Flame Conditions Research Articles

A machine learning-based approach for flames classification in industrial Heavy Oil-Fire Boilers

The burner combustion tuning is a complex problem that has been studied through flame monitoring and characterization. It has been observed that the flame electromagnetic spectrum and flickering contain specific flame information in combustion processes. This information is helpful for combustion stoichiometry tuning on burners. This paper described a method for selecting the best flame feature subset that can be computed from the scanner signal, in order to get the flame index and induce combustion stoichiometry on burners under specific combustion conditions. We propose a method for selecting a reduced subset with only the useful flame features for flame index classification. To extract the most relevant flame features we use a feature subset selection (FSS) algorithm and to determine the combustion state in burners, five flame indices were defined that represent the most common flame states in oil fuel-fired boilers. FSS includes complete, sequential, and random searches in order to eliminate redundant and noisy flame features to decrease the flame feature set dimension. A probabilistic neural network (PNN) algorithm was implemented for flame feature clustering. Signals from the actual flame scanner system and relevant variables from the boiler data acquisition system were used by the algorithms to calculate the burner flame index. A set of parametric tests was done in a heavy oil-fired boiler under well-known flame and index conditions to train and test the flame classifier. The results showed that only the four more relevant features are enough to classify flames with a good performance (92.3% accuracy), which is useful for burner combustion monitoring and optimization.

Elucidating the polycyclic aromatic hydrocarbons involved in soot inception

Polycyclic aromatic hydrocarbons are the main precursors to soot particles in combustion systems. A lack of direct experimental evidence has led to controversial theoretical explanations for the transition from gas-phase species to organic soot clusters. This work focuses on sampling infant soot particles from well-defined flames followed by analysis using state-of-the-art mass spectrometry. We found that PAH molecules present in soot particles are all stabilomers. Kinetic Monte Carlo simulations and thermodynamic stability calculations further identify the detected PAHs as peri-condensed and without aliphatic chains. Van der Waals forces can easily link PAHs of such size and shape to form PAH dimers and larger clusters under the specified flame conditions. Our results provide direct experimental evidence that soot inception is initiated by a physical process under typical flame conditions. This work improves our understanding of aerosol particulates, which has implications for their environmental and climate change impacts.

Simultaneous planar laser-induced fluorescence of nitric oxide and hydroxyl radical with a single dye laser in hydrogen flames

Simultaneous, planar laser-induced fluorescence (PLIF) of nitric oxide (NO) and hydroxyl radical (OH) is evidenced with a single dye laser and two intensified CCD (ICCD) cameras. The technique is demonstrated on three premixed turbulent hydrogen-air flames, at atmospheric conditions, with several laser excitation wavelengths within the A2Σ+ − X2Π (0,0) and A2Σ+ − X2Π (1,0) vibrational bands of NO and OH, respectively. Via adjustments of the grating angle of the dye laser, the output wavelengths can be optimized so that both LIF signals are obtained simultaneously, or it can be tuned to reach the most common excitation wavelengths for OH-PLIF (i.e., near 283 or 284 nm) or NO-PLIF (i.e., 225 and 226 nm) alternatively. The flame visualization results, obtained with the different excitation strategies, are consistent for the three flame conditions and enable to characterize both the reaction zones and the nitric oxide formation. With appropriate selection of the wavelength pair, the technique shows its great potential to enhance the ability of existing lasers to simultaneously visualize and quantify distributions of two key species in reacting flows with only a single laser.

On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pecr2/Pecr1≈2. The temporal acceleration exponents (αR) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as SF/SL,b∼Pedt and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (dt) of turbulent flames continuously increase with u’/SL in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the dt of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H2/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.

