Methane Flames Research Articles

On the Flow Structure and Dynamics of Methane and Syngas Lean Flames in a Model Gas-Turbine Combustor

The present paper compares the flow structure and flame dynamics during combustion of methane and syngas in a model gas-turbine swirl burner. The burner is based on a design by Turbomeca. The fuel is supplied through injection holes between the swirler blades to provide well-premixed combustion, or fed as a central jet from the swirler’s centerbody to increase flame stability via a pilot flame. The measurements of flow structure and flame front are performed by using the stereo particle image velocimetry and OH planar laser-induced fluorescence methods. The measurements are performed for the atmospheric pressure without preheating and for 2 atm with the air preheated up to 500 K. The flow Reynolds numbers for the non-reacting flows at these two conditions are 1.5 × 103 and 1.0 × 103, respectively. The flame dynamics are analyzed based on a high-speed OH* chemiluminescence imaging. It is found that the flame dynamics at elevated conditions are related with frequent events of flame lift-off and global extinction, followed by re-ignition. The analysis of flow structure via the proper orthogonal decomposition reveals the presence of two different types of coherent flow fluctuations, namely, longitudinal and transverse instability modes. The same procedure is applied to the chemiluminescence images for visualization of bulk movement of the flame front and similar spatial structures are observed. Thus, the longitudinal and transverse instability modes are found in all cases, but for the syngas at the elevated pressure and temperature the longitudinal mode is related to strong thermoacoustic fluctuations. Therefore, the present study demonstrates that a lean syngas flame can become unstable at elevated pressure and temperature conditions due to a greater flame propagation speed, which results in periodic events of flame flash-back, extinction and re-ignition. The reported data is also useful for the validation of numerical simulation codes for syngas flames.

Imaging-based harmonic frequency analysis of a bluff-body premixed flame under acoustic excitations

The dynamic responses of a bluff-body premixed methane flame under acoustic excitations were studied using high speed color and schlieren imaging techniques. Two excitation frequencies at different amplitudes were tested and analyzed (50 Hz-105, 110, 115 dB and 200 Hz-120, 125, 130 dB). The fast Fourier transform (FFT) analysis reveals the dominant excitation frequency, as well as several harmonic frequencies as nonlinear responses. The dynamic mode decomposition (DMD) analysis was conducted using the time-resolved color and schlieren images to extract the spatial pattern at the dominant and multiplication harmonic frequencies. The results indicate that under these frequencies, the energy distribution has exhibited axisymmetric patterns to the nozzle axis, with positive and negative energy bands appearing alternatively. The spatial distribution has shown great variances under different excitation frequencies and amplitudes. An energy band sub-dividing phenomenon at multiplication frequencies was observed from the DMD results of both color and schlieren images.

Improving the mixing of a turbulent jet diffusion flame of methane via linear quadratic regulator control

The purpose of this article is to show the improvement of the mixture of a turbulent jet diffusion flame of methane by means of linear quadratic regulator control. The model is based on low Mach and high Damköhler numbers. Large eddy simulation is used to simulate a turbulent flow with the Smagorinsky submodel for turbulent viscosity. The control employs a pair of actuators that act transversally to the flow direction on the walls of the burner. They consist of rectangular openings added along the depth of the duct. Numerical tests were performed by changing the position of one of these actuators. The results show that the controllers alter the jet diffusion flame, generating vortices of different sizes and dispersion patterns, which quickly leads to highly complex flow patterns.

PAHs and fullerenes as structural and compositional motifs tracing and distinguishing organic carbon from soot

