Flat Premixed Flames Research Articles

Comprehensive Assessment of STGSA Generated Skeletal Mechanism for the Application in Flame-Wall Interaction and Flame-Flow Interaction

In this study, we conduct a thorough evaluation of the STGSA-generated skeletal mechanism for C2H4/air. Two STGSA-reduced mechanisms are taken into account, incorporating basic combustion models such as the homogeneous reactor model, one-dimensional flat premixed flame, and non-premixed counterflow flame. Subsequently, these models are applied to more complex combustion systems, considering factors like flame-flow interaction and flame-wall interaction. These considerations take into account additional physical parameters and processes such as mixing frequency and quenching. The results indicate that the skeletal mechanism adeptly captures the behavior of these complex combustion systems. However, it is suggested to incorporate strain rate considerations in generating the skeletal mechanism, especially when the combustion system operates under high turbulent intensity.

Investigation of temperature correction methods for fine wire thermocouple losses in low‐pressure flat premixed laminar flames

Accurate temperature measurements in reactive combustion systems are crucial for kinetic and heat transfer studies. Intrusive temperature measurement methods, such as thermocouples, are still widely used in experimental combustion research applications. Thin wire customized type B thermocouples, (Pt Rh6%-Pt Rh30%), coated with BeO (Beryllium oxide) - Y2O3 (Yttrium oxide) anti-catalytic ceramic, are used in this study to measure the flame temperature at low pressure of three H2/CH4/CO-air flat flames at different equivalence ratios. In the range 500–1700 K, the measured flame temperatures are corrected for radiation losses with two different well-known approaches, the experimental electrical compensation method (ECM) and the numerical heat transfer correction method (HTM). Both methods are applied to the measured flame temperatures, and their performance and limits are assessed, highlighting the primary sources of uncertainties. This work aims to shed light on the crucial issues related to the temperature correction methods primarily used in the experimental combustion diagnostic. A strategy to reduce the uncertainties of the two methods is proposed and applied to the measured temperature flames. The propagation of the errors on the main parameters involved in the numerical correction method is carried out to estimate the final uncertainty of the corrected gas temperatures. Finally, a new approach is proposed to quantify the error incurred by neglecting conductive heat transfer between the thermocouple bead and the connecting thermocouple wires.

Combined effect of experimental and kinetic uncertainties on NO predictions in low-pressure premixed laminar H2/CH4/CO-air and H2/CH4/CO/C6H6-air flames

Accurate predictions of nitrogen oxides (NOx) are crucial for the design of low-emissions combustion technologies and require a truthful description of chemical kinetics and underlying thermo-chemical field. Nowadays, uncertainty remains in the reaction rates of different pathways of NOx formation. Experiments in simple systems, such as 1D flat premixed laminar flames, are necessary to shed light on the NOx formation process. However, experimental uncertainty cannot be avoided and influences the subsequent NOx formation modelling. In this context, forward propagation of experimental and kinetic uncertainties can be beneficial to assess their impact on the predictions of NOx emissions. In this work, forward Uncertainty Quantification (UQ) is performed via Polynomial Chaos Expansion (PCE), based on a sparse grid, to quantify the influence of the variability of temperature measurements and kinetic parameters on NO predictions in H2/CH4/CO-air 1D premixed laminar flames at low pressure, without and with 1.12% of benzene (C6H6) in the fuel blend. A significant uncertainty affects the prediction of NO for all the investigated flames. The measured temperature, with its uncertainty, is proven to be one of the most sensitive factor affecting the prediction of NO, particularly for the flames operated in lean and stoichiometric conditions. The results demonstrate that an accurate quantification of the uncertainty affecting the temperature measurements is critical for the development of predictive NOx formation models.