Nonlinear dynamic characteristics of self-excited thermoacoustic instabilities in premixed swirling flames

The nonlinear behavior of thermoacoustic oscillation for premixed swirl flames fueled with CH4/air mixtures is experimentally studied by changing the equivalence ratio Φ, combustor inlet bulk velocity Uin and swirl number S. The nonlinear dynamic characteristics of the system are analyzed by the methods of phase space reconstruction, recurrence plot, Poincaré section and 0–1 test for chaos. Five kinds of flame modes are observed in the combustion system with the change of Φ, Uin, and S, which are lean/rich blow-off, stable combustion, quasi-periodic oscillation, and limit cycle oscillation. Subcritical Hopf bifurcation of the oscillation is captured either by changing Φ or Uin, and Φ and Uin are found to have similar effects on the flame mode transition under specific operating conditions. It is observed that the appearance of flame in the outer circulation zone (ORZ) and the enhancement of its burning intensity are matched with the occurrence of quasi-periodic and limit cycle oscillations, respectively. It is examined that the lean/rich blow-off limits and oscillation range are extended with a larger S, and the stable combustion regime is wider with a smaller S. This study contributes to understanding the nonlinear thermoacoustic instability of swirl flames under broad flame conditions, which helps to develop effective instability prediction and control methods in stable, efficient, and low-emission gas turbines.

Ultraviolet electronic spectroscopy of heavily substituted naphthalene derivatives

Context. The so-called bump spectral signature observable on interstellar extinction curves, peaking at 217.5 nm, is commonly assigned to π* ← π transitions from carbonaceous carriers, but the exact nature of the carbonaceous carriers remains debated. Aims. To constrain the chemical structures associated with the bump carriers, we record and compare the UV spectra of a large variety of carbonaceous molecules to this interstellar feature. Methods. Large carbonaceous molecules, such as polycyclic aromatic hydrocarbons (PAHs), were produced by a combustion process stabilized at low pressure under rich flame conditions. Species were extracted and probed through resonance enhanced multiphoton ionization spectroscopy coupled to a time-of-flight mass spectrometer. Masses and absorption profiles of the carbonaceous molecules were measured, and their spectra were compared to the bump feature. Results. Species showing a specific mass progression starting at mass 128 u visible up to mass 394 u with a characteristic progression of +14 u present a common electronic absorption band profile peaking asymptotically around 220 nm. The first masses were assigned to a naphthalene C10H8 molecule and two of its derivatives: C10H7CH3 and C10H7C2H5. The mass progression of +14 u is explained by successive H atom substitutions by CH3 functional groups. This mass distribution was thus assigned to naphthalene derivatives with large aliphatic carbon substitution. This derivative family shows an electronic band assigned to S3 ← S0 transitions involving electron promotion within the π aromatic orbitals of the naphathlene chromophore. More importantly, after a few substitutions, the position of the band converges asymptotically to a value close to the interstellar bump signature, independent of the molecule size. Conclusions. Based on the asymptotic behavior of the larger members in the species distribution, a similar band position is expected from double aromatic ring substructures within hydrogenated amorphous carbons (HACs). Similar to the conclusions of previous works, we find substituted naphthalene units as substructures of interstellar HACs to be good candidates as carriers of the bump feature.

Strontium monofluoride – a new molecule to measure fluorine using high-resolution continuum source flame molecular absorption spectrometry and its application for gasoline analysis

