Effects Of Flame Stretch Research Articles

Modeling the displacement speed in the flame surface density method for turbulent premixed flames at high pressures

We propose a model of the local displacement speed for the large-eddy simulation (LES) of turbulent premixed combustion with the flame surface density (FSD) method. This model accounts for flame stretch and curvature effects in order to improve the prediction of LES–FSD for turbulent premixed flames with the non-unity Lewis number at high pressures. The proposed model is validated by a priori and a posteriori tests, with the direct numerical simulation (DNS) and LES–FSD of statistically planar turbulent premixed flames of lean hydrogen at 1 and 10 atm and a range of turbulence intensities. The a priori test confirms that the proposed model accurately estimates the negative correlation of the displacement speed and the flame curvature. In the a posteriori test, the comparison of DNS and LES–FSD results demonstrates that the proposed model improves the prediction on the turbulent burning velocity and the turbulent flame thickness, compared with those from the existing displacement–speed model. The stretch effect in the present model leads to the rise of the turbulent burning velocity and the suppression of the “bending” phenomenon in LES–FSD, consistent with the DNS result.

Numerical Studies of the Pilot Flame Effect on a Piloted Jet Flame

ABSTRACT In a piloted jet flame, the pilot flame has an effect of stabilizing the main flame. Detailed mechanisms of pilot flame/main flame interaction are however not well studied. It is expected that the pilot flame affects the main flame through the following mechanisms: (a) the pilot flame provides the heat and radicals to the reaction zone of the main flame, (b) the pilot flame prevents the cold ambient air from being entrained into the main flame, and (c) the pilot flame modifies the stretch rate of the main flame. In this paper, detailed numerical simulations of piloted laminar methane/air jet flames are carried out to elucidate the effect of pilot flame on the structure and burning velocity of the main jet flame. One-dimensional (1D) freely propagating flame is also simulated to investigate the effect of hot gas mixing with the unburned fuel/air mixture, and 1D counter-flow flame is simulated to study the diffusion of the hot gas from the pilot flame to the reaction zone of the main flame and the effect of flame stretch. The results showed that heat transfer from the pilot flame to the main flame has a more significant effect on the structures and propagation of the main flame than the mass transfer from the pilot flame to the main flame. The heat and mass transfer from the pilot flame affects the local equivalence ratio and temperature of the unburned mixture, which gives rise to a significant enhancement of burning velocity. When the hot gas from the pilot flame is at sufficiently high temperatures, an ultra-lean fuel/air mixture can burn at equivalence ratio below the flammability limit. The reaction rate and burning velocity of ultra fuel-lean flames are enhanced by the strain rate, whereas for a main flame with the equivalence ratio closer to that of pilot flame, the reactivity and burning velocity of the main flame decrease with increasing strain rate.

Impact of chamber volume on the measurement of laminar burning velocity using constant volume spherical flame method

The objective of this work was to investigate the impact of chamber volume on the measured value of laminar burning velocity using constant volume spherical flame method. Experimental unsteady pressure-time data of stoichiometric methane air mixture recorded in spherical chambers of different volumes ranging from 0.5 to 20 L was obtained from different research groups. All the collected data in the reduced normalized pressure range of 0.25–0.5 were processed with a two zone thermodynamic model. One dimensional unsteady spherical flame model at constant volume in COSILAB along with GRI-Mech3.0 reaction mechanism was used for predicting laminar burning velocity. The values of stretched burning velocity at elevated pressures and temperatures differed with chamber volume due to flame stretch effects. The extrapolated stretched laminar burning velocity at 1 bar and 300 K using power law found to be independent of spherical chamber volume. From the present comparison of stretched burning velocity data between 0.5 and 20 L, it was found that the constant volume spherical flame method may deliver laminar burning velocity with less stretch effects if a spherical combustion chamber of 4.4 L range was used.

