to better understand the inception process. In order to scrutinize the transition from non sooting to sooting flames, three premixed atmospheric n-butane/oxygen/nitrogen flames surrounding the nucleation flame conditions were investigated: the flame at ϕ=1.6 is a fuel-rich non-sooting flame, the flame at ϕ=1.75 is a nucleation flame and the flame at ϕ=1.95 is a lightly sooting flame. The soot volume faction (SVF) profiles were previously measured by laser-induced incandescence as well as the soot size distributions. In the present study, mole fractions of stable species up to benzene were measured by gas chromatography while those of naphthalene and pyrene were obtained by jet-cooled laser-induced fluorescence. It is found that acetylene, propyne, benzene, naphthalene and pyrene species show the highest sensitivity with the equivalence ratio, while a two orders of magnitude increase of SVF is observed. The chemical flame structure of these flames was modeled using three kinetic mechanisms of the literature to which a recently developed soot code, based on a sectional approach, was coupled. The chosen model is representative of the current state-of-the-art of models describing all steps of soot formation and growth and fully coupled with the gas phase. The ability of such a model to predict soot volume fraction with reasonable predictions of gas-phase species is validated here based on the new experimental database presented in this work. It also highlights the improvement required on particle inception modeling to correctly predict particle size distributions.

Low-temperature chemistry triggered by probe cooling in a low-pressure premixed flame

Previous studies on sampling probe effects in the premixed flat flame showed that the temperature in the preheat zone can be lowered down to low-temperature oxidation regime (e.g., 400–800 K). In order to investigate the contribution of the low-temperature chemistry in flame-sampling experiments, a stoichiometric laminar premixed flat flame of ethylene/oxygen/argon was investigated at 30 Torr using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) in this work. Ethyl hydroperoxide (C2H5OOH) was identified and quantified in the present experiment, providing an experimental evidence for the existence of low-temperature chemistry in flame-sampling experiments. In addition to C2H5OOH, the formation of several other intermediates like ethanol and formaldehyde was also influenced by the low-temperature chemistry in the present flame-sampling experiment. A literature kinetic model (Hashemi et al., 2017) was used for predictions and analyses. The difference between the predicted maximum mole fractions of C2H5OOH with the perturbed and unperturbed temperature profiles can reach up to five orders of magnitude. The great improvement of the predictions with the perturbed temperature profiles indicates that the observed low-temperature chemistry in the present flame sampling experiment originates from the probe-induced perturbations, which lowers down the temperature window of the preheat zone and leads to a temperature drop of more than 400 K compared with the unperturbed temperature profile. Through modeling analysis, the low-temperature oxidation chemistry of ethylene involved in the present flame-sampling experiment was discussed. The influence of low-temperature chemistry in the present experiment has also been demonstrated by comparing the model predictions with/without key reactions at low temperatures. It is concluded that predicted maximum mole fractions of several low-temperature chemistry related intermediates, i.e. C2H5OOH, ethanol and formaldehyde, are strongly reduced without these reactions, while low-temperature chemistry only has negligible influence on the predictions of the fuel and the majority of flame intermediates. Furthermore, preliminary experiments were also conducted in ethane, propene and n-butane flames under similar conditions, where hydroperoxides can also be observed.

Experimental and numerical investigation of atmospheric laminar premixed n-butane flames in sooting conditions

Experimental and numerical investigation of atmospheric laminar premixed n-butane/O2/N2 flames operating at two equivalence ratios (ϕ=2.16 and 2.32) has been performed. Both flames produce soot particles. Mole fraction of reactants, stable intermediate and combustion products have been obtained using a quartz microprobe coupled to gas chromatography and infrared spectroscopy technique analysis. The temperature profiles were obtained by Laser induced fluorescence of nitric oxide. The experimental results showed that a weak variation of equivalence ratio in sooting conditions introduces an increase of benzene and some unsaturated aliphatic species peak mole fraction. Except for benzene, the peak species mole fraction increase does not exceed a factor of two. A new reaction kinetic model is proposed to predict gaseous species mole fraction. In addition to the good agreement observed with our experimental data base, the proposed mechanism predicts well the detailed data obtained in low pressure rich n-butane premixed flames and high temperature ignition delay times reported in the literature.Reactions path analysis revealed that the first aromatic ring is controlled by C4+C2 (nC4H5+C2H2=C6H6+H) and C3+C3 (self recombination of C3H3 radicals) reactions routes. C4+C2 reaction path predominates in the reaction zone while C3+C3 path tends to predominate in the post flame region. The impact of the equivalence ratio on both reactions and the main differences between our and three selected literature models are discussed.