Dependence of Soot Primary Particle Size on the Height above a Burner in Target Ethylene/air Premixed Flame

ABSTRACT McKenna burner providing premixed, laminar, steady flat flame is widely used in the combustion community. The ethylene/air flame with equivalent ratio of 2.34 is considered as one of the target flames for the development and calibration of optical diagnostics of soot particles. In this work, the accurate soot particle sizing in dependence on the height above a burner was performed using the methods of transmission electron microscopy and laser-induced incandescence. It was found, that shielding co-flow gas type and rate together with small variation of the height of stabilization plate in flat premixed flame burner is not influenced on the center flame temperature and on the soot particle size measurement results. Also, the small variation of the velocity of the cold gas in the 5% range at the same equivalence ratio is not influenced by the results of flame temperature and soot particle size measurements. From the other side, the variation of the burner stabilization plate configuration and material of the sintered plate results in the essential differences of the measured values. The accomplished data analysis resulted in a reliable dependence of soot size in the target flame on the height above a burner, which can be applied for validation of soot formation models.

Determination of the volume fraction of soot accounting for its composition and morphology

The soot volume fraction, fv, is essential for combustion engineering, air quality and climate modeling. It is commonly obtained from mobility or optical data assuming soot spheres of constant composition that tend to overestimate fv by up to an order of magnitude. Here, a method is presented for estimation of the fv from soot mobility or optical diagnostics data accounting for the realistic soot morphology and composition. With mobility data, the fv is determined by a relation for soot morphology (fractal-like structure) and the average primary particle diameter from microscopy, gas adsorption or power laws. Such fv and the corresponding soot volume/mass size distributions are in excellent agreement with accurate but tedious soot mass-mobility measurements in premixed flames along various heights above the burner (HAB) as well as from the exhaust of diffusion flames and diesel engines. With light extinction data, the fv is obtained by explicitly accounting for soot composition (through its optical band gap and refractive index, RI) and the light absorption enhancement (due to multiple light scattering within soot agglomerates) in the absorption function, E(RI). So as nascent soot matures by surface growth and agglomeration, its optical band gap decreases while its mobility diameter, carbon to hydrogen (C/H) ratio and fv increase, increasing the soot E(RI) from 0.22 to 0.40, in excellent agreement with laser induced incandescence data from flat premixed ethylene flames. Using this variable E(RI), the soot fv from light extinction data in these flames is up to 50% lower than that using standard constant E(RI) for soot spheres.

Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene

Pinenes and pinene dimers have similar energy densities to petroleum-based fuels and are regarded as alternative fuels. The pyrolysis of the pinenes is well studied, but information on their combustion kinetics is limited. Three stoichiometric, flat premixed flames of the C10H16 monoterpenes α-pinene, β-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomer-resolved identification and quantification of the flame species formed during the combustion process. Flame-sampling molecular-beam mass spectrometry even enables the detection of very reactive radical species. Myrcene was chosen because of its known formation during β-pinene pyrolysis. The quantitative speciation data and the discussed decomposition steps of the fuels provide important information for the development of future chemical kinetic reaction mechanisms concerning pinene combustion. The main decomposition of myrcene starts with the unimolecular cleavage of the carbon-carbon single bond between the two allylic carbon atoms. Further decompositions by β-scission to stable combustion intermediates such as isoprene (C5H8), 1,2,3-butatriene (C4H4) or allene (aC3H4) are consistent with the observed species pool. Concentrations of all detected hydrocarbons in the β-pinene flame are closer to the myrcene flame than to the α-pinene flame. These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during β-pinene combustion indicate that β-pinene undergoes isomerization to myrcene under the studied flame conditions. Aromatic species such as phenylacetylene (C8H6), styrene (C8H8), xylenes (C8H10), and indene (C9H8) could be clearly identified and have higher concentrations in the α-pinene flame. Consequently, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes α-pinene, β-pinene, and myrcene give insights into their combustion kinetics.

