Laminar Flat Flame Research Articles

Laminar flame speed measurement and combustion mechanism optimization of methanol–air mixtures

AbstractLaminar flame speeds of methanol/air mixtures at 338–398 K are measured by the heat flux method, extending the range of equivalence ratio up to 2.1. And a new optimized methanol mechanism with 94 reactions is proposed by using the particle swarm algorithm, adjusting 20 Arrhenius pre‐exponential factors in their uncertainty domains. The optimized model is compared with eight methanol combustion mechanisms and experimental data published in recent years, covering a wide range of initial temperatures (298–1537 K), pressures (0.04–50 atm) and equivalence ratios (0.5–2.1). The results show that the optimized mechanism not only improves the accuracy of ignition delay time with rapid compression machine at low temperature but also moderately improve the description of laminar flame speed in lean and stoichiometric conditions. Meanwhile, the optimized model significantly enhances the prediction accuracy of CH3and CH2O radical, and perfectly captures the evolution trend of HCO radical in laminar flat flame. Overall, the optimized mechanism provides the best overall description of the currently available measurements, leading to more accurate and comprehensive prediction of ignition delay time, laminar flame speed and species concentration.

LIF-imaging of temperature and iron-atom concentration in iron nitrate doped low-pressure aerosol flat flames

To enable the investigation of the flame chemistry of precursor-laden nanoparticle forming spray flames in quasi-one-dimensional geometry, a matrix burner was developed in which a laminar flat flame is uniformly supplied with liquid aerosol. In the present work, the burner was used to study the interaction of iron(III) nitrate as precursor dissolved in 1-butanol with the flame. Distributions of temperature and relative iron-atom concentrations were measured via OH and Fe laser-induced fluorescence imaging, respectively. Elastic laser light scattering was used to determine the spatial distribution of particles and droplets to characterize the burner and to obtain insight into the particle formation process. The matrix burner was operated with hydrogen or methane. The measured flame temperatures were compared with one- and three-dimensional CFD simulations supporting the development of reaction mechanisms for precursor-laden flames.

Soot formation of renewable gasoline: From fuel chemistry to particulate emissions from engines

Renewable fuels are essential to meet climate protection goals, especially for decarbonizing the existing vehicle fleet. In addition, optimized and specially designed fuels allow emissions to be avoided before they occur. This paper presents fuel effects on emission characteristics of six chemically complex gasoline blends compliant with the DIN EN 228 standard and one ethanol/gasoline blend, focusing on particle emissions. First, the sooting propensity of the fuels was examined using different soot indices, such as the Particulate Matter Index (PMI), the Oxygen Extended Sooting Index (OESI), the Yield Sooting Index (YSI), and the soot threshold (φST), which is drawn on measurements of the particle number (PN) density in the exhaust gas of a laminar flat flame. Second, PN emissions (particle number concentration) from a production-calibrated turbo gasoline direct injection engine with four cylinders were investigated. Third, soot precursor chemistry was investigated using a laminar flow reactor at atmospheric pressure. Finally, correlations to fuels’ volatility and chemical characteristics were made for the complete data set. Based on the soot precursor chemistry in the homogeneous gas phase reaction, the analysis shows that engine PN emissions are primarily influenced by fuel chemistry for the investigated engine conditions. In particular, the composition of the hydrocarbon fraction is decisive, with the heavy C9+ aromatics and the upper distillate significantly influencing soot formation. In order to reduce PN engine emissions, future fuels should contain fewer soot-increasing hydrocarbon fractions.