Influence of the biofuel isomers diethyl ether and n-butanol on flame structure and pollutant formation in premixed n-butane flames

Diethyl ether (DEE) and its isomer n-butanol are both considered as promising fuel additives or neat biofuels. While some effects of their addition to hydrocarbon fuels under engine conditions have been reported, fundamental studies that aim at understanding the joint reaction pathways of such fuel mixtures remain quite scarce. Here, we have chosen n-butane as a well-studied hydrocarbon base fuel, and we have added these oxygenated isomers individually under identical conditions in premixed low-pressure flames. Different combustion behavior of the respective alkane-biofuel mixtures must then be related to the fuel structure. Analyses were performed in five fuel-rich flames including flames of n-butane, DEE, and n-butanol as well as two flames of n-butane doped with 50% DEE or 50% n-butanol. In this series, the carbon-to-oxygen ratio, argon dilution, pressure, and gas velocity were kept constant. More than 40 species in the range of C0–C8 were identified and quantified in each flame by electron ionization (EI) molecular-beam mass spectrometry (MBMS) coupled with gas chromatography (GC). The experiments were partially complemented by tunable synchrotron vacuum ultraviolet (SVUV) photoionization (PI)-MBMS. To assist in the interpretation of the data, a kinetic model was established by combining different sub-mechanisms for these fuels available in the literature. As expected, the formation of toxic carbonyls, such as formaldehyde and acetaldehyde, increased significantly upon addition of both oxygenated fuels to n-butane. Blending n-butane with DEE noticeably reduces the formation of soot precursors, because primary reactions of DEE mainly release C1–C2 hydrocarbon species to the system. n-Butanol addition, however, shows no significant reduction effects or even higher formation of soot precursors. These trends were observed both in the experiments and model predictions, and the higher ability of n-butanol to form soot precursors compared to DEE indeed results mainly from the differences in the fuel structure.

Effects of various densities and velocities on gaseous hydrocarbon fuel on near nozzle flow field under different laminar coflow diffusion flames

An experimental study on the flow characteristics under various laminar coflow diffusion flames was conducted with a particular focus on the buoyancy force exerted from gaseous hydrocarbon fuels. Methane (CH4), ethylene (C2H4), and n-butane (C4H10) were used as the fuels. A coflow burner and the Schlieren imaging technique were used to observe the flow field of each fuel near the nozzle exit as well as the flow characteristics in the flames. The results show that a vortex with a density heavier than air appeared in n-butane near the nozzle exit with a strong negative buoyancy on the fuel steam. As the Reynolds number increased through the control of the fuel velocity of the n-butane flame, the vortices were greater and the vortex tips were moved up from the nozzle exit. In addition, the heated nozzle affected the flow fields of the fuel steam near the nozzle exit.

Direct and Convenient Mass Spectrometry Sampling with Ambient Flame Ionization.

Recent innovations in ambient ionization technology for the direct analysis of various samples in their native environment facilitate the development and applications of mass spectrometry in natural science. Presented here is a novel, convenient and flame-based ambient ionization method for mass spectrometric analysis of organic compounds, termed as the ambient flame ionization (AFI) ion source. The key features of AFI ion source were no requirement of (high) voltages, laser beams and spray gases, but just using small size of n-butane flame (height approximately 1 cm, about 500 oC) to accomplish the rapid desorption and ionization for direct analysis of gaseous-, liquid- and solid-phase organic compounds, as well as real-world samples. This method has high sensitivity with a limit of detection of 1 picogram for propyphenazone, which allows consuming trace amount of samples. Compared to previous ionization methods, this ion source device is extremely simple, maintain-free, low-cost, user–friendly so that even an ordinary lighter (with n-butane as fuel) can achieve efficient ionization. A new orientation to mass spectrometry ion source exploitation might emerge from such a convenient, easy and inexpensive AFI ion source.