Examining the features distinguishing organic carbon from soot is crucial for understanding the source, the effect on the environment and their respective role in aerosol chemistry and soot formation. Beside to the obvious PAH picking-out in the low-mass mode (C number < 40), the challenging identification of PAHs in the high-mass mode (C number > 40) of organic carbon, separated by carbon particulate matter extraction from young and mature soot thermophoretically sampled in premixed flames, was done by laser-desorption-time-of-flight mass spectrometry, exploiting the laser power increase. The perusal of organic carbon mass spectra through mathematical tools in comparison to aromatic and alkyl-substituted PAH-laden samples and the persistence of high-mass mode at high laser power led to exclude the contribution of dimers and alkyl-bridged PAHs attributing the second mode to both fully-benzenoid and cyclopenta-PAHs.Profound differences between mass spectra of organic carbon and soot were noticed as neither molecules nor radicals of PAHs could be drawn out from soot, even at high laser power, and only small radicals and carbon clusters like fullerenes were observed, especially for young soot. These inferences evidenced the importance of analysing separately organic carbon and soot especially if insights into soot particles nucleation are to be obtained. In the case of benzene flame, already at the inception, soot consists of strongly tangled aromatic motifs crosslinked each other, presumably deriving from reactive coagulation/clustering of relatively small aromatic hydrocarbons/radicals early formed. In methane and ethylene flames, coalesced liquid-like material composed of soot and PAHs is formed and transformed later on undergoing some carbonization and molecular growth, respectively.

One zirconia-based ceramic coating strategy of combustion stabilization for fuel-rich flames in a small-scale burner

From the perspective of the thermal/chemical performance management of a slot burner, three kinds of blended coatings with the low thermal conductivity and certain chemical activities, including 30wt.%Y2O3/ZrO2, 30wt.%MgO/ZrO2 and 30wt.%Al2O3/ZrO2, were prepared on stainless steel walls plated by atmospheric plasma spraying. The effects of coating material, wall temperature, and equivalence ratio on fuel-rich methane flame characteristics were investigated experimentally. The OH planar laser-induced fluorescence (OH-PLIF) technique was used to obtain OH intensity and flame anchoring location. The physical and chemical properties of different coatings before and after spraying were analyzed by a series of material and surface characterization techniques. Results show that compared with pure ZrO2 coating, the quenching distances of three ZrO2-based blended coatings with Y2O3, MgO, and Al2O3 decrease to some extent, resulting in the minimum quenching distance of Al2O3/ZrO2 coating, and the maximum one for Y2O3/ZrO2 coating. Meanwhile, the blended coating walls result in the higher flame anchoring location than that of pure ZrO2 in most cases, in which the coating of Y2O3/ZrO2 displays the highest location. The blended coatings have higher thermal conductivity, which is beneficial to balance part of the heat loss through the axial heat recirculation of wall structures, and thus expanding stability limits at low temperatures. In addition, partial lattice solid solutions are formed in mixture systems after the ultra-high-temperature thermal spraying, leading to more surface lattice oxygen and stronger chemical promotion. In summary, reasonable allocation of ZrO2-based ceramic components as coating wall is an effective alternative strategy to control flame stabilization in small-scale combustion devices.

Large Eddy Simulation of rich ammonia/hydrogen/air combustion in a gas turbine burner

Ammonia and hydrogen are promoted as potential energy carriers for centralized energy restitution. This article investigates ammonia/hydrogen/air premixed turbulent combustion, using Large-Eddy Simulations, in an academic atmospheric gas turbine swirled burner. A one-dimensional flame analysis demonstrates the existence of a trade-off in NOX and NH3 emissions for ammonia/hydrogen blends, and the possibility to obtain 1D flame propagation characteristics close to that of a lean methane flame by adjusting the amount of H2. Large-Eddy Simulations of the PRECCINSTA burner exhibit stable combustion, while the optimized trade-off equivalence ratio is pinpointed at φ=1.46 for XH2Fuel=0.46. Corresponding emissions are XNOX≈XNH3≈300 ppmv. Large amounts of hydrogen are found in the exhaust gases, inducing a low combustion efficiency. The flame structure, combustion dynamics, influence of kinetics modelling and mesh resolution are discussed. This work paves the way for future studies, in the perspective of applications to industrial systems.