The effects of various flame retardants on the combustion of polypropylene: Combining optical diagnostics and pyrolysis fragment analysis

The assessment of the efficacy and mode of action of flame retardants in polymers requires a thorough knowledge of the coupling of the presence of chemical inhibitors with the underlying heat and mass transport processes. In this experimental work, we combine a thermal decomposition analysis with optical combustion diagnostics in different test environments to gain a comprehensive understanding of the relevant sub-processes and demonstrate how this approach provides future opportunities for the research regarding the mode of action of flame retardants. Four different composites of polypropylene (PP) and 10 wt% of selected flame retardants as well as neat PP are prepared. Flame retardants include pentaerythritol spirobis(methylphosphonate) (PSMP), zinc diethylphosphinate (DEPZn), aluminum hydroxide (i.e. aluminum trihydrate, ATH) and ammonium polyphosphate (APP). The released substances in the gas phase are investigated by using thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy (TGA-FTIR), and the burning behavior regarding carbon monoxide concentration and heat release rate (HRR) is studied in cone calorimeter experiments. The ignition and combustion of flame retarded micrometer-sized particles of PP are experimentally investigated in a laminar flat flame burner by means of simultaneous scanning planar laser-induced fluorescence of OH radicals (OH-PLIF) and diffuse back-illumination (DBI) at a repetition rate of 10 kHz. Particles with gas-phase activity show a decreased normalized OH signal intensity during combustion. A dependency of the dimensionless flame radius on the particle size is found which can be linked to the faster heating and pyrolysis of smaller particles compared to large particles and agglomerations. Eventually, the interaction with an external premixed flame and the subsequent extinction of self-sustaining flames of polymer sticks is investigated using OH-PLIF in an adapted horizontal burning test suited for the application of optical diagnostics. This approach allows to link the findings of investigations encompassing multiple scales, resulting in a thorough understanding of both polymer chemistry and combustion dynamics in flame retarded polymers.

Visible chemiluminescence of ammonia premixed flames and its application for flame diagnostics

We report a spatially resolved spectroscopic study of the visible chemiluminescence emission from different premixed ammonia-air-oxygen flames stabilized on a laminar flat flame burner, with equivalence ratio ranging from 0.7 to 1.35 and an O2/N2 ratio of 0.4. In the reaction zone of the observed flames, the visible emission was recognized to be the chemiluminescence of excited NH2* radicals, while in the post-flame zone, two types of chemiluminescence were observed: NO2* chemiluminescence dominated in the fuel-lean flames and NH2* chemiluminescence dominated in the fuel-rich flames. The high-resolution spectra of the NO2* and NH2* chemiluminescence in the visible region (400-700 nm) were recorded. The intensity of both spectra increased gradually with wavelength. However, the NO2*-chemiluminescence spectrum appeared to be continuous and unstructured, while the NH2*-chemiluminescence spectrum consisted of groups of distinct emission lines. Based on the spectral feature, the ratios of the integrated chemiluminescence intensities over the 598-603 nm wavelength range to the intensities over the 586-592 nm range and 447-453 nm range were used to sense equivalence ratio. In addition, slightly different colors of the fuel-lean and fuel-rich flames were observed, due to the fact that NO2* chemiluminescence had a relatively stronger signal in the blue region than NH2* chemiluminescence. The difference was used to infer flame equivalence ratio using the flame images recorded by a RGB digital camera, where the ratio of the signal from the red channel to the signal from the blue channel was calculated.

Measurement of acetylene concentration in laminar flat flames by diode laser cavity ring-down spectroscopy

Measurement of acetylene concentration in laminar flat flames by diode laser cavity ring-down spectroscopy

Experimental and modeling study of NO formation in methyl acetate + air flames

Premixed laminar flat non-stretched flames of methyl acetate + air have been studied at atmospheric pressure. The heat flux method was employed to stabilize the flames over equivalence ratios ɸ = 0.7–1.5 at initial gas temperatures from 298 to 348 K. The laminar burning velocities of methyl acetate + air mixtures have been measured at these conditions and compared with the literature data. Laser-induced fluorescence was used to measure NO concentrations in the post-combustion region of the flames at 298 K. The detailed kinetic mechanism of the authors was extended by the reactions of methyl acetate, mostly relying on the model developed by Ahmed et al. (2019). The mechanism was tested against selected literature data from measurements in a shock tube and jet-stirred reactor. Moreover, it was compared with new measurements of the laminar burning velocity and NO concentrations in methyl acetate flames. In all cases, good agreement between the experimental and modeling results was observed. A comparison with LIF measurements obtained for methane + air flames at the same conditions shows that NO concentrations in methyl acetate flames are lower than those in methane + air flames both in stoichiometric and rich mixtures. These differences were interpreted using sensitivity and rate-of-production analyses of NO formation via different routes.