Evolution of size distribution of nascent soot in n- and i-butanol flames

The impact of fuel bound oxygen on the sooting behavior of butanol fuels was examined by following the evolution of the particle size distribution function (PSDF) of nascent soot produced in atmospheric pressure burner stabilized stagnation (BSS) flames of n-butanol and i-butanol. Similar experiments were carried out for i-butane and n-butane flames to better understand the influence of fuel structure and the presence of the alcohol group on detailed processes of soot nucleation and growth. In terms of fuel structure, the branched chain functionality has the most observable effect on soot formation. The onset of soot nucleation is faster in the branched fuels in comparison to the straight-chain counterparts. Under the same C/O ratio, however, the butanol flames were found to nucleate soot earlier and have higher soot volume fraction than the butane flames. A combustion reaction model for i-butanol and n-butanol was used to explore the precursor chemistry. Similar to the measured PSDF, benzene is computed to rise earlier in flames of the branched fuels than the straight-chain fuels.

Role of chemical kinetics on the detonation properties of hydrogen /natural gas/air mixtures

The first part of the present work is to validate a detailed kinetic mechanism for the oxidation of hydrogen–methane–air mixtures in detonation waves. A series of experiments on auto-ignition delay times have been performed by shock tube technique coupled with emission spectrometry for H 2/CH 4/O 2 mixtures highly diluted in argon. The CH 4/H 2 ratio was varied from 0 to 4 and the equivalence ratio from 0.4 to 1. The temperature range was from 1250 to 2000 K and the pressure behind reflected shock waves was between 0.15 and 1.6 MPa. A correlation was proposed between temperature (K), concentration of chemical species (mol m - 3 ) and ignition delay times. The experimental auto-ignition delay times were compared to the modelled ones using four different mechanisms from the literature: GRI [Smith PG, Golden DM, Frenklach M, Moriarty NW, Goldenberg M, et al. 〈 http://www.me.berkeley.edu/gri_mech/ 〉 ], Marinov et al. [Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combust Flame 1998; 114:192–213], Hughes et al. [ 〈 http://www.chem.leeds.ac.uk/Combustion/Combustion.html 〉 ], Konnov [Detailed reaction mechanism for small hydrocarbons combustion. Release 0.5 〈 http://homepages.vub.ac.be/ ∼ akonnov/ 〉 , 2000]. A large discrepancy was generally found between the different models. Konnov's model, which auto-ignition delay times predictions were the closest to the measured ones, has been selected to calculate ignition delay times in the detonation waves. The second part of the study concerned the experimental determination of the detonation properties, namely detonation velocity and cell size. Effect of the initial composition, hydrogen to methane ratio and the amount of oxygen in the mixture, as well as the initial pressure on the detonation velocity and on the cell size were investigated. The ratio of methane/(methane + hydrogen) varied between 0 and 0.6 for two different equivalence ratios (0.75 and 1) while the initial pressure was fixed to 10 kPa. A correlation was established between the characteristic cell size and the ignition delay time behind the leading shock of the detonation. It was clearly shown that methane has an important inhibitor effect on the detonation of these combustible mixtures.

STUDIES ON THE LIFTOFF PROPERTIES OF DIMETHYL ETHER JET DIFFUSION FLAMES

The liftoff properties of the DME jet diffusion flame were investigated experimentally and analytically with Emphasis on the influences of flame stretch and fuel oxygen. The present experiments showed that the DME jet diffusion flame exhibited a distinct liftoff phenomenon that differed from other hydrocarbon fuels. This unique phenomenon was analyzed theoretically by taking into consideration the effects of flame stretch and the fuel oxygen. The results showed that the stretch effect had a significant impact on the critical liftoff Schmidt number and the flame liftoff height. Based on these observations, a new criterion for the lifted flame at the blowout limit was presented. The results also demonstrated that the appearance of fuel oxygen in DME increases the fuel mixture fraction at the stoichiometric condition and changes the flame liftoff phenomenon. The effect of fuel oxygen was further investigated by adding air into propane and n-butane diffusion flames. It was found that with the increase of oxygen addition, both propane and n-butane flames change from the direct liftoff regime to the direct blowout regime. The results well described the unique liftoff phenomenon of DME and also applicable to other oxygenated and air diluted hydrocarbon fuels.