Large-Eddy-Simulation Modeling of the Flame Describing Function of a Lean-Premixed Swirl-Stabilized Flame

The prediction of thermo-acoustic instabilities is of paramount importance for gas-turbine combustion systems to meet the emission and efficiency targets. To predict the frequencies and amplitudes of pressure oscillations, and as a consequence, their impact on engine reliability, the nonlinear behavior of the system should be studied. An atmospheric lean-premixed combustor has been numerically investigated for validation purposes. A decoupled approach, separating the acoustic behavior and the nonlinear flame response, which is represented in this work by the flame describing function, has been used. As the first step, the flame describing function calculation from large-eddy simulations has been validated, including the effects of flame stretch and heat loss into the turbulence combustion model. Then, frequency-domain calculations using a three-dimensional Helmholtz solver have been carried out, and results have been validated against experimental measurements and a self-excited large-eddy simulation.

Effects of the external turbulence on centrally-ignited spherical unstable Ch4/H2/air flames in the constant-volume combustion bomb

In the present study, we conducted experiments to investigate the effects of external turbulence on the development of spherical H2/CH4/air unstable flames developments at two different equivalence ratios associated with different turbulent intensities using a spherical constant-volume turbulent combustion bomb and high speed schlieren photography technology. Flame front morphology and acceleration process were recorded and different effects of weak external turbulent flow field and intrinsic flame instability on the unstable flame propagation were compared. Results showed the external turbulence has a great influence on the unstable flame propagation under rich fuel conditions. For fuel-lean premixed flames, however, the effects of external turbulence on the morphology of the cellular structure on the flame front was not that obvious. Critical radius decreased firstly and then kept almost unchanged with the augment of the turbulence intensity. This indicated the dominating inhibiting effect of flame stretch on the turbulent premixed flame at the initial stage of the flame front development. Beyond the critical radius, the acceleration exponent was found increasing with the enhancement of initial turbulence intensity for fuel-lean premixed flames. For fuel-rich conditions, however, the initial turbulence intensity had little effect on acceleration exponent. In order to evaluate the important impact of the intrinsic flame instability and external turbulent flow field for spherical propagating premixed flames, intrinsic flame instability scale and average diameter of vortex tube were calculated. Intrinsic flame instability scale decreased greatly and then stayed unchanged with the propagation of the flame front. The comparison between intrinsic flame instability scale and average diameter of vortex tube demonstrated that the external turbulent flow filed will be more important for the evolution of wrinkle structure in the final stage of the flame propagation, when the turbulence intensity was more than 0.404 m/s.

The characteristics of flame propagation in ammonia/oxygen mixtures.

As a carbon free fuel and a hydrogen-energy carrier, ammonia is expected to be one of the promising energy carriers and to be widely used in industries. In this study, the parameters of ammonia/oxygen flame, such as laminar flame speed, the Markstein length, laminar flame thickness, and critical radius of flame instability onset have been investigated experimentally and numerically. A spherical ammonia /oxygen flame propagated in a constant volume chamber and a high speed digital Schilieren photograph system were used in the study. The influences of initial pressure and equivalence ratio on flame propagation have been investigated. It was found that the maximum laminar flame speed in ammonia/oxygen mixture is 1.09 m/s. The flame thickness decreases with the initial pressure increase. Generally, Markstein length increases with the increasing of equivalence ratio while it decreases with the increasing of initial pressure. The minimum critical radius in ammonia/oxygen is 1.8 cm. And the critical radius decreases with the increase of initial pressure. The mechanisms of flame instability in ammonia/oxygen mixture were analyzed, and the stabilize effect of flame stretch in ammonia/oxygen have been observed.

