Flame Radicals Research Articles

The comparative and combined effects of hydrogen addition on the laminar burning velocities of methane and its blends with ethane and propane

Laminar burning velocities of hydrocarbon blends of relevance to natural gas combustion, with addition of 0, 10, 35 and 50% hydrogen, were measured using the heat flux method. Hydrocarbon blends were methane (80%)/ethane (20%), methane (80%)/propane (20%) and methane (80%)/ethane (10%)/propane (10%), and in addition experiments were performed using pure methane as a fuel. For the first time it was shown experimentally that hydrogen promotes laminar burning velocity of blends with heavier hydrocarbons to a smaller extent than the well-studied effect on methane. Measurements show that enrichment by hydrogen results in 20–40% lower increase in laminar burning velocity for hydrocarbon blends compared to pure methane, depending on stoichiometry. Modeling points at the importance of increasing concentrations of OH, O and H radicals in H2 enriched flames. At lean conditions increase in H atom concentration is of particular importance. The results are rationalized based on asymptotic flame theory analysis.

Correlation between the Gas Temperature and the Atomization Behavior of Analyte Elements in Flame Atomic Absorption Spectrometry Estimated with a Continuum-light-source Spectrometer System

In flame atomic absorption spectrometry (FAAS), the gas temperature for two types of the gas compositions, which was estimated based on a two-line method by using a simultaneous multi-wavelength spectrometer, on which a line pair of ruthenium, Ru I 372.692 nm and Ru I 372.803 nm having different excitation energies, was measured at the same time. Also using the spectrometer system, the absorption signals of both iron and ruthenium, whose oxides had different thermodynamic properties: the latter oxide was decomposed much more easily than the former one, were investigated with a nitrous oxide - acetylene flame, in comparison with an air - acetylene flame. The fuel/oxidant ratio of both the flames as well as the height of the optical path was varied as an experimental parameter. The atomization behavior of iron and ruthenium, which could be deduced from a variation in their absorption signals, was considered to be dependent not only on the gas temperature but on reducing atmosphere of the flame gas, which might be attributed to reducing radicals in a fuel-excess flame consisting of nitrous oxide. In the nitrous oxide - acetylene flame, a broader optical path having a constant and higher temperature was obtained, thus contributing to formation of analyte atoms with a stable atomization efficiency and eventually to better precision in the analytical result in FAAS.

An experimental study on the spectroscopic characteristics in coal-water slurry diffusion flames based on hot-oxygen burner technology

The characteristics of spectral emissions in coal-water slurry (CWS) diffusion flames are investigated in this paper. Using Praxair's improved hot-oxygen burner (HOB) technology, CWS can be ignited directly in the ambient atmosphere. Moreover, the spectral emissions in methane and CWS diffusion flames are obtained by a fiber-optic spectrometer and a high-spatial-resolution ultraviolet imaging system, respectively. The results indicate that the spectral emission lines of the excited radicals in CWS flames can be used to evaluate whether the coal has been ignited. Besides, the peak intensity evolutions of several main radicals under different conditions were analyzed. Based on the two-dimensional OH* distributions, the axial and radial OH* intensity distribution in CH4 and CWS flames were discussed. Moreover, the areas of the flames and reaction regions show the proportions of the reaction region in the flame. Additionally, for the CWS diffusion flames, the OH* radicals can be considered as a good indicator of heat release rate. The OH*/CH* and OH*/C2* intensity ratios can be used to estimate the O/C or CWS flow rate.

Single-shot 3D flame diagnostic based on volumetric laser induced fluorescence (VLIF)

We demonstrate single-shot three-dimensional (3D) flame measurements based on volumetric laser-induced fluorescence (VLIF). For this effort, the excitation laser beam was expanded into a “slab” with a height of 40mm and thickness of 10mm to excite the CH radicals in the target flames. The volumetric LIF signals were then simultaneously captured by five intensified cameras, and the images were fed into a tomographic algorithm as inputs to obtain the 3D distribution of CH. The results show that single-shot 3D measurements with sufficient accuracy can be obtained in a volume of 9.3×9.3×32.7mm3 with an excitation pulse energy of ∼10mJ. With a total of five intensified cameras, 3D measurements were obtained with a voxel size of 0.15mm over a total of 64×64×224≈106 voxels, which can be further improved by the use of additional cameras or advanced tomographic inversion techniques. Comparison with PLIF measurements suggested that the spatial resolution of the CH-based VLIF technique to characterize the flame surface was about 0.4mm. Measurements were first performed on stable laminar flames, for comparison and validation against PLIF, and then on turbulent jet flames, to illustrate the feasibility and accuracy of VLIF for instantaneous 3D flame measurements and motivate further investigation.