Acoustic-Convective Interference in Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

Abstract We investigate the occurrence and source of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff body stabilized CH4/H2 and pure H2 flames. The modulations are shown to be caused by flow disturbances originating from the upstream geometry, in particular the grub screws used to center the bluff body, indicative of a more generalized phenomenon of convective wave propagation. Velocity measurements are performed at various locations around the injector dump plane, inside the injector pipe, and in the wake of the bluff body to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the grub screws appear in the unforced velocity spectra and it is found that the magnitude of these convective modes depends on their location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear approximately axisymmetric. However, the urms and vrms fields capture a strong asymmetry due to convective disturbances. To further quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. The reason behind the excitation of the first mode is due to a condition that states that for acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cutoff frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cutoff frequency.

Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

Hydrogen and its derivatives, including ammonia, are gaining increasing attention as carbon-neutral fuel alternatives. An intermediate step in the transition to hydrogen and ammonia is the blending of these fuels with hydrocarbons, introducing the challenge of soot formation. The impact of ammonia on soot formation has recently been the focus of several studies, but a complete understanding of its chemical effects is lacking. Hydrogen, by comparison, has received significant attention from the soot community. However, controversy remains with regards to hydrogen’s chemical impact, and the dependence of this impact on fuel and flame configuration. This work investigates the effect of both hydrogen and ammonia on soot formation in laminar coflow flames of both ethylene and methane. Hydrogen or ammonia are introduced either by addition or substitution, with parallel studies of helium and argon, in order to isolate their chemical effects. Time- and spectrally-resolved laser-induced emissions from UV and IR excitation are used to quantify differences in soot and soot precursor formation. Additionally, chemical kinetics calculations and analyses are used to elucidate the effects of ammonia introduction to ethylene flames. Ammonia is found to chemically inhibit soot when mixed with either ethylene or methane, with increasing effects on larger precursors. Calculations suggest that this suppression is due to carbon consumption in the formation of HCN and CN. Hydrogen is found to chemically enhance soot formation in both ethylene and methane flames.

Thermoacoustic coupling of premixed flames in mesoscale diameter tubes

In the present experimental work, we investigate the thermoacoustic coupling of methane flames propagating inside 4 and 6 mm diameter tubes (slightly larger than the quenching diameter). Multiple tube’s lengths were selected (between 275 and 900 mm) so typical system’s fundamental and harmonic frequencies ranged between 190 and 600 Hz. Experimental results were obtained for stoichiometric and lean methane-air mixtures. The results show that even in such small diameter tubes, open at both ends, thermoacoustic instability still develops and impacts the flame propagation dynamics. We show that depending on the tube length (acoustics) the flame either fully propagates or extinguishes. At 350 Hz (short tubes) both stoichiometric and lean flames propagated over the entire length of the tube in an oscillatory mode, whereas at 190 Hz (long tubes) the flames always quenched within a short distance from the tube’s entrance, even inside the 6 mm diameter tube. These results thus show that the quenching diameter of methane flames can be highly dependent on the tube’s natural acoustics. Moreover, at 250 Hz (medium length tubes), we identified a transition regime where the flame propagation is a stochastic event: combustion, wall-heat transfer and acoustics are in fierce competition in this regime. We have also observed, in tubes with an aspect ratio of 95, a flame shape inversion resembling the tulip flame. In this transition regime, a mode transfer from the fundamental (250 Hz) to subharmonic frequency (125 Hz) of the peak of CH* emission intensity was also observed. This mode transfer is believed to occur when the tulip-flame cusped structure collapses, promoting the mixture of burnt gases with fresh gases, and the consequent increase of the ignition delay and therefore the delay of the peak of CH* emission intensity.

Experimental Investigation of Lean Methane-Air Laminar Premixed Flames at Engine-Relevant Temperatures.

Lean premixed combustion is one of the most effective methods to constrain pollutant emissions for modern industrial gas turbines. An experimental study was performed on its propagation speed and internal structure at engine-relevant temperatures. A Bunsen burner was employed for the measurement with an optical schlieren system. The results show that the increase of preheating temperature dramatically accelerates the propagation of methane flames. The numerical results predicted by GRI-Mech 3.0, FFCM-1, and USC Mech II were also compared. The GRI-Mech 3.0 seems to overestimate the laminar flame speed at high operating conditions, while FFCM-1 underestimates the laminar flame speed compared to the present experimental data. The prediction by FFCM-1 shows good agreement with the overall existing data. The USC Mech II seems to overestimate the laminar flame speed at fuel-lean conditions while shows good agreement with present experimental measurements at stoichiometric conditions when the inlet temperature increases. It is also indicated that the flame is thinned at high-temperature conditions and the importance of CO production to the propagation speed increases. Finally, based on the experimental data, an empirical correlation of the laminar flame speed was developed in the range of Tu = 300–800 K and ϕ = 0.7–1.0, the maximum deviation of which was less than 8%. The results of this study may contribute to the optimization of advanced gas turbine combustors.