Flame morphology and self-acceleration of syngas spherically expanding flames

The self-acceleration of spherically expanding flames were investigated using a constant volume combustion chamber for CO/H2/O2/N2 mixtures over a wide range of initial pressure from 0.2 to 0.6 MPa, CO/H2 ratio from 50/50 to 10/90 and equivalence ratio from 0.4 to 1.5. The adiabatic flame temperature was kept constant by adjusting O2/N2 ratio at different equivalence ratios. Schlieren images were recorded to investigate the flame front evolution of spherically expanding flames. Local acceleration exponents were extracted using a proper equation to study the process of flame self-acceleration. Results show that the flame cells develop on the smooth flame fronts and finally reach fractal-like structures due to the hydrodynamic and diffusional-thermal instabilities, resulting in flame self-accelerative propagation. The critical Peclet number corresponding to the onset of self-acceleration, Pecr increases nonlinearly with the Markstein length, Ma. The observation further reveals that the onset of self-acceleration is mainly controlled by the diffusional-thermal effect. There exists two distinct flame propagation regimes in the self-acceleration, namely quick transition accelerative and quasi self-similar accelerative regimes. The quick transition regime is controlled by the destabilization effect of hydrodynamic perturbation and stabilization effect of flame stretch. While the quasi self-similar regime is primarily affected by the cascading process of flame front cells controlled by hydrodynamic instability. The self-similar acceleration exponent, αs varies with the initial pressure and Lewis number, Le. The values of αs are measured to be 1.1–1.25 (smaller than 1.5), indicating the flame dose not attain self-turbulization.

Unsteady propagation of premixed methane/propane flames in a mesoscale disk burner of variable-gaps

Unsteady flame propagation of air-premixed methane and propane flames was investigated in a new mesoscale disk burner, of which disk-gap could be precisely varied. To begin with, the quenching disk-gaps on the flammability limits were measured. In most cases, with the slight increase of the disk-gap, cellular flame structures could be generated. The initiation of such cellular structures could be explained by the thermally induced hydrodynamic instability, and it could be enhanced if the Lewis number was sufficiently small. When the disk-gap was sufficiently larger than a critical value (approximately 1.5 times the quenching distance), the cellular flame structure changed into a smooth one in the azimuthal direction. With a further increase of the disk-gap, the flame propagation velocities approached to constant values. These values were comparable to the laminar burning velocities except for the propane-rich conditions, in which much larger propagation velocities were observed. The flame stretch effects (coupled with Le-effects) within a narrow space were suspected as the reason. The structural transition of the premixed flame could be investigated successfully through various disk-gaps, from the smallest quenching-scale to the ordinary large scales via whole mesoscales including Hele–Shaw scales.

Effect of Le on criteria of transition to secondary acoustic instability of downward-propagating flame in a tube with controlled curvature induced by external laser

An experimental study is conducted to investigate the effect of Le on the transition to secondary acoustic instability when the curvature of the flame front in a tube is induced and controlled by using external laser irradiation. Once a downward-propagating flame in the primary acoustic instability region is exposed to a specific laser irradiation condition, the flame is transferred to the secondary acoustic instability region. The transition limit is decreased, that is, transition occurs is an easier manner, with increasing laser power input. While the flame propagates with increasing laser irradiation, the flame first exhibits a convex curvature owing to laser irradiation and then a concave structure is formed owing to buoyancy-induced flow. Two types of transition behavior caused by the concave structure and the convex structure are observed. The conflicting thermal-diffusive effect depending on Le leads to the differing transition behaviors. Based on an evaluation of the flame stretch effect attributed to the flame front curvature, it is confirmed that the Lewis number effect influences the transition criteria.