Strategies for Quantitative Planar Laser-Induced Fluorescence of NH Radicals in Flames

ABSTRACTLaser-induced fluorescence imaging of the NH radical using excitation in the (0-0) and (1-0) vibrational bands of the A3Π-X3Σ– electronic transition is characterized in premixed NH3-air flames. Filtered detection for excitation of the (1-0) band is found to be beneficial for measurement conditions with a challenging background and/or interferences. Concentration evaluation for excitation in the (0-0) and (1-0) bands is feasible by means of fully or partially saturated fluorescence, respectively. Detection limits of a few tens of ppm for averaged data and 100 ppm on a single-shot basis was achieved for 3-cm imaging size using both excitation alternatives. Thinning and broadening of the NH layer in a turbulent flame indicate turbulence/flame chemistry coupling, which occurred at lower turbulence levels compared with hydrocarbon combustion. Fluorescence imaging of the NH flame-front marker in such flames thus provide an experimental configuration for detailed studies of turbulence/flame chemistry interactions.

Prediction of NOx Emissions from a Biomass Fired Combustion Process Based on Flame Radical Imaging and Deep Learning Techniques

ABSTRACTThis article presents a methodology for predicting NOx emissions from a biomass combustion process through flame radical imaging and deep learning (DL). The dataset was established experimentally from flame radical images captured on a biomass-gas fired test rig. Morphological component analysis is undertaken to improve the quality of the dataset, and the region-of-interest extraction is introduced to extract the flame radical part and rescale the image size. The developed DL-based prediction model contains three successive stages for implementing the feature extraction, feature fusion, and emission prediction. The fine-tuning based on the prediction is introduced to adjust the process of the feature fusion. The effects of the feature fusion and fine-tuning are discussed in detail. A comparison between various image- and machine-learning-based prediction models show that the proposed DL prediction model outperforms other models in terms of root mean square error criteria. The predicted NOx emissions are in good agreement with the measurement results.

Prediction of Pollutant Emissions of Biomass Flames Through Digital Imaging, Contourlet Transform, and Support Vector Regression Modeling

This paper presents a method for the prediction of NO x emissions in a biomass combustion process through the combination of flame radical imaging, contourlet transform and Zernike moment (CTZM), and least squares support vector regression (LS-SVR) modeling. A novel feature extraction technique based on the CTZM algorithm is developed. The contourlet transform provides the multiscale decomposition for flame radical images and the selected operator based on Zernike moments is designed to provide the well-defined structure for the images. The resulted image features are a variable structure, which is originated from the CTZM. Finally, the variable features of the images of four flame radicals (OH*, CN*, CH*, and $\text{C}^{\ast }_{2}$ ) are defined. The relationship between the variable features of radical images and NO x emissions is established through radial basis function network modeling, SVR modeling, and the LS-SVR modeling. A comparison between the three modeling approaches shows that the LS-SVR model outperforms the other two methods in terms of root-mean-square error and mean relative error criteria. In addition, the structure of the image features has a significant impact on the performance of the prediction models. The test results obtained on a biomass-gas fired test rig show the effectiveness of the proposed technical approach for the prediction of NO x emissions.

Structure of CH4/O2/Ar flames at elevated pressures studied by flame sampling molecular beam mass spectrometry and numerical simulation

Experimental data are reported on the structure of laminar premixed methane/oxygen/argon flames stabilized over a flat burner at 1, 3, and 5 atm with different equivalence ratios ϕ (0.8–1.2). Mole fraction profiles of the reactants (CH4, O2), major stable products (CO2, H2O, H2, CO) and intermediates such as H, OH, CH3 radicals, as well as ethylene and acetylene, were measured by molecular-beam mass spectrometry. The temperature profiles in the flames were measured by thermocouples in the presence of a sampling probe to take into account the flame cooling effect due to the probe. The structures of stoichiometric flames at 1, 3 and 5 atm were compared to elucidate the effect of pressure on the mole fractions of the flame species. Fuel-lean (ϕ = 0.8) and fuel-rich (ϕ = 1.2) flames at 5 atm were also investigated in this work. All the experimental data were compared with the numerical simulations using the Premix code and three detailed chemical kinetic mechanisms for methane combustion available in the literature: the GRI-Mech 3.0, AramcoMech 1.3 and USC Mech II. The absolute mole fractions of CH4, O2, H2O, CO, CO2, H2, H, OH, CH3 in the flames and their dependences on pressure were captured by both mechanisms reasonably well. An analysis of the reaction mechanisms was performed to gain insights into the kinetics of methane combustion in stoichiometric conditions in the range of pressures from 1 to 5 atm and to explain the observed pressure effects on peak mole fractions of flame radicals. The decrease of peak mole fractions of acetylene and ethylene with pressure increase, which was observed in the experiments, was not reproduced by the mechanisms. Both mechanisms predicted the increase in their peak mole fractions with pressure (in the range from 1 to 3 atm). The kinetic analysis indicated the need to revise the pressure-dependent chemistry of acetylene and ethylene formation in the mechanisms.