In this work, for the first time, strontium monofluoride was generated in an air-acetylene flame of a high-resolution continuum source molecular absorption spectrometer with the aim of fluorine determination via molecular absorption. The particular task was the determination of organic fluorine in gasoline/its components dissolved in xylene. Sequential measurements in the area of the most intensive absorption of the molecule were executed to select the spectral range for analysis. The acquired spectra differed from the ones from the literature, especially the emission spectra. The best and comparable sensitivity has been registered near 651 nm and 663 nm in measurements of rotations from the range of the X2Σ+ → A2Π electronic transition of the SrF molecule. Unfortunately, SrOH molecules were also spontaneously formed and absorbed radiation in the selected area. No other spectral interferences were found, except absorption by the SrBr molecule, which could have been formed in the flame despite a relatively low energy bond of 330 kJ/mol. The dependence of generation of the SrF and the SrOH molecules on the flame conditions was studied for pure solvent (xylene) and solutions of investigated samples of gasoline type. The effect of the SrOH molecule was overcome by blank subtraction or the least square background correction. The last method had to be used to overcome the SrBr effect. Due to the various combustion properties of various hydrocarbons and their different behaviour in flame, applying the simplified standard addition method became indispensable. As a source of strontium, an oil Sr standards or Sr acetylacetonate (SrAA) were investigated. The crucial benefit of SrF molecule application was the relatively good solubility (and relatively low price) of SrAA. Under the selected conditions, the characteristic concentration of fluorine was 1.9 and 2.0 mg/L, while the instrumental detection limit was 0.5 and 0.8 mg/L for the 651 and 663 nm ranges, respectively. The analytical technique enables the analysis of samples diluted to a small degree, e.g. 1:3 v:v and the best method detection limit is 1.3 mg/L. The proposed method is fast and cheap.

The evolution of flame geometrical characteristics and air entrainment of inclined jet flames

This paper reported the jet fire issuing from different angles, a common hazard in pipeline leakage accidents during the gas storage and transportation process. The flame geometrical profiles including flame projection length in the horizontal direction, vertical height from the nozzle position and flame trajectory length as the essential parameters to evaluate jet fire risk and design safety distance between process equipment were measured experimentally. The result shows that flame projection length in the horizontal direction first increases to a maximum value at the horizontal jet flame condition and then decreases, flame vertical height from the nozzle position increases monotonously and the length of flame trajectory first decreases and then increases with the increasing inclined angle from downward to upward. Then, the normalized flame trajectory and its length for different inclined jet flames were obtained by plotting the flame tip location parameters normalized by a characteristic length representing the interaction between jet flow momentum and buoyant force generated by temperature variation. Finally, the air entrainment coefficients for different inclined jet flames were quantified by an integral model derived from the conservation equations showing a trend of first increase and then decrease with the increasing inclined angle from negative to positive. This work provides a good reference for quantifying the flame size of jet flame in inclined conditions.

Cell-level thermal runaway behavior of large-format Li-ion pouch cells

Lithium-ion batteries are widely used in industry for their high energy density and long cycle life; however, they are sensitive to abuse, such as external heating. This study examines 63 Ah, NMC pouch cells at 100% state of charge being forced into thermal runaway (TR) through external side heating at rates ranging from 0.2 to 0.5 °C/s. The fastest heating rate was found to initiate TR earliest, but the peak surface temperatures achieved by all cells were found to be approximately equal. A delay between TR initiation on the heat-exposed surface of the cell and the opposite surface was observed as TR spread across the cell, with the duration found to increase with heating rate. A flaming condition occurred for all cells during TR. For the utilized range of heating rates studied, the radiant heat flux of the flame was found to be unaffected, and the peak chemical heat release rate showed no statistical trend due to the changes in cell heating. Cell mass loss throughout TR also appears to be independent of heating rate with more research required to further explore this parameter. This study provides an in-depth analysis of TR behavior in the context of single, large-format pouch cells, the results of which will be used to develop a future 1D model for cell-level TR that predicts TR propagation behavior in a module.

Effects of impinging object on geometrical flame features of horizontal jet flames

Accidental release of hydrocarbons can result in jet fires which have the potential to escalate accidental events to catastrophes. Jet fire can impinge on nearby equipment surface or a pasive firewall to make the fire plume temperature and the radiant heat flux complex. The present work experimentally investigated the flame characteristic of free horizontal jet fires and the impingement effect using propane as fuel, with different nozzle diameter, and exit velocity. To analyse the effect of flowrate to jet flame condition, the flow rates range between 30 and 600 g/min were employed with horizontal separation distance from the main source to target was at 0.8-m and 1.2-m. The results show that, the incremenet of flame features are faster for free release than impinging release. The presence of object downstream affect the flame profiles at some point. Thus, new correlations were proposed for the prediction of projected flame length and trajcetory length.