Combustion synthesis of SiO2 nanoparticles using flat premixed flame

Recently, numerous researches have actively progressed for producing nanoparticles such as SiO2, TiO2, and Al2O3 used in various commercial applications. In particular, SiO2 nanoparticle is widely used for the fiber optics, pigment and dye, ceramic semiconductor industries. Synthesis of SiO2 by combustion process using gas-phase silane as a precursor is very attractive because the flame aerosol method can be easily scaled up to mass production and economically feasible. Therefore it is necessary to investigate the fundamental research such as nuclei inception, coagulation, coalescence, and morphology during combustion synthesis for the nanoparticle formation. In this study, we performed experimental research to generate the SiO2 nanoparticle using the methane - hydrogen flat premixed flame which can show the effect of hydrogen on the formation of SiO2 nanoparticles especially. We found out the effect of the H2 content in the fuel mixture on the characteristics of SiO2 particle formation when the optimum condition of the flat surface flame is satisfied. Measurements of the particle morphology and size by TEM (Transmission Electron Microscopy) showed that the hydrogen enhancement in fuel mixture was likely to prevent aggregation process of SiO2 primary particles effectively.

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.

Structural response of different Lewis number premixed flames interacting with a toroidal vortex

Simultaneous measurements of temperature, CH* and OH* chemiluminescent species are carried out to explore the impact of stretch rate and curvature on the structure of premixed flames. The configuration of an initially flat premixed flame interacting with a toroidal vortex is selected for the present study and reasons for this choice are discussed. Lewis number effects are assessed by comparing methane and propane flames. It is emphasized that the flame structure experiences very strong variations. In particular, the flame is shrunk (broadened) in the initial (final) period of the interaction with the vortex where strain rate (curvature) contribution of the stretch rate is predominant. By further analysing independently the thickness of the preheat and reaction zones, it is shown that for propane flames, not only the former but also the latter is significantly altered in zones where the flame curvature is negative. Changes in the reaction zone properties are further emphasized using CH* and OH* radicals. It is demonstrated that higher thermal diffusivity plays a significant role around curved regions, in which the enhanced diffusion of heat leads to a strong increase of CH* compared to OH* intensity. As an overall conclusion, this study suggests that it would be interesting to reassess the internal flame structure at lower and moderate Karlovitz numbers since changes might appear for a moderate vortex intensity with typical size much larger than the flame thickness.

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII, HIM, and 1 nm-SMPS

ABSTRACTLaser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermo...

A laminar flame investigation of 2-butanone, and the combustion-related intermediates formed through its oxidation

2-butanone (methyl ethyl ketone) is a recently identified high-octane, next-generation biofuel candidate, synthesized through microbiological pathways from biomass. Its high octane number (RON = 117), low boiling point (80°C), and relatively high vapor pressure (10.8kPa at 20°C) make 2-butanone a potentially good candidate for use in spark-ignition engines. The flame structure and species formed in 2-butanone combustion are of interest when further considering this compound for use as a fuel. To this end, the present study has, for the first time, quantitatively measured species profiles within a fuel-rich (ϕ = 1.6) laminar premixed flat flame of 2-butanone. Two experiments which used different facilities and measurement techniques were combined to provide a cross comparison of the measured species. One experiment was performed using electron ionization (EI) molecular-beam mass spectrometry (MBMS), and the second relied on synchrotron-generated vacuum ultraviolet (VUV) photoionization (PI) MBMS. Quantitative mole fraction profiles for about 45 species were obtained from the two independent experiments that are found to be in good agreement. The experiments have identified the formation of a number of toxic oxygenated intermediates, such as methyl vinyl ketone (MVK), acetaldehyde, and formaldehyde. In addition, the measurements have also provided quantitative species profiles for a number of soot precursor species, the concentrations of which are notably low.These measurements provide useful validation targets for the further development of detailed chemical kinetic models. As a first test, an existing 2-butanone model was examined against the experimental data with the main aim to identify elements of the model that may need improvement.

A comprehensive experimental and kinetic modeling study of tert-butanol combustion

The combustion of tert-butanol was studied experimentally in this work, including flow reactor pyrolysis at 30–760Torr, laminar flat premixed flame at 30Torr, coflow methane/tert-butanol diffusion flame at atmospheric pressure, and laminar flame speed at 1–10atm. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was applied in the investigation of pyrolysis and flame, and the schlieren technique was used in the measurement of laminar flame speed. A detailed kinetic model consisting of 209 species and 1486 reactions was developed in this work, which can predict the decomposition of tert-butanol and the formation of aromatics over wide temperature and pressure ranges. Kinetic analysis illustrates that the unimolecular decomposition of tert-butanol is the most important and sensitive reaction class in different combustion modes, which readily produce the most important aromatic precursors: tert-butanol→iso-butene→2-methylallyl→allene→propargyl. As a result, the production of benzene, phenyl radical, and benzyl radical are all enhanced in the combustion of tert-butanol compared to those of other butanol isomers. The self-combination of propargyl and the combination of propargyl with other resonantly stabilized radicals are the key steps of aromatics formation in the combustion of tert-butanol.