Laser-induced spark ignition for DME–air mixtures with low velocity

This work studied laser-induced spark ignition for dimethyl ether (DME)–air mixtures with low velocities ranging from 10 cm/s to 100 cm/s, with an emphasis on the minimum ignition energy (MIE) and flame kernel development. Part of ignition experiments were conducted in a microgravity environment that was simulated in a laboratory-scale drop tower to eliminate natural convective flow. MIE was significantly affected by flow velocity. MIE first decreased with increasing flow velocity but then increased with further increasing flow velocity. The role of the flowing mixture was to act as a fuel supplier in a low-velocity flow, and in contrast, to increase convective heat loss in a high-velocity flow. The competition between these roles determined MIE. The effect of flow direction on MIE was also investigated by inclining the combustion chamber. The minimum value of MIE was the same in all flow directions. Therefore, changing flow directions was merely intended to vary the flow velocity by adding or removing buoyant flow to or from the forced flow, which caused the MIE distribution to shift from side to side. The flame kernel hardly developed against a forced flow and was often locally quenched. Flame deformation was noticeable as flow velocity increased. Quenching is comparable to the heat loss from the flame kernel. Therefore, flame kernel development should have a significant effect on MIE. Flame stretch was also found at a high velocity. A high-velocity flow did not allow the flame kernel to stably develop into a self-sustaining flame. Thus, higher energy was required to form a large-volume of flame kernel to overcome both conductive heat loss and the effect of flame stretch. Accordingly, flame kernel behaviors were very much a concern regarding the mechanism via which to determine MIE.

Experimental and Kinetical Study of Component Volumetric Effects on Laminar Flame Speed of Acetone–Butanol–Ethanol (ABE)

Acetone–Butanol–Ethanol (ABE), as a clean and economical alternative transportation fuel, its component ratio has been reported to have a significant impact on spray combustion characteristics. To fundamentally clarify the interactions between components in ABE, the laminar flame speeds (LFS) and stretch effect of ABE mixtures with component ratios (6:3:1, 3:6:1, 1:6:3) and their individual components under a wide range of equivalence ratios (0.8–1.6) at 1 bar, 400 K, were measured, and the chemical kinetics were further analyzed. The results indicate that the LFSs of the three pure fuels follow the order of ethanol > n-butanol > acetone, while ethanol only propagates marginally faster than n-butanol. The LFSs of the three ternary mixtures distribute in between that of the ethanol and acetone. In addition, their LFSs follow the order of ABE631 < ABE361 < ABE163. The comparison among all the tested fuels shows that the addition of ethanol or n-butanol improves burning velocity, while acetone suppresses it. It is noteworthy that ABE163 exhibits nearly the same LFS as pure n-butanol does, but with a lower Markstein length. The reason is that the Markstein lengths of acetone flames are the lowest among the three pure fuels at all equivalence ratios. The sensitivity analysis shows that, in addition to the commonly important methyl radical (CH3), the kinetics of the ketenyl radical (HCCO) also plays an indispensable role in reducing the global reaction rate of acetone and ABE631. Judging from the sensitivity coefficients and the concentration distribution of key species, the combustion characteristics of the three ternary mixtures are the combinations of that of the three individual components, moreover, determined by the dominant component.

空気流速の周期変動に対する伸長円筒状拡散火炎の応答特性（速度変動振幅が火炎応答に及ぼす影響）

An experimental study investigated the frequency characteristics of a stretched cylindrical diffusion flame in response to sinusoidal air flow velocity oscillation. The flame has a convex curvature with respect to the air stream. The fuel was methane diluted with nitrogen, and oxidizer air. The flame was formed in the air stream side from the stagnation surface. The oscillation frequency f ranged from 10 Hz to 250 Hz. In order to clarify the influence of an oscillation amplitude A of the air flow velocity on the flame responses, three kinds of amplitude were chosen. The flame was photographed by the high speed video camera, and flame radius rf, flame thickness δ, and flame luminosity Lf were analyzed. The air flow velocity va at the nozzle outlet was measured by using particle image velocimetry. Though velocity gradient of the air flow ga responds quickly to the changing va regardless of f and A, the gradient of the fuel flow gf delays with increasing f and A. Here, ga correlates to the flame stretch effect, and gf impacts the fuel transportation. Response of rf to varying va also delays with increasing f and A. δ/rf, which relates to the flame curvature effect, responds complicated with the change in f and A. Oscillation amplitudes of Lf have maximum values with respect to f regardless of A. These maximum values are greater than those of a steady flame, over the same air velocity fluctuation ranges. This complex change in the Lf with respect to f is closely related to the phase difference in the respective time variations in δ/rf and gf, and the magnitude of these values.