T-cut slotted quartz tube-atom trap strategy for the on-line preconcentration of thallium in well water samples

In this study, an on-line preconcentration system was developed for the quantitative determination of thallium in well water samples using hydrogen-assisted T-cut slotted quartz tube atom trapping (HA-T-SQT-AT) coupled to flame atomic absorption spectrometry (FAAS). The method is based on trapping the thallium atoms on the inner surface of the T-SQT for a short period using sample aspiration and lean flame conditions. Then, hydrogen gas as a reducing agent was released instantly to free the trapped analyte atoms, resulting in a sharp and narrow signal. The system analytical performance was enhanced by performing comprehensive optimization of several factors. The linear range and LOD/LOQ values of the optimized method were found to be 7.5–––200 ng/mL and 2.6/8.7 ng/mL, respectively. Accuracy/practicability of the presented method were verified using well water samples, and the percent recoveries calculated using the external calibration approach were all above 80%.

Comparative study of NH3 and H2 impacts on combustion and emission characteristics of n-Hexadecane under different flame conditions

The addition of renewable fuel to hydrocarbon fuel mitigated the carbon monoxide, carbon dioxide and soot particle emissions of internal combustion engines. A comparative study of the ammonia and hydrogen effect on combustion and emission characteristics of n-hexadecane flames was executed at various flame conditions. The results showed that alternative fuels had a stronger impact on peak temperature at diffusion flame conditions rather than at premixed flame conditions. The premixed flame released carbon monoxide emission lower than the diffusion flames with/without additional fuels. The carbon monoxide emissions decreased highly with ammonia enrichment instead of hydrogen. At premixed and coflow diffusion flame conditions, the ammonia highly reduced carbon dioxide emission compared to hydrogen. Hydrogen addition reduced the nitrogen oxide emission, whereas it was increased by ammonia. The soot particle number density was reduced highly by hydrogen addition instead of ammonia at the premixed flow flame and diffusion coflow flame conditions. The impact of hydrogen on the reduction of soot mass concentration emissions was greater than that of ammonia at all combustion conditions. It is suggested that H2 ≥ 70% should be mixed in n-hexadecane fuel because a higher burning velocity can be achieved with a higher emission reduction. A reduction in the emission of carbon-related species can be realized with a small reduction in burning velocity at NH3 < 70%.

Deposition of graphenic nanomaterials from elevated temperature premixed stagnation flames

The work examines the unique nanostructure of carbon nanoparticles deposited from sooting premixed flames with flame temperatures exceeding 2200 K. This flame temperature regime has previously been shown to transition from typical soot formation conditions to a regime whereby the flame-form carbon adopts a nanostructure considerably more ordered than soot. Graphenic carbon deposits observed by High-resolution TEM (HRTEM) are reported here corroborating previous Raman spectroscopy evidence. The use of premixed stretch-stabilized flames enables particle production in the high-temperature regime under a flow field amenable to low-dimensional flame modeling. Although the flame flow configuration is relatively simple, three sample preparation methods are used to assess the representation of true carbon properties as they exist in the flame. HRTEM imaging is carried out on carbon particle samples prepared by rapid-insertion deposition, aerosol dilution probe deposition and carbon particle film deposition. Images from rapid-insertion samples show amorphous particles in the lightly sooting flame and turbostratic particles in the heavy sooting flame. There is trace evidence of graphenic structure in rapid-insertion samples but the most striking particles on the TEM grid are graphite nanocrystals presumably formed by a new artificial crystallization process. HRTEM images of particles collected over time by diluted aerosol deposition and film deposition show clear graphenic structures. Overall, the carbon nanostructure observed by HRTEM is a mixture of amorphous, turbostratic and graphenic carbon lattices depending on the flame condition and sampling method. The current work highlights potential impacts of higher flame temperatures and higher equivalence ratio on deposited flame-formed carbon. Namely, graphenic particle structure is observed in rapid-insertion deposition samples but graphene portions are most abundant in aerosol dilution and carbon particle film deposition samples. This may indicate that graphene structures grow on the deposition surface over time.