Numerical investigation and assessment of flamelet-based models for the prediction of pulverized solid fuel homogeneous ignition and combustion

The homogeneous ignition and volatile combustion of pulverized solid fuel in single-particle and particle group configurations were studied numerically in a laminar flat flame burner. Simulations with increasing particle streams were performed to investigate the influence of the interactions in particle groups on homogeneous ignition and combustion. An extensive set of simulations are conducted considering models with different levels of detail for both the gas-phase and solid fuel chemistry. The reference simulations employ the chemical percolation devolatilization model coupled with a detailed chemistry model for gas-phase reactions. The particle-fluid interactions were modeled with a fully coupled Eulerian-Lagrangian framework. Increased ignition delay times for higher particle streams were successfully validated against available experimental measurements. Furthermore, the transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions near the flame base in particle group combustion was observed in both experiments and simulations. The subsequent detailed investigations revealed that the increased heat transfer to particles and, therefore, lower gas temperature for higher particle number densities together with the local oxygen depletion are the primary reasons for this transition. Based on the reference simulation, different simplified model combinations were assessed. The systematic model reduction investigation started with assessing the fixed volatile composition as a required assumption for flamelet models. Finally, the effects of gas-phase chemistry and different simple devolatilization models on ignition and combustion chemistry were studied. Overall, all model combinations provide reasonable predictions of volatile combustion with minor local deficits in the studied conditions.

Experimental characterization of ozone-enhanced n-decane cool flames and numerical investigation of equivalence ratio dependence

Low temperature combustion behavior is critical to understand for advanced combustor design and safety, but cool flames are challenging to study under chemistry-transport coupled conditions because of the generally transient nature of these flames. A series of studies have utilized a laminar flat flame Hencken burner platform and ozone enhancement at sub-atmospheric pressures to stabilize dimethyl ether, propane and heptane cool flames and measure flame characteristics such as propagation speed, flame temperature and product species. This work builds upon those prior studies to investigate n-decane, first through experimental characterization of ozone-enhanced n-decane cool flames, including measurement of propagation speeds and flame temperatures and imaging of formaldehyde planar laser induced fluorescence (CH2O PLIF) over a range of equivalence ratios and, second, through comparison of experimental results with one-dimensional cool flame simulations. Experimental findings showed moderate equivalence ratio dependence of cool flame propagation speeds and temperatures at lean equivalence ratios, and no significant equivalence ratio dependence for richer mixtures. Experimental CH2O profiles showed equivalence ratio dependence and no downstream consumption, as is characteristic of cool flames. Simulated n-decane cool flames showed significant dependence of propagation speeds and temperatures on equivalence ratio, as well as downstream consumption of CH2O, at the experimental conditions. Experimental and numerical discrepancies were first investigated by examining the influence of different ozone enhancement strategies on equivalence ratio dependence and the results indicate a minimal influence of ozone enhancement strategy for n-decane, while also providing insights to into equivalence ratio dependencies observed in previous ozone-enhanced and ozoneless cool flame studies. The effect of ozone concentration on simulated n-decane cool flames was then investigated, revealing a strong influence of ozone enhancement on equivalence ratio dependence. Simulated propagation speed and temperature dependence on equivalence ratio were shown to develop with increase in ozone enhancement levels. A kinetics analysis revealed enhancement of low temperature peroxy chemistry and other cool flame heat releasing reactions with ozone addition, increasing simulated flame temperatures and promoting the onset of early intermediate temperature chemistry (ITC), inducing equivalence ratio dependence which was not seen in experimental results. Finally, insights from this analysis were used to investigate reconciliation of differences in experimental and simulated cool flame temperatures and identify potential kinetics model discrepancies from experimental findings.