On‐line identification of biomass fuels based on flame radical imaging and application of radical basis function neural network techniques

In biomass fired power plants a range of biomass fuels are used to generate electric power. It is desirable to identify the type of biomass fuels on-line continuously in order to achieve an improved combustion efficiency, and reduced pollutant emissions. This paper presents the recent investigations into the on-line identification of biomass fuels based on the combination of flame radical imaging and radical basis function (RBF) neural network (NN) techniques. The characteristic values of flame radicals (OH*, CN*, CH* and C2*), including the intensity ratio, intensity contour, mean intensity, area and eccentricity, are computed to reconstruct two types of RBF NN, that is, accurate and probabilistic RBF networks. Experimental results obtained for three types of biomass fuels (flour, willow sawdust and palm kernel shell) firing on a laboratory-scale combustion test rig are presented to demonstrate the effectiveness of the proposed method.

Hydrocarbon flame inhibition by C3H2F3Br (2-BTP)

A kinetic mechanism for hydrocarbon flame inhibition by the potential halon replacement 2-BTP (2-Bromo-3,3,3-trifluoropropene) has been assembled, and is used to study its effects on premixed methane–air flames. Simulations with varying CH4–air stoichiometry and agent loading have been used to understand its flame inhibition mechanism. In particular, the response of lean methane–air flames is examined with addition of 2-BTP, CF3Br, C2HF5, and N2 to illustrate the effect of agent heat release on these flames. The results predict that addition of 2-BTP or C2HF5 can increase the burning velocity of very lean flames, and 2-BTP is less effective for lean flames than for rich. The flame inhibition mechanism of 2-BTP involves the same bromine-species gas-phase catalytic cycle as CF3Br, which drives the flame radicals to equilibrium levels, which can be raised, however, by higher temperatures with added agent (for initially lean flames). Simulations for pure 2-BTP–O2–N2 mixtures predict burning velocities on the order of 1cm/s at 300K initial temperature.

Stretch and Curvature Effects on NO Emission of the Lean H2/Air Premixed Flames

NO production is simulated and analyzed in three lean H2/air premixed flames: the classic one-dimensional (1D) flame, the opposed jet flame, and the tubular flame. The 1D flame is simulated with the PREMIX code followed by the SENKIN code to determine the NO production for a long residence time. The opposed jet flame and the tubular flame are simulated with the OPPDIF and TUBEDIF codes, respectively. The major factors influencing NO production are flame temperature, radical concentration, and post-flame residence time. For the 1D flame, almost 100% of NO is produced in the post-flame region and the reactant residence time in this region is the only factor determining NO concentration. For the opposed jet flame, flame stretch influences all three factors and the competition among them controls the NO concentration response to stretch rate. For the tubular flame, flame curvature influences flame temperature and post-flame residence time, and it makes the NO concentration response different from that of the opposed jet flame.

Nitric oxide PLIF measurement in a point-to-plane pulsed discharge in vitiated air of a propane/air flame

The effect of a point-to-plane pulsed discharge on the vitiated downstream of a propane/air flame has been investigated by phase-locked NO planar laser-induced-fluorescence (PLIF) measurements. Phase-locked NO PLIF measurements with the variation of pulsed plasma energy, equivalence ratio and applied voltage rise time have been performed. Fast rise time (25 ns) and slower rise time (150 ns) high-voltage pulsers are used to produce NO radical densities greater than the ambient flame-produced NO radicals in lean, balanced and rich premixed flames. The pulsed plasma produced excess NO radical densities were found to decay to 50% level with time constants greater than 250 µs in the burnt gas regions with gas temperatures greater than 1000 K. The super-equilibrium NO populations were dependent on energy deposited and overall equivalence ratio, but independent of voltage pulse rise time for similar energy deposition per pulse. Due to long NO radical density decay lifetimes, super-equilibrium NO populations are convected away from production regions with the ambient flow and observed in downstream exhaust gas regions.

