Showing papers on "Diffusion flame published in 2021"

Structure and Laminar Flame Speed of an Ammonia/Methane/Air Premixed Flame under Varying Pressure and Equivalence Ratio

Abstract: This paper presents a joint experimental and numerical study on premixed laminar ammonia/methane/air flames, aiming to characterize the flame structures and NO formation and determine the laminar flame speed under different pressure, equivalence ratio, and ammonia fraction in the fuel. The experiments were carried out in a lab-scale pressurized vessel with a Bunsen burner installed with a concentric co-flow of air. Measurements of NH and NO distributions in the flames were made using planar laser-induced fluorescence. A novel method was presented for determination of the laminar flame speed from Bunsen-burner flame measurements, which takes into account the non-uniform flow in the unburned mixture and local flame stretch. NH profiles were chosen as flame front markers. Direct numerical simulation of the flames and one-dimensional chemical kinetic modeling were performed to enhance the understanding of flame structures and evaluate three chemical kinetic mechanisms recently reported in the literature. The stoichiometric and fuel-rich flames exhibit a dual-flame structure, with an inner premixed flame and an outer diffusion flame. The two flames interact, which affects the NO emissions. The impact of the diffusion flame on the laminar flame speed of the inner premixed flame is however minor. At elevated pressures or higher ammonia/methane ratios, the emission of NO is suppressed as a result of the reduced radical mass fraction and promoted NO reduction reactions. It is found that the laminar flame speed measured in the present experiments can be captured by the investigated mechanisms, but quantitative predictions of the NO distribution require further model development.

Stabilization mechanisms of CH4 premixed swirled flame enriched with a non-premixed hydrogen injection

Abstract: High-fidelity Large Eddy Simulations (LES) are performed to study the effect of hydrogen injection on a lean turbulent CH4/Air premixed flame. An Analytically Reduced Chemistry (ARC) mechanism is used to achieve a detailed description of CH4/Air-H2 chemistry. First, a validation of this kinetic scheme against the detailed GRI-Mech 3.0 mechanism is presented considering both simplified and complex transport properties. When hydrogen is added to the mixture, large variations of the mixture Prandtl and of the N2 Schmidt numbers are observed depending on the local species concentrations, features that are missed by simplified models. LES is then applied to study the structure and stabilization mechanisms of a lean (ϕ = 0.8) premixed CH4/Air swirled flame enriched with hydrogen by using different transport modeling strategies. First, the fully premixed CH4/Air case is considered and results are found to validate the LES approach. In agreement with experiments, a classical V-shape flame is stabilized in the low-velocity region near the flame holder created by a central recirculation zone (CRZ). Then, hydrogen enrichment is achieved injecting 2% of the CH4 thermal power with a central fuel injection lance. Both premixed and diffusion flame branches are present in this case, impacting flame stabilization and flame angle. The flame root of the main premixed flame is stabilized by a diffusion flame kernel created by the injected hydrogen reacting with the oxygen in excess of the premixed stream. Moreover, the H2 consumed with the remaining oxygen in burnt gases leads to the formation of a second flame branch inside the CRZ which is responsible of an increase of the flame angle. Given the high concentration of hydrogen, an impact of the molecular transport models is observed on the flame lift-off height highlighting the importance of using complex transport properties in any LES involving hydrogen combustion.

Stereoscopic high-speed imaging of iron microexplosions and nanoparticle-release

Abstract: In this work, the combustion behavior of seeded iron particles (d50 = 70 µm) in a laminar diffusion flame was studied in a modified Mckenna flat-flame burner. Two high speed cameras in stereo configuration allowed 3D position and 3D velocity measurements of burning iron particles as well as 3D evaluation of particle microexplosions. Microexplosive processes are important since it can affect both combustion stability and formation of product components. The observed microexplosions happened before particle extinction resulting in change of trajectories, velocities, radiation intensities and fragmentation into smaller particles. It was observed for the first time that fragments of these microexplosions tend to produce planar structures. A frequent release phenomenon was observed during the iron particle combustion using magnified thermal radiation imaging and high-speed shadowgraphy. This release phenomenon was indirectly confirmed with scanning electron microscopy of combust products, revealing multiple cracked particle shells and hollow structures. Black body radiation characteristics was observed indicating the release being in condensed phase and emission spectroscopy identified FeO as intermediate species during combustion. The observed release is believed to mainly consist of iron-oxide nanoparticles formed in the homogenous reaction between vapor iron and oxidizers.

Simultaneous in-situ measurements of gas temperature and pyrolysis of biomass smoldering via X-ray computed tomography

Abstract: In-situ X-ray computed tomography (XCT) imaging is employed to investigate the smoldering dynamics of biomass at the sub-millimeter scale. This technique provides simultaneous and spatially-resolved information about the gas temperature and the biomass density, thereby enabling tracking of the pyrolysis and char oxidation fronts. To achieve well-controlled heating and flow conditioning, oak biomass samples are instrumented above a diffusion flame inside a tube, with total oxygen concentrations of 6% and 11% per volume. Experiments are performed on a laboratory XCT system. The flow is diluted with Kr to increase X-ray attenuation in the gas phase thus allowing for simultaneous 3D measurements of sample density and surrounding temperature. XCT scans are acquired every 90 s at a spatial resolution of 135 µm. The high spatial resolution enables the volumetric visualization of the smoldering process that is associated with pyrolysis and char oxidation. These measurements show how the grain structure affects flame stabilization and induces fingering of the pyrolysis front, while crack formation accelerates the char oxidation locally. Evaluations of the sample mass via XCT are compared with load cell measurements, showing good agreement. A low-order model is developed to evaluate the propagation speeds of pyrolysis and oxidation fronts from the X-ray data over time, and comparisons are made with the surface recess speed.