Stability Characteristics of a Turbulent Nonpremixed Conical Bluff Body Flame

The thermal characteristics of turbulent non-premixed methane flames were investigated by four burner heads with the same exit diameter but different heights. The fuel flow rate was kept constant with an exit velocity of 15 m/s, while the co-flow air speed was increased from 0 to 7.6 m/s. The radial profiles of the temperature and flame visualizations were obtained to investigate the stability limits. The results evidenced that the air co-flow and the cone angle have essential roles in the stabilization of the flame: An increase in the cone angle and/or the co-flow speed deteriorated the stability of the flame, which eventually tended to blow off. As the cone angle was reduced, the flame was attached to the bluff body. However, when the cone angle is very small, it has no effect on stability. The mixing and entrainment processes were described by the statistical moments of the temperature fluctuations. It appears that the rise in temperature coincides with the intensified mixing, and it becomes constant in the entrainment region.

Hydrogen enhancement on a mesoscale swirl stabilized burner array

This manuscript presents a study of hydrogen-enhanced methane flames in a compact mesoscale burner array, where even a minute addition of hydrogen can significantly improve flame stability with reduced combustion length scale thereby enabling volumetric flexibility in small-scale combustion systems. The effect of hydrogen enhancement is demonstrated through combustion reaction length scale and operation limit analysis using methane fuel. The added hydrogen lowered lean blow off equivalence ratios, increased flame temperatures, and shorten the flame length scale, ultimately producing a denser flame array. 10 kHz OH∗ chemiluminescence and OH planar laser induced fluorescence imaging techniques were used to estimate heat release rate and visualize flame structures. Hydrogen addition increased the OH intensity and decreased global heat release rate fluctuation, and also showed more stable operation under acoustic perturbations. Hydrogen enhancement can be a promising solution for reducing geometric constraints and improving operating capabilities for compact propulsion and power generation systems.

Experimental investigation of reverse flow porous medium reactor with premixed and non-premixed flames

Reverse flow porous medium reactor with premixed and non-premixed flames of methane and oxidant was experimentally investigated to evaluate its suitability for hydrogen (H2) and syngas production. Methane-air premixed, non-premixed, and non-premixed with steam were the tested cases for different inlet configurations. Thermal profiles, wave velocities, and H2 and carbon monoxide (CO) production for equivalence ratios (ϕ) of 2, 3 and 4 were analyzed. Methane-air premixed flames at ϕ = 2 obtained good performance with 1574 K average and 31% and 68% of H2 and CO yield, respectively. The H2/CO ratios in syngas registered were 1.1, 1.7 and 1.5 at equivalence ratios of ϕ = 2, 3, and 4, respectively. Methane-air non-premixed flames obtained higher temperature in the zone of methane injection but no higher syngas production when compared to the premixed case. The H2/CO ratios in syngas registered were 2 and 2.4 at equivalence ratios of ϕ = 2 and 3, respectively. Methane-air non-premixed with steam shows a similarity between the temperature of without steam cases, which implies the lower influence of the steam on the reaction mechanism. Results show that separate supply of reagents allows obtaining synthesis gas with a higher H2/CO ratio than premixed case.

Reaction Mechanism of Ozone with Methane Flame Soot: Langmuir–Hinshelwood or Unimolecular Decomposition?

Reaction Mechanism of Ozone with Methane Flame Soot: Langmuir–Hinshelwood or Unimolecular Decomposition?