A Numerical Study on Premixed Turbulent Planar Ammonia/Air and Ammonia/Hydrogen/Air Flames: An Analysis on Flame Displacement Speed and Burning Velocity

The economic storage and transportation of ammonia (NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3$$\\end{document}), and its capability to be thermally decomposed to hydrogen (H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2$$\\end{document}) make it a potential carbon-free synthetic fuel for the future. To comprehend the fundamental characteristics of NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3$$\\end{document} as a primary fuel enriched with H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2$$\\end{document} under low turbulent premixed flame conditions, three quasi direct numerical simulations (quasi-DNS) with detailed chemistry and the mixture-averaged transport model are conducted under decaying turbulence herein. The Karlovitz number is fixed to 4.28 for all the test conditions. The blending ratio (α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document}), specifying the hydrogen concentration in the ammonia/hydrogen mixture, varies from 0.0 to 0.6. The results reveal that the mean value of the density-weighted flame displacement speed (Sd∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$S_{\ extrm{d}}^{*}$$\\end{document}) is similar to (higher than) the unstrained premixed laminar burning velocity (SL0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$S_{\ extrm{L}}^{0}$$\\end{document}) for NH3/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3/$$\\end{document}air flame (NH3/H2/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3/\\hbox {H}_2/$$\\end{document}air flames). Furthermore, the performance of two extrapolation relations for estimating Sd∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$S_{\ extrm{d}}^{*}$$\\end{document} as linear and non-linear functions of flame front curvature is discussed thoroughly. The performances of both models are almost similar when evaluating the data near the leading edge of the flame. However, the non-linear one offers more accurate results near the trailing edge of the flame. The results show that the mean flame stretch factor increases with increasing the blending ratio, suggesting that the mean flamelet consumption velocity deviates from SL0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$S_{\ extrm{L}}^{0}$$\\end{document} by enriching the mixture with H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2$$\\end{document}. The mean value of the local equivalence ratio (ϕ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\phi$$\\end{document}) for the turbulent NH3/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3/$$\\end{document}air flame is almost equal to its laminar counterpart, while it deviates significantly for NH3/H2/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3/\\hbox {H}_2/$$\\end{document}air flames. In addition, the local equivalence ratio for the flame front with positive curvature values is higher than the negatively curved regions for NH3/H2/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {NH}_3/\\hbox {H}_2/$$\\end{document}air flames due to H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2$$\\end{document} preferential diffusion. Furthermore, the results indicate that hydrogen is consumed faster in positively curved regions compared to the negatively curved zones due to enhanced reaction rates of specific chemical reactions.

Quantifying acetylene mole fraction in rich flat laminar premixed C2H4/air flames using mid-infrared polarization spectroscopy

Mid-infrared laser polarization spectroscopy (IRPS) has been applied to measure the mole fraction of acetylene in rich premixed laminar C2H4/Air flat flames at equivalence ratios (Φ) of 1.7, 2.1, and 2.3, and under atmospheric pressure. The detection was conducted by probing the ro-vibrational P(19) transition at ~ 3.1 μm. The total collisional broadening coefficient of C2H2 was approximately 0.074 cm−1 atm−1 and varied within a range of 0.5% under different flame conditions, which made the effect of linewidth not obvious in the CH4/air flame. The calculated mole fraction of C2H2, using the Chemkin model, at Φ = 1.3 and 1.5 was used to calibrate the recorded IRPS signal intensities at different Height Above Burner (HAB). A single scaling factor was then used to quantify the measured C2H2 at highly sooting conditions, Φ = 1.7, 2.1, and 2.3, with a Limit of Detection (LoD) of 35 ± 5 ppm. The first observed C2H2 mole fraction appeared at HAB of 3 mm and measured as 2003 ppm, 2217 ppm, and 2495 ppm, for Φ = 1.7, 2.1, and 2.3, respectively. The mole fraction increased as the HAB increased to reach the maximum value of 2296 ppm, 2807 ppm, and 3478 ppm, for Φ = 1.7, 2.1, and 2.3, respectively, up to HAB of 5 mm. It was observed that the C2H2 mole fraction reaches a plateau region at HAB of ~ 8 mm. The production of C2H2 has been observed to be subject to a critical gas temperature of 1400 ± 30 K. The critical soot inception temperature, where the first incepted soot particles are observed, is the same as the gas temperature where {chi }_{{{mathrm{C}}_{2}mathrm{H}}_{2}}^{mathrm{max}} was detected, namely at 1500 ± 30 K. These measurements and calibration procedure demonstrate a plausible technique to probe other flames and to better understand soot inception and its correlation with C2H2.