Radiation Emissions from Turbulent Diffusion Flames Burning Vaporized Jet and Jet-like Fuels

This study measured and compared the size and radiation emissions from turbulent diffusion flames burning large hydrocarbon fuels, including jet fuels. This effort is motivated by the need to understand how burning conventional or alternative jet-like fuels changes radiation emissions. Radiative heat transfer is significant because it is can alter pollutant formation and the lifetime of combustion devices. Eleven fuels were evaluated, consisting of traditional and alternative aviation fuels (e.g., Jet-A) and 1–3 component fuels. The flames were burned on a reduced-scale piloted Sydney turbulent diffusion burner with Reynolds numbers (Re) ranging from 7500 to 45 000. The vaporized fuel exited the burner near 300 °C. The radiative heat flux and the radiation intensity were measured using a radiometer (Medtherm, Model 64-0.2-20) and a mid-infrared camera (FLIR, Model SC6700), respectively. The radiant fraction (χR) is reported for the different fuels. χR typically varied by <15% at similar Re for the different flames, despite the aromatic content ranging from 0% to 31%. The χR had a tendency to be relatively insensitive to aromatic content, but was sensitive to the weight percent of hydrogen in the fuel. Differences in the molecular structure of the fuels, even for fuels with similar hydrogen contents, can significantly affect χR. χR values initially increased as the Re was increased, but then decreased monotonically for Re > 20 000. This trend is attributed to competing effects of flame stretch, entrainment, and surface-to-volume ratio on radiation emissions. The peak radiation intensity occurs near the location of the peak heat flux, which corresponds to the visible flame height. Radiation emitted from soot and unburned hydrocarbons is predominantly observed from within the flame sheet; significant variations in radiation emissions was observed between the different fuels. In contrast, emissions from CO2 for all the fuels varied by <10% and were more broadly distributed across the flame. Fluctuations in radiation emissions have a tendency to be greatest near the flame sheet for CO2 and within the flame sheet for soot and unburned hydrocarbons. Total radiation emissions from these flames have a tendency to be dominated by emissions from gaseous combustion products instead of emissions from soot or unburned hydrocarbons.

Effects of Initiation Radius Selection and Lewis Number on Extraction of Laminar Burning Velocities from Spherically Expanding Flames

ABSTRACTExperimental flame trajectories of hydrogen/air and n-butane/air spherically expanding flames were measured over a wide range of equivalence ratios at pressures of 0.1 MPa and 0.05 MPa. An experimental radius range selection method was developed to determine the critical initiation and end radii for extraction of laminar burning velocity. The results showed that both flame radius range selection and nonlinear flame stretch have significant effects on laminar burning velocity extraction. As the selected initiation radius increases, the flame radius range effect increases. Thus, the widest unaffected radius range should be processed using a proper method to reduce the extraction induced uncertainty. Additionally, the extraction method, which reflects a linear relationship between the flame propagation speed and the flame curvature, well fits the experimental data for a wide radius range when Lewis number, Le > 1. And the performances of all tested extraction methods are almost equal when Le < 1, especially for Le slightly smaller than unity. The nonlinear method is still recommended in extraction for very small Le. The critical initiation radius depends strongly on the Lewis number and initial pressure, and increases linearly with the absolute value of Markstein length. At large and small Le, the initiation radius used in measurements should be large enough to ensure reliable extraction results. Furthermore, the critical initiation radius is affected by transition and spark-assisted propagation regimes for Le > 1 and Le < 1, respectively. It is concluded that the proper flame radius range for extraction is narrowed primarily by the strong flame stretch effects, which increase the critical initiation radius for stable flames.