Continuous hydroxyl radical planar laser imaging at 50 kHz repetition rate.

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of hydroxyl radicals (OH) in flames at a continuous framing rate of 50kHz. A frequency-doubled dye laser is pumped by the second harmonic of an Nd:YAG laser to generate laser radiation near 283nm with a pulse width of 8ns and rate of 50kHz. Fluorescence is recorded by a two-stage image intensifier and complementary metal-oxide-semiconductor camera. The average power of the 283nm beam reaches 7W, yielding a pulse energy of 140μJ. Both a Hencken burner and a DC transient-arc plasmatron are used to produce premixed CH4/air flames to evaluate the OH PLIF system. The average signal-to-noise ratio for the Hencken burner flame is greater than 20 near the flame front and greater than 10 further downstream in a region of the flame near equilibrium. Image sequences of the DC plasmatron discharge clearly illustrate development and evolution of flow features with signal levels comparable to those in the Hencken burner. The results are a demonstration of the ability to make high-fidelity OH PLIF measurements at 50kHz using a Nd:YAG-pumped, frequency-doubled dye laser.

The role of radical + fuel-radical well-skipping reactions in ethanol and methylformate low-pressure flames

Although the reactions of fuel-radicals with other dominant flame radicals such as H and CH3 are important reactions in low-pressure flames, they have not been well studied. These reactions may occur through either recombination to form stabilized molecular complexes or direct abstractions and chemically activated addition–eliminations to yield bimolecular products. Here, the role of such reactions in low-pressure flames of ethanol and methylformate is studied through a combination of theoretical characterizations of key reactions and detailed kinetic modeling. In particular, H and CH3+fuel-radical reactions have been characterized theoretically in this work and these are shown to make a pronounced impact on the formation of intermediates. Theoretical calculations for H+CH3CHOH and CH3+CH3CHOH predict that at low pressures recombinations are minor processes with well-skipping (addition–eliminations) dominating the reaction flux. Direct abstraction was also considered in H+CH3CHOH and theory suggests that abstraction at the CH3-site forming CH2CHOH is the only important channel. Notably, this result is counter to analogy based predictions that CH3CHO should be the dominant abstraction product. Low-pressure ethanol flame simulations indicate that addition–elimination reactions from H+CH3CHOH and CH3+CH3CHOH are a major source for C2H4 and C3H6 profiles, respectively. Similar results are observed in simulations of a low-pressure methylformate flame, where addition–elimination reactions of H+CH2OCHO and CH3+CH2OCHO have a significant impact on CH3OH and C2H4 mole fraction profiles, respectively. The present results suggest that the well-skipping reactions of relatively stable fuel-radicals with ubiquitous flame radicals such as H, O, OH, and CH3 should be considered extensively in combustion models.

Experimental study on the performance of transition metal ions modified zeolite particles in suppressing methane/air coflowing flame on cup burner

This study investigated the performance of three kinds of transition metal ions (Mn2+, Cu2+, and Zn2+) modified zeolite 4A particles in suppressing methane/air coflowing flames on cup burner. Ion-exchange method was employed to incorporate different amount of transition metal ions into parent zeolite 4A. Fire suppression effectiveness of zeolites containing varied molar percentage of metal ions was tested by cup-burner method. Results showed that transition metal ions modified zeolites outperformed the parent zeolite 4A with a ranking order of Mn2+ > Cu2+ > Zn2+. Performance of the zeolite particles was affected by the percentage of transition ions loaded, which dominated the number of active sites and Brunauer–Emmett–Teller surface area of the particles. Increasing molar percentage of metal ions loading increased the capacity to scavenge flame radicals, but decreased Brunauer–Emmett–Teller surface area of the particles. There existed an optimized molar percentage of transition metal ions loading where the best fire-suppressing effectiveness was exhibited.

100 kHz thousand-frame burst-mode planar imaging in turbulent flames

High-repetition-rate, burst-mode lasers can achieve higher energies per pulse compared with continuously pulsed systems, but the relatively few number of laser pulses in each burst has limited the temporal dynamic range of measurements in unsteady flames. A fivefold increase in the range of timescales that can be resolved by burst-mode laser-based imaging systems is reported in this work by extending a hybrid diode- and flashlamp-pumped Nd:YAG-based amplifier system to nearly 1000 pulses at 100 kHz during a 10 ms burst. This enables an unprecedented burst-mode temporal dynamic range to capture turbulent fluctuations from 0.1 to 50 kHz in flames of practical interest. High pulse intensity enables efficient conversion to the ultraviolet for planar laser-induced fluorescence imaging of nascent formaldehyde and other potential flame radicals.