Noise analysis of hydrogen addition to a lean-premixed combustion with imposed inlet oscillating velocity

Combustion instability is one of the main disadvantages of power generation systems. Indirect noise or acceleration of entropy waves are sound sources that cause the combustion chamber unstable. This subject, however, has been less investigated due to the complex nature of entropy waves. A deep understanding of the physics of entropy waves is crucially important in order to reduce noise and instability. This study considers entropy generation in premixed flame conditions. In this situation, the hydrodynamic and thermal effects on the entropy wave performance in a combustion chamber are studied. To model turbulent flow, the large eddy simulation has been used. The effects of adding hydrogen to the input fuel mixture on entropy wave generation and deterioration in thermal convective and adiabatic combustion have been investigated. Comparing the acoustic noise produced by entropy waves with the studied cases shows the thermo-hydrodynamic effects on the entropy wave in the low-order models of combustion instability prediction, which has been limitedly noticed so far. Although the combustion efficiency is improved by increasing the percentage of hydrogen in the fuel, significant instabilities may occur.

Revisiting the initial reaction rates for TMS combustion and a new evidence for metastable silica nanoparticles in the gas-phase synthesis

Nanomaterials from gas-phase synthesis are tapping into new fields of application in research and industry due to their unique size-dependent properties. To produce tailored nanoparticles, the synthesis starting from the precursor decomposition to the particle formation must be fully understood. Tetramethylsilane is used as a precursor for the synthesis of silica nanomaterials in flames. The precursor starts to decompose by H-abstraction. Reaction rates for H-abstraction of tetramethylsilane (TMS) by +O/+H/+OH radicals are determined by means of quantum chemical calculations. The rate expressions are obtained in the temperature range from 300 to 1400 K: kH = 6,025,735 (T/K)2.1731exp(-(+2613.840 K)/T) cm3mol−1s−1 for the total bimolecular reaction coefficient of TMS with hydrogen atoms, kOH = 10,179,818 (T/K)1.7790 exp(-(+152.739 K)/T) cm3mol−1s−1 for TMS with OH radicals, and kO = 93,100 (T/K)2.4797 exp(-(+889.166 K)/T) cm3mol−1s−1 for TMS with O(3P) radicals, respectively. The reaction rates are implemented into the TMS reaction mechanism of Janbazi et al. [1], and a better agreement of the TMS reactivity in the flame is achieved. Experiments are conducted to obtain the mass deposition rates with a Quartz-Crystal-Microbalance (QCM) in a wide range of different equivalence ratios. The equivalence ratio is varied between ϕ = 0.6–1.2, and precursor amounts of 400, 600 and 800 ppm are used. These QCM-experiments are complementary to the MBMS-studies from Karakaya et al. [1–3] but use the same flame conditions to extend the data set. The results reveal that metastable particles exist in the reaction zone of the flame. Depending on flame conditions, their concentration decreases towards the end of the reaction zone, but particles subsequently grow again in the recombination zone of the flame. The mechanisms, which describe the reactivity of the metastable nanoparticles, are tentatively proposed. The understanding of the mechanisms can open up the way for tailored nanoparticles with different structures and stoichiometries.