Electrochemical Power Generation Directly from Combustion Flame of Gases, Liquids, and Solids

The performance of a solid oxide fuel cell fueled with flames of combustible gases, liquids, and solids was studied. A cell structure in which cells were serially integrated within a disk was also examined. Open-circuit voltages indicated with a single cell and the integrated cell were ∼0.8 and ∼3.5 V, respectively. Maximum power density obtained with the flames of n-butane, kerosine, paraffin wax (candle), and wood were respectively 75, 65, 62, and 5 mW/cm2. The integrated cell gave a maximum power density of 318 mW/cm2 with n-butane flame. Fuel to air ratio distinctly influenced the output. © 2004 The Electrochemical Society. All rights reserved.

Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Laminar Premixed n-Butane Flame

Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane–oxygen–argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings. Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the n-butane flame. Reaction flux analysis showed the propargyl recombination reaction was the dominant pathway to benzene formation. The consumption of propargyl by H atoms was shown to limit propargyl, benzene, and naphthalene formation in flames as exhibited by the large negative sensitivity coefficients. Naphthalene and phenanthrene production was shown to be plausibly formed through reactions involving resonantly stabilized cyclopentadienyl and indenyl radicals. Many of the low molecular weight aliphatics, combustion by-products, aromatics, branched aromatics, and PAHs were fairly well simulated by the model. Additional work is required to understand the formation mechanisms of phenyl acetylene, pyrene, and fluoranthene in the n-butane flame.

Effects of oxygen on soot formation in methane, propane, and n-Butane diffusion flames

Overventilated coflow axisymmetric laminar diffusion flames of methane, propane, and n-butane were used to study the influence of oxygen addition to the fuel side on soot formation. The line-of-sight soot volume fractions and the visible flame profiles were measured as a function of axial location along the centerlines of pure fuel flames, and the flames in which the fuel was diluted either with oxygen or nitrogen at selected temperatures of the reactants, to maintain a constant adiabatic flame temperature. The relative influences of dilution and direct chemical interaction effects of oxygen in the fuel gas mixture were quantified. It was found that, when allowance was made for the influence of dilution and thermal effects, the addition of oxygen to the methane diffusion flame chemically suppressed soot formation. This suppression was argued to be due to the reduction in acetylene concentration in the pyrolysis products as the oxygen mole fraction in methane was increased. The chemical influence of oxygen addition to methane decreased when the adiabatic flame temperature was decreased by decreasing the temperature of the reactants. In propane and n-butane flames, on the other hand, oxygen addition chemically enhanced soot formation. The degree of enhancement was small for low mole fractions of oxygen, but increased with increasing oxygen. When oxygen is added to the fuel side of a diffusion flame, two counteracting chemical effects are expected: Oxygen promotes fuel pyrolysis and hence production of hydrocarbon radicals and H atoms which enhance soot formation. On the other hand, aromatic radicals and critical aliphatic hydrocarbon radicals are removed by reactions with molecular oxygen and oxygen atom. The net chemical influence is the difference of these two counteracting effects.

Two-stage N-butane flame: A comparison between experimental measurements and modeling results

Two-stage autoignition of unburned end-gases is one of the important processes considered to be responsible for homogeneously-initiated engine knock phenomena. Two-stage flame processes were investigated to achieve a better understanding of the low temperature and high temperature chemistry responsible for this type of autoignition behavior. For the first time, a stabilized n-butane two-stage flame was successfully simulated, utilizing a detailed kinetic mechanism involving 141 species and 850 elementary reactions, and results are compared with experimental data obtained by Sochet, et al. from a one dimensional, two-stage flame burner. With the exception of those for butenes and hydroperoxyl radicals, calculated and experimental profiles are found to agree within a factor of two over the entire flame region. A decrease in the estimated rate of reactions R+HO 2 →RO . +OH (R=C 3 H 5 , CH 3 , C 2 H 5 and C 4 H 7 ) is required to obtain agreement of fuel profiles between the two flame zones. In the second stage region, predicted reaction profiles are in general agreement with the experiment, with the exception of formaldehyde and C 4 -oxygenated species, for which consumptions are substantially underestimated. This study suggests that the reaction channels for direct metathesis and stabilized hot adduct formation for the overall process C 4 H 9 +O 2 →C 4 H 8 +HO 2 need further attention.