Prediction of Flame Burning Velocity at Early Flame Development Time With High Exhaust Gas Recirculation and Spark Advance

Diluting spark-ignited (SI) stoichiometric combustion engines with excess residual gas improves thermal efficiency and allows the spark to be advanced toward maximum brake torque (MBT) timing. However, flame propagation rates decrease and misfires can occur at high exhaust gas recirculation (EGR) conditions and advanced spark, limiting the maximum level of charge dilution and its benefits. The misfire limits are often determined for a specific engine from extensive experiments covering a large range of speed, torque, and actuator settings. To extend the benefits of dilute combustion while at the misfire limit, it is essential to define a parameterizable, physics-based model capable of predicting the misfire limits, with cycle to cycle varied flame burning velocity as operating conditions change based on the driver demand. A cycle-averaged model is the first step in this process. The current work describes a model of cycle-averaged laminar flame burning velocity within the early flame development period of 0–3% mass fraction burned. A flame curvature correction method is used to account for both the effect of flame stretch and ignition characteristics, in a variable volume engine system. Comparison of the predicted and the measured flame velocity was performed using a spark plug with fiber optical access. The comparison at a small set of spark and EGR settings at fixed load and speed, shows an agreement within 30% of uncertainty, while 20% uncertainty equals ± one standard deviation over 2000 cycles.

Numerical investigation on flame blow-off limit of a novel microscale Swiss-roll combustor with a bluff-body

A micro Swiss-roll combustor with a bluff-body was proposed for the first time. It is demonstrated that this improved design can greatly extend flame blow-off limit by reducing the flame stretch effect. The flame stabilization ability of this new combustor made of quartz, stainless steel (SS) and silicon carbide (SiC) was numerically investigated using a three dimensional, steady state, k-epsilon turbulence model. The results show that at an equivalence ratio of 0.5, the blow-off limits of quartz combustor, SS combustor and SiC combustor are 40 m/s, 50 m/s and 35 m/s, respectively. Comprehensive analyses were conducted under an inlet velocity of 20 m/s. It was revealled that the heat loss rates for quartz, SS and SiC combustors were 153 W, 108 W and 185 W, respectively. The SS combustor has the best heat-recirculation effect, followed by quartz combustor and SiC combustor. The lengths of recirculation zones are 4.35 mm, 3.4 mm and 3.37 mm for SiC combustor, quartz combustor and SS combustor, respectively. In summary, the heat loss rate is the chief factor responsible for the flame blow-off limit of this novel Swiss-roll combustor. It acts together with the heat-recirculation effect and flow-recirculation effect to determine the flame blow-off limit.

Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory

Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially, for highly reactive fuels, boundary layer flashback (BLF) is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task. In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen–air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near-wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.

Laminar flame speed of CO2 and N2 diluted H2/CO/air flames

The laminar flame speeds of H2/CO/air mixtures with N2 and CO2 dilution were experimentally and numerically studied over a wide range equivalence ratios (0.8–4.0) with N2 (dilution ratio 0–150 vol%) and CO2 (dilution ratio 0–66.7 vol%) dilution in the fuels. The experimentation was conducted using the Bunsen flame method incorporating the Schlieren technique, and the corresponding cases were computed using Chemkin-Pro with four different reactions mechanisms, viz, Li-Mech, Frassoldati-Mech, Davis-Mech and Keromnes-Mech. To eliminate the flame stretch effect on the laminar flame speed measurement based on the Bunsen flame method, a non-linear extrapolation approach was proposed and proved to give comparable accuracy with those from the stretch-corrected spherical flame and counterflow flame methods. A comparison between the experimental and numerical results showed that Davis-Mech and Li-Mech provided better estimates of the laminar flame speed. Keromnes-Mech over-estimated the laminar flame speed especially for fuel-rich mixtures. Meanwhile, Frassoldati-Mech under-estimated the laminar flame speed when the H2 content was high and the disparity decreased as the dilution ratio increased. Detailed analysis of the computation results indicated that the accurate prediction of H and OH mole fractions was crucial to the calculation of laminar flame speeds of H2/CO/air mixtures with N2 and CO2 dilution. CO2 exhibited a stronger dilution effect than N2 due to CO2's stronger thermal and chemical kinetic effects in reducing the flame temperature and radical concentrations. The preferential importance of the thermal, direct reaction and the third body effects caused by the dilution was addressed using numerical computation with two artificially created species. The results revealed that the thermal effect, i.e., the flame temperature reduction by the dilution, dominated the reduction of laminar flame speed, especially under high dilution conditions.