An optimized evaluation strategy for a comprehensive morphological soot nanoparticle aggregate characterization by electron microscopy

For a comprehensive understanding of nanoparticle formation in gas phase processes, such as soot formation, morphological parameters of fractal-like particle aggregates, like the radius of gyration, the fractal dimension and the primary particle size, have to be determined. Often transmission electron microscopy (TEM) is employed for the investigation of particle characteristics as it not only allows to investigate ensemble averages but single particle aggregates and thus also to determine statistical properties, such as the size distribution within a sample. Many different evaluation methods can be found in the literature. We investigated different approaches for the determination of morphological parameters from TEM images of soot and compared the results to derive an optimized evaluation strategy for TEM nanoparticle characterization. We compared four methods for the determination of the radius of gyration – three length- and one pixel-based methods – showing good agreement within 9% deviation for the median of the recovered lognormal size distribution. Furthermore, the fractal dimension was determined via a sample-based and various box counting methods with different limiting box sizes. Here, we could show that the upper and lower bounds (aggregate size in terms of radius of gyration and primary particle size in terms of its radius, respectively) of self-similarity of fractal-like aggregates should be accounted for by choosing corresponding upper and lower box sizes. Using box counting, we could show that for small aggregates the fractal dimension as well as its span are increased, yet with increasing aggregate size the fractal dimension converges towards 1.6. Furthermore, we could show the potential of semi-automatic aggregate detection through Trainable Weka Segmentation. However, image noise resulting in erroneous aggregate splitting often leads to smaller aggregate sizes by automatic detection compared to manual segmentation. Generalized Hough Transformation for the semi-automatic determination of primary particle sizes performs well for large soot particle aggregates as those often show spherical primary particles. For leaner combustion conditions, the primary particles of the formed clumpy soot aggregates cannot be detected well via semi-automatic detection. TEM images were taken on soot samples from premixed laminar flat flames (burner type: McKenna) under various conditions to provide comprehensive reference data.

Quantitative SO2 Detection in Combustion Environments Using Broad Band Ultraviolet Absorption and Laser-Induced Fluorescence.

Spectrally resolved ultraviolet (UV) absorption cross sections of SO2 in combustion environments at temperatures from 1120 to 1950 K were measured for the first time in well-controlled conditions through applying broad band UV absorption spectroscopy in specially designed one-dimensional laminar flat flames. The temperature was observed to have a significant effect on the absorption cross-section profiles at wavelength shorter than 260 nm, while at the longer wavelength side, the absorption cross-section profiles have much less dependence on temperature. The absorption cross section at 277.8 nm with a value of 0.68 × 10–18 cm2/molecule was suggested for the evaluation of the SO2 concentration because of the weak dependence on temperature. To make spatially resolved measurements, laser-induced fluorescence (LIF) of SO2 excited by a 266 nm laser was investigated. Spectrally resolved LIF signal was analyzed at different temperatures. The LIF signal showed strong dependence on temperature, which can potentially be used for temperature measurements. At elevated temperatures, spatially resolved LIF SO2 detection up to a few ppm sensitivity was achieved. Combining UV broad band absorption spectroscopy and LIF, highly sensitive and spatially resolved quantitative measurements of SO2 in the combustion environment can be achieved.

Temperature and water measurements in flames using 1064 nm Laser-Induced Grating Spectroscopy (LIGS)

Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH4/air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4–2%. These results confirm the potential for 1064 nm LIGS-based thermometry for high-precision temperature measurements of combustion processes.

In situ photoacoustic measurement of soot profiles in laminar flames using a high-repetition-rate pulsed fibre laser

We present a photoacoustic technique for in situ sensing of particulates in reacting flows. The method is demonstrated by measuring soot distribution in laminar flat flames. The excitation source is a high-repetition-rate pulsed fibre laser. The resulting acoustic signals are measured using a uni-directional microphone and lock-in amplifier. Based on an Allan–Werle plot, the detection limit is estimated to be a soot volume fraction of 3 parts-per-billion. Fluence curves are presented, showing a plateau region similar to the behaviour typically observed for laser-induced incandescence. A dependence of the photoacoustic signal on pulse repetition rate is also demonstrated.