Quantitative measurements of one-dimensional OH absolute concentration profiles in a methane/air flat flame by bi-directional laser-induced fluorescence**Project supported by the National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2012YQ040164), the National Natural Science Foundation of China (Grant Nos. 61275127 and 91441130), the China Postdoctoral Science Foundation (Grant No. 2014M560262), and the Postdoctoral Fellowship in Heilongjiang Province,

The one-dimensional (1D) spatial distributions of OH absolute concentration in methane/air laminar premixed flat flame under different equivalence ratios at atmospheric pressure are investigated by using bi-directional laser-induced fluorescence (LIF) detection scheme combined with the direct absorption spectroscopy. The effective peak absorption cross section and the average temperature at a height of 2 mm above the burner are obtained by exciting absorption on the Q1(8) rotational line in the A2Σ+ (ʋ′ = 0) ← X2Π (ʋ″ = 0) at 309.240 nm. The measured values are 1.86×10−15 cm2 and 1719 K, respectively. Spatial filtering and frequency filtering methods of reducing noise are used to deal with the experimental data, and the smoothing effects are also compared using the two methods. The spatial distribution regularities of OH concentration are obtained with the equivalence ratios ranging from 0.8 to 1.3. The spatial resolution of the measured result is 84 μm. Finally, a comparison is made between the experimental result of this paper and other relevant study results.

Wavelength dependence of extinction in sooting flat premixed flames in the visible and near-infrared regimes

A diode-laser-based setup was used for extinction measurements at various heights in two flat premixed sooting ethylene/air flames in order to investigate the wavelength dependence of the extinction. Twelve laser wavelengths were used in the interval 405–1064 nm, and with this setup, a sensitivity of ~10−4 was achieved for the extinction. Soot volume fractions inferred from the extinction measurements were always higher in the visible spectral region than in the infrared, an effect associated with additional absorption by large hydrocarbons, such as polycyclic aromatic hydrocarbons. For wavelengths greater than around 700 nm and at high heights above burner (HAB), the extinction shows an inverse dependence with wavelength, in concurrence with wavelength-independent optical properties of soot. In the same spectral region at lower HAB, the wavelength dependence is observed to be stronger, indicating that the soot optical properties show variation with wavelength. Furthermore, a continuous change in the wavelength dependence of the absorption function E(m) is shown from nascent soot at lower heights to more mature soot at higher heights. The presented results are of importance for soot diagnostics relying on the efficiency of light absorption by soot such as laser-induced incandescence (LII).

Comprehensive kinetic modeling and experimental study of a fuel-rich, premixed n-heptane flame

An existing comprehensive kinetic hydrocarbon oxidation model has been augmented and revised for a detailed analysis of n-heptane flame chemistry. The analysis was enabled by experiments in which the detailed species composition in a fuel-rich flat premixed (ϕ=1.69) n-heptane flame at 40mbar has been studied by flame-sampling molecular-beam mass spectrometry using electron impact ionization. Mole fraction profiles of more than 80 different species have been measured and compared against the new detailed kinetic model consisting of 349 species and 3686 elementary reactions. For all major products and most of the minor intermediates, a good agreement of the modeling results with the experimentally-observed mole fraction profiles has been found. The presence of low- and intermediate-temperature chemistry close to the burner surface was consistently observed in the experiment and the simulation. With the same kinetic model, n-heptane auto-ignition timing, flame speeds and species composition in a jet-stirred reactor have been successfully simulated for a broad range of temperatures (500–2000K) and pressures (1–40bar). The comprehensive nature and wide applicability of the new model were further demonstrated by the examination of various target experiments for other C1 to C7 fuels.