Experimental and numerical investigations on extinction strain rates in non-premixed counterflow methane and propane flames in an oxygen reduced environment

The aim of this work is to contribute to the better understanding of the oxidation process of light hydrocarbons, low calorific value gas and components of natural gas in the presence of inert gases and the influence of oxygen reduction in exhaust gas recirculation situations. The extinction behavior of laminar counterflow diffusion flames of methane (CH4), ethene (C2H4) and propane (C3H8) under nitrogen diluted condition has been investigated experimentally and numerically. In the experimental set-up, a counterflow burner was employed to investigate the extinction behavior under atmospheric conditions (1 bar and 298 K). The present study highlights the non-linear influence on the decrease in the extinction strain rate (ESR) that were found with decreasing fuel content and decreasing oxygen content. The oxygen content on the oxide side was stepwise diluted with nitrogen in order to distinctly show the possible oxygen-depleted influence in the recirculation areas. Additionally, it has been shown that the distance between the nozzles has an impact on the ESR but not on the general rising trend with the increasing fuel content in the non-premixed flames. Moreover, to clarify the influence, the flame zones were visualized using a system of high-speed OH* chemiluminescence camera. It was possible to investigate the width, intensity and position of the flame front from the radical detection. This provided a comprehensive data base for the subsequent numerical validation. In order to complement the experimental investigations, extensive uncertainty analyses were conducted and a new laser-based approach for the detection of flame extinction were demonstrated for the low-visible flames. Besides the experimental study, ESRs of CH4 and C3H8 combustion obtained from the experimental measurements were compared with those predicted by the numerical simulations. The San Diego-2014 reaction mechanism was used to represent the detailed chemical reaction, in which the predicted ESRs agree well with the experimental data was shown. Additionally, the effects of radiative heat loss, molecular transport model and chemical reactions on the ESRs were studied numerically. The present results suggest that the radiative heat loss plays a minor role on the ESRs, the molecular transport model is more significant and the ESRs are quite sensitive to several key reactions.

Combustion enhancement and inhibition of hydrogen-doped methane flame by HFC-227ea

Hydrogen enriched with compressed natural gas is an efficient and environment-friendly gaseous fuel. However, the safety issues of mixture and the method to control or weaken their combustion are highly concerned. To explore the inhibition effect of halogenated fire suppressants on the mixture, the effect of HFC-227ea on the laminar premixed methane/air flames, with different fractions of H2, have been studied. Burning velocities have been measured with constant-volume combustion chamber and kinetically modelled a recently assembled kinetic mechanism. The fractions of H2 influence the enhancement and inhibition effect of HFC-227ea, and it is less effective with the lean mixture. In stoichiometric condition, HFC-227ea showed good inhibition effect on the mixture flames. The HFC-227ea increased the burning velocities of CH4-0% H2-air and CH4-10% H2-air flames at leanest condition, whereas the increased burning velocity arising from HFC-227ea not occurred as the addition of H2 above 20%. Experimental results coincided well with numerical results, however the agreement was poor for the leanest flames at low agent loading. Lastly, kinetic mechanism analysis was used to interpret the combustion enhancement and inhibition effect of hydrogen-doped methane flame by HFC-227ea.

Identification of the extinction mechanism of lean limit hydrogen flames based on Lewis number effect

The conventional flame stretch analysis showed some technical limitations in describing the observed extinction mechanism without considering the heat transfer characteristics related to flame flow patterns. To improve the flame stretch analysis, this study investigated the extinction mechanism of lean limit hydrogen flames with the stabilized flame method. Multidimensional lean limit hydrogen flames stabilized in a tube of inner diameter of 25 mm were simulated numerically. Unlike the methane flames, the positive effect of a stretched hydrogen flame (Le≪1) on the flame tip dominated the negative stretch effect by increasing the preferential diffusion. Hence the primary extinction of hydrogen flames occurred at the trailing edge by heat loss mechanism. The simulation confirmed that the radiation heat loss rate did not attain 5% of the conduction heat loss rate at the trailing edge. When the flame skirt length was shortened enough by the primary extinction, an enhanced recirculation flow was generated. The combined effect of conduction and recirculation flow associated with the Lewis number effect was the key to understand the extinction mechanism of lean limit hydrogen flames. Finally, generation of recirculating flow causing the stretch extinction plays a dominant role in final flame extinction.