Experimental Study and a Short Kinetic Model for High-Temperature Oxidation of Methyl Methacrylate

ABSTRACTSynthetic and natural polymeric esters find applications in transport and construction sectors, where fire safety is an important concern. One polymer that is widely used is poly (methyl methacrylate) (PMMA), which almost completely undergoes thermal decomposition into methyl methacrylate (its monomer) (MMA) at –C. In order to analyze the high-temperature gas-phase oxidation of PMMA, and thereby predict its fire behavior (such as burning rate, temperature of the material, and heat fluxes) with less computational effort, a compact kinetic model for the oxidation of its primary decomposition product, MMA, is most essential. This is accomplished in the present work by obtaining a reduced mechanism for MMA oxidation from a detailed mechanism from the Lawrence Livermore National Laboratories group.To extend the available data base for model validation and present validation data at atmospheric pressure conditions, for the first time, (i) detailed measurements of species profiles have been performed in stoichiometric laminar flat flames using flame sampling molecular beam mass spectrometry (MBMS) technique and (ii) laminar burning velocities have been obtained using the heat flux method for various unburnt mixture temperatures. Evaluating the model against these data sets point to the need to revise the kinetic model, which is achieved by adopting rate constants of key reactions among analogous molecules from recent literature. The updated compact kinetic model is able to predict the major species in the flat flame as well as the burning velocity of MMA satisfactorily. The final “short MMA mechanism” consists of 88 species and 1084 reactions.

The significance of beam steering on laser-induced incandescence measurements in laminar counterflow flames

Beam steering is often encountered in laser diagnostic measurements, especially in flame environments, due to changes in refractive index caused by thermal and species gradients. It can negatively affect the accuracy of the results. In this work, the effects of beam steering on laser-induced incandescence (LII) measurements of pre-vaporized-liquid counterflow flames are assessed. The focus on counterflow flames is to facilitate future detailed experimental campaigns on one-dimensional nonpremixed sooty flames. It is found that the temperature and species gradients in the counterflow configuration have a much more significant impact on the beam profile than in laminar flat flames, especially for heavier fuels. As a result of the changes in the beam profile, for the same applied laser energy, the local fluence shifts markedly with fuel type, therefore, having a direct impact on the LII measurements. A procedure is developed for ensuring accurate measurements and it is shown that, for a specific fuel, it is possible to tailor the laser energy, such that the collected LII signal in the counterflow flames is nearly independent of beam-steering effects.

Characterization of ozone-enhanced propane cool flames at sub-atmospheric pressures

A recently developed experimental setup for the study of cool flames was employed to investigate low temperature combustion of propane. Cool flames were stabilized though ozone activation using a laminar flat flame Hencken burner at sub-atmospheric pressures. Propane cool flame stability was observed to be highly sensitive to pressure, and a pressure stability map is presented for a range of equivalence ratios (ϕ) from 0.17 to 1.0 in 0.17 increments and a range of reactant flow rates. Based on the stability map, a propane cool flame of ϕ = 0.17 at 17.3 kPa was chosen for further study. Flame lift-off height above the burner surface was measured and two operating regimes of burner-stabilized and freely propagating flames were observed. The intersection of these two regimes was used to estimate the cool flame propagation speed. Temperature measurements of the cool flame were taken and used in fixed-temperature numerical flame simulations. Additionally, flame temperatures and propagation speeds are presented for all equivalence ratios and pressures where stable, freely propagating flames were observed in the stability map. In order to enable non-fixed temperature, freely propagating cool flame simulations to be performed, reduction of a chemical kinetics model using the Directed Relation Graph method (DRG) in conjunction with reaction rate sensitivity analysis was performed. Experimentally determined propagation speed, temperature and species concentrations at partial equilibrium were compared to numerical simulations with reasonable agreement and results are discussed.