Extinction characteristics of ammonia/air counterflow premixed flames at various pressures

Ammonia has promising features as a carbon-free fuel without greenhouse gas emission. However, due to its low combustion intensity, the combustion characteristics of ammonia have been rarely investigated. The design of ammonia based industrial applications requires the development of effective turbulent ammonia combustion models. Thus, a study of the flame stretch effect in ammonia/air premixed combustion is necessary. The objective of this research was to study the ammonia flame extinction stretch rate both experimentally and numerically and to investigate the effects of pressure on its extinction characteristics. Experiments were conducted using a counterflow flame burner. Numerical simulations were run on CHEMKIN-PRO, using the PREMIX opposedflow model, for various detailed chemistry mechanisms. The effects of pressure on the extinction stretch rate of ammonia/air premixed flame were compared with that of methane/air flame, and the effects of pressure on the detailed reaction paths were clarified. It was found that the extinction stretch rate of ammonia/air flame is low compared with that of methane/air flame, as could be expected from its low laminar burning velocity. However, the increase of extinction stretch rate with pressure was found to be greater in the case of ammonia/air flame. From detailed chemistry analysis, it was found that the different dependence on pressure of the reaction path of the two fuels could explain this difference. Indeed, the heat release process and flame strength are affected by a greater dependence on pressure of the reactions contributing the most to heat released in the case of methane/air flame. For methane/air flame, CH3 is consumed in the main heat releasing reactions, and they experience competition by the third body recombination, 2CH3+M⇋C2H6+M at high pressure. In the case of ammonia/air flame, the heat release process is mainly related to NH3+OH ⇋ NH2+H2O, NH2+OH ⇋ NH+H2O and NH2+NO⇋N2+H2O, which remain preponderant when pressure increases. Thus, the decrease of characteristic reaction time with pressure was found to be greater in the case of ammonia/air flame, explaining a larger increase in extinction stretch rate when pressure increases.

Nonlinear analysis of an acoustically excited laminar premixed flame

The nonlinear effects of the fully coupled two-way interactions between a disturbance field defined by an amplitude variation of the inflow velocity distribution and a laminar premixed flame incorporating gas expansion effects are investigated by numerically solving the conservation equations of a compressible fluid. Results of the higher harmonics of the flame front perturbation for two burnt to unburnt temperature ratios show how the nonlinear transfer of flame response is affected by the hydrodynamic instability and the shear layer effect. The flame response at the fundamental forcing frequency, i.e., the flame-describing function can be well predicted by the reduced-order model (ROM) for both investigated temperature ratios at low Markstein length values, where the balance between the flame stretch effects and the hydrodynamic instability is insignificant. However, at high Markstein lengths the balance between the flame stretch and the hydrodynamic instability influences the flame response characteristics at all excitation amplitudes. The nonlinear study of the flame front perturbation indicates that differences in the higher harmonic content of the overall nonlinear flame response between the detailed computation and the ROM predictions are located at the flame front just before the flame pockets separate. Thus, the shear layer and the hydrodynamic instability effect determine the nonlinear character of the flame response through the nonlinearity introduced at the flame base, the location of flame pocket separation, and the annihilation process at the flame tip especially at high Markstein length values. In addition, the flame base movement response is compared to a recent developed reduced-order flame base model to understand the findings with respect to the assumption of flame attachment at the burner lips.