Quantification of the size, 3D location and velocity of burning iron particles in premixed methane flames using high-speed digital in-line holography

Due to its low emission and high energy density, iron powder has been proposed as a promising recyclable metal fuel for a future low-carbon society. The comprehensive understanding of combustion behavior of iron particles is crucial for studying fundamental mechanisms, developing suitable combustion technologies, and designing efficient iron powder combustor. In this work, iron particles are combusted in a modified Bunsen burner with a stable metal powder supplying system. As a versatile three-dimensional (3D) imaging technique, high-speed digital in-line holography (DIH) is employed to reconstruct the 3D particle field and simultaneously quantify the size, 3D location and velocity of burning iron particles. The statistical results of three cases with oxygen volume fraction varying from 24% to 40% are obtained and compared at different heights above the burner. Along the height, some typical features of the burning iron particles were observed. The violent combustion of iron particles accelerates the ejection of the particles radially outward from the central region of the flame, resulting in non-uniform spatial distribution of the particles and reducing the particle number density in the measurement volume. Such trend is enhanced with increased oxygen concentration. Besides, the observed particle size enlarged as the height increases, which validates the swelling phenomenon of iron particle oxidation. The results demonstrate that DIH is a powerful tool for in-situ, quantitative characterization of particle dynamics in flames.

Effects of jet diameter and spacing in a micromixer-like burner for clean oxy-fuel combustion in gas turbines

The effects of jet diameter and spacing on flame structure and stability are examined in an oxy-methane micromixer-like burner for potential implementation in zero-emission gas turbines. Three burner headend geometries (HE1, HE2, and HE3) are compared at fixed flame-base velocity of 5.2 m/s and over ranges of equivalence ratio and oxygen fraction (O2 vol% in the O2/CO2 oxidizer). HE1 has 61 jets of 3.175-mm diameter, arranged on a hexagonal matrix with a uniform jet spacing of 5.5 mm. HE2 has 37 jets of 3.970-mm diameter (25% increase), also arranged hexagonally with a spacing of 5.5 mm. HE2 has fewer jets to maintain almost the same combustor power. HE3 is similar to HE2, but with a jet spacing of 7.0 mm (27% increase). The three blowout limits were found to be only ∼45 K apart, despite the considerable differences in jet diameter and spacing, which demonstrates the remarkable geometry flexibility of micromixers. HE1 outperformed HE2 and HE3. Increasing the jet diameter adversely affects flame stability, because the burner deviates from the stable lean-direct-injection concept. Increasing jet spacing is also unfavorable. The adiabatic flame temperature is the primary parameter controlling the structure and stability of premixed methane flames for different burner geometries.

Semi-Crystalline Hydrophobic Polyamidoamines: A New Family of Technological Materials?

The hitherto known polyamidoamines (PAAs) are not suitable as structural materials because they are usually water-soluble or swellable in water. This paper deals with the synthesis and characterization of semi-crystalline hydrophobic PAAs (H-PAAs) by combining different bis-sec-amines with bis-acrylamides obtained from C6–C12 bis-prim-amines. H-PAAs were initially obtained in a solution of benzyl alcohol, a solvent suitable for both monomers and polymers. Their number average molecular weights, , which were determined with 1H-NMR by evaluating the percentage of their terminal units, varied from 6000 to >10,000. The solubility, thermal properties, ignitability and water resistance of H-PAAs were determined. They were soluble in organic solvents, semi-crystalline and thermally stable. The most promising ones were also prepared using a bulk process, which has never been previously reported for PAA synthesis. In the form of films, these H-PAAs were apparently unaffected by water. The films underwent tensile and wettability tests. They showed similar Young moduli (260–263 MPa), whereas the maximum stress and the stress at break depended on the number of methylene groups of the starting bis-acrylamides. Their wettability was somewhat higher than that of common Nylons. Interestingly, none of the H-PAAs considered, either as films or powders, ignited after prolonged exposure to a methane flame.