Simultaneous 10-kHz PLIF and Chemiluminescence Imaging of OH Radicals in a Microwave Plasma-Enhanced Flame

This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. A tunable, MW waveguide plasma source-operating at 2.45 GHz and delivering 90-130 W to the flowfield-ignites and sustains a CH4/air flame, whereas laser-induced fluorescence and chemiluminescence are acquired at a sustained framing rate of 10 kHz, using two intensified CMOS cameras and a synchronized laser. Multiple geometries and flames (premixed and nonpremixed) are studied by adjusting gas flow compositions and the plasma applicator nozzle components. A stoichiometric premixed flame configuration produces a divergent flame with large-scale fluctuations and vortex shedding into ambient air and is capable of feedstock flow velocities for combustion-to-plasma power ratios . Another arrangement produces plasma along the initial mixing layer of a nonpremixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with a little flamefront fluctuation. The high-speed diagnostics capture OH signals in cinematic sequences, providing new understanding of the plasma-assisted flame holding mechanism and allowing for the tracking of individual flow feature development.

Quantitative Radar REMPI measurements of methyl radicals in flames at atmospheric pressure

Spatially resolved quantitative measurements of methyl radicals (CH3) in CH4/air flames at atmospheric pressure have been achieved using coherent microwave Rayleigh scattering from Resonance enhanced multi-photon ionization, Radar REMPI. Relative direct measurements of the methyl radicals were conducted by Radar REMPI via the two-photon resonance of the $$ 3p^{2} A_{2}^{\prime \prime } 0_{0}^{0} $$ state and subsequent one-photon ionization. Due to the proximity of the argon resonance state of 2s 22p 54f [7/2, J = 4](4+1 REMPI by 332.5 nm) with the CH3 state of $$ 3p^{2} A_{2}^{\prime \prime } 0_{0}^{0} $$ (2+1 REMPI by 333.6 nm), in situ calibration with argon was performed to quantify the absolute concentration of CH3. The REMPI cross sections of CH3 and argon were calculated based on time-dependent quantum perturbation theory. The measured CH3 concentration in CH4/air flames was in good agreement with numerical simulations performed using detailed chemical kinetics. The Radar REMPI method has shown great flexibility for spatial scanning, large signal-to-noise ratio for measurements at atmospheric pressures, and significant potential to be straightforwardly generalized for the quantitative measurements of other radicals and intermediate species in practical and relevant combustion environments.

Simulation of NO Formation in Hydrogen-Air Counterflow Diffusion Flame

The combustion of hydrocarbon fuels produces large amounts of carbon dioxide. In order to cope with the challenge of greenhouse effect and global environmental protection. H2, as a cleaner and more energy-burning fuel, is being considered in many of the practical applications of combustion equipments. However, H2 fuel combustion will still produce pollutant NOx. Thus, the study of NOx emission is one of the most important topics in H2 combustion. With the classical counterflow burner, a numerical simulation of NO emission was carried out on the H2 diffusion flames. The stretch effects on flame temperature, NO concentration, and EINO are analyzed; the contribution of different NO generation routs are also quantified and analyzed. The major parameters influencing NO emission are flame temperature, radical concentration, and residence time, the observed relationship between stretch rate and NO emission is explained with the three major parameters.

Decomposition of water-insoluble organic waste by water plasma at atmospheric pressure

The water plasma was generated in atmospheric pressure with the emulsion state of 1-decanol which is a source of soil and ground water pollution. In order to investigate effects of operating conditions on the decomposition of 1-decanol, generated gas and liquid from the water plasma treatment were analysed in different arc current and 1-decanol concentration. The 1-decanol was completely decomposed generating hydrogen, carbon monoxide, carbon dioxide, methane, treated liquid and solid carbon in all experimental conditions. The feeding rate of 1- decanol emulsion was increased with increasing the arc current in virtue of enhanced input power. The generation rate of gas and the ratio of carbon dioxide to carbon monoxide were increased in the high arc current, while the generation rate of solid carbon was decreased due to enhanced oxygen radicals in the high input power. Generation rates of gas and solid carbon were increased at the same time with increasing the concentration of 1-decanol, because carbon radicals were increased without enhancement of oxygen radicals in a constant power level. In addition, the ratio of carbon dioxide to carbon monoxide was increased along with the concentration of 1-decanol due to enhanced carbon radicals in the water plasma flame.