Measurements and modeling of laser-induced incandescence of soot at different heights in a flat premixed flame

Laser-induced incandescence (LII) measurements were conducted in a laminar flat rich premixed CH4/O2/N2 flame at atmospheric pressure using a Nd:YAG laser operated at 1,064 nm. Time-resolved LII signals were measured at 610 nm (20 nm FWHM) at different heights above the burner exit surface and at different laser fluences. The averaged soot temperature over 10 ns around the laser peak was determined from spectrally resolved LII signals over the spectral range of 500–630 nm. Flame temperature and soot primary particle diameter and aggregate size at several flame heights were obtained using the NO-LIF thermometry and from image analyses of sampled soot. The time-resolved LII signal and the averaged soot temperature were modeled using two LII models. To achieve reasonable agreement between the modeled and experimental LII signal and soot temperature at low laser fluences, it is necessary to use flame height-dependent soot absorption functions. Such practice can be attributed to the continuous variation of soot thermal and/or radiative properties during the surface growth process after inception. At high laser fluences, both LII models are unable to reproduce the overall shape of the normalized time-resolved LII signal profiles and the averaged soot temperatures.

Highly flexible burner concept for research on combustion technologies with recirculation of hot combustion products

This paper reports the development and testing of a research coflow burner that generates laminar flames in a hot and diluted environment, which is adequate for studying the operating conditions found in practical combustors that use flue gas recirculation techniques. The burner has two flame zones; the first is an annular laminar premixed flat flame stabilized by a perforated plate, which generates a hot oxygen-rich flue gas mixture. The second is a non-premixed laminar flame, which uses the hot oxygen-rich flue gas mixture as an oxidizer. A methodology based on coflow calculations, which is the most significant component, was designed to provide different temperatures and dilution levels. The burner was built based on this design and tested using both intrusive and non-intrusive measurement techniques. The device emulated several features of practical combustor systems but with a simpler geometry, well-defined boundary conditions and using the simplest configuration found in literature (laminar flames). In a preliminary study, the MILD combustion regime was emulated by generating methane laminar non-premixed flames at oxygen concentrations between 3% and 9% and temperatures between 973 K and 1173 K.

Experimental evaluation of DC electric field effect on the thermoacoustic behaviour of flat premixed flames

One promising approach to eliminate thermoacoustic instabilities in combustion appliances is the use of adaptive control of the flame/burner acoustic transfer function (TF). Application of a DC electric field (EF) as a spatially distributed, easily and quickly adjustable and low-energy method to affect the flame behaviour may be considered as a possible actuation method to control the flame TF. Experimental evaluation of such a possibility is the main goal of the present study. The effect of a DC EF on the acoustic TF of premixed flat burner-surface stabilized flames is studied systematically as a function of the following parameters: flow velocity, equivalence ratio, applied voltage and burner geometry. It is established that the response of the flame TF on the DC EF can be characterized as a TF shift towards higher frequencies. The mechanism of TF alteration is related to the decrease of the flame stand-off distance and the related increase of the burner deck surface temperature. From a practical point of view, the efficiency of the EF control of the flat flame TF is restricted to a relatively narrow frequency range around the position of the TF resonance peak. To get insight into the physics of the EF–flame interaction, the method of [1] to measure the EF effect on the adiabatic flame speed is improved and the measurement range is extended. The new measurements allow a revision of previous results and allow an explanation for the ambiguity in the old measurements.

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.

Numerical and experimental study of the combustion of acetic acid in laminar premixed flames

Experimental mole fraction profiles of chemical species (stable, radical and intermediates) have been measured in three CH 3COOH/O 2/Ar flat premixed flames burning at low pressure (50 mbar) and with equivalence ratios equal to 0.77, 0.9 and 1.05, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionisation (EI). Experimental results have highlighted an important role played by ketene (CH 2CO) as an intermediate during the combustion of acetic acid. In order to simulate the experimental results, a sub-mechanism concerning the combustion of CH 3COOH and CH 2CO has been built and added to a global mechanism recently proposed. This ensures a reasonably good modelling for the structures of acetic acid flames.