Optical investigation of gas-phase KCl/KOH sulfation in post flame conditions

A counter-flow reactor setup was designed to investigate the gas-phase sulfation and homogeneous nucleation of potassium salts. Gaseous KOH and KCl were introduced into the post-flame zone of a laminar flat flame. The hot flame products mixed in the counter-flow with cold N2, with or without addition of SO2. The aerosols formed in the flow were detected through Mie scattering of a 355 nm laser beam. The temperature distribution of the flow was measured by molecular Rayleigh scattering thermometry. From the temperature where nucleation occurred, it was possible to identify the aerosols formed. Depending on the potassium speciation in the inlet and the presence of SO2, they consisted of K2SO4, KCl, or K2CO3, respectively. The experiments showed that KOH was sulphated more readily than KCl, resulting in larger quantities of aerosols. The sulfation process in the counter-flow setup was simulated using a chemical kinetic model including a detailed subset for the Cl/S/K chemistry. Similar to the experimental results, much more potassium sulfate was predicted when seeding KOH compared to seeding KCl. For both KOH and KCl, sulfation was predicted to occur primarily through the reactions among atomic K, O2 and SO2, forming KHSO4 and K2SO4. The higher propensity for sulfation of KOH compared to KCl was mostly attributed to the lower thermal stability of KOH, facilitating formation of atomic K. According to the model, sulfation also happened through SO3, especially for KCl (KCl → KSO3Cl → K2SO4).

Underresolved absorption spectroscopy of OH radicals in flames using broadband UV LEDs

A broadband absorption spectroscopy diagnostic based on underresolution of the spectral absorption lines is evaluated for the inference of species mole fraction and temperature in combustion systems from spectral fitting. The approach uses spectrally broadband UV light emitting diodes and leverages low resolution, small form factor spectrometers. Through this combination, the method can be used to develop high precision measurement sensors. The challenges of underresolved spectroscopy are explored and addressed using spectral derivative fitting, which is found to generate measurements with high precision and accuracy. The diagnostic is demonstrated with experimental measurements of gas temperature and OH mole fraction in atmospheric air/methane premixed laminar flat flames. Measurements exhibit high precision, good agreement with 1-D flame simulations, and high repeatability. A newly developed model of uncertainty in underresolved spectroscopy is applied to estimate two-dimensional confidence regions for the measurements. The results of the uncertainty analysis indicate that the errors in the outputs of the spectral fitting procedure are correlated. The implications of the correlation between uncertainties for measurement interpretation are discussed.

Determination of rate parameters based on NH2 concentration profiles measured in ammonia-doped methane–air flames

The experiments of Rahinov et al. (Combust. Flame, 145 (2006) 105–116) measuring NH2 concentration profiles in methane–air laminar flat flames doped with ammonia were re-evaluated. The flames were simulated with the FlameMaster code using a modified NOx combustion mechanism of the CRECK Modeling Group. Based on local sensitivity analysis results, Arrhenius parameters A, n, E of reaction steps NH2 + H = NH + H2 and NH3 + OH = NH2 + H2O were selected for optimization, which took into account not only the experimental data of Rahinov et al., but also related direct measurements and theoretical determinations as optimization targets. The optimized mechanism described the measured concentration profiles better than the original one, while the new rate parameter values were within the prior uncertainty limits obtained from the evaluation of literature data. The optimization process also provided new posterior uncertainty limits, which are within the prior uncertainty limits.

Infrared absorption spectrometer for the determination of temperature and species profiles in an entrained flow gasifier.

An absorption spectrometer utilizing a tunable distributed feedback diode laser at 2.3μm and an interband cascade laser at 3.1μm has been developed to measure temperature and concentrations of CO, CH4, C2H2, and H2O under gasification conditions. A wavelength division multiplexing approach using a single ZrF4-fiber was used to measure both wavelength regions simultaneously. The performance of the spectrometer has been tested in laminar flat flames and a heated cell and then applied for measurements at an atmospheric entrained flow gasifier (REGA). A water-cooled optical probe was used to provide optical access at two measurement positions. By moving the burner, axial profiles of temperature and species concentration could be obtained. These profiles were compared with numerical simulations and can be used to validate the simulation.