Tailoring crystal structure and morphology of MnOx nanoparticles via electrospray-assisted flame spray pyrolysis

In this work, MnOx nanoparticles were produced by a continuous electrospray-assisted flame spray pyrolysis (EAFSP) method. In this regard, the precursor solutions containing manganese (II) nitrate hydrate (MnN) dissolved in 1-propanol precursor solutions are initially atomized into charged droplets by an electrospray in the microdripping mode. Then, the electrically charged precursor droplets enter the flame in a controlled manner without requiring a dispersion gas. The synthesis was carried out with a variation in horizontal injection locations from different heights above the burner (HAB) to control the high-temperature residence times of the charged droplets without changing the flame conditions for producing MnOx nanoparticles with tunable particle properties. Various diagnostics such as TEM, SMPS, BET, and XRD were applied for analyzing the formation of primary and agglomerated MnOx nanoparticles as well as their crystal phases with varying temperature histories. Furthermore, phase-Doppler anemometry (PDA) experiments have been conducted to track the evolution of droplet size and droplet axial velocity in the electrospray flame, allowing for the study of combustion of electrosprayed MnN droplets and the presence of µ-explosions in the flame.

Diffusion flames and a semi-empirical method for estimating the distribution of hydrogen molecules in propane flames

Relevance. The greatest effectiveness in determining the main characteristics in gas burning was shown by optical methods due to their high speed and accuracy. Despite all the advantages of these methods, their main disadvantage is the price and folding of implementation. Therefore, today it is necessary to improve approaches to solve this problem. Purpose. The research is devoted to the study of diffusion flame by a semi-empirical method. Methods. The data of the experimental study of the diffusion flame of hydrocarbons on the example of propane are presented. To visualize the invisible part of the structure of this flame, namely, the afterburning zone of hydrogen molecules formed in the flames in nonequilibrium quantities and, due to the large value of the diffusion coefficient, leaving the flame zone and creating a new combustion zone, molecules containing atoms of alkali metals (NaCl and Na2CO3) are vaporized in the flame zone. Results. The method of delivery of molecules of alkali metal salts from outside was applied for the first time, which allowed the investigation of this phenomenon more thoroughly. Based on the research results, a method for determining the concentration of hydrogen atoms and the relative distribution of the concentration of hydrogen molecules along the axis of propagation of the flame after the burning zone was proposed. The research method combines experiments with mathematical modeling. The application of the method described in the article makes it possible to determine the distribution of hydrogen molecules over the glow zone of the main fuel. Conclusions. The results obtained will help to better understand the phenomena of hydrocarbon combustion under diffusion flame conditions, as well as to search for new ways of obtaining hydrogen fuel from domestic waste treatment

Modelling the electrophoretically-enhanced in-flame deposition of carbon nanoparticles

Flame synthesis of Carbon NanoParticle (CNP) films is gaining strong interest for novel industrial applications because of the easy tuning of the operating conditions that enables accurate control of the chemical and physical properties of the produced CNPs. This work proposes a novelty in the synthesis of CNP films, namely the possibility of applying electric fields in flame to trigger electrophoretic deposition phenomena. In this way, it is possible to add another degree of freedom to the harvesting process and potentially modify the properties of the CNP films, without changing the operating flame conditions. To investigate the physical mechanisms governing the thermo-electrophoretic deposition of CNPs, a numerical model to simulate the particle dynamics close to the collecting substrate has been developed, and experiments have been carried out to provide highly controlled test conditions that can be used to support model validation. The experimental results consist of Atomic Force Microscopy (AFM) measurements to determine the number of particles deposited after a controlled harvesting condition as a function of the applied electrophoretic force, imposed on the substrate by means of a DC voltage varied from 0 to -3kV. The AFM shows that the amount of deposited material increases up to six times when passing from an uncharged to the -3kV charged case. The model predictions are highly consistent with the AFM measurements and pointed out that the electric field in flame significantly alters the CNP deposition velocities and impact angles, which are likely to affect the properties of the film.