Experimental and Kinetic Investigation of Stoichiometric to Rich NH3/H2/Air Flames in a Swirl and Bluff-Body Stabilized Burner

Abstract: Ammonia (NH3) is an inorganic substance considered as a promising fuel for power sector decarbonization. As a result of the absence of carbon in its structure, ammonia is capable of producing energy with zero CO2 emissions when burned. However, the combustion of NH3 presents several challenges as a result of its low reactivity and low flame speed as well as the formation of large quantities of nitrogen oxides (NOx) and frequent ammonia slip. A suggested solution for gas turbines is the use of rich-to-lean approaches, with a fuel-rich first-stage combustion, which mitigates NOx formation, followed by a lean phase for the oxidation of the remaining reactants, improving combustion efficiency. To help assess this concept, the present work investigates experimentally and computationally the combustion of ammonia/air mixtures, enriched by hydrogen (H2) for enhanced burning characteristics, in a swirl and bluff-body stabilized burner, at stoichiometric to fuel-rich conditions. Stability tests were performed for a fixed thermal input (2.8 kW), and flames of fuel/air equivalence ratios of 1.0, 1.1, and 1.2, with molar fractions of ammonia in fuel of 0.7 and 0.8, were studied. Temperature profiles along the combustor axis were measured, and flue gas measurements for NOx emissions and unburned NH3 and H2 concentrations were performed for the six studied flames. Computational simulations were performed using a chemical reactor network coupled with recent kinetic mechanisms to compare species trends and further understand the NOx formation and NH3 conversion into hydrogen, through rate of production analyses. It was found that the present laboratory combustor performed well in terms of flame stability, also generating low levels of NOx emissions in all fuel-rich conditions. H2 was detected in high concentrations in the flue gas, partially originated from ammonia dissociation, and is followed by high unburned ammonia emissions. Both H2 and NH3 emissions increase with the equivalence ratio. A secondary, spontaneous diffusion flame was observed above the combustor, proving that the flue gas may subsequently be burned. Higher fractions of hydrogen in the fuel generate more unburned ammonia but also higher H2 concentrations in the flue gas. The predictions based on a reactor network model coupled with the evaluated kinetic mechanisms presented good agreement with the experimentally observed species trends and fair agreement with species values. (Less)

The role of DME addition on the evolution of soot and soot precursors in laminar ethylene jet flames

Abstract: Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of C C bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (Dp). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts.

Numerical and experimental studies of extinguishment of cup-burner flames by C6F12O

Abstract: The extinguishment of propane cup-burner flames by a halon-replacement fire-extinguishing agent C6F12O (Novec 1230) added to coflowing air in normal gravity has been studied computationally and experimentally. The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (up to 141 species and 2206 reactions), molecular diffusive transport, and a radiation model, is used to reveal a unique two-zone flame structure. The peak reactivity spot (i.e., reaction kernel) at the flame base stabilizes a trailing diffusion flame, which is inclined inwardly by a buoyancy-induced entrainment flow. As the volume fraction of the agent in the coflow is increased gradually, the total heat release increases up to three times due to unwanted combustion enhancement by exothermic reactions to form HF and CF2O in the two-zone trailing flame; whereas at the base, the flame-anchoring reaction kernel weakens (the local heat release rate decreases) and eventually the flame blows off. A numerical experiment, in which the C6F12O agent decomposition reactions are turned off, indicates that for addition of inert C6F12O, the maximum flame temperature decreases rapidly due to its large molar heat capacity, and the blow-off extinguishment occurs at ≈1700 K, a value identical to that for inert gases previously studied, while the reaction kernel is still burning vigorously. The calculated minimum extinguishing concentration of C6F12O in a propane flame is 4.12 % (with full chemistry), which nearly coincides with the measured value of 4.17 ± 0.30 %.

Interpreting diffusion flame structure by simultaneous mixture fraction and temperature measurements using optical and acoustic signals from laser-induced plasmas

Abstract: The measurement of mixture fraction and temperature is of great importance in non-premixed flame structure studies and combustion optimization. However, obtaining profiles of mixture fraction and temperature simultaneously for a wide range of fuels and compositions has proven to be challenging, especially when suitable spatial and temporal resolutions are desired. In this study, the acoustic and optical signals from laser induced plasmas were simultaneously used for the first time to obtain acoustic-based laser-induced breakdown thermometry (LIBT) combined with the laser induced breakdown spectroscopy (LIBS). The system was first calibrated in an ethylene-air premixed flame. Then the atomic-ratio and temperature distributions along the centerline of an ethylene counterflow diffusion flame were measured. The strong compositional and temperature gradients in diffusion flames represents a potential challenge, but simultaneous measurements were successfully performed within a 1.5 mm wide physical region, where the equivalence ratio ranged from 0.5 to 12 and temperature ranged from 1100 K to 2000 K. The elemental mass fractions and mixture fraction distribution were inferred based on measured atomic ratio distributions. The preferential diffusion of H relative to C was directly observed by the measurement of C/H ratio. Lastly, the physics behind the LIBT technique is discussed and analyzed. This work demonstrates that the combination of LIBT and LIBS holds promise as a simple tool to measure the mixture fraction and temperature in a broader range of combustion conditions; and facilitates a better understanding of flame structure.